A retrofit to enable dynamic $B_1^{+}$ steering for transmit arrays without multiple amplifiers

Purpose: $B_1^{+}$ shimming is an important method for mitigating B₁ inhomogeneity in high-field MRI. Using independent power amplifiers for each transmit (Tx) element is the preferred method for B₁ shimming but comes with a high cost. Conversely, the simplest approach to control a Tx array is by using coaxial cables of varying length in the Tx chain, but this approach is cumbersome and impractical for dynamic shimming. In this article, a system is described that enables dynamic, phase-only, eight-channel $B_1^{+}$ steering on a 7T MR scanner with only two power amplifiers.

Methods: Power dividers were utilized to first split the existing two-channel Tx signal into eight channels. Digitally controlled phase shifters on each channel were designed to provide independent phase shifts with a resolution of 22.5° (from 0°, 22.5° … 337.5°). To validate the system, an eight-channel body dipole array was simulated and constructed for bench and 7T imaging and evaluation.

Results: The phase conjugate $B_1^{+}$ steering method was employed at three different spatial positions in simulation, bench measurements, and scanner measurements-all with matching results. At the desired points, regions with homogenous $B_1^{+}$ were generated, indicating good Tx steering to the selected region.

Conclusion: The described system can be used as a simple retrofit to existing hardware to provide phase control while avoiding the need to manually switch cables and without requiring independent power amplifiers for each channel, thus demonstrating the ability to perform dynamic $B_1^{+}$ shimming with increased degrees of freedom but without significantly increased hardware cost.