A retrofit to enable dynamic steering for transmit arrays without multiple amplifiers

Abstract

Purpose B1+ 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 B1+ 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 B1+ 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 B1+ 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 B1+ shimming with increased degrees of freedom but without significantly increased hardware cost.

A Review of Current Control and Decoupling Methods for MRI Transmit Arrays

“…The implementation of B1 shimming can vary greatly depending on the application. An inexpensive way is to split the power from the amplifier to N ways, and then perform the phase modulation and/or amplitude modulation on each of the N channels [28,64].…”

“…The phase shift for each channel is achieved by adjusting a commercially available phase shifter and the cable length. The difficulty is to control the phase shift remotely, though an add-on system has been demonstrated recently [64].…”

Hybrid‐pair ratio adjustable power splitters for add‐on RF shimming and array‐compressed parallel transmission

“…[4][5][6][7][8][9][10][11][12][13][14] The desired amplitude/phase applied to individual coils in RF shimming can be realized by independent RF power amplifiers, or using add-on RF shimming devices, such as power splitting circuits, phase shifters, and/or power combining circuits. 7,9,15,16,17 The latter design typically provides lower RF shimming ability compared to independent power amplifiers, but with the benefit of much simpler structure, lower cost and smaller footprint. In particular, the combination of add-on RF shimming devices and multiple independent RF power amplifiers can provide additional degrees of freedom to manipulate the transmit field beyond the scanner's hardware limitation via array-compressed parallel transmission (acpTx) and related techniques.…”

“…To address this, RF shimming was proposed and widely used to manipulate 4‐14 . The desired amplitude/phase applied to individual coils in RF shimming can be realized by independent RF power amplifiers, or using add‐on RF shimming devices, such as power splitting circuits, phase shifters, and/or power combining circuits 7,9,15,16,17 . The latter design typically provides lower RF shimming ability compared to independent power amplifiers, but with the benefit of much simpler structure, lower cost and smaller footprint.…”