Planar-pair local coils for high-resolution magnetic resonance imaging, particularly of the temporomandibular joint.

The two-loop--one-gap loop--gap resonator (planar pair) has been investigated as a local receiving coil for high-resolution proton imaging at 1.5 T (63.8 MHz). Sensitivity contours were determined on the bench by measuring the voltage induced in the coil when it is placed on the surface of a tank containing 0.1-M NaCl, as a function of the coordinates and orientation of a radiating dipole inside the tank. Contours were obtained as a function of the diameter D of the two loops, and their separation d. For comparison, considerable data also were obtained for single-turn circular loops as a function of loop diameter. Coil quality factors, Q's, are reported in free space and the saline tank as a function of geometric parameters. Contours also were obtained using phantoms in a whole-body imager. Planar-pair coils couple to slabs of approximately (2D + d) X D cross section, and are superior to circular coils for imaging anatomic structures that are somewhat elongated. A particularly important feature of planar-pair local coils is that they are intrinsically isolated from the whole-body coil of the imager, and no special passive or active circuits are required when the whole-body coil is used for excitation. Planar-pair local coils have been optimized for imaging of the temporomandibular joint, and an image is shown.