CT Colonography

CT colonography (CTC), also called virtual colonoscopy, is a low dose CT of the abdomen and pelvis after bowel cleansing, faecal tagging with oral contrast, and gentle insufflation of the colon with carbon dioxide (or room air) through a rectal catheter.

Scans are acquired in both supine and prone positions (or supine and decubitus when prone is not possible) in 10 to 15 second breath holds, with slices of 0.625 to 1.25 mm.

The dose is low: effective dose about 3 to 6 mSv for both acquisitions with low dose settings and iterative reconstruction.

Images are read in two dimensional multiplanar mode and with three dimensional endoluminal flythrough. The examination takes 15 to 20 minutes and requires no sedation. Intravenous contrast is given when the study is done for symptoms in frail patients or for staging.

CTC has a sensitivity of about 90% for polyps of 10 mm or more and 85 to 90% for polyps of 6 mm or more, with specificity of 85 to 95%.

Sensitivity for colorectal cancer is over 95% (about 96%, comparable to optical colonoscopy).

Colorectal cancer is the third commonest cancer worldwide, with about 1.9 million new cases each year. Extracolonic findings, important and unimportant, are found in about 5 to 10% of cases, and CTC shows the whole colon in 98 to 99% of cases, including after incomplete colonoscopy, with a very low perforation rate (about 0.01 to 0.03%).

Indications. Incomplete or failed colonoscopy (obstructing cancer, tortuous or fixed colon, patient intolerance). Symptomatic patients (bleeding, change in bowel habit, iron deficiency anaemia, abdominal mass) who are unfit or unwilling to undergo colonoscopy, or in whom colonoscopy is unavailable. Colorectal cancer screening where colonoscopy is declined, or as an alternative test in some programmes (every 5 years).

Surveillance after polypectomy in selected cases. Staging of obstructing cancer (assessment of the proximal colon before surgery). Diverticular disease with stricture.

Frail or anticoagulated patients in whom colonoscopy carries a high risk. Not for acute diverticulitis, active colitis (inflammatory bowel disease), recent colonic surgery or biopsy (within 1 to 2 weeks), symptomatic bowel obstruction or suspected perforation, and not as a primary test for rectal bleeding in high risk young patients.

Contraindications and precautions. Acute colitis or diverticulitis, toxic megacolon, recent deep biopsy or polypectomy (wait 4 to 6 weeks), suspected perforation, large symptomatic hernia containing colon, pregnancy, and inability to cooperate or lie in position. Relative: severe constipation, symptomatic stricture, and large abdominal wall hernia. Bowel preparation precautions in renal impairment, heart failure and electrolyte disorders (avoid sodium phosphate preparations).

Iodinated contrast is avoided unless needed. Carbon dioxide is preferred to room air because it is absorbed quickly and reduces discomfort.

Preparation and technique. A low residue diet for 1 to 2 days, then a clear fluid diet on the day before.

A cathartic laxative (low volume polyethylene glycol, magnesium citrate, or sodium picosulphate) or a reduced preparation with faecal and fluid tagging (barium sulphate 2% and a water soluble iodinated agent 20 to 50 mL taken in divided doses with meals the day before).

Hyoscine butylbromide 20 mg intravenously (or glucagon) may be given to relax the colon, unless glaucoma, prostate disease or cardiac disease contraindicates it. A thin flexible rectal catheter is inserted, and carbon dioxide is delivered by an automatic insufflator at a controlled pressure (about 20 mmHg) until the colon is distended and the patient reports mild fullness.

A scout view confirms distension, and the supine and prone acquisitions are obtained; repeat if segments are collapsed. Low tube current (30 to 50 mAs) is used. Images are reviewed with computer aided detection as a second reader in some centres.

Reports include polyp size (largest diameter), shape, position and the number, as well as extracolonic findings.

Normal appearances. The colon is distended, with a smooth wall under 3 mm thickness and regular haustral folds (1 to 3 mm thick and 3 to 5 mm apart in the distended colon).

The diameter is 3 to 6 cm (the caecum up to 7 to 9 cm).

Tagged faecal residue is bright and moves between supine and prone scans, with no soft tissue masses fixed to the wall. The ileocaecal valve is seen as a lip or fatty prominence at the caecum, the appendiceal orifice as a small dimple, and the rectum is distended with the catheter balloon (if used) visible.

There are no diverticula in a young patient. Extracolonic structures, solid organs and vessels appear normal.

Interpretation.

• Polyp: a fixed soft tissue filling defect of 6 mm or more attached to the colonic wall (sessile, pedunculated with a stalk, or flat lesion under 3 mm in height).
• Features are non-mobile on position change, soft tissue density (about 20 to 50 HU, not tagged), and a fixed shape.
• Large polyps (10 mm or more) and those with a villous surface, lobulation or high density carry a higher risk of advanced neoplasia.
• Reporting categories by size: no polyp or polyps under 6 mm (not usually reported and not followed); 6 to 9 mm (1 to 2 polyps: surveillance CTC at 3 years or colonoscopy; 3 or more: colonoscopy); 10 mm or more: colonoscopy and polypectomy.
• Colorectal cancer: a mass with a fixed irregular nodular surface, apple core stricture (circumferential narrowing with shouldering and shelves), or wall thickening with loss of folds; extracolonic extension (stranding of pericolic fat), enlarged nodes, liver metastases, and complications of obstruction.
• Flat lesions and carpet lesions: subtle plaque like thickening with faint folds, best seen with fluid tagging and in the prone view.
• Diverticular disease: outpouchings with muscular thickening and stricture; distinguishing diverticular stricture from cancer may require colonoscopy.
• Colitis: loss of haustra, wall thickening, narrowing and pseudopolyps in inflammatory bowel disease (CTC is avoided in active disease).
• Lipoma: well defined mass with fat density (-50 to -100 HU) and a smooth surface, often at the ileocaecal valve; ileocaecal valve prominence is normal; inverted appendix stump and appendiceal mucocele are mimics.
• Mimics of polyps: retained faeces (changes position, contains gas, heterogeneous), folds, prominent ileocaecal valve, rectal tube, haemorrhoids, and impacted stool in diverticula.
• Extracolonic findings: abdominal aortic aneurysm, renal masses and cysts, adrenal nodules, liver and pancreatic lesions, gallstones, pelvic masses (ovarian), lung base nodules, and hernias.
• They are graded by importance: highly important findings need action, moderately important need follow up, and low importance findings are not reported or are only noted.
• Complications: perforation (extraluminal gas or fluid), most often in patients with severe diverticulitis, stricture, or recent biopsy; the scan can also show post-procedural leak.

Critical values and findings needing immediate action.

• Suspected colorectal cancer (mass, apple core stricture): urgent colonoscopy with biopsy, staging CT (chest, abdomen and pelvis), and multidisciplinary referral within the local time limit.
• Polyp of 10 mm or more (or 3 or more polyps of 6 to 9 mm): referral for colonoscopy and polypectomy.
• Large bowel obstruction or perforation (free gas, extraluminal contrast): immediate surgical review.
• Highly important extracolonic findings (aortic aneurysm of 5.5 cm or more, suspicious renal or adrenal mass, lung mass, ovarian mass with ascites): urgent referral to the relevant specialist.
• Acute diverticulitis, colitis, or complication detected: stop insufflation, antibiotics and clinical assessment.

Limitations and artefacts.

• Small polyps (under 6 mm) and flat lesions are missed; the sensitivity falls with lesion size, and flat serrated lesions in the right colon have lower detection rates than adenomas.
• Residual stool and fluid hide lesions or mimic polyps; faecal tagging, prone and supine scanning and fluid subtraction help, and a repeat scan may be needed after adequate preparation.
• Collapsed segments from spasm or poor distension (the sigmoid colon in diverticular disease) lead to false negatives or false positives; additional positions and further insufflation are used.
• Reader experience is a major factor; a specialised, trained reader with audit achieves higher accuracy, and a computer aided detection second read improves sensitivity.
• Radiation dose is small but not zero; it is acceptable for older patients but is a consideration for repeated surveillance of young patients.
• Lesions cannot be biopsied or removed; a positive study means a second invasive procedure in about 10% of those tested.
• Colonic preparation can be unpleasant and may cause electrolyte disturbance in vulnerable patients.
• Extracolonic findings generate additional investigations, anxiety and cost, and most are of low significance.
• Intravenous hyoscine can cause urinary retention, glaucoma attacks, tachycardia and blurred vision.

Special groups.

• Older and frail patients: well tolerated, often preferred to colonoscopy, with better completion rates.
• Patients on anticoagulants: avoids stopping treatment and bleeding risk of polypectomy.
• Patients with incomplete colonoscopy: CTC the same day or soon after, with no need for repeat bowel preparation if tagging was used.
• Patients with high risk family history, polyposis syndromes and inflammatory bowel disease: colonoscopy is preferred.
• Pregnancy: avoid.
• Patients with a stoma: CTC can be done with adaptations.
• Regions: colorectal cancer is increasing in younger people and presents late in many places; colonoscopy capacity is limited, so CTC can extend access where scanners and trained readers exist; bowel preparation, local diet, and parasitic and amoebic infection of the colon may affect study quality.

Follow up tests. Optical colonoscopy with polypectomy or biopsy, histology of lesions, faecal immunochemical test and occult blood test, staging CT chest, abdomen and pelvis for cancer, MR of the rectum for rectal cancer, carcinoembryonic antigen, full blood count and iron studies for anaemia,

genetic counselling and testing for polyposis and hereditary non-polyposis colorectal cancer, surveillance colonoscopy at set intervals, ultrasound or CT for extracolonic findings, and referral to colorectal surgery, gastroenterology or oncology.