# Wearable 256-Element MUX-Based Linear Array Transducer for Monitoring of Deep Abdominal Muscles
## 4 Discussion  
Our wearable ultrasound platform consists of a MUX-based linear array probe specifically designed for monitoring muscle motion in the context of training combined with a flexible multichannel research system. Until recently, this wireless version of our previous platform, MOUSE, was the only system combining native multichannel capabilities in transmit and receive with full access throughout the entire data pipeline and a wireless data link. It has only recently been outperformed by the new state-of-the-art wearable ultrasound TinyProbe [19], which comes in a significantly reduced size and weight while still providing 32 TX/RX and a wireless data link. Our system is derived from a wired system with a focus on maximum flexibility for research applications (adjustable transmit voltage, arbitrary delay profiles, 12 Bit analog-to-digital conversion (ADC) with 50 MSamples/s (MSPS), adjustable number of samples per receive channel), TinyProbe comes with more integrated components with optimal performance-per-mW, resulting in an unprecedented compactness and wearability. The combination of our 32 ch electronics with our new MUX-based linear array transducer showed an imaging performance very similar to that of a 256 ch system and a fully populated array in terms of resolution and SLL but with a decrease in contrast, most likely due to the lower number of transmit elements. A penetration depth of 11 cm was achieved with our 5.8 MHz linear probe, and lesions could be visualized up to 7 cm. In  
Figure 7: Investigation of acoustic coupling stability and analysis of change of TrA thickness during SSE. While static images such as C could be obtained with the wireless data link, the USB link was used for ideal temporal sampling of the TrA thickness change in (**F**). Figure (**A**) shows the setup with the probe in the custom holder, ensuring reliable acoustic contact. Figure (**B**) shows the signal amplitude over time as a metric for acoustic coupling in two proband positions. (**C**) the gives a B-mode image of the abdomen showing the different muscle layers. (**D**) The flow field image shows the displacement of specific landmarks in the B-mode image during the breathing cycle and SSE. (**E**) Segmented image with specific points at which the thickness of the muscle layer is followed over time. (**F**) Analysis of the muscle layer thickness over more than 1000 frames (corresponding to 30 s). The reddish area defines the SSE contraction. the in-vivo setting for which the system was designed, we could demonstrate stable and reliable acoustic coupling during segmental stabilization exercise by means of a customized probe holder, eliminating the need for the transducer to operate in a handheld manner. The goal of in-vivo imaging was a proof-of-concept of our probe's capability for wearable use and monitoring of deep abdominal muscles during exercise. The validation of the system's effectiveness as a biofeedback instrument on a larger target population was beyond the scope of the work and will be the object of a follow-up study. Nevertheless, we could demonstrate suitable image quality for identification and tracking of the muscle structures of interest in a realistic setting. The implementation of a muscle tracking algorithm, which was performed offline on image data reconstructed with a plane wave compounding scheme adapted to the specific transmit/receive setting, allowed measuring the thickness of the muscle structures of interest (Figure 7). At the same time, the in-vivo experiments showed the main limitation of the system, which is the Wi-Fi data link, which consists of a bottleneck. As reported, 40 frame/s of pre-beamformed channel data could be transmitted, which, however, results in 10 frames/s for a full aperture, given the need for switching between the four MUX positions. Although this seems acceptable for applications with low temporal dynamics, it has to be mentioned that multiple transmit angles are necessary for ideal image quality, resulting in a further decrease in frame rate.