Noninvasive bladder volume measurements based on nonlinear wave distortion.

The purpose of this study is to suggest a new approach to measure noninvasively the fluid content of a human cavity; in particular, the bladder volume. Determination of bladder volume is important clinically for patients suffering from bladder dysfunction. Several ultrasonic methods were proposed in the past for such a purpose. Most of these methods are based on multiple cross-sectional images and detection of echoes from the bladder wall. We propose here, in a first step, a simple approach that is able to provide an indication of whether or not the bladder volume has exceeded a certain threshold volume. Second, the possibility of accurate volume assessment will be discussed. The approach takes advantage of the difference in harmonic generation of liquids (urine) and tissues. We know that nonlinear effects occur most strongly when ultrasound (US) propagates through liquids with relatively low acoustic attenuation, such as water or urine. However, within soft tissues, the tendency for wave distortion to occur is limited as a result of different acoustic characteristics, most notably the attenuation. Our method is based on measuring the presence of harmonics in an echo from a region-of-interest (ROI) at fixed depth beyond the bladder. The harmonic content in the echo will increase with increase of fluid in the echo path. Phantom measurements were carried out with a single-element transducer and a phased-array transducer. Phantoms containing different volumes were used. Furthermore, measurements on volunteers were performed. The results confirmed that the harmonic content of an echo measured at a deep ROI increases for a full bladder and decays strongly after the volunteer has voided. These preliminary results demonstrate the feasibility of the approach.