ABSTRACT
PURPOSE: The purpose of the study was to determine the volumes of pleural fluid (PF) required to produce visible menisci in the lateral and posterior costophrenic angles (CPA) and obscure the hemidiaphragms (HD) on upright frontal and lateral chest radiographs (CXRs), using volumetric analysis of chest computed tomography (CT). MATERIALS AND METHODS: A total of 98 patients with small pleural effusions on chest CT, in whom CXRs were obtained within a 24-hour interval, were selected for retrospective analysis. PF within each hemithorax was quantified using a semiautomatic method of image segmentation. A cardiothoracic radiologist scored each hemithorax on each CXR from 0 to 3 (0-normal CPA, 1--fluid meniscus below the HD, 2--fluid meniscus at the level of the HD, 3--fluid opacity obscures the HD). Each CXR category was correlated with CT-determined PF volumes. RESULTS: A mean of 20 mL of PF was present on CT without a visible correlate on CXR. A meniscus below the HD on CXR correlated with roughly 100 mL; a meniscus occurring at the HD correlated with roughly 250 mL; a meniscus obscuring the HD correlated with a mean of approximately 650 mL. There were large standard deviations for all PF volumes. CONCLUSIONS: We provide guidelines for estimating PF volumes on upright frontal and lateral CXRs. We also confirm that the lateral radiograph is more sensitive for detection of small pleural effusions, with blunting of the posterior CPA only correlating with a mean of 26 mL of PF.
