In such cases, the lateral projection may be helpful, especially if it exhibits the "spine sign," which is an interruption in the progressive increase in lucency of the vertebral bodies from superior to inferior.
Among patients with pneumonia, the lateral view was helpful when the spine sign was present, but it was misleading when the spine sign was absent.
Among pneumonia patients with the spine sign, however, the lateral view was often helpful.
Although left lower lobe pneumonias may be obscured by the heart on the PA projection, they are often visible on the lateral view.[8,9] On the normal lateral chest view, the apparent density of the vertebral bodies gradually decreases from superior to inferior (Figure 1). Any interruption in this progression, making the lower vertebrae appear more dense, may indicate a pathologic process in the lungs, even if the process is not seen on the PA view (Figure 2).[2,8-10] This finding on the lateral view is known as the "spine sign" or "vertebral fade off sign," and is often present in patients with left lower lobe pneumonia.[8,10]
The purpose of this study was to determine the usefulness of the lateral projection and the usefulness of the spine sign to family physicians who interpret chest radiographs with left lower lobe pneumonia.
At the beginning of the study, a different faculty radiologist ("second radiologist"), who was blinded to the original readings, repeated the interpretations and judged whether the spine sign was present.
The physician readers were asked to judge the probability of left lower lobe pneumonia on a standard 5-point scale, ranging from 1=definitely or almost definitely no left lower lobe pneumonia to 5=definite or almost definite left lower lobe pneumonia.[13-15] In addition, each reader was asked to state whether the spine sign was present.
We repeated the analysis after including only pneumonia patients who had the spine sign (n=43) and all normal patients (n=83).
Among pneumonia patients with the spine sign, the lateral view was helpful in 13 cases and mis-leading in 3 cases; whereas in pneumonia patients without the spine sign, the lateral view was helpful in 5 cases and misleading in 7.
In most pneumonia patients with helpful lateral views, the pneumonia was retrocardiac on the PA view, and a classic spine sign was seen on the lateral view (Figure 2).
At the beginning of each reading session, we showed examples of the spine sign and a normal lateral chest view to the readers.