Transverse field limited localized coil for magnetic resonance imaging
The present invention is a continuation-in-part of prior pending application ser. No. 931,726, filed Nov. 17, 1986, now U.S. Pat. No. 4,752,738 which in turn is a continuation-in-part of application ser. No. 765,708, filed Aug. 14, 1985, now U.S. Pat. No. 4,793,356. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to the magnetic resonance arts. It finds particular application in conjunction with localized coils for medical imaging equipment to receive electromagnetic signals from resonating nuclei and will be described with particular reference thereto. It is to be appreciated, however, that the invention may also find utility in other magnetic resonance applications, such as exciting resonance, chemical analysis, well logging, and the like. 
     Heretofore, various types of coils have been positioned to receive electromagnetic signals for magnetic resonance imaging and spectroscopy, including whole body, body portion, and localized coils. The whole body and body portion receiving coils had standard sizes which were selected for readily receiving the patient&#39;s whole body or a selected body portion. Due to the standardized coil size and variable patient size, a significant void or empty region was commonly defined between the coil and the portion of the patient to be imaged. 
     Localized or surface coils were configured from rigid and flexible non-conductive sheets of plastic or nylon on which wire loops were mounted. Rigid flat coils were constructed in a variety of sizes to facilitate positioning adjacent the selected area of the patient to be imaged. When a flat coil was positioned adjacent a relatively flat area of the patient, the intervening air gap was relatively small as the air gap associated signal-to-noise degradation and aliasing. Flexible coils could be bent or wrapped around curved portions of the patient for a better, substantially air gap-free fit. 
     The wire or other conductors on the localized coils were commonly wound in circular loops on the surface of the plastic sheet. In use, the loops were positioned parallel to the surface of the patient. With a patient positioned along a z-axis magnetic field, coil loops disposed adjacent the patient&#39;s back were positioned in an x-z plane. An image plane transversely through the patient, e.g. an x-y plane, would normally intersect the surface coil at two points. Radio frequency magnetic resonance signals originating within this plane induced a like radio frequency current signal within coil segments at the intersection of the x-y plane, particularly segments that extend perpendicular to the x-y plane. However, because the coil was connected in a loop, the currents induced at opposite points were in the opposite direction around the loop and, hence, tended to cancel. 
     To receive signals from deeper within the patient, larger diameter loops were utilized. The depth of the coil&#39;s region of sensitivity has been adjusted by selecting more complex winding patterns, such as a planar arrangement of concentric loops through which current flows in different directions. However, the complex loop arrangements still had the high magnetic energy losses of the single loop. 
     At high frequencies, coils placed against the subject interacted strongly with the subject. The coils were most sensitive to the regions of the subject which were in immediate proximity to the coil segments. Increasing the complexity of the coil winding placed additional coil segments adjacent the subject, each of which was more sensitive to the immediately contiguous region. The more windings in the coil pattern, the more the signals from regions adjacent the subject surface dominated the signals from more remote regions. Other problems attributable to high sensitivity near the conductors were also aggravated. 
     A localized coil is provided which overcomes the above referenced problems and others by interacting with the subject in an optimal manner. In a transmit mode, it produces a more homogeneous field pattern within the subject and a receive mode provides an improved signal-to-noise ratio. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a localized coil is provided. A subject is disposed within a main magnetic field which is directed along a main magnetic field axis. The localized coil includes an inner conductor extending parallel to the magnetic field axis and an outer conductor extending parallel to the magnetic field axis but further displaced from the subject than the inner conductor. The inner and outer conductors are electrically connected adjacent a first end by an electrical connecting member. 
     In accordance with a more limited aspect of the present invention at least the outer conductor is a wide sheet or plate of electrically conductive material. 
     In accordance with another more limited aspect of the present invention, a capacitive coupling is provided between the other end of the conductor. 
     In accordance with another more limited aspect of the present invention, the two conductors are parallel plates. The outer plate is larger than the inner plate. 
     In accordance with yet another more limited aspect of the present invention, the first conductor is surrounded along at least one portion of its length by an electrically conductive shield. The electrically conductive shield is electrically connected to the outer conductor. 
     A first advantage of the present invention resides in its improved signal-to-noise ratio. 
     A further advantage of the present invention is that it optimizes patient loading. 
     Another advantage of the present invention resides in a higher intrinsic Q of the coil which leads to less conductor loss. 
     Yet another advantage of the present invention is an increase in the field uniformity. Particularly, dark areas in sagittal views, known as phase reversal areas, are eliminated. 
     Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the detailed description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may take form in various parts and arrangements of parts or in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. 
     FIG. 1 is a diagrammatic illustration of a magnetic resonance imaging apparatus incorporating the present invention; 
     FIG. 2 is a perspective view illustrating a localized coil construction in accordance with the present invention; 
     FIG. 3 is an alternate embodiment of the localized coil; 
     FIG. 4 is another alternate embodiment of the localized coil; 
     FIG. 5 is yet another alternate embodiment of the localized coil; and, 
     FIG. 6 is still another alternate embodiment of the localized coil. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to FIG. 1, a magnetic resonance imaging apparatus includes a main magnetic field generating means A for establishing a generally uniform, main magnetic field extending longitudinally through an imaging region along a z-axis. A gradient field means B selectively produces magnetic field gradients transversely across the main magnetic field of the image region. A magnetic resonance excitation means C excites magnetic resonance in selected nuclei of a patient or subject disposed in the image region. The resonating nuclei generate radio frequency magnetic resonance signals which are received by a surface or localized coil D. More specifically, the surface coil is disposed adjacent a preselected region of the subject to be imaged such that it receives magnetic resonance signals from nuclei disposed in the preselected region of interest. Electronic processing circuitry E converts the received radio frequency signals into an image representation indicative of nuclei density, relaxation time, position, nuclei characteristics, or the like. 
     The main magnetic field means A includes a magnetic field control circuit 10 and a plurality of high power magnets 12. A gradient magnetic field control circuit or means 14 applies current pulsed to gradient coils 16 to create gradients across the main magnetic field. The current pulse to each gradient coil is controlled by the gradient field means B to select the angular orientation of the transverse gradient fields generated by a gradient field coil 16. The resonance excitation means C includes a radio frequency transmitter 18 and a radio frequency coil 20 to broadcast radio frequency (RF) signals that excite and manipulate magnetic resonance. Optionally, the localized coil D may be connected with the RF transmitter to apply the RF pulses to adjacent regions of the subject. A central computer 22 coordinates the sequence and timing of the gradient field pulses and the RF pulses. 
     The signals emitted by the relaxation of nuclei in the subject have components only in a plane that is normal to the direction of the static magnetic field, i.e. in the x-y plane which is normal to the z-axis. Accordingly, only those conductors of the localized coil which intersect the x-y plane are capable of detecting the magnetic resonance signal. RF signals from the x-y plane will try to induce a like RF current signal along the z-axis in the intersecting coil segment. 
     In high frequency magnetic resonance, the conductors contribute to a heavy interaction with the subject which results in Q loading and reduced sensitivity. When used as a transmitter, RF current pulses through a z-axis conductor of the localized coil induce magnetic fields in the x-y plane. Magnetic resonance signal components not in the x-y planes, i.e. z-axis components, are attributed solely to noise. Accordingly, it is desirable to suppress the reception of the z-axis RF field components. 
     The localized coil D includes a first or inner conductor 30 which defines a current path parallel to the z-axis and a second or outer conductor 32 which also extends parallel to the z-axis but is further displaced from the subject than the inner conductor. An electrical interconnecting means or conductor 34 interconnects first ends of the inner and outer electrical conductors. The interconnecting means is disposed parallel to the x-y plane to inhibit the induction of RF currents therein. A capacitive coupling means 36 couples second ends of the inner and outer conductors with a flexible cable 38. The coupling means also capacitively connects the second ends with the capacitors selected to set the resonance frequency of the localized coil. 
     The flexible cable 38 connects the localized coil with the remotely located processing circuitry E. More specifically, the cable conveys the received radio frequency signals from the surface coil to a radio frequency receiver 40. An imager 42 reconstructs the image representation from the received magnetic resonance signals and the magnetic field control and timing information from the central processer 22. The image representation may be converted to a man-readable display by a video monitor or other display means 44. 
     With reference to FIG. 2, in a preferred embodiment, the inner conductor 30 is a flat plate of conductive material, e.g. a rectangular copper plate or sheet. The second conductor 32 is a larger plate, preferably also a rectangular copper sheet. The copper sheets may be supported on a dielectric reinforcing means. The electrical connecting means 34 is a conductive plate or sheet which connects one end of the conductors. The connecting conductive sheet is disposed substantially perpendicular to the inner and outer conductors and to the surface of the patient. When the first and second conductors are aligned with the z-axis, the connecting plate is disposed parallel to the x-y plane. In this orientation, radio frequency magnetic fields generated in the x-y plane will induce radio frequency currents flowing along the z-axis. Because high frequency coils tend to be dominated by subject generated noise, the coil sensitivity to areas outside of the region of interest is reduced. To this end, the inner conductor in closer proximity to the subject is reduced in size relative to the outer conductor of the surface coil. Making the outer plate larger than the inner increases the sensitivity of the localized coil D to signals originating deeper within the subject as well as reducing the sensitivity to magnetic resonance signals induced near the surface. 
     The coupling means 36 includes a series of evenly spaced capacitors 50 that are evenly distributed along the second end of the z-axis current path defined by the conductors. The total value of the capacitors is chosen so that the response occurs at a preselected frequency of operation. A matching and tuning circuit 52 matches the coil to the flexible transmission line 38, e.g. a 50 ohm coaxial conductor. The matching and tuning circuit also adjusts or tunes the capacitive coupling to tune the localized coil to the subject. 
     With reference to FIG. 3, the inner and outer conductors are again conductive sheets that define current paths in the z direction. The conductive connecting means 34 is again a conductive sheet extending parallel to the x-y plane. The inner conductive plate is relatively flat and the outer conductive plate 32 is curved. The curvature parallels a curved body portion such as a wrist, knee, or neck to be imaged. Alternately, the outer plates may be parabolic to focus at a selected point or plane within the subject. Capacitors 50 again interconnect the second ends of the plates. 
     The embodiment of FIG. 4 is similar to the embodiment of FIG. 3 except that both the inner and outer plates are arcuate. Preferably, the inner and outer plates are defined by concentric arc segments. The embodiment of FIG. 4 is particularly advantageous for curved or cylindrical areas of the body. 
     In the alternate embodiment of FIG. 5, the inner conductor 30 is again a flat plate extending along the z-axis. The outer conductor 32 is a tube of conductive material which surrounds the patient. Preferably, the second conductor is a bore liner, such as copper foil on a plastic cylinder, which conventionally lines the bore of a magnetic resonance imager. The conductive connecting portion 34 is again a sheet extending perpendicular to the z-axis and the bore liner. 
     In the embodiment of FIG. 6, a plurality of inner conductors 30, 30a and 30b are provided. The conductive connecting means includes a plurality of wire segments 34, 34a, 34b, which interconnect the conductors with the outer conductor plate 32. The inner conductors and the connecting conductors are surrounded by first tubular conductive Faraday shield portions 54, 54a, 54b. The other ends of the outer conductors are surrounded by second Faraday shield portions 56, 56a, 56b. The first and second Faraday shield portions define gaps 58, 58a, 58b therebetween through which the outer conductors are exposed. All Faraday shield portions are electrically connected with the outer conductive plate 32. The outer conductors and surrounding shield portions may be sections of coaxial cable which may merge integrally or with the matching circuit 36 to the cable 38. The outer coaxial cable conductor may be removed while the dielectric material is retained to define the gaps. Preferably, the gaps are aligned with an x-y image plane. Alternately, a single Faraday shield may surround a plurality of inner conductors or a wide conductive plate. The one or both of the conductor may also be a combination of plates, parallel wires, flat or curved, conductive strips, perforated plates, conductive tubes, or the like. 
     The invention has been described with reference to the preferred embodiments. Obviously modifications and alterations will occur to others upon reading and understanding the preceeding specification. It is intended that the invention be construed as including all such alterations and modifications insofar as they come within the scope of the appended claims or the equivalents thereof.