Pressure Wave Velocity Using High-Frame-Rate Ultrasound Imaging for Urodynamic Study

Abstract

Benign Prostatic Hyperplasia (BPH) is a noncancerous urologic condition in aging males where an overgrowth of the prostate gland occurs. It notably leads to lower urinary tract symptoms ranging from inconvenient nocturia to severe damage of the bladder from blockage, infection, and bladder stones. High-frame-rate ultrasound (HiFRUS) has enabled significant advances in urology through visualization of high temporal urinary flow dynamics within the male urinary tract during voiding. Such work has shown that characteristic flow patterns for pathological states found within the diseased male urinary tract, such as urethral obstruction, can be used for non-invasive assessment. However, complementing urinary flow are pressure wave dynamics propagating locally along the urethral tissue wall that has not been studied before. Here HiFRUS is utilized again to visualize and study in-vitro these local dynamics for a new metric to assess the male urinary tract state called the pressure wave velocity (PreWV). This research presents the first investigation on how significant the initiation of voiding in the collapsed and tortuous geometry of the urinary tract, which is common to BPH, are on the regional and subregional PreWV response. A new tortuous flow phantom platform was devised to facilitate an in-vitro urodynamic study to collect PreWV data using HiFRUS and perform tissue Doppler analysis. To study the effect in changing the tortuous geometry, the male urinary tract is modelled by two deformable bent geometry phantoms, one with 30° bend and another with 45° bend. These phantoms were fabricated using a lost-core strategy protocol. To study the effect in changing the collapsed state, the phantoms were deflated and encased in a tissue mimicking slab where the opening of the collapsed lumen was controlled in a flow circuit setup. In this setup, initiation of voiding by a diseased patient was mimicked with a flow pump set at 15% duty cycle, 7 mL/s peak flow rate and 4 second period. HiFRUS imaging views near the inlet, at the bend, and near the outlet in each phantom were acquired through plane-wave imaging using a 192-channel linear array transducer connected to an ultrasound open research platform. Tissue Doppler analysis was employed on the beamformed frames to obtain the arrival times of the wall velocity at each lateral position in 0.1 mm lateral steps along the segmented phantom walls. Global PreWV results from this analysis were validated with the ground truth PreWV measured by pressure sensors that were connected to the inlet and outlet ports of the imaging setup. Regional and subregional PreWV results were validated through statistical analysis and agreed with theory regarding pressure wave propagation in human tissue. For both the 30° and 45° phantoms, no statistical significance was observed on the PreWV near the inlet and outlet when compared to the ground truth (p>0.14; N=11). The subregional PreWV after the bend site showed a relative decrease in PreWV of 44% and 58% for the 30° and 45° phantoms respectively when compared to the subregional PreWV before the bend (p<0.001; N=11). Furthermore, a noticeable difference in the bend regional PreWV was observed when comparing the 30° and 45° phantoms (p<0.001; N=11). It was found that a 15° increase in the phantom bend angle translated to 4 times increase in the difference between the subregional PreWV before and after the bend, thus highlighting the significance of the tortuous and collapsed geometry effects on the PreWV mapping. Overall, this work establishes PreWV as a novel metric to assess the urinary tract at sub-millisecond temporal resolution using HiFRUS and may serve as a complementary method to non-invasively distinguish pathological states due to BPH in future urodynamic studies.