We have integrated real time volumetric ultrasound imaging and ultrasound ablation in the same intracardiac catheter. This single device could be used to visualize ablation sites in three dimensions immediately prior to inducing necrosis to eliminate cardiac arrythmias. After the course of therapy, the ablated tissue could be examined ultrasonically. The 12 Fr catheter includes a 2D transducer array for imaging and a single element piston for ablation. The imaging transducer consists of 38 active elements built on a multilayer flex circuit operating at 5.2 Mhz. The ablation piston is a 4 mm by 2 mm piece of air backed PZT-4. Our real time 3D scanner (Volumetrics Medical Imaging) and the 2D array were used to image phantom targets. The spatial peak, temporal average intensity (I<sub>SPTA</sub>) and acoustic power of the ablation beam were measured using a hydrophone. A 7 mm thick slab of beef was imaged and then ablated for 1 minute. The ultrasound ablation piston produced an I<sub>SPTA</sub> of nearly 30 W/cm<sup>2</sup> and a corresponding acoustic power of 2.6 W. The electrical to acoustic power efficiency of the transducer was 39%. The minute long ablation produced a transmural lesion in the beef 2mm by 4 mm by 7 mm deep.
The Integrated Force Array (IFA) is a metallized polyimide actuator made up of a large array of capacitive cells that deform when voltage is applied. The deformations of the individual cells add to produce an overall muscle-like compression of the array. In previously reported work deformations of up to 30% have been realized and the IFAs have been used as mechanical scanners in ultrasound imaging systems. The gaps of the capacitive cells are etched directly into the polyimide and oriented perpendicular to the plane of the array. Metal is deposited on the sidewalls of the etched features in order to form the plates of each capacitor. The force associated with the IFA motion is directly proportional to the height of the sidewall metal and thus to the thickness of the membrane. Until now, the thickness has been 2μm with gap widths of 1μm. In recent work, much higher aspect ratio IFAs (thicker but with the same gap width) have been fabricated in order to produce devices that operate with greater force and are much more robust devices.
There is much interest in the biomedical community in mechanically steering both high frequency ultrasound transducers and various optical beams. We are currently investigating the use of two different types of MEMS actuators, integrated force arrays (IFAs) and spiral wound transducers (SWTs). The IFA is a linear actuator that is a parallel network of hundreds of thousands of flexible capacitors that electrostatically contract, and the SWT is a patterned tape that is wound to form a circular network of flexible capacitors that can be electrostatically compressed to tilt desired structures. Using ANSYS finite element analysis, we have developed tilting polyimide support structures, which are fabricated on silicon wafers. High frequency ultrasound transducers (20-30 MHz) have been built on these structures and IFAs used to tilt them to steer the ultrasound beam in fluids. Prototype structures have produced 20 degree sector scans scanning at frequencies up to 30 Hz. IFAs have also been used along with similar support structures to steer optical laser beams up to 45 degrees at frequencies up to 60 Hz. The SWT is a more recent development that operates with much greater force than the IFA that could steer ultrasound and optical beams for similar applications.