ESTRO 2024 - Abstract Book

S3780

Physics - Image acquisition and processing

ESTRO 2024

587

Digital Poster

Exploring signal multiplexing for next-generation high-density MRI-coil arrays on the MR-linac

Tom Bosma, Jan G.M. Kok, Cornelis A.T. van den Berg, Martin F. Fast

UMC Utrecht, Radiotherapy, Utrecht, Netherlands

Purpose/Objective:

Ideally, MR-guided online-adaptive radiotherapy treatment (MRgRT) should be applicable to lesions throughout the human body. However, thoracic MRI remains very challenging, as cardiorespiratory motion necessitates fast image acquisition to achieve tight margins for the treatment volume, testing the sensitivity limits of today’s low-density MRI coil arrays on the MR-linac. An established way to speed up MRI acquisitions is to increase the number of coil channels in the scanner’s receive array, since it allows for more averaging and sensitivity encoding (SENSE). In this regard there is ample room for improvement in clinical MRgRT systems: a factor five speed improvement was demonstrated in earlier work [1] by replacing the default 8-channel coil array (Fig. 1a,c) of the 1.5 T Unity MR-linac (Elekta AB, Sweden) with a 32-channel coil array (Fig. 1b,d). Unfortunately, clinical implementation of this improved array is currently impossible due the severely limited number of read-out channels on the 1.5 T MR-linac: only 12 digitizers are present whereas generally one per coil channel is required. In this study we prototype and investigate a novel way of circumventing the mismatch between the number of desired MRI coil elements (32) and the number of readout channels (12) on the MR-linac by multiplexing signals from four MRI coils into a single read-out channel. We developed a printed circuit board (PCB) with a time-division-based 4-to-1 multiplexer (mux, N mux =4) where up to four radiofrequency (RF) input channels can be passed onto a common output (Fig. 1e, three inputs used). A set of cascaded switches determined which channel was passed onto the common output at any given time. The MRI system’s internal clock was used to control the switches. In this way a 62.5 kHz channel repetition rate was used, synchronized with the system’s 10 MHz clock. Finally, a trigger input allowed for synchronizing the mux timings with any given MRI sequence. To demonstrate the functionality of the mux, we installed the prototype coil array in a Unity MR-linac and scanned the tilted square in a standard Philips image quality (PIQT) phantom twice, once with and once without mux. In one acquisition, signals from three selected coil channels were passed directly to three separate digitizers and in the other scan the same coil channels were combined in the mux and passed onto a single digitizer. The fourth mux channel was terminated internally for reference. A 2D T2-weighted fast-field-echo sequence was used with 1 MHz sampling rate. Demultiplexing was done in post-processing where the raw k-space data was binned according to the four mux transmission windows along the time axis. The three bins corresponding to the connected coil channels were then separated and reconstructed individually. Material/Methods: