# Wearable 256-Element MUX-Based Linear Array Transducer for Monitoring of Deep Abdominal Muscles
First, routine use of ultrasound in physiotherapy rehabilitation would require low-cost ultrasound imaging systems, which provide imaging quality that is good enough to assess the change of the thickness of the TrA reliably in real-time. Second, such a system should ideally be wearable or at least compact and portable, such that it is usable during exercise. In particular, probes must be designed such that they can be worn or attached to patients during exercise, in contrast to standard sonography probes, which are made for handheld use. Finally, since expertise in the interpretation of ultrasound data is not always given (for physiotherapists and especially for patients), methods of image interpretation and conversion of image data into easy visual feedback are required. In the last years, a number of portable or even wearable ultrasound probes have been developed [11; 12; 13; 14; 15; 16]. However, from a closer look, many of them are wearable ultrasound transducers, which still require bulky table-top multichannel electronics to drive them [11; 12; 13]. Nevertheless, fully integrated, miniaturized and wearable ultrasound systems have been reported [14; 15; 17]. In wearable ultrasound, especially when the applications require (B-Mode) images, the huge amount of data that needs to be acquired and processed represents a main challenge. Since B-mode relies on driving array transducers with typically 100 to 200 elements, a digitization scheme either relying on parallel channels or on multiplexing needs to be implemented. Due to the size and power consumption constraints, most so far presented wearable ultrasound systems rely on a reduced channel count combined with multiplexers (in contrast to table-top multichannel ultrasound research systems with dozens to hundreds of native TX/RX channels). In addition to challenges with respect to the form factor, the large number of digitization channels needed for imaging represents a burden for the implementation of a wireless data link. On the other hand, since ultrasound imaging is typically based on very low-duty cycles (DC), thermal dissipation does not play a major role as a design constraint. With respect to the data handling issue, most so far presented potentially portable systems with many native transmit and receive channels that had no wireless data link [18]. However, very recently, an ultra-compact system with 32 TX/RX channels, a wireless data link (Wi-Fi up to 21.6 MBps), low power mode and less than 60 mm in the largest dimension has been presented [19]. Here, a design optimized for wearable applications was realized by using a 32 ch IC (Integrated Circuit) focusing on energy efficiency and size rather than flexibility (e.g., max. 16 delay profiles, 2048 samples/ch and 64 Vpp excitation). Up to now, imaging performance comparable to established sonography devices in a handheld format has only been achieved in so-called POCUS (Point of care ultrasound) devices such as the Butterfly IQ (Butterfly Network, Burlington, VT, USA), where the entire data processing pipeline (from channel data to images) has been integrated into the probe by means of customized ASICs [20]. However, such systems, despite being highly compact, provide no interfaces for use in research tasks and have limited suitability for wearable applications (since probes are optimized for handheld use). In previous work, we presented a compromise between such miniaturized wearable devices and table-top systems consisting of a small portable system with native 32 TX/RX channels and full access to pre-beamformed channel data [16]. Beyond the availability of all data formats throughout the pipeline, from pre-beamformed channel data to image data, the flexible architecture allows combination with all kinds of transducers, representing an advantage in research applications.