Medical imaging techniques for organs and tissues in a human or animal body have changed considerably over the last 20 years, in good measure because of adoption of nuclear magnetic resonance imaging for medical imaging. Damadian(Science 171(1971) 1151-1153), Weisman(Science 178 (1972) 1288-1290), Lauterbur(Nature 242(1973) 190-191), Eggleston et al(Cancer Research 33 (1973) 2156-2160) and Damadian et al (Proc. Nat. Acad. Sci. 71 (1974) 1471-1473; Science 194 (1976) 1430-1431) were among the first to recognize the value of, and to apply the techniques of, NMR to distinguish between normal and abnormal developments in human and animal bodies.
Nuclear magnetic resonance ("NMR") is a relatively young research area and was first discussed and experimentally investigated by Bloch and his co-workers in 1946 (Phys. Rev. 70 (1946) 460-474 and 474-485). The Bloch articles are incorporated herein by reference. In NMR, an approximately constant magnetic field B.sub.0 =B.sub.0Hi.sub.z is applied in a fixed direction, which defines the z-axis of the associated coordinate system, to the target(organ, tissue, etc.), and a time-varying field B.sub.1 =B.sub.1 (i.sub.x cos .omega.t+i.sub.y sin .omega.t) is applied in a plane perpendicular to the z-direction, where the amplitude B.sub.1 is also approximately constant. The magnetic polarization vector M satisfies the magnetization torque equation EQU dM/dt=.gamma.(M.times.B)+(M.sub.0 /T1)i.sub.z -.OMEGA..multidot.M (1)
where .gamma. is the gyromagnetic ratio, B=B.sub.0 +B.sub.1 is the total impressed magnetic field, M.sub.0 is an equilibrium magnetization established by the polarization field, T1 is a characteristic time interval for return to equilibrium of the transverse component of magnetization, T2 is a characteristic time interval describing de-phasing of the magnetization, and .OMEGA. is a diagonal second rank tensor or dyadic that phenomenologically accounts for relaxation of the three magnetization components that is of the form ##EQU1## For protons, the ratio .epsilon. is 42.57 MHz per Tesla. The spin-lattice relaxation time T1 and the spin-spin relaxation time T2 are often of the order of 600-1,000 msec and 20-100 msec, respectively. The observable quantities are M.sub.x and M.sub.y.
These equations can be solved under various driving and receiving field conditions to obtain the magnetization components for the system. When the system is excited by a radiofrequency ("RF") magnetic field intensity B.sub.1 at or near the resonant frequency f.sub.0 =.omega..sub.0 /2.pi.=.gamma.B.sub.0, the spin system will draw energy from the RF exciting field. Conversely, if the spin system is near resonance, energy can be returned to a structure positioned to receive this RF energy. Analysis of the system of equations is Eq. (1) is discussed by A. Abragam, The Principles of Nuclear Magnetism, Oxford University Clarendon Press, 1961, pp. 37-75, and is incorporated herein by reference. Medical imaging is concerned generally with receipt and interpretation of the fields produced by this given-back energy.
In subsequent discussions, it will be assumed that the frame of reference is one that rotates with the RF rotating magnetic field B.sub.1 at the resonant frequency f.sub.0. The magnetization components M.sub.x' and M.sub.y' are of particular interest here. In a frame rotating with the field, the magnetic field directed along the x'-axis in the rotating frame produces a magnetization only along the y'-axis. In this frame, the broadband RF pulse and various gradient magnetic fields (discussed below) that perturb the spin system are easily visualized and analyzed.
One problem that faces any approach to excitation, selective or otherwise, of a tissue, organ or other biological component of a human or animal body (herein referred to simply as "tissue" for convenience), or parts thereof, is that the "noise", which arises from tissue not within the desired volume element, is often substantial because of the relatively large surrounding tissue volume that produces such noise. A time-varying magnetic field B.sub.1 in the tissue produces a corresponding electric field E.sub.1 by Maxwell's equations, and because the tissue has non-zero conductivity, this produces a corresponding non-zero current vector J. The volume integral of the scalar product of J and E gives rise to power dissipation in the entire tissue volume element, and this produces noise at the signal sensing apparatus unless the field of view of the tissue volume element can be somehow limited. This process can be represented by a "body noise" resistor whose contribution is proportional to tissue conductivity. Noise sets a lower limit on the resolution, expressed as the smallest volume of tissue that can be sensed by the receiver, and sets a lower limit on the length of the time interval over which signal acquisition is possible. Noise is produced by uncontrolled electronic action in the receiver circuits ("Johnson noise"), by the "body noise" resistor noise source, and by thermally induced magnetization in the tissue being imaged.
Three volume elements, of quite different sizes, are involved here: (1) tissue volume, which can be a few hundred to a few hundred thousand cm.sup.3 in size; (2) RF signal interrogation volume from which the receiver receives the sensed response signals; and (3) magnetic resonance excitation volume or "voxel volume" within the tissue, which can be much less than 1 mm.sup.3 in size. The interrogation volume is defined by the volume surrounded by the coil, applicator or other transmitter used to generate the RF magnetic field and by the extend of the unwanted electric field generated in the body itself. In conventional approaches, this interrogation volume can be 50,000-100,000 cm.sup.3, which is much larger than the tissue volume for cardiac monitoring. Preferably, the interrogation volume should be about the same size as the tissue volume, or smaller.
According to one well known relation in magnetic resonance physics, the signal-to-noise ratio (SNR) is proportional to the product of B.sub.0 and a volume ratio: EQU SNR.varies.B.sub.0 [voxel volume/interrogation volume][.DELTA.t].sup.1/2,
where .DELTA.t is the data acquisition time and B.sub.0 is the primary magnetic field strength. Increase of B.sub.0 causes a proportional increase in the system's resonant frequency. Increase of .DELTA.t is often constrained by throughput requirements. Increase of B.sub.0 and/or reduction of the interrogation volume is thus a primary concern, if the signal-to-noise ratio is to be increased.
What is needed here is an approach that (1) minimizes or suppresses the body noise per unit acquisition time that issues from the tissue volume, and (2) increases the available signal per unit acquisition time.