Methods for Characterizing Convective Cryoprobe Heat Transfer in Ultrasound Gel Phantoms

Abstract

While cryosurgery has proven capable in treating of a variety of conditions, it has met with some resistance among physicians, in part due to shortcomings in the ability to predict treatment outcomes. Here we attempt to address several key issues related to predictive modeling by demonstrating methods for accurately characterizing heat transfer from cryoprobes, report temperature dependent thermal properties for ultrasound gel (a convenient tissue phantom) down to cryogenic temperatures, and demonstrate the ability of convective exchange heat transfer boundary conditions to accurately describe freezing in the case of single and multiple interacting cryoprobe(s). Temperature dependent changes in the specific heat and thermal conductivity for ultrasound gel are reported down to À150 C for the first time here and these data were used to accurately describe freezing in ultrasound gel in subsequent modeling. Freezing around a single and two interacting cryoprobe(s) was characterized in the ultrasound gel phantom by mapping the temperature in and around the "iceball" with carefully placed thermocouple arrays. These experimental data were fit with finite-element modeling in COMSOL Multiphysics, which was used to investigate the sensitivity and effectiveness of convective boundary conditions in describing heat transfer from the cryoprobes. Heat transfer at the probe tip was described in terms of a convective coefficient and the cryogen temperature. While model accuracy depended strongly on spatial (i.e., along the exchange surface) variation in the convective coefficient, it was much less sensitive to spatial and transient variations in the cryogen temperature parameter. The optimized fit, convective exchange conditions for the single-probe case also provided close agreement with the experimental data for the case of two interacting cryoprobes, suggesting that this basic characterization and modeling approach can be extended to accurately describe more complicated, multiprobe freezing geometries. Accurately characterizing cryoprobe behavior in phantoms requires detailed knowledge of the freezing medium's properties throughout the range of expected temperatures and an appropriate description of the heat transfer across the probe's exchange surfaces. Here we demonstrate that convective exchange boundary conditions provide an accurate and versatile description of heat transfer from cryoprobes, offering potential advantages over the traditional constant surface heat flux and constant surface temperature descriptions. In addition, although this study was conducted on Joule-Thomson type cryoprobes, the general methodologies should extend to any probe that is based on convective exchange with a cryogenic fluid. [DOI: 10.1115/1.4023237] Keywords: cryosurgery, cryotherapy, cryoablation, thermal properties, heat transfer modeling, clinical modeling IntroductionCryosurgery is a minimally invasive technique in which freezing temperatures are used to destroy tissue in the body to treat various conditions, including cancer (prosta...

An efficient thermal evolution model for cryoablation with arbitrary multi-cryoprobe configuration

“…For the spherical tissue, the blood perfusion is set as q b c b x b = 1.8 kW/m 3°C for T > 0°C and zero for T < 0°C, and Q m = 0. The thermal properties qc (J/kg°C) and k (W/m°C) are considered as [6]:…”

“…The position of cryoprobe is determined by its orientation n and the center position r 0 (x o , y o , z o ). The convective heat transfer boundary condition of the cryoprobe wall is considered as k w oT/on = h p (T p À T) [6,11], where k w is the thermal conductivity of the tissue contacting with cryoprobe, T p is the temperature of cryoprobe wall, h p is the convective heat transfer coefficient. Considering the stable-state temperature field of the infinite cylinder with the inner wall R p (fixed by temperature T p ) and outer boundary R (fixed by temperature T R ), its solution can be analytically solved (shown in Fig.…”

“…However, FDM is difficult to accurately characterize the heat transfer of the thin cylindrical cryoprobe based on cubic voxels, where the temperature changes dramatically during the freezing process. The thermal boundary condition of the cryoprobe wall could be assumed as the fixed temperature or convective heat transfer type [6,11]. A direct method of the heat flux estimation from cryoprobe is to compute the temperature difference between cryoprobe wall and the intersected voxel, where the smaller voxel should be adopted for the accurate calculation [10,23].…”

Transient Thermal Mixed Boundary Value Problems in the Half-Space

“…Cryosurgery is the application of extreme cold to destroy abnormal tissue, often using liquid nitrogen, and it holds great promise for the treatment of a variety of medical problems in tissue, including cancer [14]. Its appeal is that it is minimally invasive and can treat local tissue while ensuring that surrounding tissue is relatively unaffected.…”

“…First, similar to the application regarding heat loss from buildings described above, a more complicated boundary condition regarding convective heat flow inside the probe should be incorporated so that boundary conditions of the form (6.3) could be considered [14]. Second, in reality this problem is three-dimensional, although as stated it would generally be assumed axisymmetric about the vertical line of the probe.…”