A coil-channel multiplexer for high-density receive arrays compatible with the MR-linac

<i>Objective.</i>We present a prototype add-on multiplexer for retro-fitting high-channel count receive coils onto systems with analog readout chains, such as the MR-linac. This particular system is currently equipped with two coil elements, containing only four channels each.<i>Approach.</i>We developed a 4:1 multiplexer board based on time-division. CMOS-based single-pole double-throw switches were used to accommodate for fast switching and adequate encoding bandwidths. Coil-channel images were separated after reconstruction by solving a system of equations based on the square-wave modulation imposed by the multiplexer. The prototype was tested at a 1.5 T scanner on an image-quality phantom and a slice of bovine muscle using four coil channels from a previously characterized 16-channel anterior coil prototype.<i>Main results.</i>The image data received through multiplexing could be reliably divided into separate coil-channel images, preserving the coil sensitivity maps. Leakage between channels remained below -30 dB. In each coil channel the signal-to-noise ratio was penalized by a factor of two compared to direct digitization due to the inflicted data compression. Furthermore, it was shown that the<i>g</i>-factor for parallel imaging remained largely unaffected by multiplexing. Imaging speedup is expected with this multiplexer, since a fourfold channel-count increase typically yields<i>g</i>-factor improvements that are superior to the signal-quality reduction.<i>Significance.</i>A 4:1 coil-channel add-on multiplexer was developed and its functionality demonstrated in the MR readout chain at 1.5 T. We expect that this solution will enable significant imaging speedup for clinical MR systems that suffer from compatibility with a limited number of channels for the receive coil array.