Effect of pulse parameters on cavitation and acoustic streaming in ultrasonic surgical devices

Over the last 15 years, ultrasonic surgical devices have become the preferred instruments for cataract surgery within the ophthalmologic community. This approach, called phacoemulsification, uses needle tips which vibrate longitudinally at frequencies between 28 and 50 kHz (depending upon the manufacturer). Mechanical impact and inertial cavitation at the tip both act to erode and liquefy the lens material, which is then aspirated out through the needle core. Surgeons have reported that despite the aspiration flow through the needle, which would tend to bring the lens material into contact with the needle tip, lens material seemed to be repulsed from the tip region. It was theorized that acoustic streaming caused by sound radiation from the tip was the reason for this effect. One recent advance in phaco technology was the introduction of microbursts, or very short (less than 10ms) pulse lengths. These short bursts appeared to reduce the repulsion effect, while at the same time retaining the cutting effectiveness of continuous wave (CW) excitation. The purpose of this study was to investigate both the potential acoustic streaming and cavitation effects as a function of burst length. Measurements of cavitation energy were made using PVDF membrane hydrophones and custom designed analysis software; visualization of flow patterns in the region of the needle were made using a high resolution diagnostic ultrasound imaging system in color Doppler mode. The results show that cavitation levels from these short burst modes are similar to CW excitation at the same excursion levels, while the streaming patterns in the tip region were markedly reduced. This matches with clinical results, which indicate superior cutting effectiveness when compared to CW operation, with better "hold" on the cataract.