CT and MR Imaging in Colorectal Carcinoma: A Tool for Diagnosis, Staging, Response Evaluation, and Follow-Up.

The present review highlights the role of computed tomography (CT), CT colonography (CTC), and magnetic resonance imaging (MRI) in the diagnosis, staging, response evaluation, and follow-up of colorectal cancer. For a CT scan, prior bowel preparation is required. This is done to enhance imaging of the colon with the use of oral or rectal contrast agents. Negative contrast like air or carbon dioxide are helpful in detecting polyps and masses by distending the colon. Virtual colonoscopy offers a lower-radiation alternative for polyp and cancer detection. Intravenous contrast administration with arterial and venous phase CT images is also important in complete staging of a known case of colon cancer and for evaluation of residual/recurrent disease. With respect to MRI, high-resolution T2-weighted images in multiple planes are important, with diffusion-weighted imaging (DWI) sequences being important for restaging. Intravenous contrast is not generally recommended. Contrast-enhanced CT and MRI are used for nodal and distant metastasis staging, with special attention to the pelvic side wall nodes. Positron emission tomography (PET) CT is to be considered for further evaluation if the findings are unclear and recurrence is suspected.