A cross-shaped combined ultrasound array patch for continuous bladder volume monitoring

Continuous monitoring of bladder volume is important for the assessment and management of lower urinary tract dysfunction. The study aims to develop a wearable non-invasive ultrasound patch for continuous bladder volume monitoring, addressing operator dependence, limited field of view (FOV), and manual probe rotation required in conventional handheld ultrasound. A cross-shaped combined ultrasound array patch (CCUA-Patch) was proposed using three 64-element linear arrays, including two vertically aligned arrays for extended sagittal imaging and one horizontal array for transverse imaging. Time-division scanning enabled multi-plane acquisition, and a feature-based stitching algorithm reconstructed a complete sagittal image. Bladder contours were automatically extracted, and volume was estimated using the prolate ellipsoid method. A 2 mm self-adhesive hydrogel layer was used to ensure stable acoustic coupling for hands-free continuous monitoring. The fabricated transducers exhibited a center frequency of 2.60 ± 0.02 MHz and a -6 dB bandwidth of 86.67 ± 2.73%. In phantom experiments, the estimated bladder volume was 126.14 mL versus a calibrated volume of 137 mL, with only 0.89 mL difference from a commercial ultrasound probe. In a pilot study of 10 healthy volunteers, the CCUA-Patch showed good agreement with the commercial device, with a mean Bland-Altman bias of 17.48 mL and most measurements falling within the 95% limits of agreement. The proposed CCUA-Patch expands the effective sagittal FOV, eliminates manual probe reorientation, and provides a practical solution for automated monitoring of the bladder volume. However, the present device used a benchtop Vantage 256 platform and only ten healthy volunteers; it therefore demonstrates the wearability and feasibility of the patch structure, not a fully wearable clinical system. Larger studies in target patient populations and integration with miniaturized, low-power readout electronics are required before clinical or home use.