Experimental measurement of microwave ablation heating pattern and comparison to computer simulations

Abstract

Introduction For computational models of microwave ablation (MWA), knowledge of the antenna design is necessary, but the proprietary design of clinical applicators is often unknown. We characterized the specific absorption rate (SAR) during MWA experimentally and compared to a multi-physics simulation. Methods An infrared (IR) camera was used to measure SAR during MWA within a split ex vivo liver model. Perseon Medical’s short-tip (ST) or long-tip (LT) MWA antenna were placed on top of a tissue sample (n=6), and microwave power (15W) was applied for 6 min, while intermittently interrupting power. Tissue surface temperature was recorded via IR camera (3.3 fps, 320×240 resolution). SAR was calculated intermittently based on temperature slope before and after power interruption. Temperature and SAR data were compared to simulation results. Results Experimentally measured SAR changed considerably once tissue temperatures exceeded 100 °C, contrary to simulation results. The ablation zone diameters were 1.28 cm and 1.30 ± 0.03 cm (transverse), and 2.10 cm and 2.66 ± −0.22 cm (axial), for simulation and experiment, respectively. The average difference in temperature between the simulation and experiment were 5.6 °C (ST) and 6.2 °C (LT). Dice coefficients for 1000 W/kg SAR iso-contour were 0.74 ± 0.01 (ST) and 0.77 (± 0.03) (LT), suggesting good agreement of SAR contours. Conclusion We experimentally demonstrated changes in SAR during MWA ablation, which were not present in simulation, suggesting inaccuracies in dielectric properties. The measured SAR may be used in simplified computer simulations to predict tissue temperature when the antenna geometry is unknown.

Sensitivity analysis of critical parameters affecting the efficacy of microwave ablation using Taguchi method

“…During microwave ablation the dominant physical processes are: (1) the radiation of electric field within the biological tissue through the slot of antenna applicator, (2) absorption of the electric field in the tissue leading to heating, and (3) distribution of thermal energy by thermal conduction and blood perfusion . In the present study, a coupled electromagnetic‐heat transfer model has been incorporated using FEM based COMSOL software using “heat” and “radiofrequency” modules to compute the electric fields and transient temperature profiles within the tissue.…”

“…During microwave ablation the dominant physical processes are: (1) the radiation of electric field within the biological tissue through the slot of antenna applicator, (2) absorption of the electric field in the tissue leading to heating, and (3) distribution of thermal energy by thermal conduction and blood perfusion. 26 In the present study, a coupled electromagnetic-heat transfer model has been incorporated using FEM based COMSOL software using "heat" and "radiofrequency" modules to compute the electric fields and transient temperature profiles within the tissue. The Helmholtz harmonic wave equation 24 was used to compute the FIGURE 1 Three-dimensional homogeneous cylindrical domain of biological tissue with an embedded single slot coaxial antenna generated using COMSOL multiphysics radiated electric fields within the biological tissue from the microwave applicator…”

“…Although, in vivo animal studies and clinical trials are useful for evaluating the efficacy of any device, but numerical modeling could play a crucial role, specifically, during the design of devices for MWA; serving as a quick, convenient and inexpensive evaluation to isolate and optimize the device during prototyping . Several computational studies are available in the literature on mathematical modeling of MWA, specifically in one compartment model of the hepatic tissue . Most of these reported studies are based on axisymmetric model, with few incorporating temperature dependent parameters.…”

“…17 Several computational studies are available in the literature on mathematical modeling of MWA, specifically in one compartment model of the hepatic tissue. [18][19][20][21][22][23][24][25][26] Most of these reported studies are based on axisymmetric model, with few incorporating temperature dependent parameters. Importantly, the outcome of all these studies is quantified by the qualitative assessment of the ablation volume at the end of MWA procedure.…”

A numerical investigation of microwave ablation on porous liver tissue

“…The effect of tumor diameter on the SAR profile within the porous liver tissue at P = 10 W, f = 2.45 GHz, f n = 0.6, f t = 0.7, and t = 300 s are shown in Figure 5. Figure 5 40 MW power delivered from the antenna propagates inside the tumor and the normal tissue and is transformed into the thermal energy by electromagnetic heating. The SAR profile curves form a nearly oval distribution around the slot and it has the highest value near the antenna slot for all tumor diameters.…”