Frame Rate Exposimetry for Pulmonary Capillary Hemorrhage During Lung Ultrasound

Abstract

Objectives: 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).

Methods: 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.

Results: 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.

Conclusions: 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.

Keywords: B lines; Mechanical Index; Thermal Index; bioeffects of ultrasound; neonatal lung ultrasound safety; pulmonary ultrasound; ultrasound dosimetry.