Recent findings suggest that shear stresses, induced by urine flow, may stimulate the proliferation and maturation of bladder cells such as urothelial cells, while abnormal shear stresses could have detrimental effects on these cells [1]. Changes in flow patterns may arise in urinary tract disorders (e.g., overactive bladder, ureteral reflux, and obstruction) [1,2]. Given that flow patterns within the urinary tract are still poorly understood, it is crucial to first explore these patterns in healthy bladders [2]. This will enable subsequent studies to investigate alterations in flow patterns in diseased bladders. Although advancements in imaging technologies, particularly Magnetic Resonance Imaging (MRI), have provided powerful capabilities (e.g., for dynamic non-invasive investigations of geometry and flow in the cardiovascular system), their application in the urinary tract has been very limited and primarily qualitative [2,3]. In the present study, our aim is to explore the feasibility of using Phase-Contrast MRI (PC-MRI) to quantitatively measure urine velocity in the bladder. As an example, our focus was on the ureteral jet since it has recently been proposed as an indicator of ureteral obstruction.

Following institutional guidelines, one adult male was recruited for this pilot study (inclusion criteria: healthy adult without lower urinary tract symptoms; exclusion criteria: contraindication to MRI e.g., pacemaker, metallic implants, claustrophobia). MRI was performed using a clinical 3T scanner (MAGNETOM Terra and Prisma, Siemens Healthcare, Erlangen, Germany) using a 32-channel body surface coil. The volunteer was instructed to empty his bladder approximately 90 minutes before the MRI session and to drink approximately 1.5L of water until the MRI scan in order to reach a full bladder volume for the MRI session. 
Phase-contrast velocity mapping: PC-MRI has the capability to identify flow in the three cardinal directions, aligned with the bipolar encoding gradients. Since the phase shift is proportional to the velocity, it can be used to quantify moving fluids. This enables the determination of the three components of the velocity vector: Ux, Uy, and Uz. These components are visualized individually as velocity maps (i.e. images where the pixel intensities are proportional to velocity). In this study, we focused on the in-plane velocity components Ux and Uy. Dynamic images with an update rate of 1.63 s (30 images in total) were acquired in a sagittal plane with a slice thickness of 10 mm and an in-plane resolution of 1.7 x 2.1 mm. The in-plane velocity encoding (VENC) in x- and y-direction was set to 10 cm/s.The resulting mean velocity magnitude (U) in a region-of-interest (ROI), as shown in the Figure 1, is calculated as U=√(U_x^2+U_y^2 ).

The magnitude images at three different time points (t=14.6, 17.8, and 21.1s) are illustrated in Figure 1A. Corresponding velocity-encoded images (velocity maps) for Ux and Uy are shown in Figure 1B and C, respectively. Despite the bladder being considered at rest, laminar vortices were observed in the magnitude images and propagated within the entire bladder. Notably, these complex flow patterns persisted even when the subject held his breath. Since no visible bladder wall contraction occurred, at least in two dimensions (2D), we hypothesize that these vortices primarily originated from the propagation of ureteral jets within the bladder. Periodic ureteral jets manifested as distinct dark regions in the velocity-encoded images. Velocity plots for Ux, Uy, and resulting U, associated with the ROI, are shown in Figure 2. While Uy remained relatively constant throughout the acquisition period, periodic peaks of Ux occurred approximately every 15 seconds, confirming predominant intermittent jet flow in the x-direction. The estimated wall shear stresses were in the range 0-0.02 Pa.

Previous findings by Falk et al. [2] demonstrated the existence of vortices within the bladder of healthy volunteers, even when the bladder was at rest, suggesting the influence of ureteral jets on global bladder flow patterns. Our preliminary results corroborate these qualitative observations. Additionally, we present, to the best of our knowledge, the first quantitative measurements of urine velocity within the bladder by means of MRI phase-contrast velocity mapping. This novel approach offers valuable insights into the fluid dynamics of urine within the bladder and the whole urinary tract, by providing high resolution velocity measurements (both in space and time).

Our study highlights the potential of PC-MRI as a promising alternative to traditional clinical techniques like Doppler ultrasound (US) for urinary tract investigation. Unlike US, MRI is not limited by window constraints and offers more comprehensive insight. Through dynamic high-fidelity morphological imaging and velocity measurements, PC-MRI demonstrates promising capabilities in capturing intricate fluid dynamics. We anticipate that additional data will be collected to further comprehend healthy bladder fluid dynamics, crucial for the development and validation of numerical models. Subsequent investigation into pathological bladder conditions will build upon this foundation.

Hou C, Gu Y, Yuan W, Zhang W, Xiu X, Lin J, et al. Construction of a three-dimensional urothelium on-chip with barrier function based on urinary flow microenvironment. Biofabrication. 2023 Jul 1;15(3):035002.
Falk K., Usta M., Advolodkina P. Aidun C.,Kelly R, Imaging bladder fluid dynamics using MRI, September 2019 American Urogynecologic Society PFD Week 2019, Nashville, TN.
Pewowaruk R, Rutkowski D, Hernando D, Kumapayi BB, Bushman W, Roldán-Alzate A. A pilot study of bladder voiding with real-time MRI and computational fluid dynamics. Hurst RE, editor. PLOS ONE. 2020 Nov 19;15(11):e0238404

Continence 12S (2024) 101624
DOI: 10.1016/j.cont.2024.101624