HIFU power network optimization for catheter based cardiac interventions

An optimized, self-contained system platform to support both thermal strain imaging (TSI) and HIFU is desirable for multi-functional catheter-based devices. Our catheter is designed for real-time interventional feedback during therapeutic treatment of cardiac substrates which contribute to ventricular tachycardia. We are using high frequency (8-10 MHz) catheter-based HIFU to address relatively shallow (5-20 mm) tissue target depths from the epicardial aspect of the heart. An imaging system which can provide both high acoustic power at high frequencies while also providing the signal processing needed for TSI is very attractive. Our principal objective is to produce very high focal intensities (> 2000 W/cm
²
) to ablate moving cardiac tissue quickly, implementing both electronic axial steering and mechanical angulation to scan step-wise through a 4×4×4 mm
³
target volume in several minutes. The catheter and system are designed for TSI temperature monitoring feedback during HIFU. The Verasonics (VS) Vantage 256
^TM
ultrasound system limits the total system power output which is usually 8 Watts per channel at low frequencies (e.g. 1.5 MHz). The Vantage further limits the transmit voltage for higher frequencies and as well generally limits the acceptable duty cycle operation (e.g. 25%) for long bursts. Without our cabling modifications the individual VS transmitters cannot deliver enough HIFU power for our array design, which is a 7 mm diameter annular array with 4 elements for 8.92 MHz HIFU and TSI with water cooling irrigation. We have developed an optimized system power summation approach which provides enough power with minimal changes to existing hardware and is compatible with our currently implemented TSI approach. Using a standard length Philips L7-4 cable comprised of 42 AWG coax we have constructed a summation/connection interface for the catheters. The VS system acts as both the HIFU power source and echo receiver to monitor heating dynamically. Sixteen system coax channels have been summed for each of 4 elements of the catheter array. Our method uses direct transmission line power summation without the need for power splitter/combiners which greatly enhances power transmission efficiency and simplifies the design. The system ZIF connector is designed for 0.6 μH tuning inductors for each of the 64 system channels used; these inductors provide improved power delivery efficiency and system safety. A power factor at the Vantage system output of 0.77 is achieved. Each annulus of the array receives the power from 16 system channels; each system channel produces a nominal instantaneous peak power of 1.6 W; each summed to deliver over 11 real Watts at each transducer annulus. At these output levels the summation network delivers over 40 W (electrical, real, peak, instantaneous) to the entire set of 4 transducer array elements to yield a focus intensity of more than 3000 W/cm2 to achieve very rapid heating.