This study is looking for a better way to define rectal tumors undergoing pre-operative radiation. MRI scanning generally results in more clear definition of the tumor that CT scanning. Insertion of radio opaque clips in the tumor site could also help to visualize the tumor better for radiotherapy treatment planning. The investigators believe both techniques would help the physician to define to cancer better than using CT scan alone. The investigators also will compare both techniques to define the better way to define the tumor.

1. The use of endoscopically placed mucosal metal clips to define the superior and inferior extent of the rectal tumor will improve the accuracy of tumor localization in patients undergoing non-contrast CT simulation.
2. A 3T-MRI will provide a better localization of the rectal tumor and pelvic nodes compared to non-contrast CT simulation for radiotherapy treatment planning.
3. By comparing these two novel methods to the standard procedure, we would be able to define any difference between the three methods and obtain an estimate of the accuracy and reliability of each method for the localization of the rectal tumor.

Inclusion Criteria:

• Patients with adenocarcinoma of the rectum stage as T3/4 N0/1/2 M0 undergoing pre-operative chemoradiation
• Patients must be 18 years of age or greater
• Signed study-specific informed consent
• Not pregnant or lactating

Exclusion Criteria:

• Contraindication for radiotherapy or chemoradiation
• Contraindication for MRI scanning
• Prior chemotherapy or radiation therapy to the pelvis