Counter rotating current local coils for high-resolution magnetic resonance imaging

Abstract

A type of local coil (or surface coil) for magnetic resonance imaging and spectroscopy is described in which two circular loops of the same diameter are disposed axially with respect to each other and in which counter rotating currents (CRC) are supported in the two loops. This type of coil has been used for localized reception in a uniform circularly polarized excitation field produced by a whole-body coil of an imager functioning at 1.5 T. The CRC coil is decoupled from the transmitter coil by a combination of intrinsic (or geometrical) isolation (which functions during both excitation and reception) plus passive decoupling (which functions only during excitation). CRC coils of 7.5, 10, 12.5, and 15 cm diameter have been compared on a bench test setup with conventional surface coils of these diameters. Sensitivities are very similar. Q's have been measured as a function of coil diameter and both Q's and frequency shifts have been measured as a function of distance from a saline tank. The CRC coil appears to have advantages with respect to tuning and matching. A high-resolution image of the rotator cuff is shown as an illustration of coil performance.