Superthreshold behavior and threshold estimates of ultrasound-induced lung hemorrhage in adult rats: role of beamwidth

It is well documented that ultrasound-induced lung hemorrhage can occur in mice, rats, rabbits, pigs, and monkeys. The objective of this study was to assess the role of the ultrasound beamwidth (beam diameter incident on the lung surface) on lesion threshold and size. A total of 144 rats were randomly exposed to pulsed ultrasound at three exposure levels and four beamwidths (12 rats per group). The three in situ peak rarefactional pressures were about 5, 7.5, and 10 MPa. The four 19-mm-diameter focused transducers had measured pulse-echo -6-dB focal beamwidths of 470 μm (2.8 MHz; f/1), 930 μm (2.8 MHz; f/2), 310 μm (5.6 MHz; f/1), and 510 μm (5.6 MHz; f/2). Exposure durations were 10 s, pulse repetition frequencies were 1 kHz, and pulse durations were 1.3 μs (2.8 MHz; f/1), 1.2 μs (2.8 MHz; f/2), 0.8 μs (5.6 MHz; f/1) and 1.1 μs (5.6 MHz; f/2). The lesion surface area and depth were measured for each rat as well as the percentage of rats with lesions per group. Logistic regression analysis and Gaussian-Tobit analysis methods were used to analyze the data. The effects of in situ peak rarefactional pressure and beamwidth were highly significant, but ultrasonic frequency was not significant. In addition, the estimated interaction between in situ peak rarefactional pressure and beamwidth was positive and highly significant. The ultrasound beamwidth incident on the lung surface was shown to strongly affect the percentage and size of ultrasound-induced lung hemorrhage lesions. Even though ultrasonic frequency was an experimental variable, it was not shown to affect the lesion percentage or size.