Computed Tomography of the Pulmonary Parenchyma: CT in the Evaluation of Focal Pulmonary Pathology
The Lung Nodule
One of the most common clinical problems in thoracic imaging, the pulmonary nodule always presents the challenge of defining as noninvasively as possible its etiology. CT may provide unique density or morphologic information, allowing specific diagnoses to be made in given subgroups of nodules.
The Calcified Granuloma: Benign granulomas make up the majority of solitary pulmonary nodules. The detection of calcified deposits in a lung nodule is a major diagnostic clue. When the pattern of deposition of calcium salts is typical, ie, central nidus, laminated, or diffuse calcification, a confident diagnosis of benign disease can be made.
Owing to its superior sensitivity to density differences, CT can detect amounts of calcium not detectable by conventional techniques. Further, CT allows for more objective and quantifiable assessments of nodule density, whereas only qualitative and observer-dependent visual comparisons are relied on with plain radiography or tomography. Using thin-section scans, CT numbers of nodules can be analyzed, permitting detection of calcified areas within the nodule. Source
CT densitometry of lung nodules initially suffered from a lack of standardization between scanners as well as a limited understanding of the technical issues involved in accurately measuring CT density in the lung environment. By standardizing CT densitometric measurements with either reference phantoms or stringent calibration procedures, accurate detection of calcified nodules has become possible and reliable on any scanner. In a cooperative national study, 9 to 31 percent of solitary nodules not initially considered calcified by conventional methods were found to contain calcification by CT. Other major conclusions of the same study were that only nodules less than 3 cm in size should undergo CT densitometry. A cancer may also demonstrate calcification in about 10 percent of cases, but the calcified areas are usually eccentric, small, scattered, in lesions larger than 3 cm. Thus, the pattern of calcification and size of nodules is as important as the mere detection of calcification in segregating calcified granulomas from other lesions.