Sigmoid take off = horizontal direction of bowel

Anorectal junction= situated at the level of an imaginary line between the lower margin of the sacral bone and pubic bone 

Anal verge= the butthole

Low rectum= mesorectal fascia all around

Mid rectum= Some peritoneal reflection

High rectum= 50/50 reflection and MRF

Surgery based on location of tumor height from anal verge and anorectal junction.

Straddle tumor = if the tumor crosses the peritoneal reflection level.

Sphincter on coronal. Need to find internal/external to see involvement if tumor goes low.

Measure:

1. Distance from lowest part of tumor to the anal verge. (length of A+B)

2. Distance from lowest part of tumor to the anorectal junction. (length of B)

3. Total length of tumor.

4. If tumor is beyond the muscularis propria (T3 or higher), measure shortest distance from tumor to MRF.

5. Or talk about what is being invaded (T4 or higher).

Measure shortest distance from LN to MRF.

Regional lymph nodes- part of N stage

• Mesorectal lymph nodes (yellow).

• Nodes in the mesocolon of the distal sigmoid, along the “presacral” inferior mesenteric and rectalis superior blood vessels (purple)

• Nodes in the obturator spaces (blue)

• Nodes in the internal iliac spaces (green)

Non-regional lymph nodes (all in red)- part of the M-stage:

• External iliac nodes 

• Common iliac nodes

• Inguinal nodes

“high mesorectal nodes” and are part of the regional N-stage nodes. 

The level of the highest suspicious node in this region should be mentioned in the report, as this will impact the chosen radiotherapy field. 

Restricted DWI = viable tumor still present

DWI Dark through = fibrosis and dead tumor

Diffusion-weighted imaging

DWI highlights tissue with a high cellular density in which the extracellular movement of water is “restricted”.

DWI has been shown to be a useful adjunct to T2-weighted MRI to diagnose the presence of viable residual tumor within the fibrotically changed tumor bed after CRT.

In case of residual tumor, a high signal can typically be observed at the inner margin of the fibrosis on high b-value diffusion-weighted images, with a corresponding low signal on the ADC map. 

Top Row= no diffusion, dead tumor

Bottom Row= persistent diffusion, live tumor

The anal sphincter is comprised of three layers:

• Internal sphincter: continuance of the circular smooth muscle of the rectum, involuntary and contracted during rest, relaxes at defecation.

• Intersphincteric space.

• External sphincter: voluntary striated muscle, divided in three layers that function as one unit.
  These three layers are continuous cranially with the puborectal muscle and levator ani (figure).

The puborectal muscle has its origin on both sides of the pubic symphysis, forming a 'sling' around the anorectum. 

Classification

The most widely used classification is the Parks Classification which distinguishes four kinds of fistula: intersphincteric, transsphincteric, suprasphincteric and extrasphincteric.

The most common fistulas are the intersphincteric and the transsphincteric.

The extrasphincteric fistula is uncommon and only seen in patients who had multiple operations.

In these cases the connection with the original fistula tract to the bowel is lost.

A superficial fistula is a fistula that has no relation to the sphincter or the perianal glands and is not part of the Parks classification.

These are more often due to Crohns disease or anorectal procedures such as haemorrhoidectomy or sphincterotomy.

1. Position of the mucosal opening on axial images (using the anal clock).

2. Distance of the mucosal defect to the perianal skin on coronal images.

3. Secondary fistulas or abscesses.