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GASTROINTESTINAL Table of Contents   
Year : 2006  |  Volume : 16  |  Issue : 4  |  Page : 537-541
CT colonography - a practical guide


Consultant Gastrointestinal Radiologist, South Warwickshire Nhs Trust, United Kingdom

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Date of Submission03-Jun-2006
Date of Acceptance10-Aug-2006
 

Keywords: CT Colonography, MDCT

How to cite this article:
Sinha R. CT colonography - a practical guide. Indian J Radiol Imaging 2006;16:537-41

How to cite this URL:
Sinha R. CT colonography - a practical guide. Indian J Radiol Imaging [serial online] 2006 [cited 2020 Aug 8];16:537-41. Available from: http://www.ijri.org/text.asp?2006/16/4/537/32263
CT colonography (CTC) is a relatively modern, non-invasive imaging technique that is used to evaluate the large bowel using CT. Several studies have shown that CTC has sensitivities ranging from 75% to 100% for polyps greater than 10mm in size. Furthermore on thin-section imaging with multi-detector row CT (MDCT), the specificity for lesions greater than 10 mm in size is greater than 95% [1].

It is quite possible that in the near future, CTC may play a significant role in colorectal cancer screening by providing a safe and non-invasive examination [2].

Currently the author uses CTC for patients with contraindication to colonoscopy (e.g. severe cardiac or pulmonary disease). CTC may also be used to evaluate the colon after an incomplete conventional colonoscopic examination or evaluation of the colon proximal to an obstructing neoplasm to look for synchronous lesions. Several studies have shown that patients prefer a CTC to a conventional colonoscopy as it being less intrusive and uncomfortable. In routine practice, especially in the more elderly patients with rectal bleeding, anaemia or weight loss, the author prefers to provide a CTC examination first before a barium enema or colonoscopy.

In this article the different techniques used in CTC are described which can be used to provide a tailored, optimal examination for imaging the colon.

Patient Preparation

The aim of patient preparation is to obtain a clean, well-distended colon that can facilitate diagnosis and reduce the occurrence of false positive studies. A completely lean colon optimizes detection of colorectal abnormalities on 2-D or 3-D data and also allows accurate distinction between haustral folds, polyps and faecal matter.

Bowel cleansing is usually done using cathartics, which may include magnesium citrate (eg Picolax). Colonic lavage solutions such as polyethylene glycol are also used but their main drawback is that they are prone to leave large amount of faecal and fluid residue. Patients should also follow a low residue diet for 48 hours prior to the examination. It is imperative to warn patients about dehydration - especially the infirm or the elderly - and they should drink adequate amount of water during the process.

Although colonic lavage agents are prone to leaving retained fluid they are very useful in cases with renal or cardiac disease. This is because polyethylene glycol preparations do not cause significant fluid shift or electrolyte imbalance. The problem of retained fluid can be somewhat circumvented by performing supine and prone scans. This allows re-distribution of retained fluid to different segments of the colon. However the drawback of retained fluid is the lack of complete mucosal evaluation on both scans. Therefore ideally for a CTC examination the least amount of retained fluid is essential [Figure - 1].

Faecal tagging may be done to circumvent the problem of poor bowel cleansing and retained fluid. This is usually achieved by administering small amounts of barium or iodine containing contrast to the patient with their meals prior to the CTC examination. In this way the barium or contrast is incorporated into the patient's faecal matter which can help in their identification during the diagnostic process. Faecal tagging can also be used without bowel cleansing. The patient usually ingests small amounts of barium or contrast with their meals (low residue diet) usually 1-3 days before their examination. Modern software such as electronic-cleansing techniques can be used to remove the high-attenuation faecal material from the imaging data, leaving only the colonic data for diagnostic interpretation.

If future studies show that faecal tagging maintains high sensitivity for detection of polyps >10mm, the need for bowel cleansing may be obviated. As the bowel cleansing is deemed to be the most onerous, difficult part of a CTC examination for the patient, faecal tagging may yet prove to be the answer for optimal patient preparation.

Bowel relaxants and intravenous contrast:

The use of a bowel relaxant such as buscopan is controversial. Some studies have shown minimal benefit in the routine usage of bowel relaxants whereas others advocate the use of Buscopan routinely. In the authors' experience, the use of Buscopan improves colonic distention and thus diagnostic value of the examination.

The opinion on the use of intravenous contrast is also divided; routine use of contrast leads to increase in cost, allergy, whereas use of the contrast can make polyps more conspicuous even when there is retained fluid. However in cases of known colonic abnormalities it is justifiable to use contrast to delineate the abnormality and detect extramural and hepatic disease.

Tip: In general terms, if it is envisaged that patient may be poorly compliant and have inadequate bowel preparation, it makes good sense to administer intravenous contrast as it will generally lead to greater diagnostic confidence even in suboptimal bowel preparation. The use of intravenous contrast in all patients with a history of anaemia; weight loss; change in bowel habit and/or rectal bleeding may lead to increased diagnostic confidence [Figure - 2].

Colonic Insufflation

Colonic insufflation should be ideally performed by a trained radiographer or the radiologist. For colonic distension, either room air or carbon dioxide can be used. Rectal insufflation is done using a rectal tube or Foley's catheter. Usage of room air is easy and inexpensive whereas some studies have shown that use of carbon dioxide causes less patient discomfort as it is rapidly absorbed into the bloodstream. The patient should be put in the left lateral decubitus position and the rectal tube or catheter introduced after lubrication with acqueous or xylocaine based gel. Keeping the right side of the patient elevated in the decubitus position helps air to distend the right colon and caecum. It is important to ask the patient to let the operator know when they feel uncomfortable. This usually is a good sign of adequate distension.

Approximately 30-40 puffs with a rubber bulb is usually sufficient to distend the colon.

Tip: It is a good practice to palpate the right iliac fossa while insufflation as with practice, one can feel air entering and distending the caecum.

The author uses a mechanical carbon dioxide insufflator with which 1.5-2.0 L is slowly instilled into the rectum. It is important to remember that carbon dioxide is rapidly absorbed into the bloodstream and hence over time the colonic distension will reduce. Therefore after the first 2.0 litres, the gas should be continuously insufflated at a much-reduced rate throughout the procedure to maintain distension. On the other hand, insufflation of room air maintains distension longer.

CTC examination

A CT scout image with the patient in supine position is first obtained to check for optimal distension before proceeding to the CTC examination. The examination is performed in the supine followed by the prone positions. Additional air may have to be insufflated before the prone scans. The supine scans may be performed with intravenous contrast followed by the prone scans using a low-dose technique [3].

Current generation MDCT scanners can obtain 4- to 64- slices in a single rotation of the x-ray tube. Thus large volumes of isotropic data can be acquired in a single breath hold. As a result motion and respiratory artefacts are decreased and high resolution multiplanar (MPR) and 3-D images can be generated. Endoluminal and MPR images can be of help in trouble-shooting and accurate diagnosis of pathologies [4].

Usually a 1-1.25mm detector configuration is recommended for CTC. On a 4-detector row CT a 2.5-mm detector is more suitable as the mAs can be reduced thus reducing radiation.

Radiation dosages can be significantly reduced using reduced mAs settings [5]. Studies have shown that sensitivity of CTC for the detection of 10-mm or larger polyps was greater than 90% when an effective 50 mAs was used. As there is high tissue contrast between insufflated air and the colonic wall, substantial reductions in mAs can be achieved without sacrificing polyp detectability. Innaccone et al used 140 kVp and 10 mAs (effective) with a four-row, 2.5-mm detector configuration and showed 100% sensitivity for polyps larger than 10 mm. The resulting effective dose was 1.8 mSv in men and 2.4 mSv in women.

The routine settings the author uses are 140kV, 165-200mAs with collimation of 1.25mm on a 16-slice scanner and a collimation of 2.5mm on a 4-slice scanner. The prone scans are done on a low dose protocol using 10-30mAs.

CTC Interpretation

Most radiologists predominantly use axial 2-D data for interpretation of CTC examinations. This can be done using the scroll or cine function on workstations. MPR images can be used in problem-solving and distinguishing polyps or mass lesions from faecal matter or haustra. It is also important to closely evaluate abnormal or thickened appearing haustra [Figure - 3],[Figure - 4]. It is also useful to compare supine and prone scans when an abnormality is found.

Tip: Using bone window settings while reviewing the CTC allows simultaneous interpretation of the lumen, colonic wall and extra-colonic fat on the images [Figure - 5].

Typically, polyps or tumours appear as lobulated or rounded lesions and on cine mode or scrolling they can be seen as separate from the haustral indentations.

Tip: Faecal matter is generally rough or angular in contour and usually contains tiny pockets of air [Figure - 6]. Furthermore, faecal matter will usually change position between supine and prone scans. Endoluminal 3-D images may help in distinguishing between difficult cases.

Although studies have shown good results with reporting CTC only on 3-D data or endoluminal/virtual colonoscopy, there are several limitations of this imaging technique. It is generally agreed that there are several blind spots in the colon when 3D endoluminal views are used as in optical colonoscopy. Therefore to optimize data interpretation, the colon needs to be evaluated with four fly-through passes: supine and prone, antegrade and retrograde. This significantly increases interpretation time.

Recently 3D panoramic display techniques have been introduced that enable complete visualization of the undersurface of interhaustral folds [Figure - 7]. Although these panoramic techniques-or "virtual pathologic or virtual dissection techniques"- allow a quicker interpretation, a recent paper comparing 2-D with virtual dissection technique did not show improved lesion detection. Flat and annular lesions are best seen at 2D images and furthermore extracolonic abnormalities are only seen on non-virtual images.

Virtual techniques may be useful when a large volume of work is being done specifically for screening for polyps or a full-body screen. No such service exists in the UK and CTCs are done on a patient-need basis rather than a paid service. Therefore the bulk of the work is done to look for cancers, for staging or synchronous lesions and in patients with contraindications or failed colonoscopy. In view of the pattern of work, 2-D images form the mainstay of diagnostic work performed by the author with MPR and 3-D images used for problem-solving.


   Conclusion Top


CTC is a non-invasive imaging technique that can be effectively used to evaluate the large bowel. Optimal bowel preparation and intravenous contrast can be helpful in providing more diagnostic confidence. As far as screening for colorectal polyps is concerned, CTC may play a significant role in the future by providing a safe and non-invasive examination. The author recommends the use of CTC in patients with contraindication to colonoscopy (e.g. severe cardiac or pulmonary disease) or after a failed or incomplete conventional colonoscopic examination. The CTC is also more comfortable for patients and I would recommend its use as routine instead of barium enemas in the more elderly or infirm patients in whom the principle concern is a large bowel tumour.

 
   References Top

1.Macari M, Bini EJ, Xue X, et al. Colorectal neoplasms: prospective comparison of thinsection low-dose multi-detector row CT colonography and conventional colonoscopy for detection. Radiology 2002; 224(2): 383-392.  Back to cited text no. 1    
2.Pickhardt PJ, Choi JR, Hwang I, et al. Computed tomographic virtual colonoscopy to screen for colorectal neoplasia in asymptomatic adults. N Engl J Med 2003;349:2191-2200.  Back to cited text no. 2  [PUBMED]  [FULLTEXT]
3.Chen SC, Lu DS, Hecht JR, Ladell BM. CT colonography: value of scanning in both the supine and prone positions. AJR Am J Roentgenol 1999;172:595-600.  Back to cited text no. 3    
4.Royster AP, Fenlon HM, Clarke PD, Nunes DP, Ferrucci JT. CT colonoscopy of colorectal neoplasm: two-dimensional and three-dimensional virtual-reality techniques with colonoscopic correlation. AJR Am J Roentgenol 1997; 169:1237-1242.  Back to cited text no. 4  [PUBMED]  
5.Iannaccone R, Laghi A, Catalano C, et al. Detection of colorectal lesions: lower dose multi-detector row helical CT colonography compared with conventional colonoscopy. Radiology 2003;229(3):775-781.  Back to cited text no. 5    

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Correspondence Address:
R Sinha
Dept of Radiology, Warwick Hospital, Lakin Road, Warwick CV34 5BW
United Kingdom
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0971-3026.32263

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    Figures

  [Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6], [Figure - 7]

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