Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1993-03-05
1994-09-13
Tokar, Michael J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01R 3300
Patent
active
053472179
DESCRIPTION:
BRIEF SUMMARY
This invention relates to magnetic resonance spectroscopy and imaging and in particular to the localization of signals from a sample.
Nuclear magnetic resonance spectroscopy and imaging and electron spin resonance spectroscopy and imaging are widely-used techniques in medical investigations and with both techniques localization of the "field of view" to the particular volume of interest is important. A well-known localization technique is the selection of a slice of spins through the sample by applying a gradient magnetic field B.sub.o (thus making the resonant frequency a function of position in the sample) during the same period as a frequency-selective e.m. pulse is applied (Garroway, A. M., Grannell, P. K. and Mansfield, P., J. Phys.1C.7, L457 (1974)). However, the slice position is dependent on chemical shift (influence of the atomic and molecular electrons on the magnetic field experienced by the nucleus) and also other resonant offset effects and so for samples including species with varying chemical shifts (e.g. water and fat), the accuracy of the localization is reduced (the region of water contributing to the signal will be shifted slightly compared to the region of fat contributing to the signal).
Similar problems arise in electron spin resonance techniques (sometimes referred to as electron paramagnetic resonance) where although the resonant frequency is of the order of 1000 times higher, the signal may be spread over a wide spectral range.
Both n.m.r. and e.s.r. use electromagnetic excitation pulses to cause resonance of the respective magnetic dipoles and as is known for n.m.r. the excitation pulses are of radio frequency.
Phase encoding techniques are used to define position in imaging (see Kumar A, et al, 1975 NMR Fourier zeugmatography J. Magn. Reson. 18 69-83 and Edelstein W. A., et al 1980, Spin-warp NMR imaging and applications to human whole-body imaging, Phys. Med. Biol. 25 751-6) and a simplified example of a pulse sequence used in this technique is shown in FIG. 1 of the accompanying drawings. As can be seen, localization to a slice orthogonal to the z-direction is achieved by applying a gradient in the z-direction, marked G.sub.z, and a 90.degree. e.m. pulse to flip spins in a slice into the y-z plane. Localization in the x direction is obtained by sampling the e.m. FID signal emitted from the sample during application of a gradient in the x-direction, marked G.sub.x. Localization in the y-direction is achieved by phase-encoding using the G.sub.y gradient applied for a set time without any measurement e.m. pulse. During the application of this gradient the phase of the rotation in the x-y plane of the nuclear spins changes by an amount depending on the magnetic field which they experience. Spins in a part of the sample with higher magnetic field tend to gain phase, spins in a part of the sample with a lower magnetic field tend to lose phase. The amount of gain or lag depends on the magnitude of field and time for which it is applied and the gain or lag is, of course, relative to the other spins in the sample. Thus after application of G.sub.y the phase of the spin will be a function of the position in the sample. By repeating the entire process with different magnitude gradients, usually 256 times, and summing and performing a Fourier transform on the result it is possible to obtain localized information in the form of an image. The effects of chemical shift are lost in the phase-encoding direction because the signal depends only on the relative phase introduced between the spins in the repetitions, and the chemical shift is the same each time.
It is possible to use phase-encoding to localize In the x-direction as well, but 256.times.256 repetitions would be necessary and so the time taken to perform the scans might be several hours, which is clearly inappropriate for anything other than samples which can be kept perfectly still.
For spectroscopy the number of repetitions in each direction can be reduced to 8 or 16 and this allows Fourier phase-encoding to be used as the localizati
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Leach Martin O.
Sharp Jonathan C.
British Technology Group Limited
Mah Raymond Y.
Tokar Michael J.
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