Methodologic aspects of attenuation distributions from static and dynamic thoracic CT techniques in experimental acute lung injury

Abstract

Background: In acute lung injury, thoracic CT is used to gain information about lung aeration and consolidation. This can be done either during breath-holding by, spiral CT scanning of the entire lung or dynamically by scanning lung slices without interrupting ventilation. We hypothesized that attenuation distribution is dependent on static or dynamic scanning techniques. We also studied whether a variation in the CT cut level, corresponding to the diaphragm movement over a breath, had any effect on the attenuation distribution. Methods: Twenty-two pigs with oleic acid-induced lung injury, were randomly assigned to receive pressure-controlled mechanical ventilation with or without spontaneous breathing. Transversal dynamic CT scans of the chest were performed in apical and juxtadiaphragmatic regions, and end-expiratory and end-inspiratory slices were selected. In addition, after clamping the tube at end-expiration and end-inspiration, respectively, spiral CTs were performed. Guided by morphologic structures, spiral CT slices matching the dynamic scan slice and three additional neighbored slices above the diaphragm were selected. Distributions of CT attenuation were calculated and summarized in ranges for comparison. Results: No significant difference in attenuation distributions between the two scanning methods or an interaction with the factors ventilation mode, ventilation phase, and attenuation range were found. In addition, attenuation distributions of four neighbored juxtadiaphragniatic slices, 8 mm thick, from the spiral CT did not differ statistically. Conclusion: In an animal model of oleic acid-induced lung injury, analyses of transverse thoracic slices based on dynamic or static CT scanning showed comparable distributions of attenuation. Variations on the CT cut level of 24 mm had no significant effect on the distribution of Hounsfield unit numbers. CT attenuation distributions of transversal juxtadiaphragmatic slices were not dependent on exact position.