Just as for colonoscopy, proper bowel cleansing is necessary for CT colonography. As residual intraluminal fluids cannot be removed during the examination, a colon that is as dry and free from fecal matter as possible is required for reliable findings. Various bowel cleansing protocols have been described for CT colonography—sometimes differing quite widely in terms of the type and duration of the diet, the chosen laxative, and the schedule of laxative administration—but good bowel cleansing may be achieved with any of them. As a general rule, whichever bowel preparation scheme is chosen should be straightforward and simple to use. It is preferable to restrict patient preparation, including fecal tagging, to 24 hours. Some of the laxatives described or recommended may not be available or widely used in all countries. For this reason, several bowel cleansing protocols are described here, all of them effective and well-tolerated, so that every reader may find one that he or she can use:
Standard bowel cleansing protocols are commonly used, similar to those for conventional colonoscopy.
In addition, so-called fecal tagging protocols are increasingly being used, in which a contrast agent is administered along with the laxative in order to label residual bowel content.
Modified bowel preparation protocols are also currently being developed, in which a reduced amount of laxative, or none at all, is given. These are referred to as “minimal prep” or “prepless” protocols.
Standard Bowel Preparation Protocol
The two pillars of standard bowel preparation are dietary restrictions and laxative administration. Most bowel cleansing protocols recommend a clear liquid or low-residue diet for 1 day followed by administration of a laxative on the day before the examination.
Low-Residue and Clear Liquid Diets
Bowel preparation should include dietary restrictions to reduce fecal volume and fecal heterogeneity. There is no general agreement as to the length or content of diet in preparation for CT colonography.
Dietary schedule. In the majority of protocols, the patient begins with a clear liquid or low-residue diet 24 hours prior to the examination. Some bowel preparation methods also use a 48-hour protocol, in which the patient adheres to a low-residue diet for the first 24 hours, and for the next 24 hours, immediately preceding the examination, a clear liquid diet.
Diet. Foods that leave indigestible residues should be avoided for 48 hours before the examination. These include vegetables, fruit, nuts, and legumes. Milk, cheese, and other dairy products are also difficult to digest. Patients should also avoid eating bread, especially wholegrain bread, as well as granola (muesli) and cereals.
Foods that may be consumed in a low-residue diet prior to CT colonography include soup (clear, strained, with egg), white rice, spaghetti, pasta with butter, eggs in any form, curd cheese, steamed fish, and canned tuna. On the day of the examination, the patient should take only clear fluids. For this reason it is helpful to schedule the examination for before noon.
For a clear liquid diet, the patient avoids eating any solid food for 24 hours prior to the examination.
Plenty of fluids should be drunk to avoid dehydration and to improve the laxative effect. Clear liquids such as water, mineral water, cola, tea, and coffee without any cream or milk can be drunk. Orange, grapefruit, and tomato juice should be avoided. Sports drinks with added electrolytes (e.g., Gatorade, Lucozade) are recommended to help with hydration.
Bowel Purgation: Laxatives
Residual stool may obscure a lesion or mimic a polyp, interfering with proper evaluation of the CT colonography findings (Fig. 2.1). This is why, along with dietary restrictions, a thorough bowel cleansing using orally administered laxatives is a fundamental part of preparation of the bowel for CT colonography. Various agents are available for this purpose: Examples include sodium hydrogen phosphate, magnesium citrate, and polyethylene glycol (PEG). The choice of laxative depends on what is available as well as on the health of the patient. In the United Kingdom, sodium picosulfate (Picolax; Ferring Pharmaceuticals Ltd., West Drayton, UK) is commonly used. Elsewhere, a preparation often used (especially in the United States) is sodium hydrogen phosphate (Phospho-soda; Fleet Pharmaceuticals, Lynchburg, Virginia, USA). The use of Phospho-soda is contraindicated in some patients (see below). In such patients PEG-based preparations are commonly used instead, although these preparations have the disadvantage of often leaving large quantities of fluid in the bowel. Any of the various laxatives may be used together with an oral contrast agent for fecal tagging. In general, aggressive bowel evacuation should be restricted to 24 hours or less.
Sodium Hydrogen Phosphate
Preparation. For CT colonography, sodium hydrogen phosphate is usually given in the form of the Fleet Kit (Fleet Pharmaceuticals) or Prepacol (Guerbet GmbH, Sulzbach, Germany). These are commercially available preparations that contain sodium dihydrogen monophosphate, sodium dihydrogen diphosphate, and 20 mg bisacodyl in tablet form (four tablets of 5 mg each).
“Dry” preparation. Sodium hydrogen phosphate is an osmotic laxative. Bisacodyl increases the rate of peristalsis. Sodium hydrogen phosphate is a frequently used laxative in CT colonography. One of its advantages is that the patient only needs to drink a relatively small amount of fluid. Various studies have reported that patients who use sodium hydrogen phosphate have a sufficiently clean and—unlike with PEG preparations—dry colon. This is considered to be beneficial for evaluation in CT colonography. Because of the relatively small volume of residual fluid, the use of sodium hydrogen phosphate is also known as “dry” bowel preparation (“dry prep”).
As a result of this dry preparation, however, small amounts of stool can be found during the examination adhering to the wall of the intestine (Fig. 2.2). Some radiologists and gastroenterologists report using a double dose of sodium hydrogen phosphate, but recent studies show that this can be to the patient′s disadvantage because it can cause electrolyte disturbances, and it is therefore not recommended.
Sodium hydrogen phosphate is contraindicated in patients with impaired kidney function, electrolyte imbalances, heart failure, ascites, or ileus.
Administration. Preparation for CT colonography is in combination with a low-residue or, ideally, clear liquid diet that begins 24 hours before the scheduled examination. On the day before the examination, 45 mL of sodium hydrogen phosphate is dissolved in half of a glass of water (ca. 120 mL), which the patient should drink between 5 PM and 6 PM, followed by another glass of water (ca. 240 mL). The four bisacodyl tablets are taken at around 9 PM. Use of a bisacodyl suppository, placed in the rectum on the morning of the examination, is not advised as the suppository can mimic a polypoid lesion (see under “Bisacodyl”). The patient should be instructed to drink plenty of fluids. As a general rule, sodium hydrogen phosphate should be taken as a single dose, and any potential contraindications should always be taken into account.
FDA warning. A rare complication associated with sodium hydrogen phosphate is phosphate nephropathy, which can lead to kidney failure. The American Food and Drug Administration (FDA) has warned of potential toxicity related to taking oral sodium phosphate for bowel preparation for colonoscopy. Phospho-soda should not be administered to patients with kidney disease, impaired kidney function or bleeding, patients who are dehydrated, or those with electrolyte disturbances. The administration of double doses of sodium hydrogen phosphate should be avoided. Caution should also be exercised when prescribing this preparation for patients who are taking diuretics, ACE inhibitors, or nonsteroidal anti-inflammatory drugs (NSAIDs). See: www.fda.gov/Drugs/DrugSafety/
Polyethylene glycol (PEG) laxatives are frequently used for bowel preparation in conventional colonoscopy and for administration of a barium enema.
Preparations and administration. PEG preparations often come in powder form and may be dissolved in up to 4 L of water. The solution is then consumed over the course of an afternoon or early evening on the day before the examination. A commonly used preparation for CT colonography is Klean-Prep (Norgine Pharmaceuticals, Marburg, Germany), a macrogol/electrolyte solution. The electrolyte content of the preparation is intended to help avoid disturbances in the patient′s electrolyte/water balance. Klean-Prep is a white, crystalline, vanilla-flavored powder which when dissolved in water produces a clear solution. The powder comes in four sachets that are to be mixed with 4 L of fluid. The entire 4 L should be consumed within a 4- to 6-hour period (0.25 L every 15 minutes) on the evening before the examination.
A relatively new preparation made by the same manufacturer called “Moviprep” requires only 2 L of electrolyte solution to be drunk. According to the manufacturer, it offers the same quality of bowel cleansing. It is increasingly being used for CT colonograhy.
“Wet” preparation. Bowel cleansing methods that use PEG preparations are good at cleansing the colon of fecal material, but they leave quite large amounts of residual fluid. Such preparations are thus also referred to as “wet” preparations (“wet prep”).
Residual untagged fluids can compromise the quality of CT colonographic examination, prevent proper distension and evaluation of entire colon segments, and in the worst case even mask the presence of pathology. The use of fecal tagging can largely remedy this situation by providing additional enhancement of residual fluids to aid evaluation of the colon.
Another issue is that some patients—especially older patients—have difficulty consuming a large amount of fluids (up to 4 L) within a relatively short period of time. In addition, some preparations cause abdominal symptoms such as nausea and cramps.
Because of the larger quantity of residual fluid, and because they are more difficult and uncomfortable for the patient, some authors find PEG laxatives to be initially less suitable for CT colonography and use them only when an alternative to sodium hydrogen phosphate or magnesium citrate is required. Unlike sodium hydrogen phosphate, however, PEG preparations may be given to patients with impaired kidney function, electrolyte imbalance, heart failure, and ascites. They are increasingly being used, especially in Europe.
Preparations and administration. A very efficient preparation protocol (according to the Munich Colorectal Cancer Screening Trial) combines preparation with Klean-Prep and administration of Prepacol (four bisacodyl tablets of 5 mg each and 30 mL sodium hydrogen phosphate; Guerbet) (see above). The patient starts a liquid diet at midday on the day before the examination. At around 2 PM the Prepacol combination is taken. Three liters of the Klean-Prep solution are consumed between 5 PM and 8 PM, and 1 L the following day on the morning of the examination. This protocol also includes fecal tagging. For this part of the protocol, 50 mL of a nonionic oral contrast medium (e.g., Solutrast 300; Bracco Altana Pharma, Constance, Germany) is added to the last liter of Klean-Prep consumed on the day of the examination.
Thorough preparation. This combination has been shown to be a highly effective bowel cleansing method. Due to the use of Prepacol, however, it is contraindicated in patients with impaired kidney function, electrolyte imbalance, or heart failure. The very thorough bowel cleansing ensured by this protocol also allows colonoscopy to be performed on the same day as CT colonography, if clinically indicated.
Magnesium citrate is an effective alternative to Phospho-soda for bowel preparation before CT colonography. Because of the low risk of a negative effect on electrolyte balance, magnesium citrate is also recommended by some authors for use in screening.
Preparations. Magnesium citrate may be obtained either as a liquid (ca. 300 mL) or as a powder (ca. 24 g) which is dissolved in a glass of water (250 mL). Due to its milder laxative effect, some authors have recommended doubling the dosage (2 × 300 mL) (see protocol from the University of Wisconsin, p. 16). A recent study from that working group has shown that a double dose of magnesium citrate (2 × 296 mL; Sunmark, San Francisco, California, USA) is as effective as 45 mL sodium hydrogen phosphate in terms of quality of bowel cleansing. In addition, for a milder, reduced bowel preparation (see p. 17), magnesium citrate is available as a LoSo Prep kit (Bracco, Milan, Italy). The LoSo Prep kit consists of 18 g of magnesium citrate and four bisacodyl tablets of 5 mg each plus a 10 mg bisacodyl suppository.
Effects. Magnesium citrate prevents absorption of water in the colon and increases fluid secretion in the small intestine. The aim of combining magnesium citrate with bisacodyl is to reduce the amount of residual fluid in the bowel. One advantage of magnesium citrate is that it has fewer negative effects on electrolyte balance than Phospho-soda. It does not cause hyperphosphatemia or hypocalcemia. In addition, the amount of fluid that needs to be consumed is relatively small compared with PEG preparations. Nevertheless, care must be taken in patients with impaired renal function due to the loss of fluids in the gastrointestinal tract. The use of magnesium citrate is not recommended in patients with renal insufficiency.
Administration. When used as a preparation for CT colonography, magnesium citrate is given in combination with a low-residue diet, or, ideally, a liquid diet beginning 24 hours before the examination. Depending on the protocol, magnesium citrate may be combined with two to four bisacodyl tablets.
Magnesium citrate liquid: Magnesium citrate 2 × 296 mL is taken the day before the examination (1 × 296 mL between 2 PM and 6 PM and 1 × 296 mL between 5 PM and 9 PM). In addition, two bisacodyl tablets are given orally before 11 AM on the same day.
Magnesium citrate powder: On the day before the examination 24 g of magnesium citrate powder (or 18 g in the LoSo preparation) is dissolved in a glass of water and drunk in the afternoon. The four bisacodyl tablets are taken in the evening. The suppository should not be used in patients undergoing CT colonography. The LoSo Prep is not yet approved and available for use in the European Union).