# Wearable 256-Element MUX-Based Linear Array Transducer for Monitoring of Deep Abdominal Muscles
## 3 Results
### System Characterization
#### 3.1.4 System Imaging Performance--Phantom Study  
The system's image quality was assessed on the above-mentioned commercial multi-purpose tissue phantom and on an in-house wire phantom. The tissue-mimicking phantom has an acoustic attenuation in the range of human tissue of approximately 0.5 dB/(MHz cm) and includes scatterers for reproducing typical speckle patterns and echo-free voids approximating the acoustic behavior of vessels. While the wire phantom was used for quantitative assessment by analyzing relevant image metrics such as the FWHM and the SLL (side lobe level), the commercial tissue-mimicking phantom was used for qualitative comparison of the images and for quantitative assessment of the contrast. Figure 6A,B show the results when using the same acoustic block of 256 elements but driven by a 256-channel electronics system instead of using the MUX setup. In Figure 6A, the CIRS phantom is imaged in conventional PWC mode (with 21 angles), which represents the ideal configuration and, consequently, the best image quality. In Figure 6B, the ASICS use case is mimicked by using only 128 elements in transmit, where four adjacent uneven-numbered elements (e.g., elements 1, 3, 5, and 7) are excited with the same transmit delay, such as in the case of the MUX-based array. Finally, in Figure 6C, the array with integrated MUX, such as shown in Figure 5, is used. Here, 128 elements are used in transmit, such as in B, but the MUX is integrated into the receive path, and four acquisition steps are necessary to get data from all elements (with 20 plane waves in each step). The experiment relating to case C has been performed without averaging and with 8 times averaging. Only the averaged data, representing the best possible image quality with the MUX-based array, is shown as reconstructed B-mode. However, image metrics relating to the contrast are given in Figure 6D for all cases (with/without averaging). For assessment of the contrast, the contrast-to-noise ratio (CNR), which is typically used to quantify lesion detectability, and the contrast ratio (CR) were calculated as described in [33]. For this purpose, the mean value and standard deviation of the envelope-filtered data inside the lesion (red circle in Figure 6C) and in the speckle background (yellow circle in Figure 6C) were taken. The data shows that echo-free lesions in the phantom can be imaged with a CNR of 4 dB up to a depth of 45 mm with the MUX-based array. In terms of the CR, we see a larger difference between the MUX-based and the fully populated array, especially for the most shallow lesion, which might be due to near-field effects resulting from addressing the MUX-based array in groups of 4 transmit elements with identical delays. When it comes to the wire phantom, the data shows that the reflectors can be seen with excellent contrast over the full image depth of 12 cm. The image seems darker in cases B and C for depths larger than 10 cm, which results from the lower transmit pressure (since only half of the elements are transmitting when compared to A). Finally, all echo-free voids can be identified in all configurations. When it comes to the quantitative image metrics, the lateral resolution decreases from 600 \(\upmu\)m to approximately 1000 \(\upmu\)m in all three cases, with limited influence on the number of angles in PWC. The SLL ranging between \(-\)30 and \(-\)45 dB is very similar in all array configurations, with the number of angles having a larger impact than the array type.