(219d) Computational Fluid Dynamics to Understand Ureteroscopy Irrigation

Ureteroscopy with laser lithotripsy is a minimally invasive kidney stone treatment that is a common process by urologists and is becoming more important with the lifetime prevalence of kidney disease.[1] The treatment is primarily used for kidney stone removal when stones cannot be passed by the patient. The procedure involves the insertion of an ureteroscope, a very narrow scope, that allows urologists to travel up the ureter to the kidney that includes a camera to assist in finding the stones and monitoring the removal process. Ureteroscopy uses the energy pulse of a laser beam, applied directly to the stone, to chip it away into pieces small enough to pass through the ureter. The treatment also involves a process called ureteroscopy irrigation, in which a pressurized channel of saline solution is injected into the ureter and the kidney. Ureteroscopy irrigation is used to secure a better field of vision inside the kidney, it also helps provide a spontaneous flow passage for kidney stone fragments to move out of the kidney and into the bladder without using any additional instrumentation.[2] As a result, the irrigation aids urologists in locating the precise location of the stones in the kidney and helps decrease procedure time. The goal of this project is for the first time to compare and contrast laboratory measurements of the ureteroscopy irrigation process for flow velocity and pressure with simulated values obtained from computational fluid dynamics (CFD) simulations.

CFD is a branch of mechanics that creates simulations for a variety of applications and is becoming a useful tool in medical applications to study the physiological flow patterns of fluids in the human body.[3] We have recently found that CFD can accurately predict the flow patterns and streamlines of flow in the kidney and has correctly predicted where new stones will form based on the flow obstruction patterns when other stones are present. These findings have led to the construction of an in vitro hydrogel model of an actual patient kidney where measurements of velocity and pressure can be made at different locations and compared to the CFD simulations. Geometries were obtained from scans of patients’ anatomy and inserted into the CFD program. The ureteroscope was drawn into the geometry and placed at 5 different locations within the geometry to obtain representative fluid velocity and intrapelvic pressure values that were validated with measured values from the physical model. The main challenge of this work is to create an accurate 3-dimensional mesh throughout the geometry where the CFD software uses finite volume methods to solve the basic equations of fluid dynamics using iterative procedures to obtain several million data points throughout the geometry. The optimization and manipulation of the geometries to create an accurate mesh is generally the rate limiting step within the CFD simulation. The quality of the mesh corresponds directly to the accuracy of the simulation. The scope placement must be realistic and accurate enough for the simulation to be representative of the procedure.

This project involves the application of CFD modeling of the fluid velocity and intrapelvic pressure during ureteroscopy. The results have provided a novel way to computationally simulate a patient’s specific and unique flow patterns and pressure gradients during ureteroscopy procedures. Additionally, CFD can be used to establish a heat transfer profile during laser lithotripsy. The first goal of the project is to compare the physical and CFD models for velocity and intrapelvic pressure profiles corresponding to different irrigation velocities and pressures. Using CFD simulations extensive velocity and pressure profiles were generated within the kidney and ureter, providing optimal settings to the urologist. The second goal of the project is to examine the heat transfer mechanism of the laser lithotripsy. Optimal irrigation velocities and pressures have been a topic of debate among urologists due to being solely based on empirical results. Using CFD we are able to obtain time-dependent profiles of the heat transfer process in order to establish a safe working range for the laser lithotripsy process. These simulations could significantly advance the confidence that this procedure is being done as safely as possible.

References:

1. University of Wisconsin Hospitals and Clinics Authority. “How Common Are Kidney Stones?” UW Health, uwhealth.org/urology/how-common-are-kidney-stones/11208.
2. Wright, Anna E et al. “Ureteroscopy and stones: Current status and future expectations.” World journal of nephrology 3,4 (2014): 243-8. doi:10.5527/wjn.v3.i4.243
3. Williams, Jessica G et al. “The Fluid Mechanics of Ureteroscope Irrigation.” Journal of endourology 33,1 (2019): 28-34. doi:10.1089/end.2018.0707