Mastering rectal cancer MRI: From foundational concepts to optimal staging

Abstract

MRI plays a critical role in the local staging, restaging, surveillance, and risk stratification of patients, ensuring they receive the most tailored therapy. As such, radiologists must be familiar not only with the key MRI findings that influence management decisions but also with the appropriate MRI protocols and structured reporting. Given the complexity of selecting the optimal therapy for each patient-which often requires multidisciplinary discussions-radiologists should be well-versed in relevant treatment strategies and surgical terms, understanding their significance in guiding patient care. In this manuscript, we review the most common treatment options for managing patients with rectal adenocarcinoma, emphasizing key MRI principles and protocol characteristics for accurate staging. We also highlight important anatomical landmarks and essential factors to be described during baseline assessment. Additionally, we discuss crucial information for restaging and surveillance.

Keywords: Magnetic Resonance Imaging; Neoadjuvant Therapy; Neoplasm Staging; Rectal Neoplasms; Rectal cancer.

Fig. 1.

Differences in image resolution. (A) Oblique axial T2WI of a rectal tumor (white arrow in A), with 3 mm thickness and 18 cm FOV, but inadequate 200 × 177 matrix and consequently poor resolution. (B) Oblique axial T2WI of a rectal tumor after TNT with 3 mm thickness, 16 cm FOV and 288 × 288 matrix, which is even higher than the suggested for a 16 cm FOV. The optimal resolution enables the identification of a tiny fibrotic scar in the anterior wall (white arrow in B).

Fig. 2.

T2WI showing the use of (A) sagittal and (B) coronal planes to obtain an adequate angulation for the (C) oblique axial image of the tumor (white arrow in C). The acquired oblique coronal and oblique axial images have 3 mm thickness, 18 cm FOV and 288 × 230 matrix, the latter slightly lower than ideal, but adequate.

Fig. 3.

How to mark the oblique coronal planes on sagittal T2WI. (A) Oblique coronal angulation to include all the structures of the mesorectum in a single acquisition. (B) Oblique coronal angulation parallel to the anal canal for low rectal tumors.

Fig. 4.

Sigmoid tumor and how to identify the “sigmoid take-off”. (A) Schematic drawing and (B) corresponding axial T2WI showing the ventral projection of the sigmoid (arrow). (C) Schematic drawing and (D) corresponding sagittal T2WI showing the horizontal orientation of the sigmoid (white arrow), which moves away from the sacrum as opposed to the rectum (black arrow).

Fig. 5.

How to identify anterior peritoneal reflection (APR) on T2WI. (A, B) Male anatomy. (A) schematic drawing and (B) and corresponding sagittal showing APR at the level of the seminal vesicles (arrows). On (B) note also the virtual retrorectal space (dashed white lines) with edema, delineated anteriorly by the mesorectal fascia (white dashed arrows) and posteriorly by the presacral fascia (black arrows). Note the mesorectal fat in yellow. (C, D) Female anatomy. (C) Schematic drawing and (D) sagittal T2 depicting APR at the level of the uterocervical junction (arrows). Note the mesorectal fat in yellow. (E) Gull wing or V shape appearance on axial plane (arrow). Note the mesorectal fat in yellow.

Fig. 6.

Schematic drawing of the anal canal anatomy in (A) coronal and (B) sagittal planes. In (A), notice the difference between the anatomical anal canal, delimited by the dentate line and not seen on MRI, and the surgical anal canal, delimited by the anorectal junction at the level of the puborectalis muscle. MRI corresponding T2WI in the coronal (C) and sagital (D) planes.

Fig. 7.

Tumor growth patterns on oblique axial T2WI. (A) Circumferential. (B) Semicircular. (C) Ulcerated – the ulcerated component is in the opposite site of the bulk of the tumor (white arrow in C). (D) Polypoid.

Fig. 8.

Non mucinous vs. mucinous tumors on oblique axial T2WI. (A) Non-mucinous tumor: characterized by low to intermediate T2 signal intensity (arrow). (B) Mucinous tumor: characterized by high T2 signal intensity involving >50% of the tumor volume (arrow).

Fig. 9.

How to measure tumor extension beyond the rectal wall on oblique axial T2WI. (A) Sagittal T2-weighted image with the reference lines to obtain the coronal oblique (thin dashed line) and axial oblique planes (solid line) of the tumor (B). The dashed circle shows the limits of the muscularis propria and the solid line depicts the tumoral infiltration beyond the rectal wall. The arrows show spikes with low signal intensity related to desmoplastic reactions and that should not be measured.

Fig. 10.

Schematic drawing of the anatomy of the rectal wall and relevant adjacent structures for rectal tumor staging. A tumor extending to the submucosa (pink layers) is staged T1 (white arrow). Extension to the muscularis propria (red layer) is staged T2 (dashed white arrow). Tumoral invasion beyond the muscularis propria into the perirectal fat (yellow tissue) characterizes a T3 lesion (black arrows). T3 tumors can be further subclassified based on the depth of invasion beyond the muscularis propria. T3a: <1 mm. T3b: 1–5 mm. T3c: 6–15 mm. T3d: >15 mm (black arrows and arrowhead). T4a tumors extends to the anterior peritoneal reflection (gray arrow) and T4b invades locoregional structures (dashed gray arrow).

Fig. 11.

Tumor staging on T2WI according to wall penetration. (A) T1/T2. Axial plane showing a low rectal tumor that doesn’t extend beyond the muscular. (B) T3a. Axial plane depicting discrete discontinuity of the muscle layer with tumor signal intensity (arrow) in the upper rectum. (C) T3 b-d. Axial showing a tumor that clearly extends beyond the muscle layer into the mesorectal fat in the mid rectum. (D,E) T4a. Axial plane showing tumor invasion of the anterior peritoneal reflection (arrows). (F) T4b. Sagittal plane depicting upper rectal mucinous tumor invading the the urinary bladder (arrow).

Fig. 12.

Identifying EMVI (A and B) and tumor deposits (C and D) on T2WI. (A) Sagittal and (B) axial images showing elongated and enlarged structures originating near the infiltrative edge of the primary tumor with tumor signal intensity replacing the vessel flow void (arrow). Small arrow in (B) indicate adjacent vessel with normal caliber and flow void for comparison. (C) Sagittal and (D) axial images showing a tumor deposit with irregular contours (arrow) contiguous with a vein (small arrow in C).

Fig. 13.

Schematic illustration showing how tumor location within the rectum affects staging and mesorectal fascia involvement. In the low rectum (red dot plane), the mesorectum and mesorectal fascia (green line) fully encircle the rectum; a tumor invading the mesorectal fat and fascia without extension to adjacent structures is classified as T3, and if the tumor is within 0.1 cm of the fascia, it is considered involved. In the middle rectum (orange dot plane), the anterior part of the rectum is covered by the anterior peritoneal reflection (blue line), and a tumor with nodular thickening that clearly invades this structure is staged as T4a. Tumors that invade locoregional structures, such as the sacrum, are staged as T4b. Extensive tumors with invasive growth on both the anterior and posterior aspects of the rectum may invade both the anterior peritoneal reflection (APR) and mesorectal fascia (MRF), as represented in the plane of the gray dot.

Fig. 14.

Morphologic criteria for local nodal primary staging of lymph nodes on axial T2WI. (a, b) High signal intensity indicating mucinous content in mesorectal lymph nodes. (C) Superior rectal lymph node with mucinous content. All mucinous lymph nodes are considered positive regardless of the size. (D) Right mesorectal lymph node with round shape and heterogeneous signal intensity. (E) Right mesorectal lymph node with round shape and heterogeneous signal intensity (white arrow) and adjacent smaller right mesorectal lymph node with round shape, heterogeneous signal intensity and irregular border (black arrow).

Fig. 15.

Patient after neoadjuvant therapy showing complete response on MRI and colonoscopy. Baseline MRI axial T2WI (A), DWI (B) and ADC map (C) showing a polypoid lesion (arrows) with T2 intermediate signal intensity and restricted diffusion (arrows). Restaging MRI rectum post chemoradiation therapy demonstrating a complete response characterized by absence of tumor on T2 (D), DWI (E) and ADC map (F). Restaging colonoscopy was also consistent with complete response (G).

Fig. 16.

Patient after neoadjuvant therapy showing near complete response on MRI and colonoscopy. Baseline MRI axial T2WI shows (A) a semicircular lesion (arrow) with intermediate signal intensity. Baseline colonoscopy showing the ulcerated rectal tumor (B). Restaging MRI T2WI (C) shows new scar with low signal intensity (dashed arrows) with small areas of persistent intermediate signal intensity on T2WI (arrowhead), consistent with near complete response. Restaging colonoscopy (D) also demonstrated scaring tissue with small areas of viable tumor (circle).

Fig. 17.

Patient after neoadjuvant therapy showing incomplete response on MRI. Baseline MRI axial T2WI (A), DWI (B) and ADC map (C) demonstrate a semicircular lesion (arrows) infiltrating beyond the muscularis propria (T3b) and restriction on diffusion. Restaging MRI after total neoadjuvant therapy demonstrated semicircular persistent tumor (dashed arrows) with intermediate signal intensity on T2WI (D) and restriction on diffusion at the correspondent area (E,F), characterized by high signal intensity on DWI (E) and markedly low signal intensity on ADC map (F).

Fig. 18.

Anatomy of the pelvis side wall lymph nodes, from top (A) to bottom (D). Purple area: external iliac lymph nodes, yellow area: obturator lymph nodes, green area: internal iliac lymph nodes.

Fig. 19.

Low rectal tumor with lateral pelvic side node metastases. Baseline MRI rectum on T2WI sagittal (A) and axial (B) demonstrates the primary tumor (5.5 cm from the anal verge) and right obturator (lateral and anterior do the internal iliac vessels – arrowhead) round lymph node measuring 1.1 cm in the short-axis, suspicious for nodal locoregional metastasis. Restaging MRI after total neoadjuvant therapy sagittal (C) and axial T2WI demonstrated rectal tumor response with decreased size and new scar (arrow, C) and decreased right obturator (posterior to the internal iliac vessels) adenopathy, measuring 0.4 cm in the short-axis (arrow, D). This decreased size less than 0.6 cm was associated with lower risk of persistent lateral pelvic side disease.