What are the CT and MRI findings of colorectal cancer?
Focal (<5 cm) colonic wall thickening or mass is seen, often moderate-marked in degree, irregular, and asymmetric, with soft tissue attenuation on CT images and increased T2-weighted signal intensity and restricted diffusion on MR images. There tends to be abrupt shouldering at the transition between normal and abnormal portions of colon. Minimal to no pericolic fat stranding or fluid is seen, which is more typically seen in colitis ( Figure 27-14 ). In some patients, a polypoid intraluminal component may also be seen, and in other patients, extraluminal spread of soft tissue tumor into the surrounding fat or subjacent organs may be detected. Large bowel obstruction can occur when there is sufficient luminal narrowing. Adjacent pericolic lymph nodes may be present, which are assumed to represent regional nodal metastases until proven otherwise when they are ≥3 mm in size. Peritoneal spread of tumor as well as distant metastases may also be encountered.