10 MHz catheter-based annular array for thermal strain guided intramural cardiac ablations

A yearly global population of well over 100,000 ventricular tachycardia patients could benefit from guided cardiac ablation of otherwise untreatable intramural arrhythmogenic substrates. A guided catheter-based HIFU device used epicardially with a subxiphoid approach is proposed which is placed with electroanatomical mapping. After the HIFU device is positioned, the thermal strain feedback at low intensities can confirm myocardial contact and establish a projected power titration guideline for full HIFU ablation. This PZT based spherical array design is an early adjunct prototype of a fully beam steerable CMUT array design in development at Stanford. The multifunctional catheter is designed to direct an axially steerable 10 MHz HIFU beam into the myocardium without damaging the epicardium itself and the vitally important coronary vessels. The spherical array is 7 mm in diameter with a radius of curvature of 7 mm; it is housed in a custom 3D printed tip housing which is 10 mm in total diameter and 4 mm in profile and joined to a 7 Fr (2.3 mm) catheter shaft. The prototype array is water cooled and has a built-in thermistor to monitor transducer temperature. The ablation axial steering addresses the 4 to 10 mm depth range, while 10 MHz can produce a high focus gain to produce a 23 C rise in temperature with bursts of 200 msec durations. A unique feature of this work is the use of the Verasonics Vantage 256 system as both the HIFU power source and thermal strain echo data receiver; a custom 32 channel power combiner/splitter (4 annuli x 8:1) interface was built to permit this development. Two 10 MHz 7 mm prototype spherical devices have been built and tested. Both are made from PZT-5A with no backing material and only a 100 micron EPOTEK-301 epoxy front matching/insulation layer. The first device is a 9.6 MHz single element spherical HIFU transducer which was driven with an ENI amplifier and produced heating intensities sufficient to visibly ablate (0.5 mm ablation diameter by 1.5 mm length, or 0.3 mm3) beef tissue in a room temperature bath in less than 0.5 sec. The second device is a spherical four element annular array which has been used to collect thermal strain echo data from a laboratory phantom. Continued development is underway to demonstrate both heating and thermal strain data from in-vivo animal experiments.