**4. Tumor morphology with MRI**

By far the most common rectal adenocarcinoma, up to 90%, in MRI may appear as solid, polypoid or flat lesions within the intestinal wall, whereas the aspect of an annular or semiannular mass and growing with varying degrees of stenosis is the most frequent image.

### **Figure 4.**

*A. Axial T2w image shows a low-lying mucinous tumor of high signal (red star) disrupting the mesorectal fascia (green arrows) and extending into the dorsal bladder wall (yellow arrow). B. The very large mucinos adenocarcinoma (signet-ring) with a central scar situated in the middle third of the rectum with complete infiltration of the mesorectum, and of the dorsal bladder wall at 12 o'clock. Stage: T4, CRM+, N0, EMVI. Histology confirmed this result. Source: F. Bauer, Radiology Kaufbeuren.*

Less rectal tumors, up to 10%, may contain mucin, and mucinous tumors have a poor prognosis and a high risk of spillage during surgery [6]. MRI depicts these tumors well on T2w as well delimited high intensity masses, **Figure 4.**

As described above, T2-weighted sequences under optimal conditions can differentiate the wall layers of the rectum. The vast majority of carcinomas have a higher signal than the hypointense (not always controllable) mucosa, but a lower signal than the clearly hyperintense submucosa. Exceptions to this are, on the one hand, mucinous carcinoma and on the other hand, sigmoid ring cell carcinoma [7].

After administration of contrast medium, the entire rectal wall is clearly hyperintense and the individual wall layers can no longer be differentiated from each other. Therefore, the native T2-weighted sequences should be used for the primary diagnosis of the T category.
