CT

DR SAKHER-ALKHADERICONSULTANT RADIOLOGIST AMC

CT IMAGING OF BOWEL WALL

THICKENING

Bowel wall thickening is a common finding in imaging. CT can be helpful in the differentiation of

intestinal disease

Type 1 - White Attenuation Pattern

 -If the bowel wall is enhanced to a degree that is equal to or greater than that of venous opacification in the same scan , it should be classified in the white attenuation pattern. 

-At least two pathophysiologic events likely underlie this attenuation pattern: (a) vasodilation and/or (b) injury to intramural vessels with accompanying interstitial leakage

Causes

Normal Gastrointestinal Tract-The normal small-bowel wall is thin, measuring

between 1 and 2 mm when the lumen is well distended -If the bowel is partially collapsed, the wall measures between 2 and 3 mm and is of symmetric thickness-The normal thickness of the colonic wall varies greatly depending on the degree of distention. When the colon is distended, the wall should measure less than 3 mm;

Normal enhancement and appearance of small bowel in 77-year-old woman with normal bowel wall. Note thinly enhancing valvulae conniventes (arrow). This finding is often better seen when water alone is given as oral contrast agent.

Type 1 - White Attenuation

The normal bowel will enhance bright especially if the scan is in the late arterial phase, i.e. 35-40 seconds post injection.If the bowel wall is not thickened, this is normal enhancement.

Acute IBDHere a patient with acute inflammatory bowel disease (IBD).Notice the bright enhancement of a large segment of the small bowel with a thickened wall.This is the result of hyperemia due to the vasodilatation.Notice the dilated vessels on the ventral side.

White attenuation: enhancement in acute ulcerative colitis. On an intravenous contrast-enhanced CT scan, the thickened wall of the rectosigmoid segment demonstrates uniform increased enhancement (straight black arrows) similar to the attenuation of the external iliac vein (curved arrow). Pericolonic vessels are dilated (white arrow).

White attenuation: enhancement of the small intestine in shock bowel. On an intravenous contrast-enhanced CT scan, the jejunum demonstrates increased enhancement (black arrows). The attenuation is greater than that of the inferior vena cava (curved arrow)

White attenuation: enhancement in ischemic duodenitis and jejunitis. Intravenous contrast-enhanced CT scan shows enhanced segments of duodenum (large straight black arrow) and jejunum (curved arrow). Note absence of oral contrast material in the stomach (white arrow). Dissection is present in the aorta (arrowhead) and superior mesenteric artery (small straight black arrow).

White attenuation: enhancement in ileal Crohn disease. On an intravenous contrast-enhanced CT scan, the enhanced thickened wall of the small bowel (solid arrows) is slightly higher attenuation than the inferior vena cava (open black arrow). The vasa recta are dilated (arrowhead) and separated by increased fat deposition (“creeping fat sign”). Open white arrow = enlarged mesenteric node.

Type 2 - Gray Attenuation

-The pattern of gray attenuation is defined as a thickened bowel wall that shows little clear-cut enhancement and whose homogeneous attenuation is comparable with that of enhanced muscle-The gray attenuation pattern is the least specific of the five attenuation categories for diagnosis, and it is common in both benign and malignant diseases -Bowel wall thickening of less than 2 cm is more characteristic of benign conditions, whereas thickening greater than 3 cm is usually present in neoplastic cases.

In the gray-pattern, the bowel wall is thick and despite a nice bolus of contrast there is poor enhancement and you can not see the different layers of the bowel wall.This pattern is seen in chronic fibrotic Crohn's disease, ischemia and neoplasms like adenocarcinoma and lymphoma.

Gray enhancement pattern in a patient with chronic Crohn's disease.

Mesenteric IschemiaBowel ischemia frequently affects the colon and is more frequently seen in the splenic flexure, descending colon and sigmoid.It is mostly due to a low flow state like hypovolemic shock or congestive heart failure.Especially in elderly with bowel wall thickening you should always put ischemia in your differential diagnostic list.A special cause of ischemia in the small bowel is a closed loop obstruction, which we will discuss in a moment.

Mesenteric Ischemia

Gray enhancement pattern in a patient with SMV thrombosis.Notice the venous congestion in the mesentery (yellow arrow).

Mesenteric Ischemia

Gray enhancement pattern in a patient with closed loop obstruction.Notice the difference in enhancement between the normal non-distended loops (green arrow) and the distended strangulated loops (red arrow).

TumorThe gray enhancement pattern with loss of identification of the various layers of the bowel wall can be seen in various tumors like adenocarcinoma, metastases and GIST.Lymphoma and neuroendocrine tumors like carcinoid usually show somewhat more enhancement.

Adenocarcinoma of the sigmoid.

Gray attenuation. Intravenous contrast-enhanced CT scan of a case of colonic adenocarcinoma demonstrates the thickened wall of the mid-descending colon (straight arrow) with attenuation similar to that of adjacent muscle (curved arrow).

Type 3 - Water target sign

The target sign is caused by the enhancing mucosa and muscularis propria with the edematous submucosa in between .

7.Radiation damage

Pseudomembranous Colitis

There is ascites and hyper enhancement of the bowel wall with submucosal edema and edema in the mesocolon.

Water halo sign. Intravenous contrast-enhanced CT scan of a case of pseudomembranous colitis shows an outer enhanced layer (white arrows) surrounding a water attenuation layer (curved arrows).

CMV-colitis.

Typhlitis is a necrotizing inflammation of the cecum, which is usually seen in patients with neutropenia due to acute leukemia, AIDS or aplastic anemia.There is transmural edema and ulceration, which can cause perforation.

Infectious ColitisRight colon: Salmonella Shigella Campylobacter Yersinia enterocoliticaDiffuse colitis E.Coli CMV CryptococcusLeft colon and Rectosigmoid: Schistosomiasis Rectosigmoid: HSV Gonorrhea

Target” sign detected only after IV contrast administration in 64-year-old man with pain and bloody diarrhea. Contrast-enhanced axial CT image obtained 48 hr after A at same level shows thickened sigmoid with target configuration (arrow). Findings suggest inflammation or ischemia. Endoscopy and biopsy confirmed ischemic colitis.

“Target” sign in 37-year-old woman with history of lupus erythematosus. Contrast-enhanced axial CT image at level of mid abdomen shows diffuse marked circumferential thickening of colon. Target appearance is present, with enhancement of mucosa (short white arrow) and outer enhancement of muscular layer and serosa (long white arrow) surrounding low-attenuation edematous submucosa. Small amount of ascites is present (arrowhead).

Type 4 - Fat target sign

The pattern of the fat halo sign refers to a three-layered target sign of thickened bowel in which the middle or “submucosal” layer has a fatty attenuationThe observation of the fat halo sign in the small intestine is, for all intents and purposes, diagnostic of Crohn disease and by itself is a sign of a chronic phaseThe rare diagnoses in which this pattern occurs include cytoreductive therapy exposure and chronic radiation enteritis.

Target fat sign in chronic ulcerative colitis

Fat halo sign. CT scan of a case of ulcerative colitis shows an outer enhanced layer (straight solid arrows) surrounding a fat attenuation layer (curved arrows).

 Fat halo sign in chronic radiation enteritis. Intravenous contrast-enhanced CT scan demonstrates several segments of small bowel with walls thickened by a central band of lower attenuation, consistent with that of fat (arrowheads)

Deposition of fat in submucosa producing “target” sign in 85-year-old man with history of chronic ulcerative colitis.

Type 5 - black Attenuation

-The pattern of black attenuation is the equivalent of pneumatosis 

-All pneumatosis should be considered as part of an acute injury to the bowel. 

-Any process that is accompanied by a break in the mucosa can introduce intramural gas.

Pseudopneumatosis

Gas adjacent to the bowel wall can mimick pneumatosis.This is called pseudopneumatosis.

1- Give special attention to the non-dependent bowel wall, where there is no feces in contact with the mucosa and gas bubbles will not be seen.

TEACHING POINTS

2-The linear arrangement of the gas bubbles makes it suspective of pneumatosis.

3-String of pearls sign is indicative of small bowel obstruction (SBO) and not pneumotosis

Cases of pneumotaosis

Portal venous gas

Portal venous gas is an ominous radiologic sign and is associated with a high mortality rate.

Gas in the portal veins has to be differentiated from gas within the bile ducts, which is called pneumobilia. Portal venous gas is located peripherally in the liver as opposed to pneumobilia which is usually more centrally located.

Pneumobilia

TEACHING POINTS1-Symmetric bowel wall thickening is seen in intestinal

inflammatory conditions, infections, bowel edema, and ischemia, submucosal hemorrhage and lymphoma

2-Asymmetric or eccentric bowel thickening is mainly seen with malignant conditions.

3-Most neoplasms of the gastrointestinal tract present as a focal area of bowel wall thickening 4-Inflammatory processes that may present as focal areas ppendicitis, and, occasionaof bowel wall thickening include diverticulitis, ally, tuberculosis.

5-A segmental distribution of involvement is usually caused by an inflammatory process. Conditions associated with segmental involvement include Crohn's disease, infectious ileitis, radiation enteritis, and ischemia.

6-Diffuse thickening of the bowel wall is seen with a variety of inflammatory conditions, including ulcerative colitis, infectious enteritis, edema from low-protein states, portal hypertension associated with cirrhosis, and low-flow ischemia . Segmental or diffuse thickening may be seen in patients with small-bowel vasculitis, as often occurs in systemic lupus erythematosus .

7-Pericolonic lymph nodes adjacent to the focal area of colonic thickening are more commonly seen in patients with colon cancer. Pericolonic inflammatory changes are more commonly seen in diverticulitis .

8-CT findings of mild, symmetric bowel wall thickening with or without a target configuration in the distal ileum lead to a differential diagnosis of infectious enteritis, Crohn's disease, vasculitis, and radiation enteritis. Secondary findings that help establish the diagnosis of Crohn's disease include fistulas, sinus tracts, perienteric abscess, and fibrofatty proliferation.

THE END