The use of Optison to reduce the power requirements for focused ultrasound lesion production in the brain - an MRI/histology study in rabbits

An ultrasound contrast agent (Optison^®) was use with focused ultrasound to target the vasculature directly and produce localized lesions in the brain at low power levels. Also, MRI was tested to detect the induced temperature rise and the resulting lesions. Fifty-seven locations in the brains of 15 rabbits were sonicated using 1.5 MHz focused transducer (ROC/D=8 cm/10 cm). The 10 s or 20 s sonications were performed with either continuous wave (CW, 23 locations) or at a 50% duty cycle (PRF 1 Hz, 34 locations). Peak pressure amplitudes ranged from 1.6 to 9.8 MPa (in water). MRI was used to monitor the temperature rise during sonication and to detect tissue effects. At 4 h or 48 h after sonication, the animals were sacrificed and the brains were examined under light microscopy. Focal lesions were produced, with the necrosis appearing to result from a cessation of the blood supply within the sonicated regions. The lesions were seen in contrast-enhanced MRI and matched the shape of the MRI-mapped temperature distributions. In some cases (mostly CW sonications), damage along the ultrasound beam path was observed. This method may be useful for destroying tumors by targeting blood vessels directly. The threshold for tissue effects was reduced by approximately a factor of ten below that found in the same experimental conditions without the contrast agent. MRI thermometry appears useful for guiding such sonications.