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Purpose: For many years the lung has been considered off-limits for ultrasound. However, it has been recently shown that lung ultrasound (LUS) may represent a useful tool for the semiquantification of pulmonary edema, by the evaluation of B-lines (also called ultrasound lung comets). Our aim was to compare the ability of LUS to quantify the degree of pulmonary edema in critically ill patients, compared to the gold standard chest computed tomography (CT).Methods: Eighteen patients admitted to the Intensive Care Unit (4 females, mean age 69±7 years), who underwent a clinically-driven chest CT, were evaluated by LUS within a few hours. Semi-quantitative LUS assessment of pulmonary edema was performed by determining the number of B-lines with a linear 10 MHz probe, scanning on both right and left hemithoraxes, as previously described. Lung weight (LW), lung volume (LV) and lung physical density (LD) were calculated from CT scans using an ad hoc software.Results: A significant, good correlation was found between the number of B-lines and LW (R=0.67, p<.05). A stronger correlation was found between the number of B-lines and LD (R=0.82, p<.01, see figure), that further increases if the LD of only the first 7 mm of subpleural lung tissue is considered (R=0.85, p<.01).Conclusions: Lung ultrasound is a reliable tool for the evaluation and quantification of pulmonary edema in critically ill patients. Compared to chest CT, it is less expensive, can be easily performed and repeated at bedside, and does not employ ionizing radiation.