Threshold estimates and superthreshold behavior of ultrasound-induced lung hemorrhage in adult rats: Role of pulse duration

The study objective was to estimate the pressure threshold (ED ₀₅, effective dose, or in situ peak rarefactional pressure associated with 5% probability of lesions) of ultrasound (US)-induced lung hemorrhage as a function of pulse duration (PD) in adult rats. A total of 220 10- to 11-week-old 250-g female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonic exposure groups (10 rats/group) and one sham group (20 rats). The 20 ultrasonic exposure groups (2.8-MHz; 10-s exposure duration; 1-kHz PRF; -6-dB pulse-echo focal beam width of 470 μm) were divided into four PD groups (1.3, 4.4, 8.2 and 11.6 μs) and, for each PD group, there were five in situ peak rarefactional pressures (range between 4 and 9 MPa). Rats were weighed, anesthetized, depilated, exposed, and euthanized under anesthesia. The left lung was removed and scored for the occurrence of hemorrhage. If hemorrhage was present, the lesion surface area and depth were measured. Individuals involved in animal handling, exposure and lesion scoring were "blinded" to the exposure conditions. Logistic regression analysis was used to examine the dependence of the lesion occurrences, and Gaussian tobit regression analysis was used to examine the dependence of the lesion surface areas and depths on in situ peak rarefactional pressure and PD. Threshold results are reported in terms of ED₀₅. For PDs of 1.3, 4.4, 8.2 and 11.6 μs, respectively, lesion occurrence ED₀₅s were 3.1, 2.8, 2.3 and 2.0 MPa with standard errors around 0.6 MPa. Lesion size ED ₀₅s showed similar values. A mechanical index (MI) of 1.9, the US Food and Drug Administration (FDA) regulatory limit of diagnostic US equipment, is equivalent to the adult rat's in situ peak rarefactional pressure of 4.0 MPa. PDs of 8.2 and 11.6 μs had ED₀₅s more than 2 standard errors below 4.0 MPa, indicating that the ED₀₅s of these two PDs are statistically significantly different from 4.0 MPa. The ED₀₅ threshold levels for a PD of 1.3 μs are consistent with previous US-induced lung hemorrhage studies. As the PD increases, the ED₀₅ levels decrease, suggesting greater likelihood of lung damage as the PD increases. All of the ED₀₅s are less than the FDA limit.