Array-compressed parallel transmission was recently proposed as a way to reduce the number of RF power amplifiers required for many-coil parallel transmission [1]. This is achieved by connecting a large number of coils to a small number of amplifiers via an array compression network that implements optimized coil-to-channel combinations using ratio-adjustable power splitter (RAPS) circuits [2,3] and phase shifters. Currently, the RAPS circuit ratios are determined by tuning coaxial cable lengths within the RAPS circuit (Figure 2), but this prevents dynamic switching of the compression weights via remote tuning. Remotely tunable RAPS circuits and phase shifters would also be useful for dynamic mode switching in MR fingerprinting [4,5]. To achieve this, here we describe the design and validation of a quad hybrid-based phase shifter that can be tuned by varying terminating capacitors, and integrate it into a RAPS circuit. Bench tests and 7T imaging and B1+ mapping experiments were performed to validate the phase shifters and new RAPS circuit design.
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