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.5 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.