Diagnostic CT Colonography

Patients with positive bowel symptoms, such as change in bowel habits, lower gastrointestinal bleeding, iron deficiency anemia and abdominal pain are eligible to undergo a diagnostic CTC. The patient is scanned in both the supine and prone position, but unlike a screening CTC, the patient is injected with intravenous iodinated contrast material during the supine acquisition (CheN et al. 1999). Injection of contrast aids in the differentiation of polyps versus adherent stool. Studies have also demonstrated increased accuracy of polyp detection with the use of intravenous contrast (Morrin 2000). A contrast-enhanced scan may aid in the detection of extra-colonic causes of the patient's symptoms. Finally, diagnostic CTC has the ability to detect and stage colorectal cancer, unlike the other two alternatives, conventional colonoscopy and double contrast barium enema.

The indications for diagnostic CTC closely follow those for conventional colonoscopy (Rankin 1987). Rectal bleeding, heme positive stool, anemia and constipation are just a few examples. Indications for screening and diagnostic CTC are summarized in Table 2.1.

Diagnostic CTC may be used to further evaluate findings on conventional colonoscopy. Not infrequently, diagnostic CTC is performed in patients with suspicious intramural or extra-mural masses detected on optical colonoscopy (Fig. 2.2).

Occasionally, patients are unable to undergo conventional colonoscopy due to presence of a colonic stricture, redundant sigmoid, or contraindications to intravenous conscious sedation. Flexible sig-moidoscopy can be performed without sedation; however, the majority of the colon is not evaluated.

Table 2.1. Indications and contra-indications for CT colonography

Indications for CTC

Contra-indications for CTC

Screening

Diagnostic

Age > 50 years3

Colorectal cancer detection in patients with:

Acute abdomen

Bleeding diathesis

Lower gastrointestinal bleeding

Recent pelvic or abdominal surgery

Failed colonoscopy

Change in bowel habits

Acute diverticulitis

Polyp detection

Lower abdominal pain

Toxic megacolon

Elderly

Iron deficiency anemia

Colonic hernia

Contraindication to sedation

Obstructing colon mass

Scanner weight limitations

Post-operative colorectal cancer surveillance

Pregnancyc

High risk patientsb

Hip joint replacement0

Incompetent ileocecal valvec

Claustrophobiac

a Average risk patient b Patients with inflammatory bowel disease, familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, first degree relative with colorectal cancer or patients with prior history of colorectal cancer. c Relative contraindication a Average risk patient b Patients with inflammatory bowel disease, familial adenomatous polyposis, hereditary nonpolyposis colorectal cancer, first degree relative with colorectal cancer or patients with prior history of colorectal cancer. c Relative contraindication

Fig. 2.2a-c. Submucosal colonic lipoma: a sagittal reformatted CT image (lung window settings) acquired in prone position shows a 1.8-cm intramural lesion within the cecum (arrow); b sagittal reformatted CT image in supine position (soft tissue settings) demonstrates a fatty mass (arrow) along the posterior border of the cecum in the same location as seen on the prone image; c supine axial CT image (soft tissue settings) demonstrates again the fatty mass consistent with a submucosal lipoma

Although double contrast barium enema evalu- ventional colonoscopy. Several studies reveal that ates the entire colon, many proponents of the new the sensitivity and specificity of polyp detection is technology believe that CTC should be the study of higher for CTC compared to DCBE (Johnson et al. choice for patients whom are unable to undergo con- 2004; Fenlon et al. 1999b).

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