Patent GB-796366-A (GB)
Title: Improvements in or relating to magnetic resonance apparatus
IPC: G01R33/44
Filed: 1955-11-28
Abstract: 796,366. Electric tests. SCHLUMBERGER WELL SURVEYING CORPORATION. Nov. 28,1955 [Dec. 6, 1954], No. 34073/55. Class 37. In apparatus for observing gyromagnetic resonance phenomena in paramagnetic material exposed to a unidirectional field and an alternating field of the resonance precession frequency at right-angles to the unidirectional field, the R.F. field is suppressed and the intensity of the unidirectional field is periodically changed to a predetermined second intensity when the nutating macroscopic moment is substantially in line with the unidirectional magnetic field and is maintained at the second intensity during a time interval sufficient to reverse the components of the moments perpendicular to the unidirectional field, so that when the R.F. and D.C. field intensities are restored the phase coherence of the magnetic moments is maintained. As shown (Fig. 2) the sample 10 under test is placed in the field of magnet 11 (which may be an electromagnet). An R.F. coil 12 is placed at right-angles to the magnet field and is energized at the resonance precession frequency of the particles under observation by crystalcontrolled oscillator 13 feeding through gate circuit 14. A coil 18 on magnet 11 may be provided with a current to increase or decrease the intensity of the unidirectional magnetizing field. Synchronizing circuit 16 supplies gating pulses to open gate 14 and to change the energization of magnet 11 by means of coil 18. Resonance is detected by means of an R.F. detecting coil 20 at right-angles to the D.C. field, output signals being fed through phase-splitting transformer 21 to gate circuits 25, 26 alternately opened by pulses from the synchronizing circuit, and a phase-sensitive detector 24 fed with reference signals from the oscillator 13, the output being finally indicated at 29. In operation, gating pulses of the form shown in Fig. 3A are generated by the synchronizing circuit. During intervals t the gate 14 is open and the oscillation (Fig. 3B) passes to coil 12. During time t 1  the macroscopic moment rotates through 360 degrees. During the time t 2  coil 18 is activated with D.C. pulses 32 and gate 14 closes. Interval t 2  is of such a length that the components of the moments which have not nutated to the original position and are perpendicular to the unidirectional field are rotated through 180 degrees, so that at the end of the second period t 1  during which the R.F. field is restored and the D.C. field restored to its original value these moments are again aligned with the main macroscopic. moment. The output of pick-up coil 20 is shown in Fig. 3D while 3E shows unwanted induction in this coil from coil 12. Figs. 3F and 3G show gating pulses applied to gates 25, 26 so that signal 3E changes to 3J while 3E changes to 3K. Thus the phase reversal in the output signal is eliminated from the wanted signal and introduced into the unwanted signal. The phase-sensitive detector receives a reference signal 39 in phase with signal 37 while the unwanted signal is periodically reversed and is filtered out. Meter 29 is responsive to the amplitude of resonance signal 33. Since all the macroscopic moments are aligned in their original positions at the end of second interval t 1  the change in field does not then affect their positions. Alternate intervals t 2  may thus be omitted. Figs. 3M and 3N represent the first and second field energizations respectively. An alternative embodiment is described with reference to Fig. 4 (not shown) in which pulses derived from frequency division and shaping of sine wave are used to open gate 12 and to change the energization of the permanent magnet. The Specification states that the R.F. field need not be removed during the period when the unidirectional field changes in intensity but that time t 2  must be as short as possible relative to t 1  and that to this end the difference between the two field intensities should be increased. The Specification also indicates the possibility of combining the system with the arrangement as described in Specification 791,866 in which the phase of the R.F. energizing field is periodically reversed.