Magnetic resonance imaging of rectal cancer

Abstract

Magnetic resonance imaging (MRI)is a useful modality for the evaluation of rectal cancer, providing superior anatomic/pathologic visualization when compared with endorectal ultrasound (EUS) and computed tomography (CT). Preoperative MRI is useful for tissue characterization and tumor staging, which determines the surgical approach and need for neoadjuvant/adjuvant therapy. Important prognostic factors include the circumferential resection margin (CRM), T and N stages, and extent of local invasion. Postoperative MRI to assess the extent of tumor recurrence enables early resection, which can greatly prolong survival. MRI criteria for local recurrence include T2 hyperintensity, early dynamic rim enhancement, and nodular morphology. Future research in MRI of rectal cancer is geared toward developing optimal imaging techniques including high-resolution MRI, whole-body scans, and parallel imaging; imaging of lymph nodes by MR lymphography; and response to therapy using diffusion/perfusion-weighted MR and functional imaging.

Keywords: Magnetic resonance; postoperative; preoperative; rectal cancer; recurrence.

Figure 1

Rectal wall layers. Three rectal wall layers can be distinguished on magnetic resonance imaging: an inner hyperintense layer, representing the mucosa and submucosa; an intermediate hypointense layer, the muscularis propria; and an outer hyperintense layer, the serosa (perirectal fat).

Figure 2

Involved circumferential resection margin (CRM). (A) Axial. (B) Coronal. The CRM is said to be involved if tumor is within 1 mm of the mesorectal fascia or has visibly invaded it. Tumor-free CRM. (C) Axial. (D) Sagittal. A tumor-free CRM is assumed when the closest tumor extension, mesorectal tumor deposit, or suspicious lymph node is over 6 mm from the mesorectal fascia.

Figure 3

Overstaging of T2 rectal cancer. (A) Axial. (B) Sagittal. Desmoplastic reaction of tumor into adjacent perirectal fat.

Figure 4

Schematic of circumferential resection margin (CRM) versus T stage. The current T-staging system does not distinguish tumors with wide versus close/involved resection margins. The CRM measurement is of far more clinical significance. Short tumor-mesorectal fascia distances alter surgical planning and may require neoadjuvant treatment, due to the increased local recurrence risk.

Figure 5

Infiltration of pelvic sidewall. (A) Axial. (B) Sagittal. Invasion into uterus/cervix/vagina. (C) Axial. (D) Sagittal.

Figure 6

Liver metastases from rectal cancer. (A) Axial image demonstrates liver lesion. (B) Coronal image shows extent of rectal cancer.

Figure 7

Giant cell reaction to foreign material. (A) T1-weighted magnetic resonance imaging (MRI). (B) T2-weighted MRI. Mature fibrosis is readily differentiated from recurrent tumor based on a low T1-/T2-weighted signal intensity and lack of gadolinium enhancement.

Figure 8

Rim-enhancement pattern. (A) T1-weighted magnetic resonance imaging (MRI). (B) T2-weighted MRI. Recurrent tumor displays a central hypointense area surrounded by a strongly enhancing margin of variable thickness. Postsurgical abscesses may show a similar appearance. Enhancement can also be seen with early fibrosis, but the rim-enhancement pattern has not been described.