Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1994-03-24
1994-12-20
Wieder, Kenneth A.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324307, G01R 3320
Patent
active
053748891
DESCRIPTION:
BRIEF SUMMARY
This invention relates to magnetic resonance spectroscopy and in particular to methods of measurement in which the signals received from the sample are processed to give a spectrum from a localized region of the sample. Such techniques are particularly useful in medical applications of nuclear magnetic resonance (N.M.R.) for examining at a particular organ or region, e.g. tumour, in a body.
As is well known, N.M.R. relies on the resonant oscillation of magnetic nuclei, e.g. protons or phosphorus 31 or fluorine 19, which have been aligned by a magnetic field (usually referred to as B.sub.o), the resonant oscillations being induced by applying an oscillatory magnetic field (referred to as B.sub.1) which is usually the magnetic component of a radio wave. In modern spectroscopy, rather than measuring the absorption of the applied R.F. wave to make the measurement an R.F. pulse is applied to the sample and this causes the sample to emit a low intensity R.F. signal.
It is possible to localize the spectrum to a particular portion of the sample by adding to the uniform magnetic field B.sub.o, a field which varies in strength over the region occupied by the sample. Since the resonant frequency of the nuclei is a function of field strength B.sub.o, the resonant frequency will vary through the sample. Thus, taking a simple view, using an excitation pulse of one particular frequency will only resonate the nuclei in a particular part of the sample and the emitted signal will be representative of only that part. If the magnetic field strength is set to have a linear gradient in one direction then the shape of the region excited will be a slice orthogonal to that direction, the slice being the locus of points where the field strength is such that the resonant frequency equals the applied frequency. In practice the technique used is more complex than this. The magnetic nuclei are first oriented throughout the sample using a uniform field. Then the nuclei in a slice of the sample are inverted by applying a gradient field and selective signal, which may be an resonance signal of chosen frequency and shape. Finally a read-out resonance R.F. signal is applied to the whole sample and the weak R.F. signal emitted from the sample is detected. The signal from the inverted nuclei is reversed in phase and subtracts from the signal from the others. If this signal is subtracted from a signal obtained with a uniform field, then the result is representative of the slice of inverted nuclei only. This technique has been used to obtain a signal from a cuboidal region of the sample using sums from three orthogonal slices. The volume which is selected by the intersection of the slices is known as the "volume of interest" or VOI.
The accurate localisation of spectra to well defined regions of interest is of critical importance to the utility of many in-vivo NMR spectroscopic techniques, and consequently many methods have been advanced to solve the technical and practical problems involved. One of the most successful techniques employed for localised NMR spectroscopy is Image-Selected in-vivo spectroscopy or ISIS. This method has many beneficial features that make it suitable for in-vivo spectroscopy in general including: freely positionable volumes of interest (VOIs), relatively modest gradient switching requirements, and multi-volume capability. It is suitable for Phosphorus 31 spectroscopy in particular because of minimal T.sub.2 weighting (T.sub.2 is a relaxation time), optimum signal-to-noise ratio (SNR) and, due to the use of adiabatic pulses (pulses which are sufficiently slow that the magnetization of the sample follows the applied field), substantial tolerance to inhomogeneity in the applied oscillatory RF magnetic field B.sub.2 allowing the use of small, surface coils. Another important feature of ISIS is that it can be incorporated into many other NMR experiments such as the measurement of relaxation times or measurements using other localisation techniques including imaging sequences. The short minimum experiment time, compared to
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Leach Martin O.
Sharp Jonathan C.
Mah Raymond Y.
National Research Development Corporation
Wieder Kenneth A.
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