MRI Rectal Cancer Staging

Authors: Shanna A. Matalon, Leslie K Lee, MD, Atul Shinagare, MD, Michael H Rosenthal, MD, Ramin Khorasani

Date: May 24, 2020

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3 Segments (measured from anal verge)

Relationship to Anterior Peritoneal Reflection* and Mesorectal Fascia

  • Sigmoid: entirely intraperitoneal
  • High rectum: covered by peritoneum anteriorly & laterally
  • Mid rectum: covered by peritoneum reflection only anteriorly
  • Distal rectum: entirely extraperitoneal, surrounded by mesorectal fascia, which tapers distally and fuses with anal sphincter

   * identified in ~70-80% of pelvis MRIs

Images used with permission from Matalon et al. RadioGraphics, 2015.

Identifying the rectosigmoid junction

  • Sigmoid is entirely intraperitoneal
  • Upper/mid rectum covered anteriorly by peritoneal reflection
  • Various definitions exist:
    • Top of sacral promontory
    • 5 cm above peritoneal reflection
    • Confluence of teniae coli (anatomic landmark, not seen by imaging)


  • Mucosa (innermost layer), submucosa, muscularis propria* (outer dark black line)
    • *relevant in T-staging

Anal sphincter

  • Definitions:
    • Internal sphincter: continuation of inner circular layer of muscularis propria
    • External sphincter complex: continuation of outer longitudinal layer of muscularis propria, levator ani and puborectalis
    • Anal canal: measures between 3-5 cm; anorectal ring to anal verge (surgical definition)
    • Anal verge: distal end of anal canal, at junction of anal squamous mucosa and perianal skin
    • Anal margin: skin within 5 cm radius from anal verge

Feature Description

Craniocaudal location: (measured from anal verge to inferior most portion of mass)
Circumferential location: (clock face position).

(may be >1 type)

Size/length of rectum involved

  • 3 measurements if polypoidal (TRV x AP x CC)
  • 2 measurements if annular (thickness & length)


Signal characteristics

Describe T1, T2, DWI and enhancement characteristics
*High T2 signal suggests mucinous subtype, which tend to have higher metastatic  tendency and higher stage at diagnosis

Shortest distance of tumor to mesorectal fascia
*Only applies in T3 or greater
**Remember mesorectal fascia tapers at low rectum/anus fuses with internal anal sphincter

Extramural vascular invasion (EMVI)

Look for expanded, T2 intermediate tubular structures in the mesorectal fat or inferior mesenteric vasculature

*associated with poorer prognosis, including high risk of local recurrence and higher incidence of nodal and distant metastatic spread

T Stage


T1 (invades submucosa)/T2 (invades muscularis propria, but not beyond)

*cannot reliably distinguish by MRI, but may be distinguished by EUS

T3 (invades through muscularis propria into perirectal tissues)

T4a (invades visceral peritoneum)

T4b (tumor invades other organs)

“T” Staging of Low Rectal Tumors*

T Stage Examples

Involves IAS

confined to internal anal sphincter

Involves ISS

extends beyond internal anal sphincter into intersphincteric space

Involves EAS

invades through external sphincter
and into surrounding tissue but no other organ involvement

Invades other organs

*T-stage not currently recommended by SAR DFP – instead descriptive terms used

MRI criteria for pathologic lymph nodes:

  • Short axis ≥ 9 mm OR
  • Size 5-7 mm AND 2 abnormal morphologic features OR
  • Size <5 mm AND all 3 abnormal morphologic features

High Yield Sequences

Sequence Area of interest

Coronal large FOV T2

Screen for metastases

Sagittal T2

  • Prescribe oblique planes
  • Tumor location/distance from anal verge
  • Relationship to peritoneal reflection

Short axis oblique T2

T Stage

Long axis oblique T2/coronal T2

Relationship to sphincter complex (low cancers)


*Poor spatial resolution – should not be used for T staging

  • Localization of primary tumor, lymph nodes
  • Presence of extramural venous invasion
  • Response to neoadjuvant chemoradiation

Post contrast**

**Not currently recommended per Society of Abdominal Radiology Disease Focused Panel User’s Guide for the Synoptic MRI Report for Pre-Operative Staging of Rectal Cancer 2018

May be helpful in distinguishing abnormal morphology lymph nodes

Why and How to Prescribe Oblique Planes


  • Provides optimal anatomic information
  • Reduces T-staging error from volume averaging


  • Review patient chart to identify location of tumor (clinic notes, endoscopy report, etc, if available)
  • Review sagittal T2
    • Prescribe planes (fishbone drawing)
    • If cannot see tumor:
      • Techs can continue scanning through standard planes and review when more images are available
      • Prescribe based on described location (low 0-5cm, middle 5-10cm, high 10-15cm)

Sagittal T2

Distinguishing T2 from T3 Disease

1) Beware of volume averaging!

  • Always T-stage based off of the oblique planes (reduces potential error from volume averaging) and use other planes as reference to confirm extramural disease

Example 1

Standard Axial T2 Oblique Axial T2

Blurring of muscularis propria with possible extramural tumor (would be T3).

Clearly intact muscularis propria. Pathology upon resection confirm T2 disease.

Example 2

Consecutive Oblique Axial T2 Images Sagittal T2

Loss of dark band of muscularis propria (would be T3 disease).

Consecutive image(s) showed clearly intact muscularis propria. Attachment site anteriorly (arrow) would make extramural disease posteriorly unlikely. Pathology upon resection confirmed T2 disease.

Correlative sagittal image shows intact muscularis propria. Attachment site anteriorly (arrow) would make extramural disease posteriorly unlikely. Pathology upon resection confirmed T2 disease.

Example 3

Oblique Coronal T2 Oblique Axial T2

Blurring of the left levator ani (would be T4b disease).

Correlative axial image shows no tumor outside the internal sphincter, consistent with early T2 disease.

Example 4

Oblique Coronal T2 Oblique Axial T2

Blurring of the right levator ani (would be T3 disease).

Correlative axial image shows no tumor extending into the external sphincter, consistent with T2 disease.

2) Desmoplastic response in T2 tumors versus extramural tumor (T3)

  • Desmoplastic response = Linear, spiculated, T2 hypointense signal
    • *Often seen in villous adenomas
  • Extramural tumor (T3) = masslike, nodular, T2 intermediate signal

Example 1

Oblique Axial T2

Pathology proven T2 tumor showing desmoplastic response from 6-9 o’clock (bracket).

Example 2

Oblique Axial T2

Pathology proven T2 tumor with desmoplastic response at 3 o’clock (bracket).

Example 3

Oblique Axial T2

T3 cancer with extension of intermediate T2 signal tumor beyond the muscularis propria from 7-8 o’clock (arrows) AND desmoplastic response (bracket)

Example 4

Oblique Axial T2 Oblique Coronal T2

T3 cancer with extension of intermediate T2 signal tumor beyond the muscularis propria from 7-10 o’clock (arrows)

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