Large Bowel Imaging Presentation2

7/27/2019 Large Bowel Imaging Presentation2

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CT can demonstrate a mechanical intestinalobstruction, and it can be used to differentiatebetween mechanical obstruction and adynamic

ileus. CT scans show the site of obstruction andits severity. A volvulus or strangulation generally

causes a closed-loop obstruction, and CTcharacteristically demonstrates a U-shaped

distended bowel segment and signs of ischemia.These signs include mural thickening, infiltrationof the mesenteric fat, and pneumatosis

intestinalis. In the presence of a cecal or sigmoidvolvulus, a whirl sign may be apparent because of

a tight torsion of the mesentery that is caused bya twist between the afferent and efferent loops.12,

13, 14


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CT scan findings of sigmoid volvulus include the whirlsign, which represents tension on the tightly twistedmesocolon by the afferent and efferent limbs of the

dilated colon.11, 12

CT scanning may be useful in identifying the etiology

and site of the obstruction that result from otherpathologies, as well as in demonstrating ischemia thatresults from strangulation.

CT scan signs of ischemia include a serrated beak at thesite of the obstruction, mesenteric edema orengrossment, and moderate to severe thickening of thebowel wall.

Intramural gas or portal venous gas may be seen (graveprognostic signs), and in patients in whom aperforation has occurred, a large amount of freeintraperitoneal gas or fluid may be noted.


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MRI has been used successfully in theassessment of large-bowel obstruction (not

specifically in sigmoid volvulus). Theseexaminations were performed with theretrograde insufflation of 1000-1200 mL of airthrough a Foley catheter that was placed in

the rectum and with scopolamine to inhibitperistalsis in order to demonstrate the site ofbowel obstruction. In addition, MRI has beenused in the diagnosis of mural necrosis in

infants and, theoretically, this modality can beused in adults.


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CT scan is used for staging colon cancer beforesurgery, for assessing and staging recurrentdisease, and for detecting the presence of distant

metastases.

Preoperative CT scan is indicated if there isclinical suggestion of distant metastases or localinvasion of the adjacent organs or abdominalwall.

In older patients who may be unable to undergocolonoscopy or barium enema, modified CT scan

may be performed for primary detection ofcolorectal tumors.

Colonic tumors may be diagnosed on CT scan asan incidental finding.


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MRI provides greater contrast between soft

tissues than CT scan.

Colonic tumors have low signal intensity(similar to adjacent skeletal muscle) on T1-

weighted sequences, which facilitates their

differentiation from high-signal perirectal fat.T2-weighted images are used to detect pelvic

sidewall invasion.

Tumor enhancement can be achieved byparamagnetic agents such as gadolinium.


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A localized tumor may be seen on CT scan as anintraluminal or intramural mass of soft tissue densityadjacent to the gas-filled or contrast-filled bowel

lumen; More advanced tumors are associated with thickening

of the bowel wall (>6 mm) and infiltration of thepericolic fat. Thin strands of tissue may extend fromthe tumor into the pericolic fat .

Annular carcinomas are detected by a thickening of thebowel wall and narrowing of the lumen. This thickeningis concentric if the scanning plane is at right angles tothe long axis of the bowel

Extracolonic tumor spread is indicated by a loss oftissue fat planes between the colon and surroundingstructures. Invaded muscle may be enlarged . Colonictumors may invade the anterior abdominal wall, liver,pancreas, spleen, or stomach.

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Large Bowel Imaging Presentation2