Experimental assessment of microwave ablation computational modeling with MR thermometry

Abstract

Computational models are widely used during the design and characterization of microwave ablation (MWA) devices, and have been proposed for pretreatment planning. Our objective was to assess three-dimensional (3D) transient temperature and ablation profiles predicted by MWA computational models with temperature profiles measured experimentally using magnetic resonance (MR) thermometry in ex vivo bovine liver. Materials and methods: We performed MWA in ex vivo tissue under MR guidance using a custom, 2.45 GHz water-cooled applicator. MR thermometry data were acquired for 2 min prior to heating, during 5-10 min microwave exposures, and for 3 min following heating. Fiber-optic temperature sensors were used to validate the accuracy of MR temperature measurements. A total of 13 ablation experiments were conducted using 30-50 W applied power at the applicator input. MWA computational models were implemented using the finite element method, and incorporated temperature-dependent changes in tissue physical properties. Model-predicted ablation zone extents were compared against MRI-derived Arrhenius thermal damage maps using the Dice similarity coefficient (DSC). Results: Prior to heating, the observed standard deviation of MR temperature data was in the range of 0.3-0.7°C. Mean absolute error between MR temperature measurements and fiber-optic temperature probes during heating was in the range of 0.5-2.8°C. The mean DSC between model-predicted ablation zones and MRI-derived Arrhenius thermal damage maps for 13 experimental setups was 0.95. When comparing simulated and experimentally (i.e. using MRI) measured temperatures, the mean absolute error (MAE %) relative to maximum temperature change was in the range 5%-8.5%. Conclusion: We developed a system for characterizing 3D transient temperature and ablation profiles with MR thermometry during MWA in ex vivo liver tissue, and applied the system for experimental validation of MWA computational models.

How large is the periablational zone after radiofrequency and microwave ablation? Computer-based comparative study of two currently used clinical devices

“…Although the RFA and MWA models used in this study were similar to other experimentally validated models, and the data presented in Table 2 suggests the validity of the models in terms of predicting coagulation zone (CZ) size, reasonable doubts could arise as to the models’ periablational zone (PZ) prediction accuracy. To address this issue, we analyzed the experimental data from a recent study on MWA that compared simulated transient temperature profiles and ablation zones in ex vivo bovine liver tissue vs. 3 D transient temperature profiles and ablation zones measured by MRI thermometry [ 42 ]. The analysis of this data (presented as Supplementary material in the Appendix in the [ 42 – 44 ]) suggests a good agreement between the PZ size estimated from computer simulations and experimental measurements.…”

“…To address this issue, we analyzed the experimental data from a recent study on MWA that compared simulated transient temperature profiles and ablation zones in ex vivo bovine liver tissue vs. 3 D transient temperature profiles and ablation zones measured by MRI thermometry [ 42 ]. The analysis of this data (presented as Supplementary material in the Appendix in the [ 42 – 44 ]) suggests a good agreement between the PZ size estimated from computer simulations and experimental measurements. Although the analysis was done exclusively with MWA data, we think that the validity can be extended to RFA since the thermal damage process is governed by the same principles.…”

Microwave Ablation for Uterine Fibroids

“…Computational model predictions of ablation extents and transient temperature at the fibroid myometrium boundary were compared against observations from experiments in ex vivo tissue; such comparison of ablation profile and transient temperatures at a few discrete points is an established approach for assessing computational models of microwave ablation [165], [218], [219]. A more detailed validation of the model including transient temperatures at a wider range of points, such as that feasible with MR thermometry [220] would further add to model credibility and establish an even broader model validation. Another limitation in our experimental setup was the upper power limit of the generator of 35 W which resulted in rather small number of investigated power/time combinations where target volume was achieved.…”

Microwave ablation of lung tumors: A probabilistic approach for simulation‐based treatment planning

“…To simulate the distribution of electromagnetic field, power absorption, and bioheat transfer, we employed an approach similar to our previous studies 28,33 and that has been experimentally validated against volumetric MRI thermometry data in ex vivo tissue. 34 Specifically, we utilized the finite element method (FEM) to solve the coupled electromagnetic and bioheat transfer equations during MWA. At each time-step of the transient bioheat transfer model, the following sequence of equations was solved.…”