Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1981-11-09
1984-05-29
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324310, G01R 3308
Patent
active
044517882
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.
It has been proposed in U.S. Pat. No. 4,165,479 that image information can be derived from a series of spin echoes following a single excitation of the sample. The spin echoes are derived by employing switched magnetic field gradients or switched radiofrequency magnetic field pulses. This method may be referred to as the echo planar technique.
The echo planar technique is in some sense an improvement over the method described in U.S. Pat. No. 4,070,611. In the echo planar method imaging information is derived from a number of spin echoes following a single excitation of the sample. The echoes may be produced by successively reversing a magnetic field gradient along one direction. A much smaller gradient at right angles to the first gradient is constantly present to produce a varying phase shift along the second direction which serves the purpose of providing spatial discrimination along the second direction. This technique produces a two-dimensional image in the plane defined by these two gradient directions.
However, the echo planar method as described above seems to have certain difficulties. First, successive echoes produced by the switching of the first gradient mentioned in the previous paragraph must be time reversed with respect to each other. Failure to time reverse successive echo signals before performing the required Fourier transformations will result in mixing up information from the positive and negative directions associated with the first gradient and thus produce aliasing.
Once the necessity for time-reversing successive echoes is appreciated, it becomes evident that the time reversal will be rendered problematic if the second gradient mentioned above is kept on during the signal readout. Consider a small volume element of the sample at co-ordinates (X,Z). Assume that G.sub.x is the first gradient mentioned above and G.sub.z is the second. Then for successive intervals k and (k+1) the successive signals S.sub.k (X,Z,t) and S.sub.k+1 (X,Z,t) from the element will be X+G.sub.z Z)(t-t.sub.k)]S.sub.k+1 (X,Z,t).alpha.exp[i(k-1).DELTA..phi.(Z)]exp[i.DELTA..phi.(Z)]exp[i.gamma.( -G.sub.x X+G.sub.z Z)(t-t.sub.k+1)]
.DELTA..phi.(Z)=.gamma.G.sub.z Z.tau., where .tau. is the length of one interval (all N are the same length), and t.sub.k and t.sub.k+1 are reference times within the intervals k and (k+1). Thus G.sub.x and G.sub.z act in opposite ways in the two signals, and it is not at all clear how to sort out the imaging information in the X and Z directions.
The principal object of the present invention is to provide an improved method of gyromagnetic resonance imaging employing trains of free induction decays and spin echoes.
According to the present invention a method of deriving image information from an object using nuclear magnetic resonance signals comprises subjecting an object to a continuous static magnetic field along an axis and carrying out the following sequence of steps: direction parallel to the said plane together with a second gradient of the magnetic field having a gradient direction perpendicular to the said plane. of gradient direction, applying a third gradient of the magnetic field which has a gradient direction parallel to the said plane and orthogonal to the direction of the first gradient during each successive reversal of said first gradient, and holding the said first gradient with its successively reversed gradient directions for time intervals during which successive echoing free induction decay signals are read out.
In carrying out the invention alternate free induction decay signals are time-rever
REFERENCES:
patent: 4165479 (1979-08-01), Mansfield
patent: 4284948 (1981-08-01), Young
patent: 4290019 (1981-09-01), Hutchison et al.
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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