Considerations for theoretical modelling of thermal ablation with catheter-based ultrasonic sources: Implications for treatment planning, monitoring and control

Abstract

Purpose: To determine the impact of including dynamic changes in tissue physical properties during heating on feedback controlled thermal ablation with catheter-based ultrasound. Additionally, we compared the impact of several indicators of thermal damage on predicted extents of ablation zones for planning and monitoring ablations with this modality.

Methods: A 3D model of ultrasound ablation with interstitial and transurethral applicators incorporating temperature-based feedback control was used to simulate thermal ablations in prostate and liver tissue. We investigated five coupled models of heat dependent changes in tissue acoustic attenuation/absorption and blood perfusion of varying degrees of complexity. Dimensions of the ablation zone were computed using temperature, thermal dose, and Arrhenius thermal damage indicators of coagulative necrosis. A comparison of the predictions by each of these models was illustrated on a patient-specific anatomy in the treatment planning setting.

Results: Models including dynamic changes in blood perfusion and acoustic attenuation as a function of thermal dose/damage predicted near-identical ablation zone volumes (maximum variation < 2.5%). Accounting for dynamic acoustic attenuation appeared to play a critical role in estimating ablation zone size, as models using constant values for acoustic attenuation predicted ablation zone volumes up to 50% larger or 47% smaller in liver and prostate tissue, respectively. Thermal dose (t₄₃ ≥ 240 min) and thermal damage (Ω ≥ 4.6) thresholds for coagulative necrosis are in good agreement for all heating durations, temperature thresholds in the range of 54°C for short (<5 min) duration ablations and 50°C for long (15 min) ablations may serve as surrogates for determination of the outer treatment boundary.

Conclusions: Accounting for dynamic changes in acoustic attenuation/absorption appeared to play a critical role in predicted extents of ablation zones. For typical 5–15 min ablations with this modality, thermal dose and Arrhenius damage measures of ablation zone dimensions are in good agreement, while appropriately selected temperature thresholds provide a computationally cheaper surrogate.

• High intensity ultrasound
• theoretical model
• thermal ablation
• treatment planning
• ultrasound ablation

Declaration of interest: This work was supported by National Institutes of Health grants R01CA122276, R01CA111981, R41CA121740, and R21CA137472. The authors alone are responsible for the content and writing of the paper.