The Target Sign: Bowel vs Bone

The target sign is a well known entity in abdominal imaging. It refers to the CECT stratified appearance of bowel wall layers in the setting of inflammation. Specifically, there is high attenuation of the inner and outer layers of the bowel representing hyperemia of the mucosa and muscularis propria. The low attenuation of the submucosa is believed to be secondary to edema. This appearance is best seen when imaging is performed during the late arterial/early venous phase. The intensity of bowel wall enhancement is felt to correlate with the severity of disease. 

The target sign is a nonspecific finding as it can be seen several entities including inflammatory bowel disease, ischemic colitis, and radiation induced colitis. While the most common CECT finding in bowel inflammation is bowel wall thickening, the presence of a target sign can lead towards the diagnosis of a benign entity since the sign is uncommon in malignancy (the exception being the occurrence of the target sign in cases of infiltrating scirrhous carcinoma of the rectum).

A less well known usage of the target sign is in reference to the appearance of peripheral nerve sheath tumors (PNSTs) on axial T2 weighted MR images where a central area of low signal intensity is surrounded by an area of high signal intensity. In this context, the target sign appearance is due to the histologic composition of the tumor: the central low signal is due to the fibrocollagen tissue in the lesion's core, while the peripheral high signal is due to the presence of myxomatous tissue.

PNSTs are classified as benign (neurofibroma, schwannoma) and malignant (neurofibrosarcoma, malignant schwannoma). The clinical distinction between benign and malignant entities is difficult due to significant overlap in their manifestation. The target sign is most commonly seen in neurofibromas which tend to have a separation of cellular and noncellular component. It is thought that when the target sign is seen in regions of a malignant PNST, the areas demonstrating the sign represent benign tissue while the areas without the target sign represent tissue that has undergone malignant transformation.


REFERENCES

Ahualli J. The target sign: bowel wall. Radiology. 2005 February;234:549-50.
Banks KP. The target sign: extremity. Radiology. 2005 March;234:899-900.