Poor inspirations and AP views cause false-positive “fluffiness” similar to congestive heart failure (CHF) patterns. Normally on the PA film, the vascular markings stop short of the lung wall by 3 to 5 mm. Gravity causes subtle tapering of the vessels as they go toward the head (cephalad). “Cephalization of flow” is jargon for the phenomenon of enlarged lung vessels in the upper lung fields secondary to CHF.

The silhouette sign helps the clinician to localize the lesion. In the chest, there are anatomic structures that exist in fixed air–soft tissue relationships. Given proper rotation and penetration, the heart borders, the ascending and descending aorta, the aortic knob, and the diaphragms are visible (Figs. 231-2 and 231-3). The silhouette sign describes the situation where parenchymal pathology masks the silhouette of a common anatomic landmark. The heart and diaphragm are most commonly affected. For example, when anterior left upper lobe pneumonia obscures the border of the left heart, it is called a silhouette sign. When pleural effusion obscures the contour of the diaphragm, it is a silhouette sign.

Figure 231-2 Schematic anatomy of posteroanterior chest images.

Figure 231-3 Landmarks of the lateral chest film.

Nodules are classified by their diameter of 5 to 30 mm. Above 30 mm, these lesions are classified as masses. Small lesions (2 to 10 mm) are common. Most of these are calcified granulomas, and vessels on end. They are small and innocent, and do not grow over time. They can be followed by serial radiographs and clinical history. Positron emission tomography scans can differentiate metabolically active lesions (malignant, infectious) from those that are metabolically quiescent (benign).