Polychelating agents for image and spectral enhancement (and spectral shift)
This is a continuation of co-pending application Ser. No. 06/799,757 filed on Nov. 18, 1985, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to image-enhancing agents, contrast agents or spectral shift agents to enhance tissue or organ images or nuclear spectra obtained from live animals with radioisotope scanning or NMR imaging or spectroscopy. 
     The imaging of internal structures and organs of live animals has been an important aspect of medicine since the advent of X-ray usage for this purpose. Among the techniques more recently developed for such imaging are those involving scanning for emission of particles from an internally located radioisotope. Such radioisotopes preferably emit gamma particles and are generally isotopes of metallic elements. One problem common to the diagnostic usage of such gamma particle-emitting radioisotopes concerns the localization of these materials at sites of particular interest rather than to have them randomly dispersed or rapidly excreted, by the kidney, for example. Another problem of such radioisotope mediated imaging concerns optimizing the circulating half-life of radioisotopes. 
     NMR intensity and relaxation images have been shown in recent years to provide another method of imaging internal structures and organs of live animals. Clinical magnetic resonance Imaging (MRI) is a rapidly growing, new form of brain and body imaging. Low-field (proton) MRI detects chemical parameters in the immediate environment around the protons of body tissues (predominantly water protons because of their relative abundance). Changes in these parameters occur very early in disease and are independent of physical densities detected by ionizing radiation. In the brain and central nervous system, MRI has allowed detection of tumors at an earlier clinical stage and with fewer imaging artifacts than is possible with computerized axial tomography (CAT) (Runge et al., (1983) Am. J. Radiol V 141, p 1209). Under optimal conditions, image resolution is in the submillimeter size range. 
     Six factors make it important to develop nontoxic MRI image-enhancing agents analogous to those available for CAT. 1. They increase the specificity of MRI diagnosis. 2. Smaller lesions can be identified earlier. 3. Image-enhancing agents enhance tumor masses differently than surrounding edema fluid or abscesses. This allows the extent and invasion of tumors to be defined more precisely. Lesions with infiltrative-type growth (e.g., certain metastatic carcinomas and glioblastomas) will require contrast agents for demarcation between tumor and edema fluid (Felix et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 831). 4. Image-enhancing agents improve the distinction between recurrent tumor and fibrous tissue resulting from surgery and radiation. 5. Image-enhancing agents can decrease the time required per scan and potentially decrease the number of scans required per procedure. This increases the volume of procedures and decreases their expense; and 6. Body imaging has a significantly lower resolution (typically 0.5-1.0 cm) and sensitivity (decreased signal-to-noise ratio) than brain imaging (Wesbey et al (1983) Radiology V 149, p 175). These differences result from the greater inhomogeneity of the magnetic field; the larger radiofrequency coil; unequal phase-pulsing of deep versus shallow nuclei; and motion artefacts produced by respiration, cardiac systole, gastrointestinal peristalsis, and voluntary muscle movement. 
     The discrete intensities of a two-dimensional, Fourrier-transformed image are described by the following general equation (for spin-echo pulse sequences): 
     
         Intensity=N(H)·f (v)·exp(-TE/T2)·(1-exp(TE-TR/T1), where: 
    
     N(H)=number of protons in the discrete tissue volume (spin density); 
     f(v)=a function of proton velocity and the fraction of protons which are moving (e.g., due to following blood); 
     TE=time between the radio frequency (rf) pulse and the detection of signal (spin-echo); 
     TR=the interval between repetition of the rf pulse. 
     T1=the time interval associated with the rate of proton energy transfer to the surrounding chemical environment (spin-lattice relaxation); 
     T2=the time interval associated with the rate of proton energy transfer, one to other (spin-spin relaxation). 
     T1 and T2 times have reciprocal effects on image intensity. Intensity is increased by either shortening the T1 or lengthening the T2. Tissue contrast occurs naturally and is related to variations in the chemical environments around water protons (major contributor) and lipid protons (usually minor). Chemical agents have been used to enhance this natural contrast. The one most widely tested clinically is the paramagnetic metal ion, gadolinium (Gd +3 ) (Runge et al. (1983) Am. J. Radiol V 141, p 1209 and Weinman et al. (1984) Am. J. Radiol V 142, p 619). Although gadolinium shortens both the T1 and T2  times, at the low dose used for clinical imaging, the T1 effect generally predominates and the image becomes brighter. Also, the rf pulse sequence can be programmed to accentuate T1 changes and diminish those due to T2 (Runge et al. (1983) Am. J. Radiol V 141, p 1209). Hence, &#34;T1-weighted&#34; enhancement can be achieved by selecting the most favorable Gd dose and pulse sequence. 
     The shortening of proton relaxation times by Gd is mediated by dipole-dipole interactions between its unpaired electrons and adjacent water protons. The effectiveness of Gd&#39;s magnetic dipole drops off very rapidly as a function of its distance from these protons (as the sixth power of the radius) (Brown (1985) Mag. Res. Imag. V 3, p 3). Consequently, the only protons which are relaxed efficiently are those able to enter Gd&#39;s first or second coordination spheres during the interval between the rf pulse and signal detection. This ranges from 10 5  to 10 6  protons/second ((Brown (1985) Mag. Res. Imag. V 3, p 3). Still, because Gd has the largest number of unpaired electrons (seven) in its 4f orbital, it has the largest paramagnetic dipole (7.9 Bohr magnetons) and exhibits the greatest paramagnetic relaxivity of any element (Runge et al. (1983) Am. J. Radiol V 141, p 1209 and Weinman et al. (1984) Am. J. Radiol V 142, p 619). Hence, Gd has the highest potential of any element for enhancing images. However, the free form of Gd is quite toxic. This results in part, from precipitation at body pH (as the hydroxide). In order to increase solubility and decrease toxicity, Gd has been chemically chelated by small organic molecules. To date, the chelator most satisfactory from the standpoints of general utility, activity, and toxicity is diethylenetriamine pentaacetic acid (DTPA) (Runge et al. (1983) Am. J. Radiol V 141, p 1209 and Weinman et al. (1984) Am. J. Radiol V 142, p 619). The first formulation of this chelate to undergo extensive clinical testing was developed by Schering-Berlex AG according to a patent application filed in West Germany by Gries, Rosenberg and Weinmann (DE-OS 3129906 A 1 (1981). It consists of Gd-DTPA which is pH-neutralized and stabilized with the organic base, N-methyl-D-glucamine (meglumine). The Schering-Berlex agent is nearing completion of Phase II clinical testing at selected centers across the United States and abroad. The results of preliminary studies indicate that almost all human brain tumors undergo significant enhancement (Felix et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 831 and K. Maravilla, personal communication). These include metastatic carcinomas, meningiomas, gliomas, adenomas and neuromas. Renal tumors are also enhanced satisfactorily (Lanaido et al. (1985) Proc. Soc. Mag. Res. Med. V2, p 877 and Brasch et al. (1983) Am. J. Radiol. V 141, p 1019). The Schering-Berlex formulation is projected to be available for general clinical use by late 1985. 
     Despite its satisfactory relaxivity and toxicity, this formulation has four major disadvantages. 
     (1) Chelation of Gd markedly decreases its relaxivity (by 1/2 an order of magnitude). This happens because chelators occupy almost all of Gd&#39;s inner coordination sites which coincide with the strongest portion of the paramagnetic dipole (Koenig (1985) Proc. Soc. Mag. Res. Med. V 2, p 833 and Geraldes et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 860). 
     (2) Gd-DTPA dimeglumine, like all small chelates, suffers a marked decrease in relaxivity at the higher proton Larmor frequencies used for clinical imaging (typically 15 Mhz) (Geraldes et al. 1985) Proc. Soc. Mag. Res. Med. V 2, p 860). 
     (3) Due to its low molecular weight, Gd-DTPA dimeglumine is cleared very rapidly from the bloodstream (1/2 in 20 minutes) (Weinman et al. (1984) Am. J. Radiol V 142, p 619). This limits the imaging window (to ca. 30 minutes); limits the number of optimal images after each injection (to ca. 2); and increases the agent&#39;s relative toxicity. 
     (4) A disproportionate quantity (&gt;90%) of Gd-DTPA is cleared by the kidneys Weinman et al. (1984) Am. J. Radiol V 142, p 619). Of much greater interest to MRI, are the abdominal sites involved in the early detection and staging of tumors (particularly the liver, and also the spleen, bone marrow, colon and pancreas). 
     Three approaches have been taken in attempts to overcome these disadvantages. 
     (1) Alternative, small chelating molecules have been tested. These make Gd more accessible to water protons but still chelate the metal with a sufficient affinity to potentially control its toxicity in vivo. The most effective of these chelators is DOTA, the polyazamacrocyclic ligand, 1,4,7,10-tetraazacyclododecane-N,N&#39;,N&#34;-tetraacetic acid (Geraldes et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 860). Its relaxivity is approximately 2 times greater than that of Gd-DTPA over a wide range of Larmor frequencies. However, it is still less active than free Gd. 
     (2) Gd and Gd-chelates have been chemically conjugated to macromolecules, primarily the proteins, albumin (Brelman et al. (1981) Health Physics V 40, p 228 and Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11), asialofetuin (Brelman et al. (1981) Health Physics V 40, p 228), and immunoglobulins (Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11 and Brady et al. (1983) Soc. Mag. Res., 2nd Ann. Mtg., Works in Progress, San Francisco, Calif.). This increases the relaxivity of Gd by slowing its rate of molecular tumbling (rotational correlation time) (Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11). This improves coupling of the energy-transfer process between protons and Gd (Geraldes et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 860, Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11 and Brown et al. (1977) Biochemistry V 16, p 3883). Relaxivities are increased by multiples of 5 to  10 relative to Gd-DTPA (when compared as 1/T1 values at 1 millimolar concentrations of Gd) and by multiples of 2.5 to 5.0 (when compared as the molarities of Gd required to produce a specified decrease in the T1 relative to a control solution (physiologic saline). 
     The reasons for using the latter method of comparison are that 1) millimolar concentrations of Gd are never achieved in vivo--actual tissue concentrations achieved in the usual image enhancement are between 5 and 30 micromolar Gd; 2) the second method allows agents to be compared according to the more customary means of chemical activity radio, in other words, as the concentration required to produce a specified percentage decrease in the T1 (or T2) relaxation time. This latter method is the one used throughout the remainder of the application. The large drawback of conjugating DTPA to protein carriers for use in NMR image enhancement is that it has been difficult to stably conjugate more than 5 DTPA&#39;s (and hence Gd&#39;s) to each albumin molecule (Brelman et al. (1981) Health Physics V 40, p 228, Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11 and Hnatowich et al. (1982) Int. J. Appl. Radiat. Isot. V 33, p 327 (1982). 
     Comparably low substitution ratios (normalized for molecular weight) have been reported for immunoglobulins (Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11 and Brady et al. (1983) Soc. Mag. Res., 2nd Arn. Mtg., Works in Progress, San Francisco, Calif.) and fibrinogen (Layne et al. (1982) J. Nucl. Med. V 23, p 627). This results from the relative difficulty of forming amide bonds, the comparatively low number of exposed amino groups on typical proteins which are available for coupling, and the very rapid hydrolysis of DTPA anhydride coupling substrate which occurs in the aqueous solvents required to minimize protein denaturation during conjugation (Hnatowich et al. (1982) Int. J. Appl. Radiat. Isot. V 33, p 327 (1982) and Krejcarek et al. (1977) Biochem. Biophys. Res. Comm. V 77, p 581). The overall effect of these suboptimal conditions is that a large dose of carrier material is required to achieve significant in vivo effects on MR images. Indeed, low substitution ratios have generally limited the use of such protein-chelator-metal complexes to the more sensitive, radiopharmaceutical applications (Layne et al. (1982) J. Nucl. Med. V 23, p 627). 
     (3) Gd-DTPA has been entrapped in liposomes (Buonocore et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 838) in order to selectively enhance images of the reticuloendothelial organs (liver, spleen and bone marrow) and potentially the lungs. Liver clearance is mediated by phagocytic (Kupffer) cells which spontaneously remove these small (0.05 to 3 um) particles from the bloodstream (Buonocore et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 838). (Particles larger than 3 to 5 um are selectively localized in the lungs due to embolic entrapment in lung capillaries.) A recent report indicates that the small-sized Gd-liposomes produce effective decreases in liver T1&#39;s (as determined spectroscopically without imaging) (Buonocore et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 838). Also, insoluble Gd-DTPA colloids have recently been reported to enhance MR images of rabbit livers under in vivo conditions (Wolf et al. (1984) Radiographics V4, p 66). However, three major problems appear to limit the diagnostic utility of these devices. The multilamellar, lipid envelopes of liposomes appear to impede the free diffusion of water protons into the central, hydrophobic cores of these carriers, as assessed by the higher doses of Gd required for in vitro relaxivities equivalent to Gd-DTPA dimeglumine (Buonocore et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 838). This increases the relative toxicity of each Gd atom. 
     Even more importantly, these same lipid components cause the carriers to interact with cell membranes of the target organs. This leads to a marked prolongation of tissue retention (with clearance times of up to several weeks) (Graybill et al. (1982) J. Infect. Dis. V 145, p. 748 and Taylor et al. (1982) Am. Rev. Resp. Dis. V 125, p 610); and G. Kabala, personal communication). Two adverse consequences result. First, image enhancement does not return to baseline in a timely fashion. This precludes re-imaging at the short intervals (ca. 1-week) needed to assess disease progression and treatment effects. Second, significant quantities of the liposomally entrapped Gd-DTPA may be transferred directly into the membranes of host cells (Blank et al. (1980) Health Physics V 39, p 913; Chan et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 846). This can markedly increase the cellular retention and toxicity of such liposomal agents. The consequences for Gd toxicity have not yet been reported. Protein (albumin) microspheres with entrapped Gd and Gd chelates have been prepared and determined by the present applicant and others (Saini et al. (1985) Proc. Soc. Mag. Res. Med. V 2, p 896) to have only modest effects on T1 relaxivity in vitro. This is because most of the Gd as well as other entrapment materials (Widder et al. (1980) Cancer Res. V 40, p 3512) are initially sequestered in the interior of these spheres and are released very slowly as the spheres become hydrated (with t1/2&#39;s of hours) (Widder et al. (1980) Cancer Res. V 40, p 3512). This phenomenon has been found by the present applicant to markedly reduce the acute (30-to-90-minute) relaxivity of each Gd atom to approximately 1/10th that of Gd-DTPA dimeglumine. Hence, both the quantity of carrier material and the toxicity of Gd are both unnecessarily high. 
     Emulsions of insoluble, gadolinium oxide particles have been injected into experimental animals with significant image enhancing effects on the liver (Burnett et al. (1985) Magnetic Res. Imaging V 3, p 65). However, these particles are considerably more toxic than any of the preceding materials and are inappropriate for human use. Because of the significant disadvantages of existing MR image contrast agents, the present applicant has formulated improved, second-generation prototype agents with reduced toxicity, increased selectivity of organ uptake, as well as a significant potential for enhancing blood flow images. 
     Many of the advantages shown for the present developments concerning NMR image-enhancing agents (also referred to herein as NMR contrast agents or MR (magnetic resonance) contrast agents) are also expandable to other areas. For example, high-field NMR surface-coil spectroscopy of  1  H  13  C,  19  F,  23  Na, and  31  P nuclei in spacially localized tissue volumes is gaining in importance and is starting to be applied experimentally to the noninvasive clinical monitoring of genetic and metabolic disorders; myocardial infarcts and metabolism; brain, liver and tumor metabolism; drug distribution and metabolism; blood flow and tissue perfusion measurements; and temperature monitoring in regional hyperthermia. Gadolinium and related agents can produce characteristic changes in the NMR spectrum of adjacent NMR-susceptible nuclei. These changes include: modulation of peak positions, widths, intensities, and relaxation rates (which affect intensity). Hence, perturbation of spectra by such chemical shift-relaxation agents can be used to localize and identify the source of NMR signals with respect to organ location, tissue compartment (intravascular versus extravascular), cell type within the tissue, and potentially, the specific metabolic pathways within cells which are altered by drugs and disease. Also in certain situations body scanning of radioisotopic emissions is particularly useful in achieving insight into internal structures. Most frequently the emissions scanned are those of metallic radioisotopes emitting gamma particles. The mode of administering these radioisotopic metals may have significant consequences on the internal localization and body half-life of these radioisotopes, leading to increased diagnostic usage of these emission scannings. 
     SUMMARY OF THE INVENTION 
     The present invention includes an image-enhancing or spectral-shift agent comprising a biodegradable, water-soluble polymer, synthetic or derived from natural sources and having repeating hydrophilic monomeric units with a high frequency of amino or hydroxyl groups. This agent also includes chelating agents comprising functional groups bound to an amino or hydroxyl group of the monomeric units. These chelating agents have a formation constant for divalent or trivalent metal cations of at least about 10 8  (and typically &gt;10 13 ) at physiological temperature and pH. This image-enhancing agent is biodegradable to intermediary metabolites, excretable chelates, oligomers, monomers or combinations thereof, all of which have low toxicity. The term &#34;low toxicity&#34; used herein as meaning having little significant toxic effects at usable dosages of the image-enhancing agents. 
     These image-enhancing agents may further comprise a paramagnetic metal, transition element or rare-earth ion for enhancement of the images or spectra arising from induced magnetic resonance signals. As defined herein, the term &#34;metal ions&#34; refers to any of these materials as being capable of forming positively charged ions. 
     Images resulting from scanning of gamma particle emissions may be enhanced when the image-enhancing agent of the present invention comprises a radioisotopic metal, transition element or rare-earth ion emitting gamma particles. 
     The physical conversion of these image enhancing agents into microspheres allows further internal targeting of the image-enhancing agents to organs with phogocytic capabilities (principally liver, spleen and bone marrow). 
    
    
     BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Images of the internal structures of an animal may be obtained by a wide variety of means known to those skilled in the art. In one general field of imaging, the administration of metal, transition-element and rare-earth containing markers is utilized. These markers, because of the physical properties of their metal components, may be used to enhance the quality of images produced by numerous means. 
     Among the image-producing means where metallic transition element and rare-earth markers may be advantageously used are magnetic resonance (MR) imaging and scanning of gamma particle emissions. Foremost as a preferred embodiment of the present invention is the enhancement of images produced by nuclear magnetic resonance (NMR) imaging of whole animals or portions thereof. The terms magnetic resonance (MR) imaging and nuclear magnetic resonance (NMR) imaging are used herein as equivalent terms. 
     The present invention comprises novel ways to entrap paramagnetic metal-chelate complexes in biodegradable, hydrophilic polymeric microcarriers. First, the chelate is chemically conjugated in large numbers to hydrophilic polymers such as long-chain dextrans (1-6 linked, soluble, moderately branched polymers of glucose). These hydrophilic polymers are biodegradable and water-soluble. They are either synthetic or derived from procaryotes or plants and comprise repeating hydrophilic monomeric units having amino or hydroxyl groups. The negatively charged, naturally occurring, repeating hydroxyl groups contribute to the stabilization of binding of positively charged Gd ions, over and above the stability of binding conferred by the covalently conjugated chelators. 
     The image-enhancing agents of the present invention comprise chelates or chelating agents having functional groups bound to an amino or hydroxyl group of the monomeric units of the polymer. These chelating agents are further defined as having formation (stability) constants for divalent or trivalent metal cations of at least about 10 8  and typically greater than 10 13  at mammalian physiological pH and temperatures. 
     The whole image enhancing agents described above are characterized as being biodegradable by mammals to intermediary metabolites or excretable and chelates, oligomers, monomers or combinations thereof, all of which have low toxicity. 
     Chelating agents having the properties described above have then the two basic properties of affinity for divalent or trivalent metals and also the ability to bond to amino or hydroxyl groups. Particularly preferred chelating agents of the present invention include EDTA (ethylenediaminetetraacetic acid) DTPA (diethylenetriaminepentaacetic acid) and DOTA (1,4,7,10-tetrazacyclododecane-N,N&#39;,&#34;,N&#39;&#34; tetraacetic acid). 
     A particularly preferred image-enhancing agent of the present invention comprises dextran polymer and DTPA chelating agent. This particularly preferred agent, when in combination with gadolinium has been found to very effectively enhance internal images arising from induced magnetic resonance signals. 
     The polymer of the image-enhancing agents described herein is preferably a polysaccharide or oligosaccharide and most preferably dextran. Polyamino substances, poly-L-lysine, for example, are usable but not generally preferred because of their net polymeric (closely spaced) positive charges at a physiological pH, although in conceivable circumstances this type of polymer could be desirable. In all cases, the polymers of the present invention should be biodegradable. This term &#34;biodegradability&#34;, as used herein, indicates that internally available mammalian enzymes or conditions lead to the breakdown of the polymer, particularly to an excretable and non-toxic form. Thus non-biodegradable polysaccharides such as cellulose are not preferred for the practice of the present invention. 
     In size, the polymers of the present invention should have molecular weights of between 1,000 and 2,000,000 daltons. A more preferable size range for most uses is between about 40,000 daltons and about 75,000 daltons, this range representing a frequent optimum for the hybrid objectives of, amplifying the relaxivity of each Gd, allowing extravazation and localization of agent in tumors and inflammatory lesions, and of moderately delaying the renal excretion of these polymeric agents relative to lower moleculer weight agents such as Gd:DTPA (dimeglumine). 
     The functional groups of the chelating agents are preferably bound to the monomeric units of the polymer by a covalent linkage, although in certain cases a strong noncovalent bond may be usable. The most preferable covalent bond of chelating agent to polymer is an ester linkage, due to its ease of formation, adequate stability for biological targeting, and optimal susceptibility to enzymatic cleavage for clearance from target cells. 
     A second preferable physical form of the image-enhancing agents of the present invention is one of microspheres. The preferable size range of these microspheres is between about 0.1 um and about 250 um. An NMR image-enhancing agent may be formed into microspheres, either before or after the addition of a paramagnetic metal ion such as that of gadolinium. The resultant microspheres, when administered at diameters less than 3 um to a mammal by intravenous injection have been found to be taken up by organs such as the liver, spleen and bone marrow. Thus these organs, for example, are rendered selectively preferentially able to yield improved images arising from induced magnetic resonance signals. Because of differential organ clearance, microspheres of sizes 0.1 to 3.0 um are preferable for image enhancement of liver, spleen and bone marrow; and microspheres of sizes 3 to 250 um are preferable for image enhancement of lung. 
     Another significant aspect of the present invention may involve the further rapid coupling of chelate-polymer image-enhancing agents themselves to proteins such as hormones, polyclonal or monoclonal antibodies or to a substance which secondarily binds either native or derivatized antibodies or a substance such as biotin (e.g. avidin). This coupling may involve, for example, sodium periodate oxidation of vicinal sugar hydroxyl groups such as those of a polysascharide and reduction of Schiff-bases by cyanoborohydride to related, stable, covalent bonds with protein amino groups. The specific binding characteristics of antibodies, when combined with multiply chelatively bound metal ions may be used to produce specific localization of large numbers of paramagnetic or particle-emitting ions within internal targets of interest, thus amplifying greatly the signal-modulating effects of each specifically localized substance. 
     The image-enhancing agents of the present invention are also usable to enhance images being produced from the scanning of gamma particle emissions. In this usage, the general principles of NMR image-enhancement apply, the major difference being that now the chelated metal ion is a radioisotope which emits gamma particles. Preferable radioisotopic metals include  51  chromium,  68  gallium and  111  indium. 
     A general object of the present invention comprises formulation and use of an image-enhancing agent, most particularly for images induced by magnetic resonance. This image-enhancing agent comprises a chelating agent bound to a water-soluble biodegradable polymer. The agent may be utilized in soluble form or as microspheres. In soluble form the image-enhancing agent, when administered to an animal, is primarily distributed in circulating blood, kidney and especially at sizes of about 100,000 to 500,000 MW, should also have the capacity to exit the vascular compartment in regions of tumors and inflammations and focus these tissue lesions. 
     In small microsphere form, the image-enhancing agent, upon administration by injection into animals, is preferentially cleared by and redistributed to liver, spleen and bone marrow. Upon oral administration, microspheres may be introduced into the gastrointestinal tract for image visualization thereof. 
     The acute enhancement of blood flow images, for example in the heart or cerebral vessels, may be accomplished with the soluble polymeric image-enhancing agent and is even more efficiently performed with the microsphere form. 
     A significant advantage of image enhancement, with polymeric and microsphere chelators, in connection with the marginally toxic metals particularly paramagnetic ones such as gadolinium, is a further reduction of necessary metal dose and decrease in toxicity over that which can be achieved by simple (low molecular weight) chelating agents alone. 
     The relatively rapid biodegradation, accelerated clearance times and resultant shorter re-imaging intervals are particular advantages involved with the present invention. 
     The image-enhancing agents of the present invention, in soluble or microsphere form, are readily reconstituted for animal and patient administration. This reconstitution involves a simple vortex-type mixing, as compared to sonification in detergents used for protein-based microspheres. 
     The image-enhancing agents of the present invention are easily usable in any detection or imaging system involving administration of divalent or trivalent metallic marker ions. The appropriate metal need only be added to the polymer-chelate complex at pH&#39;s consistent with stable chelation binding. 
     The image or spectral enhancing agents of the present invention allow shorter imaging times for satisfactory internal resolutions. Shorter imaging times are generally adequate to produce satisfactory internal images because of the greater signal enhancement and image contrast produced per unit of chelated marker and total agent. 
     The potential for specific location of large numbers of marker metal ions by small numbers of monoclonal antibodies, nonpeptide and peptide hormones and receptor-binding substances tagged with one or more image-enhancing agents is contemplated as a major diagnostic advantage and future use. 
     Additionally, because of the high contrast moderate prolongation of lesional residence times (soluble polymers) and liver, spleen and bone marrow residence times (microspheres) (of several hours versus minutes for small molecular forms) the use of the present image-enhancing agents allows an increased number of serial images to be obtained in the enhanced mode after a single administration of agent. 
     From a chemical point of view, some advantages of the present invention may be summarized as follows. When NMR image-enhancing agents comprise paramagnetic metals such as gadolinium ion, each gadolinium ion exhibits an increased relaxivity for adjacent magnetic nuclei (e.g. protons) and hence gives greater T1 signal enhancement. This increased relaxivity is related to an increased dipolar correlation time of Gd due to slower molecular rotation of polymeric Gd, the hydrophilic polymer (which becomes completely hydrated and allows rapid on-off binding (hence relaxation) of adjacent magnetic nuclei (protons)), and, when microspheres are used, the small microsphere size (which allows access of hydrated magnetic nuclei to virtually all of the chelated paramagnetic ions). 
     The chemically defined nature of preferred chelator-polymer combinations allows ready batch-to-batch uniformity for improved pharmaceutical formulations and a likely greater ease of FDA approval. 
     Many preferred components of the present invention, such as certain dextrans (40,000 and 70,000 MW forms) DTPA and Gd, for example have already separately achieved preliminary FDA approval. 
     The following examples are presented to illustrate preferred embodiments of the present invention and their use in MR imaging. These examples are not intended to limit the scope of the present invention in any way unless otherwise so stated in the claims later appended hereto. 
     EXAMPLE 1 
     Preparation of DTPA-Dextran 
     A. Conjugation In Aqueous Solvent 
     The cyclic dianhydride of DTPA, prepared by the method of Eckelman et al. (J. Pharm. Sci. V 64, pp 704-706 (1975)), was obtained in a highly pure form from Calbiochem-Behring Corp. 6.0 g of the cyclic dianhydride was added stepwise to 1.72 g of Dextran T70 (average MW 70,000 daltons, Pharmacia Chemicals) in a reaction solvent comprising HEPES buffer 115 mg/100 cc distilled water, pH 7.0. The reaction was carried out with vigorous stirring at ambient temperatures for a 1 hr period with readjustment to pH 7.0 using NaOH, after each segmental addition of DTPA dianhydride. The dextran-DTPA product was separated from unconjugated DTPA by dialysis against 200 volumes of distilled water at pH 5.5. As assessed by molecular filtration, 83% of the dextran-DTPA product had a molecular weight of less than 100,000 daltons and only 1.3% had a molecular weight greater than 300,000 daltons. 
     Gadolinium in the form of GdCl 3 . 7.05H 2  O (Alfa Laboratories, 2.1 g in 10 cc distilled water was added to 1.38 g of the dextran-DTPA conjugate in distilled water adjusted to pH 5.5 with NaOH. Unbound gadolinium was removed from the dextran-DTPA gadolinium complex by molecular filtration through a 10,000 MW cutoff filter. Free gadolinium was monitored by standard complexometric titrations using xylenol orange (Lyle et al. (1963) Talanta V 10, p 1177) and minimized for each preparation. Alternatively the binding capacity of polymer was determined in advance and the quantity of Gd adjusted to be exactly stoichiometric, leaving neither free Gd nor free polymeric DTPA. This standard complexometric titration was also used to quantify total gadolinium of each preparation after oxidative acid hydrolysis of the organic matrix followed by neutralization of the released Gd. 
     B. Conjugation in Nonaqueous Solvent 
     This was carried out as above, except by suspending the initial reactants in N,N-dimethylformamide with ultrasonification. NaOH was added in the form of powdered pellets at the completion of conjugation, just prior to hydrolyzing any excess unreacted DTPA dianhydride with a 2-fold excess of water (with vigorous stirring and sonification). This resulted in a 2.5-5.0 fold increase in complexing ratios of Gd to dextran T70 and dreased the DTPA dianhydride required by a factor of 10. 
     Dextran-DTPA image-enhancing agents particularly with intrained gadolinium, were produced under a variety of conditions and with different dextrans in various batches. Each batch was lyophilized and, when stored at room temperature, found to be stable in excess of 1 year. Particular batches of dextran-DTPA image-enhancing agents were prepared having molecular weights of 10,000 daltons and 70,000 daltons although the method is usable for a size range of at least from 1,000 daltons to 2,000,000 daltons. 
     As recently prepared in N,N-dimethylformamide, one of every 7.2 sugar residues is conjugated to an active DTPA ligand, for a total of 54 Gd-binding ligands per 389 glucose units. This high derivatization ratio results from the increased ease of forming ester bonds relative to the amide bonds formed in conjugations to primary amines of proteins (see Background). Whereas ester bonds are sufficiently stable to allow initial targeting and tissue +cellular uptake which parallels that of the carrier molecule, these bonds are also more rapidly biodegraded in host cells and serum than are peptide bonds. It is expected that this will decrease toxicity by allowing faster cleavage of Gd-DTPA from the localized carrier, and hence, faster release from these sites of initial localization entrapment) and more rapid clearance of Gd-DTPA from the body by renal excretion. The increased rotational correlation time of the dextran macromolecule and its hydrophilic nature (which allows rapid on-off binding of water protons) amplify the paramagnetic efficiency (relaxivity) of each Gd by a multiple of 4.5. The net negative charge of hydroxyl groups on the glucose residues (which are slightly ionized at physiologic pH 0  contributes to stabilization of Gd+3 binding by electrostatic effects and hence, increases the Gd stability constant to significantly above 10 17 . The combination of these properties cause the dose and toxicity of Gd to be substantially decreased. The high derivatization ratio (Gd-DTPA per dextran) also minimizes the amount of carrier material required for MR image enhancement in vivo. 
     EXAMPLE 2 
     In Vivo Usage of the Image-Enhancing Agents 
     The soluble dextran-DTPA gadolinium complexes described in Example 1, have been injected directly into mice and rats. At the usual doses of 25 to 250 mg/kg, these complexes have a blood clearance t1/2 of ca. 180 minutes. This is anticipated to provide up to a 9-fold increase in the MR imaging window compared to Gd-DTPA. The flexibility exists for coupling DTPA to biocompatible carbohydrate carriers of various molecular weights, ranging from 1,000 to 2,000,000 daltons. By using shorter chain lengths, clearance times could be shortened towards those of Gd-DTPA. This would also increase the propensity of the contrast material to extravasate into tumors and inflammatory lesions. Alternative mono-, di-, oligo- and polysaccharides potentially include alpha, beta and gamma cyclodextrins poly-cyclodextrins, glucose, glycogen, maltose, starch, blood-group oligosaccharides and their derivative amines, mucopolysaccharides and their oligomers, heparins, heparan, heparan-SO 4 , chondroitin-SO 4 , dermatan-SO 4 , and related, natural and synthetic polycarbohydrates and their derivatives. 
     EXAMPLE 3 
     Production and Use of Microspheres 
     The soluble polymer of Example 1 or 2 has also been reformulated as small (0.1-0.5 um) hydrophilic microspheres, by a modification of the method reported by the Applicant in a recent issue of Science (V 227, p 182 (1985)). In summary, this method involved first the emulsification of the dextran-DTPA-Gd complex in an oil such as cottonseed oil. The emulsified complex was then sonicated to produce smaller microspheres. In contrast to the liposomes and colloids discussed earlier herein, these new, very small hydrophilic microspheres allow almost complete access and rapid exchange of water protons to all the Gd throughout the sphere matrix. This is inferred from the experimental finding that the microsphere-Gd and polymer-Gd have almost identical T1 activities in vitro, and the reported finding that increments in Gd relaxivity, which are produced by macromolecular coupling, plateau at macromolecular weights ≧65,000 daltons (Lauffer et al. (1985) Mag. Res. Imaging V 3, p 11). Hence, the slower rotation of microspheres relative to the soluble polymer, is not expected to give any further improvement in the relaxivity of microsphere-Gd over soluble macromolecular Gd (except potentially under flow conditions--see next paragraph). 
     On intravenous injection, the microspheres are cleared spontaneously by the liver, spleen and bone marrow of mice and rats (at a t1/2 of approximately 15 minutes). This selectively enhances NMR images of the preceding organs. Optimal T1 decreases have been obtained in the livers of mice using lower injected doses of Gd (0.01 to 0.02 mmoles/kg) than are normally used for standard contrast enhancement in clinical imaging (Gd-DTPA, 0.10 to 0.30 mmoles/kg). The latter agent produces minimal changes in liver T1&#39;s at the usual 30-minute imaging interval. 
     In rat studies, enhancement of liver images is achieved with microsphere doses 10 to 27 times lower than those required for Gd-DTPA. This significant dose advantage is produced by the combined effects of four design features: the increased rotational correlation time of microsphere-Gd, the improved permeation of water protons into the hydrophilic matrix and rapid on-off binding to (or near) Gd, the extremely small diameters of the microspheres, and the selective uptake of microspheres by target organs. As a result, these microspheres are effective in vivo at the lowest doses of any formulation reported (down to 0.007 mmoles/kg). 
     The following table describes many of the above described results. 
     
         __________________________________________________________________________
CONCENTRATIONS OF TOTAL MATERIAL AND                                      
GADOLINIUM REQUIRED TO PRODUCE A 50%                                      
DECREASE IN THE T1 OF WATER PROTONS (IN VITRO)                            
(IBM PC 20 MINISPECTROMETER, 20 MHz)                                      
                 Material                                                 
                         Gadolinium                                       
                                 Relaxivity                               
                 Concentration                                            
                         Concentration                                    
                                 Multiple*                                
Sample Material  (ug/ml) (m/10.sup.-5)                                    
                                 activity ratio                           
__________________________________________________________________________
Soluble Gd-DTPA dimeglumine (Schering) Microsphere-entapped but           
unconjugated Gd-DTPA (dimeglumine) Soluble Dextran DTPA-Gd polymer (1     
DTPA/22 glucose) (mw = 78,00 daltons)                                     
                  90 65 270                                               
                          ##STR1##                                        
 Soluble Dextran DTPA- Gd polymer (1 DTPA/18 glucose) (MW = 11,000        
daltons) Microspheres formu- lated from the 78,000 dalton Dextran-DTPA-   
Gd polymer        280 300                                                 
                          ##STR2##                                        
Microspheres formu-                                                       
lated from 78,000 dalton                                                  
Dextran-DTPA-Gd polymer                                                   
with 1DTPA/7glucose                                                       
                 80      4.35                                             
__________________________________________________________________________
 *Lower gadolinium concentration indicate a higher .sup.1 H relaxivity per
 Gd atom. A higher relaxivity multiple for T1 corresponds to an increase i
 the MR signal intensity achieved per gadolinium atom (and an increased   
 image intensity obtained in vivo).                                       
 **These lines indicate the relationships producing the ratio.            
 
    
     EXAMPLE 4 
     In Vivo NMR Enhancement of Normal Tissues: Liver, Spleen and Bone Marrow 
     Sprague-Dawley rats were imaged using a 0.35-Tesla, Diasonics clinical MR imaging system and a 30-cm rf coil. Three clinically relevant pulse sequences were used: 1) spin-echo with a TR of 0.5 seconds (for T1-weighted images), 2) inversion-recovery (IR) (for T1-weighted images), and 3) spin-echo with a TR of 2.0 seconds (for T2-weighted images). Diasonics software was used to calculate the area-averaged tissue intensities before and after injection of contrast agents. Dual pulse sequences (spin-echo, with TR&#39;s of 0.5 and 1.5 seconds or 1.0 and 2.0 seconds) were also used to calculate the in vivo T1 relaxation times. At the conclusion of imaging, the liver, spleen and kidneys were excised and their T1 (IR) and T2 (Carr-Purcell-Meiboom-Gill) relaxation times were determined at 37° C., using an IBM pC20 Minispectroneter. For these in vitro experiments, uninjected rats were used in place of preinjection controls. 
     Three contrast materials were compared at equivalent in vitro doses: 1) Gd:DTPA dimeglumine (0.1 mmoles/kg; Schering AG), 2) Gd:DTPA-dextran (78,000 MW) soluble polymer prepared by the Applicant (0.01 mmoles/kg; with an in vitro potency 4.1 times that of Gd-DTPA dimeglumine), and 3) the 0.1-0.5 um hydrophilic Gd:DTPA-dextran microspheres prepared by the Applicant from No. 2 (0.009 mmoles/kg; with an in vitro potency 4.5 times that of Gd:DTPA dimeglumine). Post-contrast images were obtained serially beginning immediately after the i.v. injection of contrast agents and continuing for several hours thereafter. At 30 minutes post-injection, the three enhancing agents decreased image T1 values as follows: 
     
         ______________________________________                                    
% Decrease in T1                                                          
(30 min post vs. pre)                                                     
                  liver                                                   
                       kidney                                             
______________________________________                                    
Gd:DTPA dimeglumine:                                                      
                    24.6   55.7                                           
GD:DTPA-dextran                                                           
soluble polymer     41.7   48.3                                           
Gd:DTPA-dextran                                                           
microspheres        54.4   26.0                                           
______________________________________                                    
 
    
     Image intensities were increased (enhanced) in inverse proportion to the decreases in T1 relaxation times. The organ pattern of T1 changes (Table above) documented selective liver uptake of Gd:soluble polymer but not Gd:DTPA dimeglumine (which was concentrated instead, in the kidney). As predicted, liver uptake was greatest with the Gd:microspheres. Liver enhancement persisted unchanged for 2.5 hours after Gd:DTPA microspheres and Gd:DTPA-dextran soluble polymer, but not after Gd:DTPA dimeglumine. (For this last agent, all liver enhancement was lost after 50 minutes). Thus, optimal selective enhancement of liver was achieved by Gd:DTPA-dextran microspheres at a Gd dose 10-20 times lower than that required for Gd:DTPA dimeglumine agent. These Gd-microspheres also prolonged the interval of image enhancement from minutes to hours. In the post-injection images of animals which received Gd-microspheres and Gd-soluble polymer (but not Gd:DTPA dimeglumine), the image pixels corresponding to bone marrow were comparably enhanced as assessed by visual inspection, however, the number of pixels corresponding to each rat bone was too small to numerically quantify these changes. Rat spleens could not be imaged due to their small size and proximity to liver, however, in vitro T1 changes of the freshly excised organs indicated that the spleens had enhanced T1&#39;s in proportion to those of liver (see next paragraph). Pre-administration T1&#39;s were compared to T1&#39;s from spleens 35 min after administration of the agents. 
     T1 and T2 relaxation times of freshly excised organs (read in an IBM PC20 Minispectrometer) decreased in proportion to those obtained from the imager. T1 changes uniformly exceeded the changes in T2 times. In particular, the normalized in vitro T1 changes in rat spleens were: 
     
         ______________________________________                                    
% Decrease in T1 of Spleen                                                
(35 min post vs. pre)                                                     
______________________________________                                    
1.       Gd:DTPA dimeglumine:                                             
                         14.4                                             
2.       Gd:DTPA-dextran 49.0                                             
         soluble polymer                                                  
3.       Gd:DTPA-dextran 61.2                                             
         microspheres                                                     
______________________________________                                    
 
    
     Hence, combined in vivo and in vitro analyses indicated that Gd:DTPA-dextran microspheres and Gd:DTPA-dextran soluble polymer gave markedly improved enhancement of MR images and/or T1 relaxation in the predicted target organs: liver, bone marrow and spleen. 
     EXAMPLE 5 
     In Vivo NMR Image Enhancement of a Primary Liver Tumor (Hepatoma) 
     Using a direct needle-puncture technique, cell suspensions of the 7777-strain, syngeneic, transplantable, metastasizing Morris hepatoma were injected orthotopically into the right lobes of the livers in 650 gm, Buffalo-strain rats. After two to three weeks the local tumors had reached an average diameter of 0.5 and 1.0 cm. The rats were then imaged both before and after i.v. injections of Gd-DTPA or microsphere Gd-DTPA. MR imaging was performed in a 30cm rf coil with a 0.35 Tesla, Diasonics clinical MRI system (as described above). Post-contrast images were obtained serially beginning immediately after injection and continuing for several hours thereafter. 
     The Gd:DTPA-dextran microspheres produced a selective enhancement of the tumor (by visual inspection) in relation to surrounding normal liver and all other organs of the rat. Tumor enhancement was maximal in the T1 modes (spin-echo with TR&#39;s of 0.5 and 1.0 sec; and inversion recovery) but was also observed in the T2 mode (spin-echo with TR of 2.0 sec). As predicted for the microsphere form of this agent, tumor enhancement became strong at 25 minutes post-injection and persisted unchanged over the 2.5 hour interval of post-injection imaging. Gd:DTPA-dextran microspheres (at 0.011 mmoles/kg) produced image enhancement comparable in intensity to that of Gd:DTPA dimeglumine (at 0.1 mmoles/kg). 
     The major differences between these two agents were dose (Gd-microspheres gave a more homogeneous enhancement with improved demarcation (contrast) between tumor margins and adjacent normal liver), and persistence of contrast (Gd:DTPA-dimeglumine contrast was significantly reduced by 1.5 hours after injection). In vivo quantification of the increase in tumor image intensity was difficult to obtain because of the small volume of tumor tissue and tumor inhomogeneity. However, in vitro T1 measurement performed on the excised tumor and liver at 2.5 hours corroborated the overall tumor enhancement observed in vivo, as follows: 
     
         ______________________________________                                    
Buffalo Rat Tissues In Vitro                                              
T1 (milliseconds)                                                         
        Tumor (hepatoma)                                                  
                     Adjacent normal liver                                
______________________________________                                    
Pre-injection                                                             
          782            330                                              
Post-injection                                                            
          530            320                                              
(2.5 hours)                                                               
______________________________________                                    
 
    
     The percentage decrease in T2 relaxation of tumor tissue post-injection was approximately 2/3 of that observed for T1. The result of enhancement was to brighten the tumor image in relation to surrounding normal liver and other abdominal organs. 
     EXAMPLE 6 
     In Vivo NMR T1 Relaxation Assessment of a Non-Liver Tumor (RIF Sarcoma) Following In Vivo Injection of Gd:DTPA-Dextran Microspheres 
     The selective uptake by a primary liver tumor (hepatoma) of Gd-microspheres at the expense of uptake by surrounding normal liver and other body organs was unexpected but reproducible for the 7777 hepatoma line. This fortuitous result was suspected to be atypical for most sold tumors, due to the general absence in such tumors of phagocytic cells responsible for microsphere-Gd uptake. This was further tested by injecting C3H mice in the legs with syngeneic, transplantable Rif sarcomas, allowing the tumors to grow to 1 cm in diameter, and then injecting the mice with Gd:DTPA-dextran microspheres i.v. at a dose comparable to that used above. Pre- and post-injection (45-min) tumors, livers and kidneys were excised and tested in the IBM PC20 Minispectrometer for effects of Gd-microspheres on T1 relaxation times. The results were as shown below. 
     
         ______________________________________                                    
              T1 of Control                                               
                         % Decrease                                       
Organ/Tissue  (msec)     post-injection                                   
______________________________________                                    
Tumor         804        3.9                                              
Liver         370        20.5                                             
Kidneys       434        7.9                                              
______________________________________                                    
 
    
     Although these latter tumors were in their orthotopic rather than hepatic (liver) locations, the results still suggest strongly that for the usual case of non-primary liver tumors which invade the liver, tumor tissue will, as anticipated, selectively exclude microsphere-Gd, and the surrounding normal liver will relatively concentrate the microsphere agent, leading to enhanced contrast in the reverse pattern from that observed for the preceding hepatoma, namely brighted normal liver surrounded by relatively darker tumor nodules. 
     Advantages of Gd:DTPA-dextran microspheres as an enhancing agent for liver lesions (and also for spleen and bone marrow lesions) include: 
     1. Detection of lesions at smaller (potentially millimeter) sizes; 
     2. Improved demarcation of tumor margins for evaluation of surgical resectability; 
     3. Prolonged enhancement interval (of hours) for performing serial MR images and shortening the time required for each image; 
     4. Administration of the lowest dose of Gd (0.007 to 0.024 mmoles/kg) resulting in production of the most minimal toxicity possible with a liver-specific paramagnetic enhancing agent. 
     EXAMPLE 7 
     Vitro NMR T1 Relaxation Assessment of a Non-Liver Tumor (RIF Sarcorma) Following In Vivo Injection of Two Gd:DTPA-Dextran Soluble Polymers 
     C3H mice, bearing 1 cm transplantable, syngenic Rif sarcomas in their legs (see Examples above), were injected i.v. with two soluble polymeric forms of Gd:DTPA-dextran at a Gd dose of 0.03 mmoles/kg. Tumors, livers and kidneys were excised from pre- and post-injection animals at 60-75 min after injection, and the T1 relaxation times of organs and tumor were determined in the IBM PC20 Minispectrometer for the effects of localized Gd. 
     
         ______________________________________                                    
         MW of                                                            
         Polymer     T1 of Control                                        
                                % Decrease                                
Organ/Tissue                                                              
         (/1000)     (msec)     post-injection                            
______________________________________                                    
1. Tumor 70          804        15.7                                      
         10          &#34;           3.2                                      
2. Liver 70          370         0.6                                      
         10          &#34;           7.7                                      
3. Kidneys                                                                
         70          434        39.7                                      
         10          &#34;          21.2                                      
______________________________________                                    
 
    
     These results indicate that, as predicted for a non-primary liver tumor such as Rif, the larger (70,000 MW) soluble polymeric form of Gd:DTPA-dextran gives the reverse pattern of uptake by tumor and liver relative to that just documented for the Gd-microsphere formulation. (This pattern is not seen with the 10,000 MW polymer due to its relatively rapid renal clearance (see above table). If the Rif tumor were grown in the liver rather than leg of mice, selective uptake of the 70,000 soluble Gd-dextran polymer by intrahepatic Rif tumor would be expected to produce image brightening in the tumor and an unchanged image intensity in the surrounding normal liver (a pattern of enhanced image contrast parallel to that shown above for Gd-microsphere enhancement of liver hepatoma). 
     EXAMPLE 8 
     Enhanced NMR Images of Blood Based on Cardiac Differential Flow Within Chambers 
     Studies were performed indicating that intravenously administered microspheres enhanced T1-weighted blood flow images in the chambers of rat hearts (ungated, 5 minute images), at time up to 20 minutes after injection. Gd:DTPA-dextran microspheres (at 0.03 mmoles Gd/kg) were injected i.v. at time zero into Sprayer-Dawley rats and images were obtained immediately and serially each 5 minutes×4 (spin-echo, multi-echo, TR=0.5 &amp; 1.5). Under normal flow conditions, image enhancement was most prominent in the regions of slower flowing blood adjacent to the endocardial surfaces. However, under conditions of generalized slow flow (induced by co-injecting a polycationic polymer at time o), all portions of the cardiac chambers gave enhanced T-1 weighted blood images. 
     The soluble Gd:DTPA-dextran polymer, injected at a comparable Gd dose, produced analogous but slightly weaker enhancements. The superior performance of microspheres under flow conditions suggests that factors related to flow turbulence are more effectively overcome by particles that by molecular carriers, and by larger molecules than smaller ones. This interpretation is supported by the finding that the very small MW enhancing agent, Gd-DTPA (dimeglumine) was almost completely ineffective. This ineffectiveness held true even when injections were made directly into the heart and imaged immediately with cardiac gating (R. Peshock, unpublished studies). Hence, it appears that the two new contrast agents are the only ones potent enough to produce noninvasive enhancement of blood flow images with the available methods of clinical MR cardiac imaging. 
     EXAMPLE 9 
     Toxicology 
     In toxicologic tests, the LD 40  of Gd-DTPA-dextran microspheres was &gt;900 mg/kg. To put this in perspective, image enhancement is carried out at less than 1/5th to 1/11th of the LD 20  dose, depending on the preparation used. Also, histologic assessment of the major organs excised after MR spectroscopy (in CBA mice) and MR imaging (in Sprayer-Dawley and Buffalo rats) revealed no evidence of acute (30-60min) toxicity. 
     Preliminary subacute toxicologic studies were performed on CBA mice by injecting them at time 0 with Gd:DTPA-dextran microspheres at a dose (250 mg/kg; 0.06 mmoles/kg Gd) which was approximately 2.5 times the standard dose used for imaging procedures. This was followed by minor elevations of the liver enzyme, serum glutamic-oxaloacetic transaminase (SGOT) which peaked on day 3-4 at 140% of the values for upper limits of normal and fell back to nearly the control range by day 7 post-injection. 
     Subacute histologic assessment of the liver (which was assessed by both the Applicant and a specialist in liver pathology, revealed minor zone 1 and 2 changes beginning at 6 hours post injection and comprising slight swelling and vacuolation of hepatocytes. This culminated at day 3-4 in rare single-cell dropout without changes in the quantity or appearance of supporting connective-tissue or portal tracts. These changes largely resolved by day seven. No significant changes in serum creatinine (an indicator of renal function) or renal histology were observed over the 7-day test interval. The mice remained normally active, ate and drank normal quantities of water, and gained weight at approximately the same rate as their uninjected littermate controls. 
     EXAMPLE 10 
     Preparation and Testing of Glycerol-DTPA Copolymer 
     Dried glycerol (0.4 ml, 0.55 mmole) was added to DTPA cyclic dianyhydride (296 mg) suspended (by sonification) in 0.4 ml of dried, N,N-dimethylformamide. This mixed suspension was sonicated for an additional 3 min at 20,000 Hz with a special microtip (Heat Systems, Inc.) and heated for 7 hours at 135° C. to give controlled polymerization, plus an additional 2 hours at 155° C. to drive off the reaction solvent (BP=149°-156° C.). The resulting resin was transferred segmentally with sonication into 60 ml of distilled water (pH 5) and sheared for 20 minutes with a high-speed Waring blender. GdCl 3 .7.05H 2  O (327 mg) was adjusted to pH 5, added dropwise to the DTPA-glycerol resin and again sheared for 3 hours to maximally solubilize the material. The residual larger gel-state material was separate by centrifugation at 250×g for 15 min, and the smaller soluble fraction was saved and separated from residual free Gd by molecular filtration (with 4 washes of distilled water, pH 5.0) through a 1000 MW cutoff filter under pressurized nitrogen. The retentate was saved and centrifuged for an additional 15 min at 1000×g and the supernatant of this was saved and lyophilized 16 hours. 
     Although it had the appearance of a gel, the resulting glycerol-DTPA:Gd copolymer was only minimally cross-linked as determined by molecular filtration, which gave a size range (for 95% of the material) of 1,000 to 10,000 MW, with an estimated average MW of 2,200 MW. 
     In vitro testing for T1 relaxation effects in the IBM PC20 Minispectrometer gave the following result (compare, for example the table of Example 7): 
     
         ______________________________________                                    
Dose Decreasing T1 by 50%                                                 
             total wt (ug/ml)                                             
                        Gd (M/10.sup.-5)                                  
______________________________________                                    
Gd:DTPA-glycerol                                                          
               32           5.0                                           
copolymer                                                                 
______________________________________                                    
 
    
     Thus, on the basis of Gd molarity, Gd:DTPA-glycerol copolymer was 1.9 times as active as Gd:DTPA dimeglumine. 
     In vivo tests were carried out by injecting CBA mice i.v. with 130 mg/kg of Gd:DTPA-glycerol copolymer and determining the effects of T1 relaxation times of organs freshly excised at 30 minutes after agent administration. 
     
         ______________________________________                                    
Control T1       Injected T1                                              
(msec)           (msec)    % Decrease                                     
______________________________________                                    
Liver   339          200       41.0                                       
Kidney  343          223       35.0                                       
______________________________________                                    
 
    
     Thus, Gd:DTPA-glycerol copolymer was considerably more active as a MR enhancing agent for liver than was Gd:DTPA dimeglumine on both a weight and Gd molar basis. Preliminary acute toxicologic studies were very slightly inferior to those of the Gd:DTPA-dextran soluble polymer. 
     EXAMPLE 11 
     Conjunction of Binding Groups to the Gd:DTPA-Dextran Polymers and Microspheres 
     Approximately 50 mg of the dextran-DTPA polymer or 150 mg of particles were suspended in 9.5 ml distilled water with 0.05M NaCl. Sodium periodate (0.05M, 300 ul) was added and the mixture stirred at 22° C. for 30 min. The preparation was washed with distilled water (by molecular filtration, polymer; or centrifugation, microspheres) and brought up in 15 ml distilled water or saline. To the periodate-oxidized preparation were added the materials to be covalently conjugated: antibody, avidin, or biotin hydrazide, at 1-3 mg each, depending on the number of reactive groups on the additive. This mixture was stirred again for 30 minutes, then NaBH 4  (8 mg) was added to reduce the Schiff base (or its equivalent) and stirring was continued for an additional 15 min. The pH was adjusted to 7.5, and the stabilized preparation was washed and resuspended in 1 ml of 0.02M phosphate-buffered in 0.15M NaCl containing 0.25% dextran T70. Microspheres derivatized by this method had between 2500 and 5000 available binding sites per 0.5 um sphere, as assessed by the high-stability specific binding of  125  I-avidin to biotinylated microspheres prepared with biotin hydrazide. 
     This method allows the direct covalent conjugation of antibodies and other receptor binding proteins or peptides via their reactive amino groups; and the indirect coupling of (a) biotinylated antibodies (commercially available) to avidin-derivatized polymer or spheres; or (b) native antibodies to polymer or spheres pre-derivatized with Protein A (Pharmacia chemicals) which binds the F c  region of antibodies at high affinities. 
     EXAMPLE 12 
     Preparation and Testing of Albumin Microspheres Containing the Entrapped, Noncovalently Bound Metal Ion-Chelate Complex, Gadolinium Diethylenetriamine Pentaacetic Acid (Gd:DTPA) 
     A 0.95M solution of Gd:DTPA in dimeglumine (2×N-methylglucamine) salt form (Schering AG, Germany/Berlex Laboratories, Inc., U.S.A.) was added at 0.25 ml to a maximally concentrated solution of human serum albumin (125 mg, Sigma Chemical Co.) in distilled water (0.25 ml). This was stirred for 20 minutes, added dropwise to 30 ml of cottonseed oil (Sargent Welch Scientific), and sheared for 20 minutes with a high-speed Waring-type blender to produce submicron droplets (0.1-0.6 um diameter). This emulsion was added dropwise to a preheated (140° C.) rapidly stirring, 100-ml volume of cottonseed oil, in order to heat denature (stabilize) the albumin matrix and maintain the integrity of particles and entrapment of Gd:DTPA upon subsequent suspension in injection medium. Heating at 140° C. was continued for 10 min with high-speed shearing. The emulsion was cooled to 220° C. with continued mixing. Oil was extracted with 6×60 ml of fresh diethyl ether (containing antioxidant) (Fisher Scientific Co.), and the resulting microspheres were lyophilized for 16  hrs to remove residual ether. Particles ranged from 0.1-0.5 um (diameter) with a mean of 0.3 um (monitored by light and electron microscopy). 
     Microspheres (Gd:DTPA:dimeglumine:albumin) were tested in vitro using a 20 MHz pulsed Nuclear Magnetic Resonance (NMR) spectrometer, for their capacity to reduce the T1 relaxation time of water protons in physiologic saline solution (0.02M phosphate-buffered, 0.15M NaCl). Activity was expressed as the concentration of material required to decease the T1 relaxation time to 50% of the value for phosphate-buffered saline (ID 50 ). Microspheres were suspended at a concentration of 1 mg/ml by brief sonification. Because albumin microspheres have a fast-release (surface) component of Gd:DTPA as well as a controlled-release (interior) component, the spheres were washed, resuspended, and diluted serially for testing. 
     
         ______________________________________                                    
                        ID.sub.50                                         
Material                (total weight)                                    
______________________________________                                    
Unwashed microsphere suspension                                           
                        0.25   mg/ml                                      
Fast-release supernatant                                                  
                        0.30   mg/ml                                      
Washed microspheres     3.8    mg/ml                                      
Gd:DTPA dimeglumine     0.084  mg/ml                                      
______________________________________                                    
 
    
     Microspheres (Gd:DTPA:dimeglumine:albumin) were tested in vivo by injecting them intravenously into 25 gm CBA mice (2 animals per group), allowing 30 minutes for uptake and sequestration by liver Kupffer cells, sacrificing the mice, and testing the excised organs. The acute (30-min) biodistribution was determined by injecting microspheres trace-labeled with  125  I-albumin. Radioisotope was quantified in a standard gamma counter. 
     
         ______________________________________                                    
            125.sub.I counts:                                             
                          125.sub.I counts:                               
            % of total    per gm of                                       
Organ       recovered at 30 min                                           
                          target organ                                    
______________________________________                                    
Blood       7.2           10.8                                            
Spleen      0.8           32.8                                            
Liver       57.6          119.0                                           
Lungs       31.5          369.1                                           
Kidneys     2.9           26.2                                            
Total       100.0                                                         
______________________________________                                    
 
    
     The pattern of uptake by liver and spleen (with moderate acute lung sequestration) is typical of that for small (&lt;3 um) particles. 
     The T1-weighted proton relaxation times of mouse livers were quantified by determining the whole-organ T1 relaxation time in a 20 MHz NMR spectrometer. 
     
         ______________________________________                                    
                 Liver T1                                                 
Injected material                                                         
                 (msec)   % of Control                                    
______________________________________                                    
Saline (0.15M)   332      Control                                         
Albumin microspheres                                                      
                 314      94.5                                            
(45 mg/kg, total wt;                                                      
0.1 mmol/kg Gd)                                                           
Gd:DTPA dimeglumine                                                       
                 327      98.5                                            
(0.1 mmol/kg Gd)                                                          
______________________________________                                    
 
    
     At equivalent doses (normalized to in vitro T1 potency), the formulatin of Gd:DTPA:dimeglumine albumin microspheres was slightly more potent than soluble Gd:DTPA dimeglumine. The high dose of albumin carrier needed to achieve this modest T1-relaxation, makes albumin a suboptimal matrix material for delivering Gd to the liver for applications specifically involving magnetic resonance imaging and specstroscopy. This high dose was necessitated by the marked sequestration of Gd in the interior of microspheres and the very slow release (1/2 in 8 hrs) of Gd:DTPA from spheres which are sufficiently stabilized to give effective liver targeting. 
     EXAMPLE 13 
     Preparation of Gd:DTPA:Diethylaminoethyl Dextran Soluble Polymer and Gd:DTPA:Diethylaminoethyl Microspheres Containing Noncovalently Bound Gd:DTPA (With Strong Ion Pairing Between DTPA and the DEAE Substituents of Dextran); and Unloaded DTPA:Diethylaminoethyl-Dextran Microspheres (Without Chelated Gd) 
     Solution 1. Diethylenetriamine pentaacetic acid, 0.72 gms (DTPA, Sigma Chemical Co.) was dissolved in 2.5 ml distilled water, the pH adjusted to 7.2 with NaOH, mised with GdCl 3 .6H 2  O, 0.34 gms, and the solution readjusted to pH 7.2 and stirred for 20 min to allow complete chelation of Gd. Solution 2. Diethylaminoethyl dextran (DEAE dextran, 500,000 MW with 1 positively charged group per 3 glucose residues, Sigma Chemical Co.) was dissolved by warming a saturated solution of 1 gm in 2.5 ml of distilled water. To prepare the soluble polymeric form of Gd:DTPA:DEAE-dextran, Solutions 1 and 2 were mixed with vigorous stirring; the pH was adjusted to 7.2 and the mixture washed twice with distilled water (20 ml per wash) to remove unbound Gd. The soluble polymer was concentrated to 50-100 mg/ml by molecular filtration under pressurized nitrogen through a 100,000 MW cutoff filter (Amicon Corporation, XM100). To prepare the microsphere form of Gd:DTPA:DEAE-dextran, DEAE dextran was stirred vigorously into 30 ml of cottonseed oil (Sargent Welch Scientific) until an even emulsion was produced. To this was added 1 ml (dropwise) of Solution 1. This emulsion was sonicated for 6 min. (with continuous magnetic stirring) using a 20,000 Hz ultrasonifier with a 3 mm special microtip (Heat Systems, Inc.) to disrupt the aqueous phase into 0.2-0.4 um microdroplets. Microparticles were stabilized and water removed by heating to 120° C. for 20 min with vigorous stirring. After cooling, oil was removed with 3×60 ml of fresh diethyl ether (containing antioxidant) (Fisher Scientific Co.), and the sample lyophilized for 16 hrs. Microspheres ranged from 0.1 to 0.3 um, with a mean diameter of 0.2 um. 
     Unloaded DTPA:DEAE-dextran microspheres. An alternative microsphere formulation was prepared without chelated Gd (or other metal ions), by dissolving DTPA, adjusting the pH to 7.2, and mixing this with a 1-gm solution of DEAE dextran, all prepared as described above. The aqueous phase was emulsified in cottonseed oil and processed as described above. 
     EXAMPLE 14 
     Loading (Chelation) by DTPA:DEAE-Dextran Microspheres of the Paramagnetic Metal Ions, Gd +3  and Fe +3   
     a. Chelation of Gd +3 . DTPA:DEAE-dextran microspheres, 100 mg, from Example 13, were added to 280 mg of GdCl 3 . 6H 2  O dissolved in 2 ml of distilled water and stirred for 30 min. Unbound gadolinium was removed by washing twice with 20 ml of distilled water (pH5.5) using a 300,000 cutoff, 43mm diameter filter (Amicon Corporation, XM300) under pressurized nitrogen. The microspheres were removed from the filter with 2 ml of distilled water and lyophilized to dryness (16 hrs). microspheres ranged from 0.1 to 0.5 um (diameter). 
     b. Chelation of Fe +3 . DTPA:DEAE-dextran microspheres, 100 mg, from Example 13, were added to 350 mg of FeCl 3 . 6H 2  O and processed to remove unbound Fe +3 . Particles ranged from 0.15 to 0.6 um in diameter. 
     EXAMPLE 15 
     In Vitro Testing of Soluble Gd:DTPA Polymers and Gd:DTPA Microspheres Prepared in Examples 13 and 14 
     Test materials were diluted serially and assayed for proton T1 relaxivities using a 20 MHz pulsed NMR spectrometer as described in Example 1. These materials contained very minor components of fast-released Gd and Gd:DTPA chelate (less than 2% of the totals). Thus, it was not necessary to wash and resuspend the materials prior to NMR testing. The ID 50  concentrations were: 
     
         ______________________________________                                    
Material              ID.sub.50 (total weight)                            
______________________________________                                    
Gd:DTPA:DEAE-dextran soluble                                              
                      0.125 mg/ml*                                        
polymer                                                                   
Gd:DTPA:DEAE-dextran  0.160 mg/ml                                         
microspheres                                                              
DTPA:DEAE-dextran microspheres,                                           
                      0.175 mg/ml                                         
loaded subsequently with Gd                                               
Gd:DTPA dimeglumine   0.084 mg/ml*                                        
______________________________________                                    
 *Molar concentrations of Gd = 4.6 × 10.sup.-5 M for the            
 Gd:DTPA:DEAEdextran soluble polymer and 9.0 × 10.sup.-5 M for      
 Gd:DTPA dimeglumine.                                                     
 
    
     The soluble Gd:DTPA:DEAE-dextran polymer was 1.96 times more potent than Gd:DTPA dimeglumine. This improved relaxivity was attributable to strong nonvocalent binding of the negatively charged, DTPA moiety of Gd:DTPA to the positively charged, DEAE substituent groups of dextran polymer. The large size of this polymer (300,00 MW) resulted in a longer rotational correlation time for each noncovalently bound Gd:DTPA and allowed improved transfer of energy from water protons to paramagnetic Gd ions. 
     EXAMPLE 16 
     In Vitro Histologic Staining of Fe:DTPA Microspheres as Prepared in Example 14 
     Fe:DTPA:DEAE-dextran microspheres were suspended at 1 mg/ml in a 70% ethanol-water solution, 10-50 uL aliquots were placed on cytologic glass slides, the microspheres were sedimented at 750×g for 12 min. in a cytocentrifuge, slides were air dried, and microsphere:Fe +3  was stained for histologic analysis by the Prussian blue, acidic ferro-ferricyanide procedure. Dark blue reaction product formed over each microsphere, as assessed by standard light microscopy. Hence, the chelated Fe +3 , which was initially bound to microsphere DTPA at neutral pH, became dissociated sufficiently by the acidic pH of the staining solution to allow histochemical detection in vitro. 
     EXAMPLE 17 
     In Vivo Testing of Soluble Gd:DTPA Polymers and Gd:DTPA Microspheres Prepared in Examples 14 and 15 
     a. Proton T1 relaxation times in mouse organs. 
     Test materials were injected i.v. into 25 gm CBA mice. At 30 min the mice were sacrificed by decapitation (exsanguination) and the excised livers and kidneys assessed for changes in proton T1 relaxation times (20 MHz; IR pulse sequence). Doses of test materials were made equivalent based on in vitro potency (ID 50  analysis). 
     
         ______________________________________                                    
             Dose     T1 (% of control)*                                  
Material       (mmol/kg)  Liver    Kidney                                 
______________________________________                                    
Gd:DTPA:DEAE dextran                                                      
               0.23       69       28                                     
soluble polymer                                                           
Gd:DTPA:DEAE dextran                                                      
               0.23       81       78                                     
microspheres                                                              
Gd:DTPA dimeglumine                                                       
               0.47       83       24                                     
______________________________________                                    
 *The T1&#39;s of control organs were 330 msec for liver and 385 msec for     
 kidney.                                                                  
 
    
     The soluble polymeric formulation of Gd:DTPA:DEAE dextran was the most potent substance for liver (approximately 4 times as potent as Gd:DTPA dimeglumine, which produced a significantly greater decrease in kidney). The microsphere form of Gd:DTPA:DEAE dextran was approximately 2 times as potent in liver as Gd:DTPA dimeglumine. Because of selective organ uptake by the liver, it produced a much smaller effect in kidney. Gd:DTPA dimeglumine was relatively ineffective at decreasing the T1 of liver even at very high doses which produced marked decreases in kidney. (The usual dose of the dimeglumine formulation used for Phase II clinical trials is 0.1 mmol/kg.) 
     b. Proton magnetic resonance imaging the proton T1 relaxation times in rats. 
     Test materials were injected intravenously into 500 gm Sprague-Dawley rats. at approximately 30 minutes, the rats were placed in the head coil of a 0.35 Tesla clinical imaging device (Diasonics Corp.), and T1-weighted images (using both a spin-echo pulse sequence at TR&#39;s of 0.5, 1.5 and 2.0, and an inversion recovery sequence) were obtained on image slices of 0.5 cm thickness taken through the liver and kidneys. T1 relaxation times were determined from the TR=0.5 and 2.0 data using area-averaging of signal intensities and a proprietary software program for calculations. At the completion of imaging, the rats were sacrificed by decapitation (exsanguination). Their livers, kidneys and spleens were excised, tested for in vitro correlations with the in vivo changes in proton T1 relaxation times and then placed in buffered formalin fixative for histopathologic evaluation. 
     The doses of test materials were as follows: 
     
         ______________________________________                                    
                        Gd (mmol/kg)                                      
______________________________________                                    
Gd:DTPA:DEAE dextran soluble polymer                                      
                        0.30                                              
Gd:DTPA:DEAE dextran microspheres                                         
                        0.15                                              
Gd:DTPA dimeglumine     0.30                                              
______________________________________                                    
                  T1 (% of control)                                       
Test Material Organ     In vivo  In vitro*                                
______________________________________                                    
soluble polymer                                                           
              liver     65       64                                       
              spleen     NT**    48                                       
              kidney    77        8                                       
microspheres  liver     63       53                                       
              spleen    NT       43                                       
              kidney    63       23                                       
dimeglumine   liver     83       88                                       
              spleen    NT       86                                       
              kidney    57       25                                       
______________________________________                                    
 *Control proton T1 values were: liver, 275 msec; spleen, 464 msec;       
 kidneys, 492 msec.                                                       
 **Not tested                                                             
 
    
     There was good correlation between in vivo and in vitro T1 relaxation times, except for kidney, in which are a averaging was difficult to perform due to small organ size and differing intensity regions within the kidney between the cortex and medulla. The microspheres formulation produced selective organ enhancement of liver and spleen at a dose approximately 5 times lower than that required for the standard, Gd:DTPA dimeglumine reagent. The proton T1 of bone marrow was also observed to enhance in the magnetic resonance images. 
     *Control proton T1 values were: liver, 275 msec; spleen, 464 mesc; kidneys, 492 mesc. 
     **Splenic images could not be obtained in the rat due to very small organ size and anatomic juxtaposition to the liver. 
     There was a direct correlation between in vivo and in vitro T1 relaxation times for liver. For kidney, the correlations were sporadic due to difficulties in (a) determining average image intensities on these much smaller organs, and (b) obtaining sharp demarcation between the differently enhancing anatomic subregions of kidney (cortex and medulla). Image intensities increased in inverse proportion to changes in T1. Soluble polymeric Gd and microsphere Gd gave preferential image enhancement of liver relative to kidney, as compared to the reciprocal changes obtained for Gd:DTPA dimeglumine. After normalizing for dose and image intensity, microsphere Gd was 5 times more potent for liver enhancement than Gd:DTPA dimeglumine, and soluble polymeric Gd was 2.5 times more potent. Bone marrow was also preferentially enhanced by the soluble polymeric and microsphere Gd. These changes were noted visually but could not be quantified due to the small number of image pixels corresponding to the bones of rats. 
     EXAMPLE 18 
     Histologic Assessment of the Selective Uptake By Liver and Spleen of Microspheres Containing the Paramagnetic Metal Ion, Fe +3   
     Fe +3  :DTPA:DEAE-dextran microspheres (prepared as in Example 14) were injected into a CBA mouse at a dose of 140 mg/kg. Thirty minutes after injection, the animal was sacrificed, and the liver and spleen were excised. The tissues were fixed in formalin and stained using the Prussion blue (acidic ferro-ferricyanide) iron staining technique, to identify cellular locations of microsphere iron. By microscopic evaluation, 0.1-0.6 um (diameter) heavy concentrations (3+/4+) of iron-positive particles were present in Kupffer cells of the liver and sinusoidal macrophages of the spleen. Other parenchymal cells of the liver (hepatocytes) and spleen were negative for iron staining, as were other organs (bone marrow was not tested). These results indicate that the Fe+3:DTPA chelate was bound (noncovalently) with a sufficient affinity to survive in vivo transit through the bloodstream and become cleared acutely as intact particle-associated iron by the phagocytic cells of liver and spleen. This histologic result documents that the preferential MR enhancement of liver images and the preferential T1 changes of splenic water proton relaxation times (above) were caused by the selective uptake of paramagnetically labeled particles by phagocytic cells in the &#34;target&#34; organs. 
     EXAMPLE 19 
     Acute Toxicologic Assessment of Gd:DTPA:DEAE Dextran Soluble Polymer and the Gd:DTPA:DEAE Dextran Microspheres of Example 17 
     The rats of Example 17 which were injected soluble polymeric and microsphere Gd noncovalently bound (ion) paired) to DEAE dextran, developed mild-to-moderate respiratory distress between 90 and 120 minutes after the injection of test materials. Based on these observations, histologic evaluation was performed on the formalin-fixed organs (brain, heart, lungs, liver, spleen and kidneys) from these rats and from CBA-strain mice injected with the same material at identical doses. The lungs, liver and kidneys of both the rats and mice revealed slight-to-moderate acute blood vessels with red blood cells). Additionally, the kidneys showed moderate acute cortical edema (accumulation of protein-poor fluid). These histologic changes documented an acute toxic effect of the two DEAE dextran-based formulations of Gd. The histologic changes in CBA mice were uniformly more pronounced than those in the Sprague-Dawley rats used for magnetic resonance imaging. These inter-species differences make it uncertain if similar effects would occur in humans. The nature of the major histologic change, acute congestion, strongly suggested that the multiple, positively charged DEAE groups of the dextran matrix, had probably interacted with the negatively charged surfaces of cells which line the blood vessel walls (endothelial cells) and had induced endothelial changes that led to red cell adherence and accumulation. From the standpoint of interpreting the image enhancement and T1 changes of Example 13, it is important that the histologic changes just described did not occur at the time when the images were performed (30 minutes following injection of test materials) but rather, at 90 to 120 minutes following injection. Hence, taken together, Examples 15 and 17 establish the efficacy (but not the biological compatibility) of Gd:DTPA noncovalently bound to polycationic carriers, as prototype formulations for preferential MR image enhancement of liver, spleen and bone marrow. 
     EXAMPLE 20 
     Preparation of Heparin-Stabilized Gd:DTPA Dimeglumine Microspheres 
     A 0.95M solution of Gd:DTPA dimeglumine (Schering Ag-Berlex) was concentrated by nitrogen evaporation, adjusted to pH 10, added at 1.8 ml to 100 ml of cottonseed oil, and homogenized for 15 min with a Waring high-speed blender to produce fine microdroplets (0.2-0.5 um). This emulsion was stabilized at 130° C. for 20 min with continued shearing. Heparin, 5000 units (Upjohn Co. clinical grade from beef lung) was added to neutralize the net positive charge of the outer surfaces of these particles and to confer additional particle stability upon subsequent resuspension. The positive surface charge of particles (which had been found in Example 19 to produce acute vascular toxicity) was conferred (in the present example) by the amine moieties of N-methylglucamine (dimeglumine). The reason for coating particle surfaces with heparin was to neutralize this positive charge and eliminate the related acute toxicity. The oil was extracted with diethyl ether and the particles lyophilized as in Example 1. Resulting microspheres were 0.1-0.4 um in diameter. The in vitro NMR proton T1 activity was: 
     
         ______________________________________                                    
Material          ID.sub.50                                               
______________________________________                                    
Gd:DTPA:dimeglumine:                                                      
                  0.045 mg/ml                                             
heparin microspheres                                                      
Gd:DTPA:dimeglumine                                                       
                  0.084 mg/ml                                             
______________________________________                                    
 
    
     Based on the molar concentration of Gd, the microsphere formulation was approximately twice as active as the soluble one. 
     EXAMPLE 21 
     In Vivo Testing of Heparin-Coated Gd:DTPA:Dimeglumine Microspheres 
     Microspheres were injected intravenously into CBA mice at a dose calculated to deliver 0.19 mmol of Gd/kg. The percentage decreases in proton T1 relaxations of the experimental versus control (uninjected) organs excised at 30 minutes were: 
     Liver: 6% 
     Kidney: 53% 
     Hence these microspheres were not sufficiently stabilized to remain intact long enough for clearance by the liver and spleen (requiring approximately 15 minutes). The addition of supplementary matrix materials such as 70,000 MW dextran, would be expected to confer this required stability. 
     Histologic assessment of acute vascular toxicity was performed in CBA mice as described in Example 19. No congestion nor edema were observed. 
     Based on the preceding examples of polymer and microsphere efficacy, stability, and toxicity, the preferred embodiments were covalently conjugated dextran-DTPA polymers and microspheres in conjunction with chelated Gd. 
     EXAMPLE 22 
     In Vitro T1 Effects of Fe +3  Chelated to DTPA-Dextran Soluble Polymer 
     FeCl 3 .6H 2  O was added in a stoichiometric quantity to DTPA-Dextran T10 (11,000 MW soluble polymer) and the T1 ID 50  &#39;s compared with those of comparably loaded Fe:DTPA and Fe:desferrioxamine (a low molecular weight iron chelator of bacterial origin. 
     
         ______________________________________                                    
                    T1 (ID.sub.50)                                        
Substance           Fe(M/10.sup.-5)                                       
______________________________________                                    
Fe:DTPA-dextran 11,000 MW                                                 
                    18                                                    
soluble polymer                                                           
Fe:desferrioxamine  113                                                   
Fe:DTPA             37                                                    
______________________________________                                    
 
    
     Fe:DTPA-dextran polymer was the most potent of 3 agents tested by a multiple of 2 over Fe:DTPA 
     EXAMPLE 23 
     Alternative Chemical Means for Conjugation of Chelating Substances to Carrier Polymers 
     In certain instances chemical advantages, such as increased stability of metal ion chelation or increased flexibility in the carrier polymer, may be achieved by using conjugation reactions other than direct derivatization with dicyclic DTPA anhydride. For example, the middle acetate group of DTPA may be selectively reacted with ethylene diamine before decylizing the stronger-chelating carboxylic anhydrides. This may be accomplished by conjugation in dried organic solvents such as N,N-dimethyl-formamide or N,N-diethylacetamide using standard organic-soluble carbodiimide techniques. 
     The amine-derivatized DTPA could then be reacted in aqueous solvents, using water-soluble carbodiimide reagents, with the OH groups of native dextran, the aldehyde groups of Na periodate-oxidized dextran (more reactive), or the carboxylic acid groups of succinylated dextran (most reactive) which had been prepared by prior reaction with succinic anhydride. Alternatively the simple DTPA chelate could be stabilized in its most favored chelation state by prebinding Gd, followed by derivatization to ethylene diamine in aqueous solvents using water-soluble carbodiimide. Such metal-protection techniques are common methods for protecting enzyme active sites during enzyme chemical reactions/purifications. The resulting dextran conjugate might have even higher binding stability for Gd and other paramagnetic metals that the completely acceptible conjugate described as the preferred embodiment in the present application. 
     Additional alternative methods for potentially improved or diversified conjugation include: (1) modified acid-catalyzed di-anhydride-alcohol reactions (W. C. Eckelman, et al., J. Pharm. Sci. (1975), 643:704); (2) amide coupling linkages between ethylenediamine-derivatized DTPA and succinylated dextran as momudied from (D. J. Hnatowich et al., J. Nuc. Med. (1981) 22:810) or direct coupling techniques involving pentatriethylammonium DTPA, isobutylchloroformate and triethylamine hydrochloride precipitation to form the reactive species, carboxycarbonic anhydride or DTPA, which may be used for various alternative conjugations (G. E. Krejcarek and K. L. Tucker, Biochem. Biophys. Res. Comm. (1977), 77:581). These reactions are expected to expand and potentially improve the already satisfactory techniques of the present application. 
     In the above-described studies, radionuclide quantification of Gd binding to the DTPA-dextran soluble polymer was performed using  153  Gd, in collaboration with Padmaker Kulkarni, Ph.D. (Nuclear Medical-Radiology, Imaging Center, University of Texas Health Science Center, Dallas, (UTHSCD); and magnetic resonance imaging was performed on the University&#39;s Diasonics 0.35T clinical magent in a 30-cm rf head coil using T1-weighted, spin-echo and inversion-recovery pulse sequences, in collaboration with Jeffrey Weinreb, M.D., Director, Clinical Nuclear Magnetic Resonance Imaging Center-Radiology, UTHSCD. 
     Changes may be made in the construction, operation and arrangement of the various parts, elements, steps and procedures described herein without departing from the concept and scope of the invention as defined in the following claims.