Frame Rate Exposimetry for Pulmonary Capillary Hemorrhage During Lung Ultrasound

Lung ultrasound (LUS) is a powerful and accessible clinical tool for pulmonary diagnosis, but risk of pulmonary capillary hemorrhage (PCH) presents a safety issue. The dependence of PCH in a rat model of LUS was evaluated for image frames-per-second (fps) and associated on-screen Mechanical Index (MI_OS ) and Thermal Index (TI).

A Philips iE33 machine with L15-7io probe was used to scan anesthetized rats in a warmed water bath. B mode was applied at 9 MHz with settings of 34, 61 and 118 fps. After 2 minutes of exposure at an MI_OS setting, samples were obtained for assessment of PCH areas on the lung surface. Ultrasound parameters were measured to determine the in situ MI_IS at the lung surface.

The PCH trend counter-intuitively decreased with increasing fps, with areas of 19.5 mm² for 34 fps (MI_OS  = 1.0, TI = 0.8, 4080 images), 9.6 mm² at 61 fps (MI_OS  = 1.0, TI = 0.5, 7320 images) and 7.5 mm² at 118 fps (MI_OS  = 1.1, TI = 0.4, 14,160 images). The PCH was not significantly different for 34 fps (TI = 0.5, MI_OS  = 0.8) (10.7 mm² ), compared to 61 and 118 fps, above, indicating some value for the TI as a predictive indicator of PCH. MI_IS thresholds were 0.42, 0.46, and 0.49 for 34, 61 and 118 fps, respectively.

The increase in PCH at low fps was associated with delivering more relatively high amplitude grazing pulse exposures during slower image scans. No significant PCH was found for the MI_OS setting of 0.5, corresponding to in MI_IS values of 0.35-0.39.