Supine and Prone Imaging

Thoracic CT evaluation is often performed while the patient is lying in a supine position.⁴ When imaging supine patients, opacities resulting from lung collapse or atelectasis are often seen in the dependent portions of the lung as shown by the arrows in the left image.³ These dependent opacities resolve during prone imaging as shown on the right.³ As these opacities may resemble interstitial lung disease, it is imperative to also capture a set of images in the prone position.³

While prone, atelectasis will resolve, while true opacities resulting from lung disease will not.³ As shown by the arrows, opacities resulting from lung disease persist during prone imaging.³ Protocols for evaluating interstitial lung disease also recommend obtaining inspiratory and expiratory scans.⁴⁵ On inspiratory scans, inhomogeneous attenuation may be attributed to ground glass, airway abnormalities, or vascular abnormalities.⁶ The underlying cause may be more readily distinguished on expiratory scans where, in the setting of air trapping, the mosaic areas of low attenuation persist.⁶ Evaluating air trapping will be discussed in greater detail in upcoming sections.