High-spatial-resolution MRI facilitates visualization of vascular structures in situ within the mesorectum over a series of adjacent images. Veins may be recognizable on T2-weighted images as serpiginous or tortuous linear structures [11
]. Larger vessels may appear black owing to signal void, and smaller vessels may be recognized because of tortuosity and branching (Fig. 1
). Very small unnamed vessels may radiate outward from the edge of the muscularis propria into the perirectal fat (Fig. 2
). Larger named vessels, such as the superior rectal vein and middle rectal vein, may be visualized in a consistent anatomic position, and a contralateral paired vessel may be present, helping with identification (Figs. 3
). It is not always possible, however, to determine with absolute certainty whether a structure is vascular, and this lack of clarity is an important limitation in the radiologic assessment of extramural vascular invasion.
Depth of tumor invasion can indicate the potential for extramural vascular invasion. By definition, histologically defined extramural vascular invasion must be associated with tumors that are at least category T3. Therefore, a small tumor clearly limited to within the muscularis propria (category T2 or T1) has no potential for invading extramural vessels. Assessment of MR images for features suggestive of extramural vascular invasion must include the following four components: pattern of tumor margin, location of tumor relative to major vessels, caliber of vessel, and vessel border.