Analysis of the Weinbaum-Jiji model of blood flow in the canine kidney cortex for self-heated thermistors.

The Weinbaum-Jiji equation can be applied to situations where: 1) the vascular anatomy is know; 2) the blood velocities are known; 3) the effective modeling volume includes many vessels; and 4) the vessel equilibration length is small compared to the actual length of the vessel. These criteria are satisfied in the situation where steady-state heated thermistors are placed in the kidney cortex. In this paper, the Weinbaum-Jiji bioheat equation is used to analyze the steady state response of four different sized self-heated thermistors in the canine kidney. This heat transfer model is developed based on actual physical measurements of the vasculature of the canine kidney cortex. In this model, parallel-structured interlobular arterioles and venules with a 60 microns diameter play the dominant role in the heat transfer due to blood flow. Continuous power is applied to the thermistor, and the instrument measures the resulting steady state temperature rise. If an accurate thermal model is available, perfusion can be calculated from these steady-state measurements. The finite element simulations correlate well in shape and amplitude with experimental results in the canine kidney. In addition, this paper shows that the Weinbaum-Jiji equation can not be used to model the transient response of the thermistor because the modeling volume does not include enough vessels and the vessel equilibration length is not small compared to the actual length of the vessel.