Threshold estimates of ultrasound-induced lung hemorrhage in adult rats: role of pulse duration

Abstract

The pressure threshold of ultrasound-induced lung hemorrhage has been estimated as a function of pulse duration (PD) in adult rats. A total of 220 10to 11week-old 250-gram female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonically exposed groups (10 rats/group) and one sham group (20 rats). The 20 ultrasonically exposed groups (2.8 MHz; 10-s exposure duration; 1-kHz PRF) were divided into four PD groups, and for each PD, there were five in situ (at the lung surface) peak rarefactional pressures. For PDs of 1.3, 4.4, 8.2, and 11.6 μs, respectively, lesion occurrence thresholds were 3.1, 2.8, 2.3 and 2.0 MPa. Lesion size thresholds showed similar values, thus suggesting greater likelihood of lung damage as the PD increases. A Mechanical Index of 1.9, the FDA regulatory limit of diagnostic ultrasound equipment, is equivalent to the adult rat’s in situ peak rarefactional pressure of 4.0 MPa. All of the ED05s are less than the FDA limit. Introduction The effect of exposure timing quantities (e.g., pulse duration, exposure duration, total on-time, and pulse repetition frequency) on the threshold for ultrasoundinduced lung hemorrhage has been examined to a limited extent. Our study examines the role of pulse duration (PD) in producing ultrasound-induced lung hemorrhage when pulse repetition frequency (PRF) and exposure duration (ED) are held constant. Specifically, the threshold estimates of ultrasoundinduced lung hemorrhage in adult rats are examined at four pulse durations. There appears to be only one study that has reported a dependency of ultrasound-induced lung hemorrhage on PD [1]. However, there have been numerous studies that have demonstrated that ultrasoundinduced lung hemorrhage can occur in mice, rats, rabbits, monkeys and pigs [2,3,4]. While the principal goal of the one PD study [1] was not aimed at assessing the dependency of ultrasound-induced lung hemorrhage on PD, its observations strongly suggested that PD influenced the pressure threshold value. Therefore, a directed study focused on whether ultrasound-induced lung hemorrhage is influenced by pulse duration is warranted. Methods Exposimetry Ultrasonic exposures were conducted using one focused, 19-mm-diameter, lithium niobate ultrasonic transducer. Water-based (highly degassed water, 22 C) pulse-echo ultrasonic field distribution measurements were performed and yielded a center frequency of 2.8 MHz, a fractional bandwidth of 12%, a focal length of 19 mm, a -6-dB focal beamwidth of 470 μm, and a -6-dB depth of focus of 2.7 mm. An automated procedure was developed to routinely calibrate the ultrasound fields that was based on established standards [3,5,6]. A calibrated PVDF membrane hydrophone (Marconi Model Y-34-6543, Chelmsford, UK) was used. Off-line processing yielded the water-based peak rarefactional pressure. The in situ (at the pleural surface) peak rarefactional pressure was determined by computing the attenuation of the water-based pressure using the rat’s chest wall thickness and the measured intercostal tissue attenuation coefficient at 2.8 MHz of 2.8 dB/cm. The rats in the 20 ultrasonically exposed groups were exposed as follows: 2.8-MHz center frequency, 10-s exposure duration, 1-kHz pulse repetition frequency. The two variables were pulse duration and in situ (at the lung surface) peak rarefactional pressure. Animals A total of 220 10to 11-week-old 250±12-g female Sprague-Dawley rats (Harlan) were randomly divided into 20 ultrasonic exposure groups (10 rats per group) and one sham group (20 rats); no lesions were produced in the sham group. The 20 ultrasonic exposure groups were divided into four PD groups, and for each PD group, there were five in situ peak rarefactional pressures. Each of the four PD groups were designed to have the same five in situ peak rarefactional pressures. The individuals involved in animal handling, exposure, and lesion scoring were blinded to the exposure condition. The exposure conditions for each animal were revealed only after the final results were tabulated. The rat exposure and analysis procedures have been described previously in detail [3]. Rats were weighed and anesthetized [ketamine hydrochloride (87.0 WCU 2003, Paris, september 7-10, 2003