Automated ultrasound ablation parameters predict lesion depth in a perfused thigh model

Abstract Background The accuracy and safety of lesions created during catheter ablation of cardiac tissue rely on the user's ability to predict the size of lesions that form under variable conditions during treatment. A user-directed robotic system was developed that uses Low-Intensity Collimated Ultrasound (LICU) for cardiac imaging and to produce linear lesions without tissue contact. Molecular absorption of mechanical energy induced in the tissue by LICU causes a temperature rise resulting in thermal tissue necrosis. Purpose By varying the speed of the LICU beam as it traces the desired lesion path, while considering the beam intensity, distance from catheter tip to tissue, and relative motion of the target tissue, we demonstrate the capability to deposit lesions of known depth according to a predictive model. Methods The LICU catheter was mounted in a servo-controlled, 3 axis stage suspended above a perfused porcine thigh that simulates the thermodynamic properties of the myocardium such as acoustic attenuation, thermal conduction, and specific heat capacity. Therapy was delivered along a linear path while varying the distance to tissue, speed of the beam, and motion of the catheter relative to the tissue in 3 dimensions. Programmed relative motion was representative of typical and extreme motion in humans. Pathological analysis after necropsy was used to quantify lesion depth which was then compared to lesion depth as predicted by a mathematical model from approx. 4–12 mm. Results Data from 66 samples were categorized according to typical and extreme motion at 4 different distances (3, 8, 9 and 13 mm). Measured vs predicted lesion depth was well correlated (R = 0.898) with 98.5% of the samples within ±2 mm. Assessment of LICU lesion depth Conclusion Lesion depth using LICU is well controlled in an animal model for the range of distances and relative motion typically encountered in a human subject. The range of acoustic beam speeds and acoustic power density with LICU predictably produces lesions up to 12 mm deep. The LICU thermal model may be useful to create lesions of known depth thus ensuring transmurality of lesions while potentially avoiding unintended extra-cardiac injury.