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Research Papers

Methods for Characterizing Convective Cryoprobe Heat Transfer in Ultrasound Gel Phantoms

[+] Author and Article Information
Michael L. Etheridge

Department of Mechanical Engineering,
Department of Biomedical Engineering,
University of Minnesota,
Minneapolis, MN 55455

Jeunghwan Choi

Department of Mechanical Engineering,
University of Minnesota,
Minneapolis, MN 55455

Satish Ramadhyani

Galil Medical Inc.,
Arden Hills, MN 55112

John C. Bischof

Department of Mechanical Engineering,
Department of Biomedical Engineering,
Department of Urologic Surgery,
University of Minnesota,
Minneapolis, MN 55455

Contributed by the Bioengineering Division of ASME for publication in the JOURNAL OF BIOMECHANICAL ENGINEERING. Manuscript received September 4, 2012; final manuscript received December 6, 2012; accepted manuscript posted December 22, 2012; published online February 7, 2013. Assoc. Editor: Michael Sacks.

J Biomech Eng 135(2), 021002 (Feb 07, 2013) (10 pages) Paper No: BIO-12-1391; doi: 10.1115/1.4023237 History: Received September 04, 2012; Revised December 06, 2012

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 −150 °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.

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Figures

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Fig. 3

Model setup for 2D, axisymmetric, single-probe case (a). The temperature (T) distribution around the freezing cryosurgical probe is numerically solved using the described boundary conditions and heat transfer equations. The dual-probe case is modeled using the same heat transfer conditions, but utilizes a 3D geometry with symmetry planes ((b), symmetry highlighted in green on near and left-hand faces).

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Fig. 2

Experimental setup. An array of thermocouples was positioned around the cryoprobe(s) during freezing at different radial positions (r) and on different measurement planes (z). The temperature distribution was measured for both single- and dual-probe cases using the same placement jig.

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Fig. 1

Simplified internal exchange flow for the 1.5 mm diameter IceSeed (Galil Medical Inc., Arden Hills, MN) cryoablation needle studied here. Compressed argon gas flows down through a center channel and impinges on the top of a stainless steel tip, where significant heat exchange occurs. Convective exchange continues up the walls of the probe for 12 mm, at which point the cryoprobe is insulated for the remainder of the shaft.

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Fig. 6

Fitted values for the convective heat transfer boundary conditions for case 4 (see Table 2)

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Fig. 5

Measured temperature profiles at the z = 4 mm plane and various radial distances for the single- and dual-probe cases (b–d). The single-probe case is plotted as the black curve for each radial distance. Colored curves correspond to the thermocouple grid locations (a), as first described in Fig. 2.

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Fig. 4

Measured ultrasound gel specific heat (a) and thermal conductivity (b). Reference values for water and ice were included for comparison [42].

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Fig. 7

Comparison of single- and dual-probe model results. Note the difference in isotherm shapes between the left and right sides of the probe. Isotherms on the left side are strongly influenced by the presence of the second probe and interactions between the probes results in expanded isotherms on the right side as well.

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