Abstract
The paper presents a set of experiments to characterize the ice front propagation in ex vivo bovine liver samples. Based on previous successful experiments using water and agar-gel as tested materials, the methodology infers the ice evolution in the sample from temperature measures obtained at the cooling device only, with a technique known as mirror image. No analytical or numerical solutions are proposed for the phase change propagation inside the tested material, such as for instance the Stefan approach as a classical free boundary problem for a parabolic partial differential equation. Thermal events inside the tested material, such as the heat flux released during the phase transition, have been deduced by the measured temperatures and the numerical solution of the thermal field inside the cooling device, conceived to mimic some thermal features of an actual cryogenic probe. The application domain for these experiments is in the cryoablative therapy, and the aim is to help providing an increased knowledge on the thermal effects to better deduce the effectiveness of the treatment and reduce recurrences, while at the same time avoid damages in the proximal anatomical structures. Results show that it is possible to detect the triggering of ice formation at the interface between the liver samples and the cooling device and predict the ice front propagation according to a law linear on the heat subtracted per unit of area. In a few tens of seconds, the maximum ice penetration distance is about 2 mm inside the liver tissue, with a penetration rate that goes from 0.2 mm/s to 0.02 mm/s. Moreover, adopting a special sample configured as an agar-gel coating superimposed to an ex vivo liver layer, the arrival time of the ice penetrating the liver and the temperature at the interface between these materials were detected, in order to estimate the part of the heat flux useful to the ice formation with respect to that spent for cooling the surrounding medium. Based on this preliminary result, to improve the cryoablation effectiveness it could be useful to increase the heat flux per unit of surface at the beginning, instead of the ablation duration.
Highlights
The cryoablation theme is one, among various, on which a profitable collaboration between medical doctors and engineers can be developed
During a clinical treatment the knowledge of thermophysical quantities relevant to the cryogenic probe, such as its temperature level, heat flux and cooling rate, is not the guarantee of an effective application, but it may be useful to enhance its success. Some of these quantities are directly accessible from the equipment available to the clinician, others can be attained with well oriented technical improvements, but for a successful application the major trouble is probably the lack of knowledge of what happens just beyond the Valter Giaretto and Claudio Passerone: Experimental Investigation on the Ice Formation and Growth in Ex Vivo Bovine Liver probe-tissue interface
The beginning of ice formation corresponds to a sudden release of heat that increases quite rapidly the sink temperature, as displayed in the enlargement on the top right of Figure 4
Summary
The cryoablation theme is one, among various, on which a profitable collaboration between medical doctors and engineers can be developed. During a clinical treatment the knowledge of thermophysical quantities relevant to the cryogenic probe, such as its temperature level, heat flux and cooling rate, is not the guarantee of an effective application, but it may be useful to enhance its success. Some of these quantities are directly accessible from the equipment available to the clinician, others can be attained with well oriented technical improvements, but for a successful application the major trouble is probably the lack of knowledge of what happens just beyond the Valter Giaretto and Claudio Passerone: Experimental Investigation on the Ice Formation and Growth in Ex Vivo Bovine Liver probe-tissue interface. The assessments on physical behavior and technical aspects of cryogenic probes have been addressed to preserve the clinical requirements in reducing recurrences [20,21], increasing the thermal effectiveness on treated biological tissues, avoiding damages in the proximal anatomical structures [22,23]
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