Ultrafast ultrasound imaging with a matrix array and sparse random aperture for single motor unit activation analysis

Ultrafast ultrasound imaging of muscle enables the assessment of mechanical and anatomical characteristics of contracting tissue with high temporal and spatial resolution, allowing for the identification of individual motor unit (MU) territories in 2D cross-sectional image sequences. Expanding this analysis to 3D imaging would allow a detailed characterization of MU fiber's characteristics and behavior. In this feasibility study, we compare sparse random aperture compounding (SRAC) against the standard transmit/aperture sequences (BLOCK) for ultrafast ultrasound imaging acquisition using a 2D matrix array in the reconstruction of the muscle volume encompassing fibers of single MUs. The Verasonics Vantage 256 programmable ultrasound research platform coupled with a 32x32 matrix array transducer was used to collect radiofrequency (RF) data for volume imaging of the abductor digiti minimi (ADM) muscle. A full transmission (TX) event coupled with two different multiplexing strategies, standard BLOCK and sparse random aperture compounding (SRAC), were employed and compared using a 500 Hz complete volume rate acquisition. The RF data, which was concurrently acquired with high density electromyography (HDEMG) signals during 5 second low-level isometric contractions, was reconstructed using delay-and-sum beamforming employing either 4 multiplexed receive events (4RX, all 1024 elements) or only 2 (2RX, 512 elements). The 3D tissue velocity sequence (3D-TVS) was estimated obtaining a sampled volume of ~20x10x10 mm3. The firing instants of individual MUs identified from concurrent HDEMG signal acquisition and analysis were then used for spike-trigger averaging (±100 ms) the 3D-TVS. The final volume was reconstructed computing the mean correlation calculated from the triggered signals to highlight MU related motion in 3D. The obtained results are consistent with the fusiform muscle architecture of the ADM: circular territories of individual MUs are consistently localized across cross-sectional slices and elongate longitudinally along the direction of the fibers. The study demonstrates how using SRAC with a 2D matrix array enables ultrafast ultrasound imaging either using two or four RX events for analysis of single motor unit activation when coupled with HDEMG.