Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1981-11-09
1985-03-19
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324311, 324313, 324314, G01R 3308
Patent
active
045062226
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to methods of producing image information from objects. It is concerned with producing images of samples containing nuclear or other spins whose spatial distribution of density or relaxation time is detected by magnetic resonance techniques. More particularly, it describes methods for producing images from free induction decays (FID's) and spin echoes of the sample in the presence of static magnetic fields and switched magnetic field gradients.
In U.S. Pat. No. 4,070,611 there is described a method of producing images by a series of FID's following separate excitations of the sample. During these FID's, magnetic fields in two (or three) orthogonal directions are switched on and off for specific lengths of time to yield two (or three) dimensional images.
One of the problems associated with the above method is that inhomogeneities of the static magnetic field can simulate the effect of the deliberately introduced switched field gradients and mask the effect of these switched field gradients, thereby destroying some of the information contained in the signal.
The masking effect can occur as follows. Different FID's have field gradients of fixed strength switched on for varying times. For any particular combination of gradient pulse lengths during a single FID, spins in different regions of the sample experience varying phase shifts relative to each other.
These phase shifts allow spatial discrimination and therefore enable an image to be formed. The amount of phase shift between two regions of the sample is proportional to the difference in local magnetic field of the two regions. If inhomogeneities in the static magnetic field contribute to the local field (in addition to deliberately introduced gradients), the spatial distribution information can be distorted.
Suppose for example that in one of the FID's needed to produce an image, the gradient G.sub.z in the z direction is switched on for time T. Consider a small volume element of a sample at Z=Z.sub.o. It experiences a field B(Z)=B.sub.o +Z.sub.o G.sub.z +.DELTA.B(Z.sub.o), where B.sub.o is the static magnetic field at Z=0 and .DELTA.B(Z.sub.o) is the deviation from the static field at Z.sub.o due to the inhomogeneity in the static field. Then at the end of time T the spins at Z=Z.sub.o experience a change in phase .DELTA..phi. relative to spins at Z=0 given by appears to be not G.sub.z but G.sub.z +.DELTA.B(Z.sub.o)/Z.sub.o. A Fourier transform along the Z direction will yield a distorted and non-linear scale in that direction. Moreover, the extra phase shifts could cause signals from some parts of the sample to appear incorrectly in the wrong part of the image (aliasing).
A numerical example will illustrate the seriousness of this problem.
In order to produce a well-determined NxN image, it is necessary to take N samples from N signals. For whole-body imaging, one would require a region at least 40 cm in diameter and produce a 64.times.64 element image.
Referring to equation (1), there need to be 64 different values of .DELTA..phi.. These are obtained by having 64 different values of time during which the gradient G.sub.z is applied. Ignoring the .DELTA.B term in equation (1) for the moment, an example of such a series is given by .DELTA..phi..sub.k but the time the gradients are applied varies. There is a condition, however, that .DELTA..phi.*<2.pi. across the sample. Using reasonable parameters based on a whole-body nuclear magnetic resonance imaging reference, the length of the sample can be set to L=40 cm, T=0.5 ms, .gamma./2.pi.=4260 Hz/Gauss, and the condition .DELTA..phi.*<2.pi. gives Gz<0.012 Gauss/cm. At 20 cm, the maximum distance from the centre of the field, G.sub.z .times.Z=0.24 Gauss. But the inhomogeneity in a four coil, eighth order resistive magnet (which is typical of those used for whole-body imaging) will be about 10.sup.-4 at 20 cm, or 0.1 Gauss for a 1 kGauss magnet, nearly half the contribution of the gradient. This situation is unacceptable since one is trying to resolve these 20 cm into 32
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Mansfield et al., "Biological and Medical Imaging by NMR", Journal of Magnetic Resonance, vol. 29, pp. 355-373, 1978.
Edelstein et al., "Spin Warp Imaging and Applications to Human Whole-Body Imaging", Physics in Medicine and Biology, vol. 25, pp. 751-756, 1980.
Journal of Physics E-Scientific Instruments, vol. 13, No. 9, Published Sep. 1980, (London, GB), J. M. S. Hutchison et al.,: "A Whole-Body NMR Imaging Machine", see pp. 947-948 and Fig. 1.
Edelstein William A.
Hutchison James M. S.
Johnson Glyn
Mallard John R.
Redpath Thomas W. T.
National Research Development Corporation
O'Shea Kevin D.
Tokar Michael J.
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