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Background
Magnetic Resonance Imaging (MRI) plays a critical role in staging and re-staging of rectal cancer. Accurate assessment of tumor burden is dependent upon optimal imaging technique and image interpretation. This exhibit reviews some of the challenges and pitfalls that radiologists may encounter when imaging patients with rectal cancer.

Educational Goals / Teaching Points
The goal of this exhibit is to demonstrate pitfalls and challenges related to rectal MRI technique as well as image interpretation for initial staging and re-staging of patients with rectal cancer.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
T-staging of rectal cancer requires acquisition of axial and coronal oblique T2-weighted imaging (T2WI) and measurement of extramural disease on true short-axis images through tumor. Spiculated desmoplastic reaction can mimic extramural tumor and should be differentiated from nodular appearance of true tumor. Staging of mucinous tumor can be confounded by high signal intensity (SI) of the mesorectal fat on T2WI and the presence of mucin pseudocapsule. Peritoneal reflection involvement is manifested as nodular thickening of the peritoneal reflection in contiguity with extramural tumor. Assessment of lymph node disease should include evaluation for malignant morphologic criteria in addition to size. Tumor deposits can be challenging to differentiate from mesorectal lymph nodes on imaging, but multiplanar capability of MRI may be helpful in this regard. Detection of extramural vascular invasion (EMVI) can be confounded by motion artifact on T2WI. During restaging, T2WI and diffusion weighted imaging (DWI) should be used in combination to differentiate between fibrosis and viable tumor. One of the common pitfalls seen following neoadjuvant chemoradiation is presence of submucosal edema resulting in a pseudomass, which can mimic viable tumor. Comparison with initial pre-treatment staging MRI is critical to accurately assess degree of response. Presence of artifact on DWI can also confound evaluation for viable tumor. Low SI on ADC maps with corresponding low SI on high b-value DWI and low SI on T2WI is termed “T2-dark through” and should not be mistaken for viable tumor; nonetheless, areas of “T2-dark through” may co-exist with other areas of true restricted diffusion related to underlying viable tumor.

Conclusion
Key imaging features of rectal cancer determine staging, prognosis, and re-staging of a tumor which subsequently guide patient management. Radiologists should be aware of potential challenges related to MR technique in patients with rectal cancer. Radiologists should be able to identify common pitfalls related to image interpretation and reporting in these patients. This exhibit provides tips to circumvent such challenges and pitfalls to determine accurate staging and treatment response of patients with rectal cancer.