Bladder Phantom Pressure Measurement with Contrast-Enhanced Ultrasonography
Kourosh Kalayeh, PhD, J. Brian Fowlkes, PhD, Amy Chen, Undergraduate student, Sophia Yeras, Undergraduate student, Mario L. Fabiilli, PhD, Jake Claflin, MD, Stephanie Daignault-Newton, MS, William W. Schultz, PhD, Bryan S. Sack, MD.
University of Michigan, Ann Arbor, MI, USA.
Background: One of the major challenges in urodynamic testing is the presence of an indwelling urethral catheter during voiding pressure evaluation. The space-occupying property of the catheter results in non-physiological conditions, which in turn, can cause misdiagnosis. In this study we hypothesize that ultrasound-based technique known as subharmonic-aided pressure estimation (SHAPE) can measure pressure in a bladder phantom. SHAPE is based on the principle that subharmonic emission of ultrasound contrast agents (UCAs) decreases linearly with increasing ambient pressure.Methods: Using a bladder phantom capable of inducing clinically relevant bladder pressures (Fig. 1), we simulated a cystometrogram (CMG). A typical simulated CMG consisted of one filling and four post-filling events over 30 minutes (simulating voiding without flow and emptying). This prolonged time of assessment examined SHAPE efficacy in measuring bladder pressures in clinical instances when a patient cannot void immediately after filling is stopped. During CMG events, the phantom was pressurized multiple times at different clinically relevant levels (Fig. 2). Simultaneously, the scattered subharmonic signal was acquired, correlated with catheter pressure and the SHAPE conversion factor in dB/cmH2O was determined (Fig. 3). The GE LOGIQ E10 ultrasound scanner equipped with the SHAPE mode and the C2-9 probe was used for ultrasound scanning. The experiments were repeated with n=3 vials of laboratory-made microbubbles similar to DEFINITY.Results: A strong inverse linear relationship is found between SHAPE and bladder phantom pressures for all the CMG filling and post-filling events (r2 > 0.9, RMSE < 0.3 dB, standard error < 0.01 dB, and p < 0.001). The SHAPE conversion factor varies between filling and post-filling events, as well as, by post-filling time (Figs. 3 and 4). The magnitude of the SHAPE conversion factor tends to increase immediately after filling, and then decreases with time.Conclusions: Microbubble subharmonic emission is an excellent indicator of pressure in the bladder phantom. The strong correlation between SHAPE signal and bladder phantom pressure indicates this method could measure bladder pressure during a CMG. However, our results suggest that different SHAPE conversion factors may be needed for different CMG time points. For patient studies, at the time of urinary urgency, an updated conversion factor may be needed just prior to catheter removal. These findings will help us better protocolize this method for introduction into human subjects allowing us to take the next step toward developing a catheter-free voiding CMG using SHAPE.
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