1.
ANATOMY:
The rectum is the concluding part of the large intestine that terminates in the anus. Its average length may range between 10 and 15 cm.
The course of the rectum is marked by two major flexures:
- Sacral flexure: anteroposterior curve with concavity anteriorly.
- Anorectal flexure: anteroposterior curve with convexity anteriorly.
There are additionally three lateral flexures (superior,
intermediate and inferior),
which are formed by transverse folds of the internal rectum wall.
Peritoneal Coverings: Fig. 1
In the superior third of the rectum,
the anterior surface and lateral sides are covered by peritoneum.
The middle third only has an anterior peritoneal covering,
and the lower 1/3 has no peritoneum associated with it.
In males,
the reflection of the peritoneum from the rectum to the posterior bladder wall forms the rectovesical pouch.
In females,
the peritoneum reflects to the posterior vagina and cervix,
forming the rectouterine pouch (Douglas).
Arterial supply: Fig. 2
- Superior rectal artery: terminal continuation of the inferior mesenteric artery.
- Middle rectal artery: a branch of the internal iliac artery.
- Inferior rectal artery: terminal continuation of the internal pudendal artery (a branch of the internal iliac artery).
Venous drainage: Fig. 2
- Superior rectal vein: empties into the portal venous system.
- Middle rectal vein: empties into the systemic venous system.
- Inferior rectal vein: empties into the systemic venous system (1-2).
2. IMAGING PROTOCOLS:
Most pathologies:
- Contrast-enhanced portal venous phase (70-80 s p.i.
or 50-60 s after bolus tracking).
GI Bleeding:
- Don’t use oral contrast because positive contrast material may obscure the bleeding.
- Non-contrast scan is needed to exclude any intraluminal hyperattenuating material such as foreign bodies,
clips or coproliths that might be misinterpreted as active bleeding.
- The entire abdomen is scanned following iv contrast administration in a late arterial (10-15 s after bolus tracking ) and portal venous phase (40 s later).
Perforation / Dehiscence of suture: Endorectal contrast
- GASTROGRAFIN 3%.
- Barium sulfate is not recommended (Peritonitis).
- Non-contrast scan is needed to identify the sutures and don’t confuse them with contrast extravasation.
3. PATHOLOGIES:
3.1.
RECTAL PROLAPSE: Fig. 3
Rectal prolapse is a pelvic floor disorder defined as the protrusion of the rectum through the anal canal. It is most frequently seen in elderly multiparous women,
connective tissue disorders,
obesity,
and chronic constipation.
It results in local symptoms (eg,
pain and bleeding),
bowel dysfunction,
and a diminished and disabled quality of life.
Diagnosis is often clinical if the prolapse can be seen on examination (3).
3.2.
HEMORRHOIDS: Fig. 4
Hemorrhoids are enlarged blood vessels around the anus and lower rectum.
Classification of hemorrhoid corresponds to its position relative to the dentate line:
- External hemorrhoids are located below the dentate line and their vascular outflow is via the inferior rectal veins into the pudendal vessels and then into the internal iliac veins.
- Internal hemorrhoids lie above the dentate line and their vascular outflow include the middle and superior rectal veins,
which subsequently drain into the internal iliac vessels.
Approximately 40% of individuals with hemorrhoids are asymptomatic; the most common symptoms of hemorrhoid include bleeding,
anal pruritus,
prolapse,
and pain due to thrombosis (4).
3.3.
RECTAL BLEEDING: Fig. 5 Fig. 6 Fig. 7
Rectal bleeding can be caused by:
- Hemorrhoids
- Anal fissures
- Diverticula
- Infections (bacterial and other pathogens)
- Inflammatory bowel diseases (IBD): Crohn's disease and ulcerative colitis
- Tumors
- Trauma
Imaging features:
Active hemorrhage is most frequently visible as a jet of extravasated contrast agent on MDCT.
It also can precisely localize the site of hemorrhage and identify a wide range of extraluminal sources of bleeding (e.g.,
bowel-wall tumors and diverticles) that may not be readily apparent on colonoscopy,
nuclear medicine studies,
or angiography (2,5).
3.4.
LARGE-BOWEL OBSTRUCTION DUE TO RECTAL CAUSE: Fig. 8
Acute complete large-bowel obstruction (LBO) is an abdominal emergency,
with high morbidity and mortality rates if left untreated.
Clinical presentation is often acute and includes abdominal pain,
constipation or obstipation,
and abdominal distension.
The competence of the ileocecal valve influences the response of the colon.
If the ileocecal valve is competent, a LBO will result in a closed-loop obstruction,
which cannot decompress into the small bowel.
Cecal distension will lead to increased wall tension and without intervention,
will progress to ischemia and,
if not treated,
bowel infarction and perforation.
The main causes of LBO are:
Common causes (>95%) |
Uncommon causes (<5%) |
- Neoplasm
- Volvulus: sigmoid,
cecum,
transverse colon
- Diverticulitis
|
- Intussusception
- Hernia
- IBD
- Extrinsic compression from an abscess or other masses
- Fecal impaction
- Intraluminal foreign body
|
Imaging features:
1) Plain radiograph:
- LBO is visible as colonic distension with collapsed distal colon.
Small bowel dilatation,
depends on the duration of obstruction and incompetence of the ileocecal valve.
- Air-fluid levels are less numerous than in SBO.
2) CT:
- It is the currently the most widely used modality for assessment of large bowel obstructions and is not only able to confirm the diagnosis and localize the obstruction but in most instances also is able to identify the cause.
- CT criteria for intestinal obstruction are similar to plain radiography.
- It is also the technique of choice when we suspected complications,
as intestinal ischemia,
and can provide an alternative diagnosis for the patient's symptoms when obstruction is not present (6).
3.5.
FAECALOMA: Fig. 9 Fig. 10 Fig. 11 Fig. 12 Fig. 13
A faecaloma is a mass of feces most frequently noted in the rectum and sigmoid colon.
Symptoms are nonspecific and include constipation,
abdominal pain,
anorexia,
nausea or vomiting,
overflow diarrhea,
fecal incontinence,
and urinary symptoms from pressure.
There are several causes of faecaloma and they have been described in association with:
- Hirschsprung's disease
- Psychiatric patients
- Chagas disease
- Inflammatory and neoplastic conditions
- Chronic constipation
Stercoral colitis is an inflammatory colitis caused by increased intraluminal pressure from impacted fecal material in the colonic segments.
It may present with a spectrum ranging between impacted fecaloma associated with inflammation to colonic perforation.
Imaging features:
A faecaloma should be considered when there is evidence of focal fecal material of equal or greater diameter than the colon (7,8).
3.6.
PROCTITIS: Fig. 14 Fig. 15 Fig. 16 Fig. 17Fig. 18
Proctitis is an inflammation of the rectal mucosa that can be caused by several different conditions:
- Ischemia
- Trauma
- Foreign bodies
- Faecaloma
- Neoplasms
- Iatrogenic: enema,
colonoscopy,
etc.
- Infection / inflammation
Its principal differential diagnosis is rectal cancer.
Imaging features:
At CT,
colitis shows circumferential wall thickening,
pericolonic stranding and small volume free fluid (9–11).
3.7. ISCHEMIA: Fig. 18
Ischemic colitis results when blood flow to the colon is compromised.
Imaging features:
At CT,
the findings of ischemia in the acute phase include bowel wall thickening,
low (submucosa edema) or high (intramural hemorrhage) attenuation of the wall,
mural hypoenhancement,
pneumatosis,
pneumoperitoneum,
and/or portal venous gas; mesenteric stranding and peritoneal fluid aid in the diagnosis (10).
3.8.
TOXIC MEGACOLON: Fig. 19
Toxic megacolon is a potentially lethal complication of IBD or infectious colitis that is characterized by total or segmental nonobstructive colonic dilatation plus systemic toxicity.
Although most commonly recognized as a complication of IBD,
toxic megacolon may also occur with infectious colitis of diverse etiology,
ischemic colitis,
volvulus,
diverticulitis,
and obstructive colon cancer.
Its differential diagnosis is idiopathic megacolon and Ogilvie syndrome.
Imaging features:
The transverse colon is most often affected and may dilate up to 15 cm in diameter.
Plain films reveal distension of the colon with absent haustra.
Edematous and ulcerated mucosa may give rise to pseudopolyps.
CT will demonstrate a thin bowel wall with an irregular nodular configuration.
Barium enemas are absolutely contraindicated in patients with suspected toxic megacolon due to high risk of perforation.
3.9. RECTAL ULCERS: Fig. 6 Fig. 20
Rectal ulcers can be caused by several conditions:
- Infections: CMV,
Clostridium difficile,
Yersinia enterocolitica,
...
- IBD: Ulcerative colitis and Crohn's disease
- Foreign bodies / Trauma
- Faecaloma
- Tumor: Adenocarcinoma,
lymphoma,
...
- Vascular: ischemic colitis,
LAS,
Behçet or RT
- Drugs: NSAIDs,
gold salts,
digoxin,
QT
- Amyloidosis
- Rectal solitary ulcer: is an uncommon benign disease that is frequently associated with constipation and rectal prolapse. Mostly young adults are affected.
Major complications related to rectal ulcers include perforation,
hemorrhage,
abscess,
and fistulas.
3.10. RECTAL FOREIGN BODY: Fig. 21 Fig. 22
Rectal foreign bodies are not uncommon in emergency departments.
Its symptoms can vary depending on the absence or presence of complications: rectal perforations,
intestinal obstruction,
abscesses or fistulas,
hemorrhage or sepsis.
Imaging features:
- Radiopaque foreign bodies may require a conventional radiograph or a CT for diagnosis and localization.
- Conventional radiography is usually diagnostic although CT scans are frequently obtained to rule out complications such as perforation.
- MRI is contraindicated when the foreign body is unknown (12).
3.11. RECTAL PERFORATION: Fig. 23 Fig. 24 Fig. 25 Fig. 26
Rectal perforation can be caused by several conditions:
- Ischemia
- Trauma
- Foreign bodies
- Faecaloma
- Neoplasms
- Iatrogenic: enema,
colonoscopy,
etc.
- Infection / inflammation
Imaging features:
- Discontinuity of the rectum wall.
- Extraluminal air (retropneumoperitoneum or pneumoperitoneum).
- Extravasation of oral contrast material.
- Fluid or mesorectal fat-stranding (13).
3.12. POSTOPERATIVE COMPLICATIONS:
Postsurgical complications are not uncommon after rectal surgery.
The complications vary according to the surgical procedure that was performed,
the indication for the surgery,
the length of the postoperative period,
and the presence of any underlying concomitant disease entities.
Common specific postoperative complications include wound complications, anastomotic leak, stenosis of the anastomosis,
fistula or abscess,
bleeding or hematoma, and bowel obstruction.
Multidetector CT with the use of multiplanar reformation provides good anatomic detail and allows early evaluation of the type of postsurgical complications (14).
1) Anastomotic leak: Fig. 27 Fig. 28
An anastomotic leak after bowel surgery is associated with high morbidity and mortality and may result in complications,
including peritonitis and sepsis.
The treatment of leaks often requires further invasive procedures,
including percutaneous drain creation,
diverting ostomies,
and repeat open surgery.
A low anterior rectal anastomosis carries the highest risk for developing an anastomotic leak.
Clinical symptoms suggestive of an anastomotic leak usually manifest in the first 2 postoperative weeks,
most commonly between days 5 and 7.
Symptoms include fever,
intense abdominal pain,
tachycardia,
guarding,
and rebound tenderness.
MDCT is currently the imaging modality of choice to evaluate for suspected anastomotic leak and it should be performed with rectal contrast material.
An anastomotic leak can be diagnosed with confidence at CT when there is frank extravasation of bowel contrast material with air or possibly stool.
After anterior resection,
this appearance can mimic the true rectum and has been described as the “double rectum” sign.
Less definitive but suggestive signs include extraluminal air in a higher proportion relative to fluid in the newly formed postoperative spaces.
2) Anastomotic stricture: Fig. 29
In the early post-operative period strictures are usually caused by edema and resolve on later examinations. However,
strictures can also be caused by ischemia and adhesions.
This last group is more frequent in low anastomoses than high anastomoses.
Anastomotic strictures can lead to significant complications such as bowel obstruction and need treatment.
3) Fistula and Intraperitoneal abscess:
Ongoing leaks may result in fistulas or abscesses.
Intraabdominal abscesses remain the most common cause of morbidity following colorectal surgery.
3.13. RECTAL FISTULAS
1) Rectovesical fistula: Fig. 30 Fig. 31
It is a communication between the rectum and the bladder.
The diagnosis is suspected clinically due to pneumaturia,
faecaluria,
recurrent urinary tract infections or passage of urine through the rectum (15).
Its main causes are:
Nowadays:
|
Before:
|
Diverticulitis,
trauma,
radiotherapy,
and surgery.
|
Syphilis,
tuberculosis,
and amebiasis.
|
2) Rectovaginal fistula: Fig. 32
It is an abnormal connection between the rectum and the vagina.
The diagnosis is suspected clinically due to recurrent urinary tract infections,
passing gas or stool to the vagina,
vulvar or vaginal pain,
etc.
Its main causes are:
- Obstetric trauma
- Crohn's disease
- Radiotherapy or cancer
- Pelvic surgery
- Other: diverticulitis,
fecaloma,
foreign bodies,
etc.