CT of a Bronchial Phantom: Factors Affecting Appearance and Size Measurements

In order to determine the most appropriate window settings for viewing CT of the bronchial tree, we performed CT of a bronchial phantom consisting of air-filled tubes measuring from 3.1 to 12.7 mm, oriented at varying angles relative to the scan plane, surrounded by water or air, and with scan collimation of 10 mm, 5 mm, and 1.5 mm. Using a computer program to graphically display CT number relative to the distance across the tubeʼs lumen, it was found that a window mean of −150 H accurately estimated the internal diameter of tubes surrounded by water, at all angles, when collimation was 5 mm or 1.5 mm. With 10-mm collimation, tube diameter was slightly underestimated for tubes 9.5 mm or less when oriented 30° or more from perpendicular to the plane of scan. At lower window settings and window widths of 500 H or less, all tubeʼs diameters were significantly underestimated. At –150 H, with tubes parallel to and centered in the scan plane, 5-mm and 1.5-mm collimation were most accurate; with decenter-ing of 4 mm, 10-mm collimation better showed the tubeʼs lumen. When surrounded by air, tube wall thickness was best estimated using a window mean of −450 H.