Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Patent
1990-06-15
1992-06-09
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
324318, G01R 3320
Patent
active
051210606
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to NMR spectroscopy and in particular to a method whereby the B.sub.o magnetic field may be made homogeneous.
BACKGROUND OF THE INVENTION
High resolution NMR spectroscopy requires a B.sub.o magnetic field variation across the volume being observed of less than 1 part in 10.sup.7. As the sample itself will introduce perturbation to the field, this degree of homogeneity is normally achieved by adjustment of the B.sub.o field with each sample. This process is known as "shimming" and involves adjusting the currents in as many as 18 electromagnets, each of which has a very specific contour. In volume-selected NMR spectroscopy it is normal practice to shim the whole volume using a simple pulse and acquire sequence before attempting to perform the volume selected experiment. As this procedure is an iterative process, it can be very time consuming. In the traditional high-resolution experiment the homogeneity adjustments are made using the response from the whole sample which is carefully positioned about the magnet isocenter. By comparison, for in vivo spectroscopy, the whole sample is large whereas the volume of interest is relatively small and not in general positioned at the magnet isocenter. The adjustment of the magnetic field homogeneity may now be difficult. It is only at the magnet isocenter that shim sets are not strongly coupled. This is clearly a serious consideration when high-resolution spectra are required from sites remote from the isocenter.
The major goal of magnetic resonance spectroscopy (MRS) is to obtain a high-resolution spectrum from a known position. This task appears to be best achieved using image-directed spectroscopy whereby one of a variety of gradient-encoded volume-selection techniques is implemented using excitation frequencies determined from a proton density image of the object. With the notable exception of the VOSY method referred to herein, no volume-selection technique achieves spatial selection in one acquisition and, in general, eight acquisitions are required to complete the volume-selection procedure. This add-substract cycle often places severe demands upon the preamplififer and ADC. In the present inventor's experience, however, the major experimental difficulty relates to the adjustment of the shim set for high-resolution acquisition from the voxel of interest.
Since, over small distances, the inhomogeneities approximate to a straight line, only the three first-order shim gradients (X, Y, Z) need be used to correct the homogeneity of the B.sub.o field. In addition, the spectral response from spins outside the voxel of interest may be severely broadened by the shimming procedure, thus reducing the dynamic range requirements of the preamplifier and ADC.
For this concept to be exploited it is essential that the volume selection technique which is used performs complete suppression of signals arising from outside the volume of interest in a single acquisition. Thus, in medium-bore and whole-body magnets, it should be possible to achieve high-resolution shimming on a small voxel using only the X, Y and Z shims.
OBJECT OF THE INVENTION
It is an object of this invention to use such a technique to determine the current required in each of the X, Y and Z shims to achieve the required B.sub.o homogeneity for each small volume element within the useable volume of the magnet. In this way the required shim currents can be predetermined and recorded. They can be mapped as a function of spatial position, described using cartesion co-ordinates x, y and z. It should be noted that such maps will be different for each magnet and may be difficult to predict. The maps will also be modified by each radiofrequency (r.f.) probe which is used. Once mapped for a particular probe configuration, the currents are only minimally affected by different samples. By use of the predetermined shim currents, the apparatus may be more readily set up in preparation to study a test subject.
Other objects, and various advantages, will hereinafter become app
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Journal of Magnetic Resonance, vol. 73, No. 1, Jun. 1, 1987, pp. 174-177 "Shimming" on spatially localized signals, from D. I. Hoult.
Magnetic Resonance in Medicine, vol. 7, No. 3, Jul. 3, 1988, pp. 352-357 "Nodal Inhomogeneity Mapping by Localized Excitation-The Nimble"-Shimming Technique for High-Resolution in Vivo NMR Spectroscoy from David M. Doddrell, Graham J. Galloway, Ian M. Brereton, and William M. Brooks.
Brereton Ian M.
Doddrell David M.
Galloway Graham J.
Tokar Michael J.
University of Queensland
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