Mechanisms of lesion formation in high intensity focused ultrasound therapy

The lesions generated by high-intensity ultrasound were studied in transparent tissue phantoms premixed with and without Optison ultrasound contrast agents (UCA) at 1.1 and 3.5 MHz acoustic frequencies. Generation of small bubbles was observed at the very beginning of exposure, while cigar-shaped thermal lesions began to form at the focus after a delay. After further heating, boiling occurred and changed the lesion from cigar-shape to tadpole-shape, with advancement toward the transducer. Broadband noise was detected in phantoms with UCA initially. UCA also lowered the pressure threshold and enlarged the lesion. However, the lesion was formed centimeters in front of the focus at 1.1 MHz. While thermal and cavitation effects are believed to be both important in lesion formation, tadpole-shaped transformation results from boiling activity.