Hyperlucent Thorax from Rotation: Anatomic Basis

RATIONALE AND OBJECTIVES: The authors studied a large group of patients with a computed tomography (CT)-based model to evaluate the presence, significance, and anatomic basis of unilateral hyperlucency from patient rotation. METHODS: Chest CT sections at three levels in 65 consecutive patients undergoing routine diagnostic studies were read into a microcomputer. Radiographic transmittance data were calculated across each CT section at different degrees of rotation as well as for lateral decentering. Data were analyzed for the pattern of density variation both individually as well as in stratified subgroups. RESULTS: The data showed a trend of increasing radiographic transmission on the side opposite that of the anterior obliquity. The resulting optical density difference was small and was not clinically significant. Lateral decentering produced smaller changes than rotation. The anterior breast/pectoral structures are the etiology for the phenomenon. Optical density differences of 0.1 result from a difference in tissue thickness of 4 mm. CONCLUSIONS: The changes in radiographic transmission with rotation are small and are overshadowed by soft tissue compression. Rotation alone will result in decreased radiographic transmission on the side of anterior obliquity; thus, the left side will be more lucent in an right anterior oblique projection.