Increased rf power absorption in MR imaging due to rf coupling between body coil and surface coil

Fears have been voiced that excessive tissue heating could occur in the event that first, a surface coil is placed with its axis parallel to the transmitting rf field leading to a maximal coupling of the two coils and second, the decoupling circuit of the surface coil breaks down. To avoid an rf coupling of the transmitting body coil to the receive-only surface coil, conventionally applied surface coils are equipped with an active electronic rf decoupling circuit. In extensive worst-case experiments on phantoms we have shown that no tissue heating occurs for surface coils which are equipped with semiconductor varicap diodes for tuning and matching. These coils should be safe for patient applications even if the decoupling circuit fails. Surface coils equipped with mechanically variable capacitors are generally passively decoupled. To simulate the worst-case situation phantom experiments were performed in which a surface coil of this type having no passive decoupling circuit was coupled to the transmitter coil by its geometric position. Theoretical calculations, in agreement with the experimental results achieved during a 15-min measurement in a 1.5-T MRI whole-body imager, show that a significant rf power deposition in the tissue underneath the coil wire occurs, leading typically to a local specific absorption rate of 24 W/kg and a local temperature rise of 5.2 degrees C.