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To evaluate the role of computed tomography (CT) in the investigation of pulmonary nodules, a special reference phantom that enabled CT densitometric measurements independent of variations between scanners and patients was used in ten institutions. A total of 384 nodules not considered calcified by conventional methods were examined; 118 (31%) proved to be benign, and in 65 of these (55%), unsuspected calcification was demonstrated. In 28 of the 65, definite calcification could be identified on thin-section CT scans by simple inspection of the scans at narrow windows. In the remaining 37, presence of calcification could not be clearly established without comparison with the reference CT number from the calibration phantom. CT was most effective in establishing the benignancy of nodules 3 cm or less in diameter and those with discrete or smooth margins. CT rarely yields a confident diagnosis of benign disease in larger nodules and in those with irregular or spiculated borders. After review of prior spot radiographs, low kilovolt peak spot radiographs, and conventional tomograms, the authors conclude that thin-section CT aided by a reference phantom in equivocal cases should be an integral part of the diagnostic approach to the pulmonary nodule.

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The CT density of the same pulmonary nodule can vary significantly between scanners or with the same scanner because several independent factors besides partial volume averaging can affect its determination. Hence a single CT number cannot be used to distinguish calcified from noncalcified nodules, ruling out direct extrapolation of quantitative data between scanners. The authors designed a phantom that simulates CT measurements in patients and permits comparison of CT density of each nodule with a physical standard derived from clinical experience. Tests on 35 patients using a GE 8800 showed that no malignant nodules and 65% of benign lesions were more dense than the phantom nodule. This method is independent of inter- and intra-scanner variation and facilitates standardized quantitative analysis of pulmonary nodules with current scanners.