MRI Evaluation of Rectal Cancer: Staging and Restaging.

Magnetic resonance imaging (MRI) plays an important role in the staging and restaging of rectal cancer. Multiplanar high-resolution (≤3-mm section thickness) T2-weighted images are the primary sequences used for rectal cancer staging. No preprocedural bowel cleansing regimen, intravenous contrast material, nor endorectal coil is necessary. MRI is highly accurate for differentiating T1-T2 disease from T3 and T4 disease, an important distinction as patients with T3 and T4 tumors typically undergo preoperative neoadjuvant chemoradiation before resection. At MRI, the muscularis propria appears as a thin black line encircling the outer wall of the rectum, and tumor extension through this line indicates T3 disease. Further tumor extension into adjacent organs indicates T4 disease. Endorectal ultrasound is generally preferred to differentiate T1 (submucosal involvement) from T2 (extension into but no disruption of muscularis propria) disease. MRI is also accurate in the assessment of tumor involvement of the mesorectal fascia. Tumor involvement of the mesorectal fascia increases the likelihood of recurrence following resection. MRI is less accurate for determination of lymph node status, though heterogeneous signal intensity and irregular margins are suggestive of node positive disease. Approximately 10%-30% of patients who undergo preoperative chemoradiation experience a complete pathologic response that is defined as no residual tumor found at histopathologic analysis of the resected specimen. The addition of diffusion-weighted images to T2-weighted images improves the accuracy of restaging examinations for determination of complete pathologic responders.