Ultrasound Coupled Radial Vibration Mode: Influence on Cardiac Sonothrombolysis

Sonothrombolysis is a technique that uses ultrasound waves with microbubbles in the bloodstream to reduce thrombi in the coronary arteries of patients diagnosed with infarction. The energy generated by the cavitation of microbubbles in the thrombus region is the main cause of their rupture. To generate cavitation, low frequency (<1 MHz) and high acoustic pressure (>100 kPa) waves, and, thereafter, specific transducers, are necessary; however, their geometry causes strong radial vibration modes, which can reduce their useful transmitted energy. This study aimed to measure the influence of the radial mode on the effectiveness of a rod ceramic transducer designed for a sonothrombolysis device. Five units of a rod Navy II ceramic were used to produce 1 element transducer and a four-element cell, to measure their sensitivities, bandwidth, and acoustic pressure levels using the pulse-echo method, a hydrophone receiver and computing treatment to raise their frequency spectra and transmitted acoustic fields. Once the diameter-to-thickness ratio of the ceramic used was only 1.59, the radial vibrational mode showed to be dominant for a pulse excitation burst, providing, in its resonant frequency, an amplitude response 6 dB higher (1:0, 480) than its thickness mode. Due to the geometry of ceramic and to the Poisson effect, the radial mode also produced a coupled thickness mode in its radial resonant frequency. The results showed that the typical radial vibration modes, existing in some low-frequency piezoelectric rod ceramics can generate a useful coupled thickness mode, producing the necessary acoustic pressure to be used in a sonothrombolysis equipment.