Patent GB-1525564-A (GB)
Title: Image formation using nuclear magnetic resonance
IPC: G01R33/54
Filed: 1974-09-11
Abstract: 1525564 Local investigation by NMR NATIONAL RESEARCH DEVELOPMENT CORP 10 Sept 1975 [11 Sept 1974 31 Oct 1974] 39625/74 and 47213/74 Heading G1N To examine a localized portion of an NMR sample, a magnetic field gradient is applied accompanied by RF energy having selected fre-  quency components, so that only certain regions of the sample are excited. The field gradient produces a corresponding resonance frequency gradient in the sample; thus if single frequency RF is applied it will excite only one point on the gradient, while if a spectrum of RF is applied it will excite all regions of the sample in which the resonance frequency lies within the spectrum. Several excitation techniques are proposed using single frequency or broadband energy either to excite or saturate the localized portion to be examined, or to excite or saturate all the sample except that portion. The frequency of the excitation may be scanned (stepwise) to scan the examined portion through the sample. Apparatus: in Fig. 3 the excitation is supplied by two channels A1-A3 and B1-B5 whose outputs are combined at M and fed to a probe 5. The unit M also isolates the signals induced in the probe by resonance and feeds them to a receiver R1-R3 for analysis. Channel A has a 15MHz oscillator Al whose output is gated at A2 to produce a 90 or 180 degrees pulse for reading or excitation (see Excitation below) which is amplified at A3. Channel B has a synthesizer B 1 producing a signal of about 15MHz and an inverter and gate B2, B3 which can pulse width modulate the signal under command from a controller C to give a desired broadband excitation; this is fed to M via an amplifier and attenuator B4, B5. The unit M is detailed in Fig. 5 (not shown); the signals picked up in the probe pass via M to a phase sensitive detector R2, A/D converter R3 and a computer where they may be averaged and Fourier analyzed. The probe construction, including field gradient coils, is described, Fig. 4 (not shown). Excitation: when the excitation is modulated to produce a desired spectrum it is referred to as tailored; single frequency unmodulated excitation is referred to as simple. Four basic forms of excitation are disclosed: (1) saturating all spins in the sample except for those in the region investigated, by tailored saturation; (2) tipping all spins in the sample except those in the region investigated into one plane, by tailored excitation; (3) defining the region investigated, by simple saturation; (4) defining the region by simple or tailored excitation. Variants of (3) and (4) are: (5) to follow (3) by an intense 180 degrees pulse which inverts the whole spectrum so that the magnetization of the bulk of the sample relaxes from a negative to a positive value while the magnetization of the region increases from zero; (6) to similarly follow (4) with an intense 180 degree pulse. Any of the excitation methods can be repeated with a field gradient in a different direction so that the region investigated is a line rather than a plane. Readout: the above excitation is performed by channels A and B in Fig. 3, the former being used for the intense 180 degree pulse of methods (5) and (6) while the latter is used for saturation or tailored excitation. Following excitation the controller switches the field gradient to an orthogonal direction and causes channel A to deliver an intense 90 degree pulse, the resulting free induction decay signal being processed in the R channel. In methods (5) and (6) the 90 degree pulse is applied as the bulk magnetization passes through zero.