Using ultrasound multiple scattering to determine severity of pulmonary edema

Abstract

Conventional ultrasound (US) cannot quantitatively evaluate lung tissue because of US multiple scattering (USMS). Pulmonary edema causes vertical artifacts -B-lines- which provide qualitative information about alveolar flooding. We showed that Scattering mean free path (SMFP), a measure of the density of air-filled alveoli, is longer in edematous than normal lungs. Here, we show that SMFP correlates with wet:dry weight ratio of lung tissue (W/D) and CT-scan assessment of edema. Ischemia-reperfusion injury (IRI) was created in lungs of anesthetized rats. The left lung hilum was clamped for 20, 40, or 60 minutes (n = 6/group) then reperfused for 60 minutes before ligating the apical portion of each lung to measure W/D, removing inflated lung blocks, then measuring SMFP and backscatter frequency shift (BFS) with a Verasonics scanner and L11-4v US probe. Six lung blocks were removed from healthy rats as Controls. Inflated lung blocks had ex vivo CT scans, followed by inflation-fixed histology. By logistic regression, there was a correlation between SMFP and W/D in 18 edematous and 12 Control lungs (r2 = 0.27, p < 0.004) and a significant correlation between edema extent by CT and SMFP and W/D. BFS was larger, and histology confirmed edema in IRI lungs. SMFP may be useful to quantify lung edema.