Automatically tuned and matched RF transceive head coil at 7T

Introduction The transmit efficiency and receive signal-to-noise ratio (SNR) of transceive RF coils depends on how well the resonance circuit is tuned to the Larmor frequency and matched to the 50 ohm coaxial cable. Patient size variability changes the load the coil couples with and adversely impacts both tuning and matching, and subsequently the power efficiency and SNR of the coil. The loading effects from different human bodies are unpredictable and problematic. Significantly mismatched coils stress other RF components with reflected power like T/R switches and RF power amplifiers. Conventional manual re-tuning and re-matching of coils between dissimilar body types is timeconsuming and impractical for clinic applications. Furthermore, at ultra-high fields, multi-channel coils aggravate the loading problem because: 1) the time required for re-tuning/matching is proportional to the number of channels and 2) the loading effects significantly change S11 (the reflection coefficient) and the associated RF power efficiency. Previous efforts to automatically tune and match coils focused only on receive coils because transmit or transceiver coils must handle kilowatts, not microwatts of power, a difficult engineering challenge. This study offers an electrically driven stand-alone automatic tuning and matching system for RF transmit and/or receive coils.