Utilization of the high-frequency piezoelectric ceramic hollow spheres for exposimetry and tissue ablation

Miniature, high-frequency piezoelectric ceramic hollow spheres were evaluated for potential use as hydrophones for exposimetry of high intensity ultrasound fields and as minimally invasive ablation devices. As a hydrophone, the hollow sphere has unique characteristics regarding its geometry, dimensions and sensitivity, which make it ideal for use in exposimetry of high pressure fields. Spheres with diameters ranging from 0.7 to 1.0 mm and with resonance frequencies from 1.8 to 2.7 MHz were used as hydrophones. A relative (reciprocal) calibration method was performed to determine the end-of-cable sensitivity as well as the directivity of the hollow sphere hydrophones. An almost constant sensitivity was reported for the spherical hydrophones and an omni-directional receive pattern was also demonstrated. The hollow sphere hydrophone was shown to have twice the sensitivity of a needle type hydrophone but with no pre-amplification stages and could withstand four times the pressure. As a minimally invasive interstitial ablation device, several in vitro experiments were conducted using two different size spheres. The results demonstrated an increased necrosed tissue volume for increasing exposure time. With a 1.0 mm diameter sphere (f = 1.87 MHz), the necrosed tissue diameter as a function of exposure times was 2.35 ± 0.34, 3.00 ± 0.37 and 4.61 ± 1.13 mm for 5, 10 and 15 second sonications, respectively. Using a 0.7 mm diameter sphere (f = 2.70 MHz), the size of the ablated tissue was 1.50 ± 0.01 and 1.63 ± 0.31 mm for 5 and 10 seconds sonications, respectively. As a tissue ablation device, the hollow sphere was able to produce controllable lesion diameters with sharp boundaries between the normal and necrosed tissue.