Design and evaluation of an ultrasound-based bladder volume monitor

Ultrasonic bladder volume monitors have successfully been used in the diagnosis and treatment of various urological disorders. Ultrasonic bladder monitors have been developed but they have either been too bulky or too simple and inaccurate. A new, wearable ultrasonic bladder volume monitor has been designed for urological patients. The instrument consists of seven phased-array ultrasonic transducers ergonomically arranged in a circular pattern to optimise detection of the bladder walls perpendicular to the abdominal wall. A Bluetooth radio link was used to transmit data to a laptop computer, where the main signal processing was performed. After detection of bladder surface points, a three-dimensional convex hull representing the bladder was generated, and the volume was estimated. Accuracy, precision, drift over time, temperature dependency and dynamic performance were evaluated using ultrasound phantoms. Furthermore, the system was tested on one volunteer using magnetic resonance imaging (MRI) as reference. The apparatus showed no significant drift, systematic error or temperature effects. Percentage error during static volume measurements had a 95% central prediction interval of +/- 7.5% and mean absolute percentage error of 2.9%. The dynamic performance analysis showed linearity in the analysed volume interval. The in vivo study showed a high degree of correlation (R-2 = 0.99) between the volume measured using MRI and that measured with the apparatus.