Focused ultrasound effects on blood vessels in vivo-limits for vascular interventions

The main ear vessels, both artery and vein, of 22 New Zealand white rabbits were sonicated. Pulsed ultrasound with RF power levels from 50 to 900 W and burst durations of 10 s were used. Pulse repetition frequency was varied from 5 to 20 Hz and total sonication duration from 10 to 180 s. Transducer operating frequencies of 0.68 and 2.02 MHz were used. Sonications were performed with normal flow through the vessel except for experiments to study the effect of ultrasound contrast agent, which were performed on clamped vessels. In all experiments the animals were followed 1-3 h after sonications before sacrificing and harvesting the tissue samples. Most sonications caused constriction of the artery even at the lowest power. At 0.68 MHz vessel rupture was also seen. At 2.02 MHz the main damage mechanism in the vessels and surrounding tissues is thermal coagulation. Both the arteries and veins are constricted or occluded often without rupture of the vessel wall. The injection of the contrast agent caused the veins to constrict at a lower power level than without it.