CHAPTER 18 CT colonography
Introduction
Computed tomography colonography (CTC) was first described in 1994 as dual position, helical computed tomography (CT) of a cleansed, gas distended colon (Vining et al., 1994). Since that time, the examination has steadily evolved to the point where it is not only advocated for the investigation of patients with symptoms of colon cancer but also as a screening test for asymptomatic patients (Levin et al., 2008).
Meta-analysis of published data demonstrates that CTC has high sensitivity and specificity rates (using conventional colonoscopy as the reference standard) for large and medium size polyps and a high reported diagnostic accuracy for symptomatic cancer (Pickhardt et al., 2003; National Institute for Health and Clinical Excellence, 2005). However, diagnostic accuracy falls with polyp size and, as with conventional colonoscopy, there may be low sensitivity in detecting flat colonic lesions (Hoon et al., 2003). Studies have also shown that CTC is significantly more sensitive than barium enema at polyp detection and has the additional advantage of being able to demonstrate significant extracolonic pathology in approximately 9% of patients (Yee et al., 2005; Spreng et al., 2005; Taylor et al., 2006; Tolan et al., 2007).
Research shows that the diagnostic accuracy of CTC is highly dependent on the quality of the examination and that meticulous attention must be given to both examination and interpretation techniques in order to achieve acceptable diagnostic performance (Ho Park et al., 2007; Rockey et al., 2007).
CTC technique
Bowel preparation and fecal tagging
There is currently no consensus as to the optimum bowel preparation regime for CTC, although it is generally recognized that a clean, dry colon is required. Picolax (sodium picosulphate and magnesium citrate) is widely used in the UK for barium enema and colonoscopy bowel preparation (Box 18.1) and there is evidence to support its efficacy for CTC (Taylor et al., 2003a). Patient safety and tolerance should also be considered in the choice and administration of bowel preparation as vulnerable groups, such as the elderly and those with renal impairment, may be at risk of dehydration and electrolyte disturbance. Patients with diabetes should be advised to contact departments so they can be scheduled first on the list and to contact a diabetic nurse specialist to obtain advice regarding glycemic control for the period of dietary restriction (Tolan et al., 2007).
BOX 18.1 Standard bowel preparation
The day before the procedure
0800 | Take one sachet of Picolax |
Then drink as much clear fluid as you can, including clear soups, Oxo, Bovril, jelly and sweet, fizzy drinks | |
1600 | Take the second sachet of Picolax |
Continue to drink as much clear fluid as you can, including clear soups, Oxo, Bovril, jelly and sweet, fizzy drinks until your examination |
It is recognized that full bowel preparation does not always result in a completely clean colon. Techniques have been developed that allow residue and fluid to be labeled or tagged using oral contrast agents in order to avoid them being confused with pathology. There are a number of tagging protocols in use which include barium compounds, iodinated contrast media or a combination of the two (Box 18.2). It is suggested that barium is superior at tagging solid residue and iodinated contrast media is better at tagging fluid, although there is undoubtedly some overlap (Figure 18.1). The diagnostic accuracy of CTC can be further increased by using specialized computer software to perform ‘electronic cleansing’. This allows the opacified colonic fluid and barium tagged stool to be digitally removed at the post processing stage so that it does not obscure the visualization of polyps or significant pathology (Rockey et al., 2007; Mang et al., 2007).
BOX 18.2 Full purgation with stool/fluid tagging: University of Wisconsin (standard regimen)
24 h before | Clear liquids only |
18 h before | Fleet phosphosoda (45 ml) undiluted followed by 1–2 l of clear fluids |
15 h before | Barium 2.1% (250 ml) (plus 296 ml of magnesium citrate if bowel cleansing not commenced). Further 1–2 l of clear fluid |
12 h before | 60 ml gastrograffin (Bracco Diagnostics) with clear fluids |
8 h before | Nil by mouth until examination |
The development of fecal tagging protocols and ‘electronic cleansing’ has resulted in some centers reducing the laxative regime given to patients. This undoubtedly increases patient acceptability as the bowel preparation is often considered the most intolerable part of the examination. It also allows CTC to be performed with reduced or no laxation in those patients where it may be harmful, e.g. the elderly and those with significant comorbidity (O’Hare and Fenlon, 2006; Laudi et al., 2008).
Colonic insufflation
Good colonic distension is fundamental to obtaining a high quality examination and optimal mucosal visualization. Imaging under-distended or collapsed segments of bowel can render the examination non-diagnostic, necessitating a repeat examination or referral for colonoscopy. It may also result in pathologies being missed or lead to a false positive diagnosis. There are several strategies currently used to achieve good colonic distension which include the use of different catheters and insufflation devices, administering spasmolytics and obtaining images in the prone and supine position (Burling et al., 2006a; Mang et al., 2007).
The practice of using a retention balloon