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)

Layers

• 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

T Stage	Examples
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

Why and How to Prescribe Oblique Planes

WHY:

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

HOW:

• 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)

• Gollub MJ et al. Recognition of the Anterior Peritoneal Reflection at Rectal MRI. AJR. 2013 200:97-101.
• Gollub MJ et al. Current controversy, confusion, and imprecision in the use and interpretation of rectal MRI. Abdominal Radiology. 2019.
• Hope TA et al. Rectal cancer lexicon: consensus statement from the society of abdominal radiology rectal and anal cancer disease-focused panel. Abdominal Radiology. 2019.
• Jhaveri KS & Hooseini-Nik H. MRI of Rectal Cancer: An Overview and Update on Recent Advances. AJR. 2015.
• Kaur H et al. MR Imaging for Preoperative Evaluation of Primary Rectal Cancer: Practical Considerations. RadioGraphics. 2012.
• Matalon SA et al. Anorectal Cancer: Critical Anatomic and Staging Distinctions That Affect Use of Radiation Therapy. RadioGraphics. 2015.
• Nougaret S et al. The use of MR Imaging in Treatment Planning for Patients with Rectal Carcinoma: Have You Checked the “DISTANCE”? Radiology. 2013.