Optical monitoring of ultrasound-induced bioeffects in glass catfish.

This study is an investigation of the therapeutic ultrasound (US) effects on the blood vessels of optically transparent fish in vivo. Although many investigators have characterized cavitation in vivo using remote-sensing methods (i.e., measuring the acoustic emissions caused by oscillating bubbles) very few have made direct observations of cavitation-induced damage. Anesthetized glass catfish, which are optically transparent, was injected with the contrast agent, Optison, and then insonified at pressures that ranged from 0.5-10 MPa (peak negative pressures). Two focused transducers were used in these experiments to cover a frequency range of 0.7-3.3 MHz. Sonications were pulsed with pulse durations of 100, 10, 1, 0.1 and 0.01 ms and a pulse repetition frequency (PRF) of 1 Hz. The entire length of one sonication at a specific pressure level was 20 s. An inverted microscope combined with a digital camera and video monitor were used optically to monitor and record US interaction with the blood vessels in the tail of the anesthetized fish at 200x magnification. The effects of the burst sonication were analyzed visually at each pressure level. For the 1.091-MHz sonications, the first type of damage that occurred due to the US interaction was structural damage to the cartilage rods that comprise the tail of the fish, and was characterized by a disintegration of the lining of the rod. Damage to the rods occurred, starting at 3.5 MPa, 3.1 MPa, 4.1 MPa and 5.5 MPa for the 100-ms, 10-ms, 1-ms and 100-micros sonications, respectively. The formation of large gas bubbles was observed in the blood vessels of the fish at threshold values of 3.8 MPa, 3.8 MPa and 5.3 MPa, for the 100-ms, 10-ms and 1-ms sonications, respectively. Neither gas bubble formation nor hemorrhaging was observed during 100-micros sonications. Bubble formation was always accompanied by an increase of damage to the rods at the area surrounding the bubble. At 1.091 MHz, petechial hemorrhage thresholds were observed at 4.1 MPa, 4.1 MPa and 6.1 MPa, respectively, for the three pulse durations. The thresholds for damage were the lowest for the 0.747-MHz sonications: they were 2.6 MPa for damage to the rods, 3.7 MPa for gas bubble formation and 2.4 MPa for hemorrhaging.