Induction of apoptosis in vivo in the rabbit brain with focused ultrasound and Optison

Histologic effects of focused ultrasound (FUS) exposures combined with an ultrasound contrast agent (Optison®) were investigated to examine whether the lesions were dominated by apoptosis or necrosis. The rabbit brains (n = 17) were sonicated (1.5 MHz, peak rarefactional pressure amplitude: 1.4 to 8.8 MPa) after Optison® was injected intravenously (IV). MRI and light microscopy were used to examine tissue effects. To detect apoptosis, TUNEL staining based on labeling of DNA strand breaks was used. The average number of apoptotic and necrotic cells in 300 × 220 μm microscopic fields were counted in 18 representative lesions. Lesions in the rabbit brains were created at lowered acoustic power levels when FUS was combined with Optison®. In histology, the lesions exhibited red blood cell extravasations and destruction of blood vessels. At 4 h after sonication, the lesions lost many cells, and the remaining cells exhibited both necrotic and apoptotic features. Overall, apoptosis dominated; there were, on average, 32.3 ± 13.2 apoptotic cells per microscopic field compared with only 5.1 ± 3.4 necrotic cells per field. In conclusion, FUS combined with Optison® could produce lesions that are dominated by apoptosis, presumably induced primarily via ischemia after cavitation-produced damage to the brain vasculature. (E-mail: Natalia@bwh.harvard.edu)