What is the “target” sign, and what is its significance?
On CT, the “target” sign, also known as mural stratification, refers to visualization of a bright soft tissue attenuation inner layer (representing mucosa, lamina propria, and hypertrophied muscularis mucosae, sometimes with hyperemia), a dark low attenuation middle layer (representing edematous submucosa), and sometimes a bright soft tissue attenuation outer layer (representing muscularis propria and serosa); the alternating layers correspond to the rings of a target. On MRI, a similar bright-dark-bright pattern is seen on contrast-enhanced fat-suppressed T1-weighted images, whereas a dark-bright-dark pattern is seen on T2-weighted images, since fluid and edema are high in signal intensity on T2-weighted images. This sign is not specific for a particular disease condition, as it can be seen in a variety of inflammatory processes including pseudomembranous colitis, ischemic colitis, and inflammatory bowel disease, but is highly specific for diagnosis of nonneoplastic disease when encountered in the colon.
