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Inspiratory and Expiratory Helical CT of Normal Adults: Comparison of Thin Section Scans and Minimum Intensity Projection Images

Wittram, Conrad M.B., Ch.B.; Batt, Jane M.D.; Rappaport, Daniel C. M.D.; Hutcheon, Michael A. M.D.

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To evaluate and compare thin section CT scans (TSS) and minimum intensity projection images (MinIPs) in healthy individuals, 10 nonsmokers with normal pulmonary function tests were studied using ten 1-mm collimated, helically acquired TSS images after full inspiration and expiration at two anatomic levels. Ten-millimeter-thick MinIPs were generated from the helical scans. Two thoracic radiologists compared TSS and MinIPs for artifacts and air trapping. Hounsfield unit (HU) measurements of TSS and MinIPs were obtained. The lung parenchyma on MinIPs demonstrates a smooth anterior-to-posterior attenuation gradient, accentuated by expiration. Motion and beam-hardening artifacts on TSS images resulted in regions of high and low attenuation on MinIPs, respectively. Expiratory TSS and MinIPs demonstrated air trapping (n = 31/40; range, 0–25%; mean, 7.2%). In comparison with TSS, MinIPs improved the conspicuity of air trapping (n = 20) and appeared to detect more air trapping (n = 7). No statistical differences were found when comparing the mean HU values of TSS and MinIPs. MinIPs demonstrated a smooth anterior-to-posterior attenuation gradient. Compared with TSS, MinIPs improve the conspicuity of air trapping in healthy individuals. Therefore, expiratory MinIPs may be useful in detecting air trapping as a result of disease.

From the Department of Medical Imaging (C.W., D.C.R.) and the Department of Respiratory Medicine (J.B., M.A.H.), The Toronto Hospital, Toronto, Ontario, Canada.

Address correspondence and reprint requests to Dr. Conrad Wittram, The Division of Thoracic Radiology, The Department of Radiology, The Massachusetts General Hospital, FND 202, 55 Fruit Street, Boston, Massachusetts 02114; e-mail: cwittram@partners.org

Presented at the 83rd Scientific Assembly and Annual Meeting of the Radiological Society of North America, December 1997, Chicago.

© 2002 Lippincott Williams & Wilkins, Inc.