Electricity: measuring and testing – Particle precession resonance – Using a nuclear resonance spectrometer system
Patent
1985-12-03
1987-12-01
Levy, Stewart J.
Electricity: measuring and testing
Particle precession resonance
Using a nuclear resonance spectrometer system
324314, G01R 3320
Patent
active
047107189
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention pertains to nuclear magnetic resonance apparatus employing a spatially inhomogeneous RF field and particularly relates to achieving enhanced spatial discrimination or enhanced spatial independence in respective types of applications.
LITERATURE REFERENCES
(1980). 335 (1983). (1980). (1982). (1983). (1977). (1974).
BACKGROUND OF THE INVENTION
Almost all NMR spectra may be improved by enhancing the quality of the radiofrequency pulses used for excitation or manipulation of the nuclear spins. Ideally a pulse should be intense (so that resonance offset effects are negligible), correctly calibrated and spatially homogeneous (so that it gives the correct flip angle for all regions of the sample), and free of any phase transients. Where instrumental limitations preclude such ideal conditions, considerable progress can be made by using composite pulses--clusters of simple pulses designed for mutual compensation (Refs. 1-5). In this way the tolerance of resonance offset effects may be greatly improved, leading to more reliable results in spin echo experiments (Ref. 3), spin-lattice relaxation time measurements (Ref. 2), two-dimensional spectroscopy (Ref. 6) and broadband decoupling (Refs. 7-11). The sensitivity to errors in pulse flip angle may also be greatly reduced by suitable composite pulse sequences, and quite recently simultaneous compensation of resonance offset effects and pulse length error has been achieved (Refs. 12, 13).
The question naturally arises as to how such compensating sequences are discovered--by inspired guesswork or the application of general principles? Once a sequence has been proposed, the calculation of its offset and pulselength behaviour is a straightforward application of the Bloch equations governing the motion of an isolated magnetization vector in the rotating reference frame, and is easily evaluated by numerical methods. In many cases an expansion procedure can then be found which permits the range of compensation to be systematically improved by reapplying the principles used in the first stage. This expansion process continues until the incremental advantages no longer justify the increased length of the sequence. Suppose that the instrumental imperfection is represented by a general parameter A, and that a particular value A.sub.o can be found where the performance is ideal. This might correspond, for example, to the exact resonance condition in the case of an offset dependence study, or to the exact setting of a pulse flip angle. Since many refinement processes are cast in the formalism of power series, the tendency has been to adopt expansion schemes that start with a sequence which works correctly at this particular condition A.sub.o and then gradually extend the range of operation, essentially removing higher and higher order terms in the power series expansion. The penalty for such timidity is a slow and asymptotic improvement in performance.
A less obvious approach is to seek a pulse sequence which works exactly not only at condition A.sub.o but also at two other values of A. Typically these might be symmetrically disposed on either side of A.sub.o. If the three points are not too far apart, then the `droop` in performance at intermediate values of A.sub.o should not prove too serious. This kind of rationale led to the 90.degree.(X) 180.degree.(-X) 270.degree.(X) composite inversion pulse employed in the WALTZ-16 scheme for broadband decoupling and is discussed by Refs. 5 and 10. If indeed the performance is reasonably flat over the chosen range, the same principle may then be reapplied in order to extend the compensation over a still wider range. A power series approach is still available to correct the intermediate regions if necessary.
This concept is extended here in the context of pulse flip angle errors; resonance offset effects are deliberately neglected. The practical interest lies in effects of spatial inhomogeneity of the B.sub.1 field of the transmitter coil but the result may be simulated by deliberately mis
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Levitt et al., "NMR Population Inversion Using a Composite Pulse", J. Magn. Reson. 33, 473-476, 1979.
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Berkowitz Edward H.
Cole Stanley Z.
Fisher Gerald M.
Levy Stewart J.
O'Shea Kevin D.
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