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Abstract
To extend the inherent signal-to-noise (S/N) advantage of high field (4T+) NMR to clinical imaging and spectroscopy, a new approach to designing RF surface and volume coils is required. As coils approach wavelength dimensions, the performance of conventional lumped element (L,C) designs succumbs to: (1) non uniform current distributions resulting in decreased homogeneity, fill factor, and increased electric field losses, (2) decreased conductor skin depths resulting in increased ohmic losses, and (3) high inductance resulting in self resonance near or below the desired frequency of operation. At lower frequencies the phase change due to finite propagation velocity of transmit and receive signals on coil conductors is negligible. Therefore, the conventional design approach considers a DC (Biot-Savart) field only, for an unloaded (free-space) RF coil.
This study recognizes and solves the problems of high field, clinical coil design. At higher radiofrequencies, the distributed nature of the coil and patient structure is considered in both circuit design and theory. Lumped elements are replaced by transmission line and cavity elements. Lumped element circuit theory is replaced by transmission line or transverse electromagnetic (TEM) theory. DC field analysis is replaced with fully time-dependent AC analysis for the coil and the human load. AC field losses and resultant heating in living tissues are investigated with regard to safety assurance for high frequency clinical coil design and application. By designing high frequency coils with the high frequency methods presented herein, desired B1 field characteristics are optimized, coil and patient losses are minimized, and self resonance is maximized. Clinical results obtained with these coils have verified for the first time the clear advantages of human NMR imaging and spectroscopy at 4 Tesla and above.
