Computed Tomography, Colonoscopy

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Computed Tomography, Colonoscopy

Synonym/acronym: Computed axial tomography (CAT), computed transaxial tomography (CTT), CT colonography, CT virtual colonoscopy.

Common use

To visualize and assess the rectum and colon related to identification and evaluation of large polyps, lesions, and tumors. Also used to assess the effectiveness of therapeutic interventions such as surgery and primarily used for patients who cannot tolerate conventional colonoscopy.

Area of application



Screening examinations are done without IV iodinated contrast medium. Examinations done to clarify questionable or abnormal areas may require IV iodinated contrast medium.


Computed tomography (CT) colonoscopy is a noninvasive technique that involves examining the colon by taking multiple CT scans of the patient’s colon and rectum and using computer software to create three-dimensional images. It becomes invasive when contrast medium is used. During the procedure, the patient lies on a motorized table. The table is moved in and out of a circular opening in a doughnut-like device called a gantry, which houses the x-ray tube and associated electronics. A beam of x-rays irradiates the patient as the table moves in and out of the scanner in a series of phases. Multiple detectors rotate around the patient to produce cross-sectional views or slices. The slices can be viewed individually or as a three-dimensional image. Multislice or multidetector CT (MDCT) scanners continuously collect images in a helical or spiral fashion instead of a series of individual images as with standard scanners. Helical CT is capable of collecting many images over a short period of time (seconds), is very sensitive in identifying small abnormalities, and produces high-quality images. Differences in tissue density are detected and recorded and are viewable as computerized digital images. The procedure is used to detect polyps, which are growths of tissue in the colon or rectum. Some types of polyps increase the risk of colon cancer, especially if they are large or if a patient has several polyps. Compared to conventional colonoscopy, CT colonoscopy is less effective in detecting polyps smaller than 5 mm, more effective when the polyps are between 5 and 9.9 mm, and most effective when the polyps are 10 mm or larger. This test may be valuable for patients who have diseases rendering them unable to undergo conventional colonoscopy (e.g., bleeding disorders, lung or heart disease) and for patients who are unable to undergo the sedation required for traditional colonoscopy. The procedure is less invasive than conventional colonoscopy, with little risk of complications and no recovery time. CT colonoscopy can be done as an outpatient procedure, and the patient may return to work or usual activities the same day. Tumor progression, before and after therapy, and effectiveness of medical interventions may be monitored by CT scanning.

CT colonoscopy and conventional colonoscopy require the bowel to be cleansed before the examination. The screening procedure requires no contrast medium injections, but if a suspicious area or abnormality is detected, a repeat series of images may be completed after IV contrast medium is given. These density measurements are sent to a computer that produces a digital analysis of the anatomy, enabling a health-care provider (HCP) to look at slices or thin sections of certain anatomic views of the colon and vascular system. A drawback of CT colonoscopy is that polyp removal and biopsies of tissue in the colon must be done using conventional colonoscopy.Therefore, if polyps are discovered during CT colonoscopy and biopsy becomes necessary, the patient must undergo bowel preparation a second time.

This procedure is contraindicated for

  • high alertPatients who are pregnant or suspected of being pregnant, unless the potential benefits of a procedure using radiation far outweigh the risk of radiation exposure to the fetus and mother.
  • high alertPatients who are claustrophobic.
  • high alertPatients with conditions associated with adverse reactions to contrast medium (e.g., asthma, food allergies, or allergy to contrast medium). Although patients are still asked specifically if they have a known allergy to iodine or shellfish, it has been well established that the reaction is not to iodine, in fact an actual iodine allergy would be very problematic because iodine is required for the production of thyroid hormones. In the case of shellfish the reaction is to a muscle protein called tropomyosin; in the case of iodinated contrast medium the reaction is to the noniodinated part of the contrast molecule. Patients with a known hypersensitivity to the medium may benefit from premedication with corticosteroids and diphenhydramine; the use of nonionic contrast or an alternative noncontrast imaging study, if available, may be considered for patients who have severe asthma or who have experienced moderate to severe reactions to ionic contrast medium.
  • high alertPatients with conditions associated with preexisting renal insufficiency (e.g., renal failure, single kidney transplant, nephrectomy, diabetes, multiple myeloma, treatment with aminoglycocides and NSAIDs) because iodinated contrast is nephrotoxic.
  • high alertElderly and compromised patients who are chronically dehydrated before the test, because of their risk of contrast-induced renal failure.
  • high alertPatients with pheochromocytoma, because iodinated contrast may cause a hypertensive crisis.
  • high alertPatients with bleeding disorders or receiving anticoagulant therapy because the puncture site may not stop bleeding.


  • Detect polyps in the colon
  • Evaluate the colon for metachronous lesions
  • Evaluate the colon in patients with obstructing rectosigmoid disease
  • Evaluate polyposis syndromes
  • Evaluate the site of resection for local recurrence of lesions
  • Examine the colon in patients with heart or lung disease, patients unable to be sedated, and patients unable to undergo colonoscopy
  • Failure to visualize the entire colon during conventional colonoscopy
  • Identify metastases
  • Investigate cause of positive occult blood test
  • Investigate further after an abnormal barium enema
  • Investigate further when flexible sigmoidoscopy is positive for polyps

Potential diagnosis

Normal findings

  • Normal colon and rectum, with no evidence of polyps or growths

Abnormal findings related to

  • Abnormal endoluminal wall of the colon
  • Extraluminal extension of primary cancer
  • Mesenteric and retroperitoneal lymphadenopathy
  • Metachronous lesions
  • Metastases of cancer
  • Polyps or growths in colon or rectum
  • Tumor recurrence after surgery

Critical findings


Interfering factors

  • Factors that may impair clear imaging

    • Gas or feces in the gastrointestinal tract resulting from inadequate cleansing or failure to restrict food intake before the study.
    • Retained barium from a previous radiological procedure.
    • Metallic objects (e.g., jewelry, body rings) within the examination field, which may inhibit organ visualization and cause unclear images.
    • Patients who are very obese or who may exceed the weight limit for the equipment.
    • Patients with extreme claustrophobia unless sedation is given before the study.
    • Inability of the patient to cooperate or remain still during the procedure because of age, significant pain, or mental status.
  • Other considerations

    • The procedure may be terminated if chest pain or severe cardiac arrhythmias occur.
    • Failure to follow dietary restrictions and other pretesting preparations may cause the procedure to be canceled or repeated.
    • Consultation with the HCP should occur before the procedure for radiation safety concerns regarding younger patients or patients who are lactating. Pediatric & Geriatric Imaging Children and geriatric patients are at risk for receiving a higher radiation dose than necessary if settings are not adjusted for their small size. Pediatric Imaging Information on the Image Gently Campaign can be found at the Alliance for Radiation Safety in Pediatric Imaging (
    • Risks associated with radiation overexposure can result from frequent x-ray procedures. Personnel in the room with the patient should wear a protective lead apron, stand behind a shield, or leave the area while the examination is being done. Personnel working in the examination area should wear badges to record their level of radiation exposure.

Nursing Implications and Procedure


  • Positively identify the patient using at least two unique identifiers before providing care, treatment, or services.
  • Patient Teaching: Inform the patient this procedure can assist in assessing the colon.
  • Obtain a history of the patient’s complaints or clinical symptoms, including a list of known allergens, especially allergies or sensitivities to latex, anesthetics, or contrast mediums.
  • Obtain a history of the patient’s gastrointestinal system, symptoms, and results of previously performed laboratory tests and diagnostic and surgical procedures.
  • Ensure results of coagulation testing are obtained and recorded prior to the procedure; BUN and creatinine results are also needed if contrast medium is to be used.
  • Note any recent procedures that can interfere with test results, including examinations using barium- or iodine-based contrast medium. Ensure that barium studies were performed more than 4 days before the CT scan.
  • Record the date of the last menstrual period and determine the possibility of pregnancy in perimenopausal women.
  • Obtain a list of the patient’s current medications, including anticoagulants, aspirin and other salicylates, herbs, nutritional supplements, and nutraceuticals (see Effects of Natural Products on Laboratory Values online at DavisPlus). Note the last time and dose of medication taken.
  • Note that if iodinated contrast medium is scheduled to be used in patients receiving metformin (Glucophage) for non-insulin-dependent (type 2) diabetes, the drug should be discontinued on the day of the test and continue to be withheld for 48 hr after the test. Iodinated contrast can temporarily impair kidney function, and failure to withhold metformin may indirectly result in drug-induced lactic acidosis, a dangerous and sometimes fatal side effect of metformin related to renal impairment that does not support sufficient excretion of metformin.
  • Review the procedure with the patient. Address concerns about pain and explain that some pain may be experienced during the test, and there may be moments of discomfort. Inform the patient that the procedure is performed in a radiology department, usually by an HCP specializing in this procedure, with support staff, and takes approximately 30 to 60 min.
  • Sensitivity to social and cultural issues, as well as concern for modesty, is important in providing psychological support before, during, and after the procedure.
  • Explain that an IV line may be inserted to allow infusion of IV fluids (e.g., normal saline), anesthetics, contrast medium, or sedatives.
  • Inform the patient that he or she may experience nausea, a feeling of warmth, a salty or metallic taste, or a transient headache after injection of contrast medium.
  • Instruct the patient to remove jewelry and other metallic objects from the area to be examined.
  • Instruct the patient to fast and restrict fluids for 6 to 8 hr prior to the procedure and to avoid taking anticoagulant medication or to reduce dosage as ordered prior to the procedure. Protocols may vary among facilities.
  • Make sure a written and informed consent has been signed prior to the procedure and before administering any medications.


  • Potential complications:
  • Injection of the contrast through IV tubing into a blood vessel is an invasive procedure. Complications are rare but do include risk for allergic reaction related to contrast reaction, cardiac arrhythmias, hematoma related to blood leakage into the tissue following insertion of the IV needle, or infection that might occur if bacteria from the skin surface is introduced at the IV needle insertion site.

  • Observe standard precautions, and follow the general guidelines in Patient Preparation and Specimen Collection. Positively identify the patient.
  • Ensure that the patient has complied with dietary, fluids, and medication restrictions and pretesting preparations; ensure that food and fluids have been restricted for at least 6 hr prior to the procedure.
  • Ensure that the patient has removed all external metallic objects from the area to be examined prior to the procedure.
  • Administer ordered prophylactic steroids or antihistamines before the procedure if the patient has a history of allergic reactions to any substance or drug. Use nonionic contrast medium for the procedure.
  • Avoid the use of equipment containing latex if the patient has a history of allergic reaction to latex.
  • Have emergency equipment readily available.
  • Instruct the patient to void prior to the procedure and to change into the gown, robe, and foot coverings provided.
  • Instruct the patient to cooperate fully and to follow directions. Instruct the patient to remain still throughout the procedure because movement produces unreliable results.
  • Record baseline vital signs, and continue to monitor throughout the procedure. Protocols may vary among facilities.
  • Establish an IV fluid line for the injection of contrast (if used), emergency drugs, and sedatives.
  • Administer an antianxiety agent, as ordered, if the patient has claustrophobia. Administer a sedative to a child or to an uncooperative adult, as ordered.
  • Place the patient in the supine position on an examination table.
  • The colon is distended with room air or carbon dioxide by means of a rectal tube and balloon retention device. Maximal colonic distention is guided by patient tolerance.
  • If IV contrast is used, a rapid series of images is taken during and after injection.
  • Instruct the patient to inhale deeply and hold his or her breath while the x-ray images are taken, and then to exhale after the images are taken.
  • The sequence of images is repeated in the prone position.
  • Instruct the patient to take slow, deep breaths if nausea occurs during the procedure.
  • Monitor the patient for complications related to the procedure (e.g., allergic reaction, anaphylaxis, bronchospasm) if contrast is used.
  • The needle is removed, and a pressure dressing is applied over the puncture site.
  • Observe/assess the needle site for bleeding, inflammation, or hematoma formation.


  • Inform the patient that a report of the results will be made available to the requesting HCP, who will discuss the results with the patient.
  • Instruct the patient to resume usual diet, fluids, medications, and activity, as directed by the HCP. Renal function should be assessed before metformin is resumed, if contrast was used.
  • Monitor vital signs and neurological status every 15 min for 1 hr, then every 2 hr for 4 hr, and then as ordered by the HCP. Monitor temperature every 4 hr for 24 hr. Monitor intake and output at least every 8 hr. Compare with baseline values. Notify the HCP if temperature is elevated. Protocols may vary among facilities.
  • If contrast was used, observe for delayed allergic reactions, such as rash, urticaria, tachycardia, hyperpnea, hypertension, palpitations, nausea, or vomiting.
  • Instruct the patient to immediately report symptoms such as fast heart rate, difficulty breathing, skin rash, itching, chest pain, persistent right shoulder pain, or abdominal pain. Immediately report symptoms to the appropriate HCP.
  • Observe/assess the needle/catheter insertion site for bleeding, inflammation, or hematoma formation.
  • Instruct the patient in the care and assessment of the site.
  • Instruct the patient to apply cold compresses to the puncture site as needed, to reduce discomfort or edema.
  • Instruct the patient to increase fluid intake to help eliminate the contrast medium, if used.
  • Inform the patient that diarrhea may occur after ingestion of oral contrast media.
  • Recognize anxiety related to test results. Discuss the implications of abnormal test results on the patient’s lifestyle. Provide teaching and information regarding the clinical implications of the test results, as appropriate.
  • Reinforce information given by the patient’s HCP regarding further testing, treatment, or referral to another HCP. Decisions regarding the need for and frequency of occult blood testing, colonoscopy, or other cancer-screening procedures should be made after consultation between the patient and HCP. The American Cancer Society (ACS) recommends regular screening for colon cancer, beginning at age 50 yr for individuals without identified risk factors. Their recommendations for frequency of screening: annual for occult blood testing (fecal occult blood testing [FOBT] and fecal immunochemical testing [FIT]); every 5 yr for flexible sigmoidoscopy, double contrast barium enema, and CT colonography; and every 10 yr for colonoscopy. There are both advantages and disadvantages to the screening tests that are available today. Methods to use DNA testing of stool are being investigated and awaiting FDA approval. The DNA test is designed to identify abnormal changes in DNA from the cells in the lining of the colon that are normally shed and excreted in stool. The DNA tests under development would use multiple markers to identify colon cancers with various, abnormal DNA changes and would be able to detect precancerous polyps. The most current guidelines for colon cancer screening of the general population as well as of individuals with increased risk are available from the ACS (, U.S. Preventive Services Task Force (, and the American College of Gastroenterology (
  • Depending on the results of this procedure, additional testing may be needed to evaluate or monitor progression of the disease process and determine the need for a change in therapy. Evaluate test results in relation to the patient’s symptoms and other tests performed.

Related Monographs

  • Related tests include barium enema, BUN, cancer antigens, capsule endoscopy, colonoscopy, CBC, CBC hematocrit, CBC hemoglobin, CT abdomen, creatinine, fecal analysis, KUB studies, MRI abdomen, PET pelvis, proctosigmoidoscopy, PT/INR, and US pelvis.
  • Refer to the Gastrointestinal System table at the end of the book for related tests by body system.
References in periodicals archive ?
The CT virtual colonoscopy does not require sedation and can be completed in 15 minutes.
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