Microbubbles during tissue treatment using high intensity focused ultrasound

Based on our previous animal experiments in the treatment of dog prostates and data from others, we selected HIFU parameters, (i.e. focal intensity, Isptp≈1680 W/cm², frequency 4 MHz and treatment exposure time=4 On/12 Off sec.), to produce thermally induced focal lesions. Using the same acoustic parameters, patient treatment for the benign prostatic hyperplasia (BPH) was commenced in 1992. During the treatment, in some patients, microbubble initiation at the focal site was observed. These microbubbles migrated towards the ultrasound near field as the HIFU procedure continued. To elucidate the similar response, animal experiments were carried out. This paper describes initiation of microbubbles at the focal site followed by their proliferation in the dog prostate during the treatment and finally their effects on tissue lesions. The microbubbles were observed by ultrasound imaging and monitored by subharmonics and broadband signals on the frequency spectrum analyzer. The temperature at the rectal wall was measured to observe and follow the effects of microbubbles in-vivo