GB1508438A - Analysis of materials - Google Patents

Abstract

1508438 Investigating sample regions by gyromagnetic resonance NATIONAL RESEARCH DEVELOPMENT CORP 3 April 1975 [5 April 1974 7 Oct 1974] 15280/74 and 43365/74 Heading G1N To investigate only a small region of a sample by a gyromagnetic technique, the sample is subjected to an inhomogeneous magnetic field which varies with time, the time variation in the small region being different from that elsewhere in the sample so that the resonance signals of the small region can be distinguished from those of the rest of the sample. The varying field may be established by superimposing several A.C. fields on the usual D.C. field; the A.C. fields can be chosen so that in the small region they have a zero resultant, i.e. so that the small region has only the D.C. field while the rest of the sample has the A.C. and D.C. fields. Field generation: in Figs. 2a-c the main D.C. field is along the Z axis and the small region is at the origin. Equal A.C. currents in the coils 21 of Fig. 2a produce no resultant A.C. field in the XY plane, but give a time varying field elsewhere; similarly equal A.C. currents in coils 19 of Fig. 2b give no resultant A.C. field on the Y axis, and equal A.C. current in coils 20 of Fig. 2c give no resultant A.C. field on the X axis. Hence the small region at the origin receives only the D.C. field. By varying the A.C. currents, the null of the A.C. fields, i.e. the small region, may be scanned through the sample. Scanning can also be achieved by moving the sample. The A.C. fields can also be scanned by creating them with a hand held probe. Scanning may be useful for medical samples, e.g. to map the positions of muscle, fat, blood or tumours, all of which have different proton relaxation times. Circuitry: In Fig. 1 the sample is subject to the D.C. field (not shown), to pulsed, phase modulated RF energy from coil 5, and to the A.C. fields of coils 19, 20, 21. The resonance signals are picked up by coil 11. The RF is generated at 2, and passes to amplifier 9 directly and via a 180 degrees phase shifter 8. The amplifier is controlled to accept the inputs in turn, and is gated at 3 into amplifier 4 for feeding coil 5. The generator 2 provides a reference signal to phase sensitive detector 13 via adjustable phase shifter 14 so that the resonance signals detected at 11 and amplified at 12 can be demodulated. The detected signals are amplified at 15 and are fed to an amplifier 17 via gate 16, e.g. in-between the pulses of RF excitation. This amplifier 17 is controlled so that although its input consists of alternating positive and negative signals due to the phase modulation of the RF energy, its output is always positive; averaging in low pass filter 28 will then remove non-random noise. An X-Y plotter receives the filtered signal, and plots it against the magnitude of the variable currents in A.C. field coils 19, 20 so as to map the whole sample in an X-Y plane; the Z field coil current can also be varied if desired, to allow a plot in the Z direction. The use of pulses of alternating phase also makes thermal effects tend to cancel so that high pulse repetition rates can be used. The pulses can be in pairs of width a having opposite signs and being separated by time t, with a interval T between pairs, or can be in triplets having a first pulse of width a separated by interval t from an opposite sign pulse of width 2a followed at interval t by a third pulse of width a with the same sign as the first. In the case of the triplet pulses, T may be made zero for maximum pulse rate (which is equivalent to the two pulse system of Fig. 1).