Assessment of thermochromic phantoms for characterizing microwave ablation devices.

BACKGROUND AND PURPOSE: Thermochromic gel phantoms provide a controlled medium for visual assessment of thermal ablation device performance. However, there are limited studies reporting on the comparative assessment of ablation profiles assessed in thermochromic gel phantoms against those in ex vivo tissue. The objective of this study was to compare microwave ablation zones in a thermochromic tissue mimicking gel phantom and ex vivo bovine liver, and to report on measurements of the temperature dependent dielectric and thermal properties of the phantom.

METHODS: Thermochromic polyacrylamide phantoms were fabricated following a previously reported protocol. Phantom samples were heated to temperatures in the range of 20 - 90 °C in a temperature-controlled water bath, and colorimetric analysis of images of the phantom taken after heating were used to develop a calibration between color changes and temperature to which the phantom was heated. Using a custom, 2.45 GHz water-cooled microwave ablation antenna, ablations were performed in fresh ex vivo liver and phantoms using 65 W applied for 5 min or 10 min ( n = 3 samples in each medium for each power/time combination). Broadband (500 MHz - 6 GHz) temperature-dependent dielectric and thermal properties of the phantom were measured over the temperature range 22 - 100 °C. Measured dielectric and thermal properties of the phantom were employed in a previously validated computational model of microwave ablation to comparatively assess model predicted extents of heating against experimental observations in the phantom.

RESULTS: Colorimetric analysis showed that the sharp change in gel phantom color commences at a temperature of 57 °C. Short and long axes of the ablation zone in the phantom (as assessed by the 57 °C isotherm) for 65 W, 5 min ablations were aligned with extents of the ablation zone observed in ex vivo bovine liver. However, for the 65 W, 10 min setting, ablations in the phantom were on average 23.7% smaller in short axis and 7.4 % smaller in long axis than those observed in ex vivo liver. Measurements of the temperature dependent relative permittivity, thermal conductivity, and volumetric heat capacity of the phantom largely followed similar trends to published values for ex vivo liver tissue. After incorporating measured dielectric and thermal properties of the phantom, model predictions of ablation zone linear dimensions ranged between 16 - 50% larger than those observed experimentally.

CONCLUSION: Thermochromic tissue mimicking phantoms provide a suitable, controlled, and reproducible medium for comparative assessment of microwave ablation devices and energy delivery settings, though ablation zone size and shapes may not accurately represent ablation sizes and shapes observed in ex vivo liver tissue under similar conditions.