Left lower lobe abnormality as suggested by the “spine sign”
In a lateral film, the opacity of the vertebral bodies progressively decreases from the thoracic spine to the diaphragm. The “spine sign” is an alteration in this pattern and suggestive of a left lower lobe abnormality, such as a pneumonia. This can be useful for retrocardiac opacities that can be difficult to catch on a PA view.
The “spine sign” is a useful finding in the detection of left lower lobe abnormalities that may be less obvious on PA or AP view. It can manifest as the absence of progressively decreasing opacification or as progressive increasing of opacification. These findings have positive predictive values of 91% and 97%, respectively, for left lower lobe abnormalities, such as consolidative processes, pleural effusions, or atelectasis. Sensitivity was 60% and 70%, respectively. Specificity was 64% and 84%, respectively.
For reference, see the below films of a normal lateral film, a lateral film with absence of decreasing opacification, and a lateral film with progressive increasing of opacification.