The signal-to-noise ratio of nuclear magnetic resonance surface coils and application to a lossy dielectric cylinder model. II. The case of cylindrical window coils

For pt. I see ibid., vol. 42, no. 5, p. 497-506 (1995). In pt. I the authors developed complete expressions for the power dissipated by and for the signal-to-noise ratio (S/N) of a coil of arbitrary geometry facing an infinite lossy dielectric cylinder. They now consider an example coil geometry, a "cylindrical window", and demonstrate the effects of coil size and position, tissue properties, and source location on the S/N. Frequencies ranging from 1 to 170 MHz are investigated for two coil sizes, the larger having four times the surface area of the smaller. For a dipole source of strength assumed independent of frequency, the S/N is constant for frequencies up to about 10 MHz. Both coil sizes yield similar optimal S/N values when imaging structures deep within the body, the larger coil showing less dependence on the source location. For more superficial structures, the smaller coil has a better performance at all frequencies investigated while still being more sensitive to source position. Hence, when imaging superficial structures the choice of coil size should be balanced between image uniformity and the need for a higher S/N. For each coil size, there is an optimal position away from the tissues which yields the highest S/N when imaging deep. By contrast, the coil should be placed as close as possible to the body when the source is near the surface. From an electromagnetic standpoint and aside from the increased equilibrium magnetization in the tissues, the S/N of both coils is actually improved by operating at a higher frequency when imaging superficially, whereas it is degraded when imaging deep. Experimental results gathered on a saline-filled cylinder correlate very well with these simulations and show the model will also predict with good accuracy the S/N for a finite length cylinder as long as that length is at least three or four times the coil longitudinal dimension.