Population inversion of a selected region of a spectrum is a concept which has wide application in both NMR spectroscopy and imaging. While inversion of population at any one frequency is a trivial matter, ensuring an accurate inversion over a specified bandwidth, with negligible perturbation of the magnetization outside that bandwidth, is a major problem. However, by using as a driving function a complex radiofrequency (r.f.) pulse with an envelope of the form (sech beta t)1+5i where 1/beta is the temporal width and t is time, we have found that above a critical r.f. power threshold, magnetization is accurately inverted over a very sharply defined bandwidth, while outside that region, magnetization is returned to its initial position, and population is unaffected. Within the broad limits imposed by our equipment, we have also discovered that the phenomenon is independent of the incident r.f. power.
Some of the factors involved in the choice of field strength for NMR imaging are examined. The influences of relaxation times and chemical shift upon image quality and signal-to-noise ratio are highlighted, and power deposition is introduced as a significant factor which may limit the flexibility and information available at higher fields as long as 180 degrees echo pulses continue to be necessary. Chemical-shift imaging is examined and found wanting as a means of coping with chemical-shift artifacts, and the use of multiple echoes (albeit with research) in conjunction with multiple-slice techniques is advocated as representing an efficient data-gathering scheme which can improve image signal-to-noise ratio. With such use, a medium field strength (0.5-1 T) is presented as representing, for general purpose imaging of head and torso, the best current compromise when imaging time is of major importance, with the important caveat that new techniques may always invalidate this conclusion.
An assembly of resistive paper and liquid crystal sheet, conveniently and cheaply constructed for visual detection of the electric fields associated with an rf probe, is presented. Electrical asymmetries, and "hot-spots" usually associated with conservative electric fields, are easily visualized by the color patterns displayed. The device is of considerable assistance in probe design and the minimization of dielectric loss.
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