CT and MRI of the Lower Gastrointestinal Tract

1

What is shown in Figure 27-1 ?

A topogram from a computed tomographic colonography (CTC), an examination tailored to evaluate for colorectal polyps and masses.

2

What is the adenoma-carcinoma sequence?

The adenoma-carcinoma sequence refers to a series of genetic mutations whereby small adenomatous colon polyps (<5 mm) transform into large adenomatous polyps (>1 cm), noninvasive carcinomas, and then invasive carcinomas. This entire process takes, on average, 10 years, which serves as the basis for colorectal cancer screening. Up to 90% of colorectal carcinomas, predominantly adenocarcinomas, develop through the adenoma-carcinoma sequence, rendering colorectal cancer a virtually preventable disease through regular screening.

3

What is the risk of malignancy in a colonic adenomatous polyp measuring <1 cm?

1%.

4

What is the risk of malignancy in a colonic adenomatous polyp measuring 1 cm?

10%.

5

What is the diagnostic performance of CTC compared to traditional or optical colonoscopy (OC) for adenomatous polyps?

The diagnostic performance of CTC improves with increased polyp size. For the answer, see Table 27-1 .

6

What is the protocol for CTC?

The typical preparation for CTC involves a combination of a low residue diet, cathartic bowel preparation, and fecal tagging (i.e., labeling of fecal residue) using both barium sulfate (Tagitol) and water-soluble iodinated contrast material (Gastroview). When CTC is performed on the same day as an incomplete OC, Gastroview is administered after sedation has worn off and 2 to 3 hours prior to scanning.

7

Can CTC be performed without a cathartic bowel preparation?

Yes. The preparation for laxative free CTC is accomplished using a combination of fecal tagging and low residue diet. However, the absence of a cathartic bowel preparation affects the diagnostic performance of this test and does not permit same day polypectomy. Specifically, the sensitivity and specificity of laxative free CTC is 91% and 86%, respectively, for polyps ≥10 mm, but is 59% and 88%, respectively, for polyps ≥6 mm. As such, laxative free CTC is generally reserved for patients unwilling to undergo a cathartic bowel preparation for either CTC or OC.

8

What are the three factors that should be optimized on CTC?

• Colonic cleansing/fecal tagging to ensure that adherent or retained stool can be distinguished from potential polyps to avoid false-positive results.

• Colonic distension so that polyps can be visualized against an air-filled lumen.

• Patient positioning using at least two positions to assess for lesion mobility, to shift fluid to increase exposed colonic lumen, and to shift gas to distend colonic segments. If patients are unable to lie prone, decubitus images may be performed as an alternative ( Figure 27-2 ).

  

9

What are the differences between a primary “2D” and a primary “3D” read on CTC?

For the answer, see Table 27-2 .

10

What is the “polyp” window?

The “polyp” window is used for primary 2D review, where the contrast and brightness settings of display of the images are set between those used for lung and bone windows. This window optimizes the contrast between the gas-filled colonic lumen and intraluminal polyps or masses (see Figure 27-2 ).

11

What is the differential diagnosis for a focal mass projecting into the colon lumen on 3D images (prior to correlation with the 2D images)?

The differential diagnosis for a focal mass identified on 3D images includes tagged stool, impacted fecalith, bulbous fold, lipoma, and polyp ( Figure 27-3 ). This differential diagnosis underscores the importance of correlating all focal abnormalities detected on 3D images with the 2D images.

12

What is the major differential diagnosis for a focal polypoid soft tissue attenuation colonic lesion on CTC?

Colon cancer, adenomatous polyp, hyperplastic polyp, and pseudolesion (untagged stool or mucosal fold) ( Figure 27-4 ).

13

How is a colonic diverticulum diagnosed on CTC?

The presence of a ring shadow or dark curvilinear focus surrounding the orifice of the lesion indicates a colonic diverticulum ( Figure 27-5 ). Recognition of these findings allows the reader to confidently ignore them on a primary 3D review.

14

How do diverticula of the gastrointestinal tract form?

Bowel diverticula are the result of one of three processes: (1) pulsion diverticula due to increased intraluminal pressure, (2) traction diverticula due to traction on the wall of the bowel by scarring, and (3) congenital diverticula due to abnormal development. The most common type of diverticula are pulsion diverticula, which are seen predominantly in the colon.

15

What are the three methods utilized to distinguish stool from polyps on CTC?

For the answer, see Table 27-3 .

16

What is the definition of a “clinically significant polyp” on CTC?

The size threshold for a clinically significant polyp on CTC is 6 mm or greater. This cutoff is based on large patient series which demonstrate that lesions less than 6 mm on CTC often represent hyperplastic polyps or normal mucosal skin tags. Hyperplastic polyps almost never undergo malignant degeneration and so there is minimal to no risk of cancer developing in these lesions. The majority of polyps less than 5 mm are adenomas that will not undergo the adenoma-carcinoma sequence.

17

How frequently should CTC be performed?

There is little long-term data on the optimal frequency at which CTC should be performed. However, according to joint guidelines from the American Cancer Society (ACS), the US Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology (ACR), CTC should be performed every 5 years, similar to double-contrast barium enema. This is compared to every 10 years for OC among patients with no polyps.

18

What are the most commonly observed indications for CTC?

The most common indication for CTC is incomplete OC, which can be performed the same day as OC, as well as a history of prior difficult OC. Less frequently encountered indications include sedation risk, chronic anticoagulation, evaluation of the colon proximal to an obstructing mass or stricture that precludes passage of an endoscope, and characterization of an indeterminate colorectal lesion on OC. Of note, CTC can be performed safely in patients with colonic anastomoses.

Coverage for screening CTC remains controversial due to concerns over cost-effectiveness, the ability of CTC to reach the previously unscreened population, extracolonic findings (findings outside of the colonic lumen visible on CTC but not OC), and radiation risk. In some regions of the country, CTC is reimbursed as a method of colorectal cancer screening, although most insurers including the Centers for Medicaid and Medicare (CMS) do not currently cover this modality for screening.

19

When is CTC not recommended?

CTC is not advised for surveillance of patients at high risk for cancer including patients with hereditary syndromes (e.g., Lynch syndrome, hereditary polyposis, or nonpolyposis syndrome). This examination is not advised on the same day as OC where a deep biopsy or polypectomy is performed. CTC is also not appropriate for patients with acute colitis, recent surgery, symptomatic colon-containing abdominal wall hernia, known or suspected bowel perforation, and high-grade small bowel obstruction.

20

What is the role of routine computed tomography (CT) and magnetic resonance imaging (MRI) in colon cancer screening?

None.

21

What are the complications of CTC?

The main risk of CTC is perforation, which is seen in 0 to 7 out of 1,000 elderly patients (compared to 12 out of 1,000 elderly patients who receive OC).

22

What is the radiation dose for CTC?

Both supine and prone position imaging can be performed at a dose of less than 12 mSv, using an effective tube current of 50 mAs or modulated dose protocols. This radiation dose is similar to or lower than that from a double-contrast barium enema and ≤50% the diagnostic reference level for routine abdominopelvic CT.

23

Why do patients prefer CTC to OC?

75% of patients prefer CTC to OC for the following reasons: noninvasiveness, lack of sedation, improved comfort, and avoidance of OC risks (i.e., perforation).

24

What is the normal cross-sectional imaging appearance of the colon?

The normal wall thickness of the colon is paper thin (i.e., <3 mm) when well distended. The diameter of the colon varies greatly, where the cecum generally has the greatest diameter and is usually <9 cm, the transverse colon is usually <6 cm, and the descending colon and sigmoid colon are slightly smaller in caliber. Gas, fecal material, and fluid are normally present in the colon.

25

What are the major nonneoplastic causes of colonic wall thickening?

For the answer, see Box 27-1 .

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