Potential of Determining Thermal Dose for Ablation Therapies Using Ultrasound Elastography: An Ex Vivo Feasibility Study

Thermal ablation therapy is a minimally invasive technique with the potential to allow eradication of tumors utilizing highly localized heating. Protein coagulation induced by heating is thought to induce a change at the microscopic level, thereby manifesting itself as measurable macroscopic changes of the tissue's viscoelastic properties. The objective of this study was to determine the relationship between clinically relevant thermal doses and a novel biomechanical parameter measurable using ultrasound. Ex vivo porcine liver tissue samples were administered clinically relevant thermal doses via a temperature-controlled double immersive water bath. Stress and strain data obtained from unconfined uniaxial compression and stress-relaxation tests were fit to a Kelvin-Voigt fractional derivative (KVFD) model of tissue viscoelasticity. In the KVFD model, the alpha parameter is the order of the fractional derivative (0<;a<;1) and may represent the amount of collagen microstructure change due to protein coagulation. An analysis of experimental results suggests a predictable correlation between thermal dose received during a simulated thermal ablation therapy and the increase of the alpha parameter. In short, the relationship between thermal dose and the viscoelastic biomechanical parameters of thermally treated tissue may be used to develop new imaging techniques to reliably delineate ablated tissues and prevent damage to healthy surrounding tissues during thermal ablation therapies. In the future, ultrasound elastography techniques allowing for the measurement of the proposed alpha parameter in vivo will be determined since the correlation between thermal dosage and the alpha parameter has now been demonstrated.