Gastrointestinal tract

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Gastrointestinal tract

Introduction to contrast media

Barium

Barium suspension is made up from finely ground barium sulphate (the contaminant barium carbonate is poisonous). The particles of barium must be very small (in the range of 1 µm) to make them more stable in suspension. To prevent the particles clumping a non-ionic suspension medium is used. The resulting solution has a pH of 5.3 which makes it stable in gastric acid.

There are many varieties of barium suspensions in use. Ingredients are designed to optimise mucosal coating and to make it palatable. In most situations the preparation will be diluted with water to give a lower density (Table 3.1) and must be shaken well immediately before use.

Table 3.1

Barium suspensions and dilutions with water to give a lower density

Proprietary name Density (w/v) – use
Baritop 100 100% – all parts gastrointestinal tract
EPI-C 150% – large bowel
E-Z-Cat 1–2% – computed tomography of gastrointestinal tract
E-Z HD 250% – oesophagus, stomach and duodenum
E-Z Paque 100% – small intestine
Micropaque DC 100% – oesophagus, stomach and duodenum
Micropaque liquid 100% – small and large bowel
Micropaque powder 76% – small and large bowel
Polibar 115% – large bowel
Polibar rapid 100% – large bowel

Examinations of different parts of the gastrointestinal tract require barium preparations with differing properties:

1. Barium swallow, e.g. Baritop® 100% w/v or E-Z HD® 200–250% 100 ml (or more, as required).

2. Barium meal, e.g. E-Z HD® 250% w/v. A high-density, low-viscosity barium is required for a double-contrast barium meal to give a good thin coating that is still sufficiently dense to give satisfactory opacification. It also contains simethicone (an anti-foaming and coating agent) and sorbitol (a coating agent).

3. Barium follow-through, e.g. E-Z Paque® 60–100% w/v 300 ml (150 ml if performed after a barium meal). This preparation contains sorbitol, which produces an osmotic hurrying and is partially resistant to flocculation.

4. Small bowel enema, e.g. either one 300 ml can of Baritop 100% w/v or two tubs of E-Z Paque made up to 1500 ml (60% w/v). N.B. As the transit time through the small bowel is relatively short in this investigation, there is a reduced chance of flocculation. This enables the use of barium preparations which are not flocculation-resistant. Gastrografin can be added to the mixture as this may help reduce the transit time still further.

5. Barium enema, e.g. Polibar 115% w/v 500 ml or more, as required. Reduced density between 20% and 40% w/v for single contrast examinations.

Complications

1. Perforation. The escape of barium into the peritoneal cavity is rare. If large amounts enter the peritoneal cavity it is extremely serious and will produce pain and severe hypovolaemic shock. Treatment should consist of aggressive intravenous fluid resuscitation, emergency surgery with copious washout and antibiotics. A 50% mortality rate is quoted and of those that survive, 30% will develop granulomata and peritoneal adhesions. Intramediastinal barium also has a significant mortality rate. It is imperative that a water-soluble contrast medium be the initial agent used for any investigation in which there is a risk or suspicion of perforation.

2. Aspiration. Aspirated barium is relatively harmless. Sequelae include pneumonitis and granuloma formation. Physiotherapy is the only treatment required (for both aspirated barium and low osmolar contrast material (LOCM)), and should be arranged before the patient leaves hospital.

3. Intravasation. This very rare complication may result in a barium pulmonary embolus, which carries a mortality of 80%.

For further complications (e.g. constipation and impaction), see the specific procedure involved.

Water-soluble contrast agents

There are numerous water-soluble contrast agents available. Two agents of particular note are Gastrografin® and Gastromiro®. Gastrografin is an aniseed-tasting, high osmolarity contrast agent (sodium amidotrizoate and meglumine amidotrizoate) containing a wetting agent for oral or rectal use. Although primarily used in diagnosis, its high osmolarity is exploited to help achieve bowel catharsis in CT colonography, to diagnose and treat meconium ileus and also in adhesive small bowel obstruction. Its use needs to be especially carefully monitored in the frail and the very young owing to potential profound fluid and electrolyte disturbance. It should be diluted 4–5-fold for rectal administration.

Low osmolar contrast agents may be given orally but taste unpleasant. Gastromiro, a low osmolarity contrast agent (containing iopamidol), is a more palatable orange-flavoured alternative.

Gases

1. Oesophagus, stomach and duodenum – Carbon dioxide and, less often, air are used in conjunction with barium to achieve a ‘double contrast’ effect. For the upper gastrointestinal tract, CO2 is administered orally in the form of gas-producing granules/powder (sodium bicarbonate) which when mixed with fluid (citric acid) produces gas. The requirements of these agents are as follows:

2. Large bowel – For the large bowel, room air is administered per rectum via a hand pump attached to the enema tube. Carbon dioxide can also be administered by hand pump and is said to be resorbed more quickly, cause less abdominal pain, but produce inferior bowel distension when compared to air.1 CO2 insufflating pumps are in common usage in CT colonography.

Pharmacological agents

Hyoscine-N-butylbromide (Buscopan®)

This is an antimuscarinic agent thus inhibiting both intestinal motility and gastric secretion. It is not recommended in children.

Adult dose

20 mg i.v.

The advantages of hyoscine include its immediate onset of action, relatively short duration of action (approx. 5–10 min) and its relatively low cost. Disadvantages include short-lived antimuscarinic side effects which include blurring of vision, a dry mouth, transient bradycardia followed by tachycardia and rare side effects said to include urinary retention and acute gastric dilatation. A particular side effect is that hyoscine can precipitate acute-angle closure glaucoma (AACG) in those patients who are susceptible to this because it dilates the pupil. In the UK patients who have AACG are almost always treated surgically in both eyes to prevent any recurrence. Pupillary dilatation has no role to play in the most common sort of glaucoma, open angle glaucoma, which accounts for 90% cases. Denying patients hyoscine on the basis of previous history of glaucoma is now not thought justified. Instead the following precautions are thought sufficient for the administration of hyoscine.1

Glucagon

This polypeptide hormone produced by the alpha cells of the islets of Langerhans in the pancreas has a predominantly hyperglycaemic effect but also causes smooth muscle relaxation. It is used in USA as an alternative to hyoscine which is not licensed there.

Contrast swallow

Technique

1. Start with the patient in the erect position, right anterior oblique (RAO) position to project the oesophagus clear of the spine. An ample mouthful of barium is swallowed and this bolus is observed under fluoroscopy for dynamic assessment to assess the function of the oesophagus. Then further mouthfuls with spot exposure(s) to include the whole oesophagus with dedicated AP views of the gastro-oesophageal junction.

2. Coned views of the hypopharynx should be obtained with a frame rate of 3–4 per second to include AP, lateral and oblique views whilst the patient swallows contrast.

3. The patient is placed semi-prone in a ‘recovery position’ in a left posterior oblique (LPO) position with their right arm by their side behind their back and the left arm used to support the cup containing contrast. One further swallow with a single bolus is observed under fluoroscopy to assess motility with the effect of gravity eliminated. A distended single-contrast view should be obtained as the patient rapidly sips and swallows contrast as this best identifies hernias, subtle mucosal rings and varices.

4. Modifications may be required depending on the clinical indication.

(a) If dysmotility is suspected barium should be mixed with bread or marshmallow bolus and observed under fluoroscopy correlating symptoms with the passage of the bolus in the erect position.

(b) If perforation is suspected a control film may be useful to identify pneumomediastinum and ideally the patient should be examined in four positions (prone/supine/left lateral/right lateral) with water-soluble contrast first, and if this is negative then with barium contrast.

(c) To demonstrate a tracheo-oesophageal fistula in infants, a ‘pull back’ nasogastric tube oeosphagogram may be performed if the standard oesophagogram is negative.1 This technique is particularly useful in patients known to aspirate or in ventilated patients. Suction and nursing support should be available should aspiration occur. The patient is positioned prone with the arms up and the table may be tilted slightly head down. A nasogastric tube is introduced into stomach and then withdrawn to the level of the lower oesophagus under lateral screening guidance. Ten to 20 ml of LOCM is syringed in to distend the oesophagus which will force the contrast medium through any small fistula which may be present. The process is repeated for the upper and mid oesophagus. It is important to watch for aspiration into the airway from overspill which can lead to diagnostic confusion.

Barium meal

Technique

The double contrast method (Fig. 3.1):

1. A gas-producing agent is swallowed.

2. The patient then drinks the barium while lying on the left side, supported by their elbow. This position prevents the barium from reaching the duodenum too quickly and so obscuring the greater curve of the stomach.

3. The patient then lies supine and slightly on the right side, to bring the barium up against the gastro-oesophageal junction. This manoeuvre is screened to check for reflux, which may be revealed by asking the patient to cough or to swallow water while in this position (the ‘water siphon’ test). The significance of reflux produced by tipping the patient’s head down and simultaneously drinking water is debatable, as this is non-physiological – 24-hour pH probe monitoring is the best current investigation. If reflux is observed, images are taken to record the level to which it ascends.

4. An i.v. injection of a smooth muscle relaxant (Buscopan 20 mg or glucagon 0.3 mg) may be given to better distend the stomach and to slow down the emptying of contrast into duodenum. The administration of Buscopan has been shown not to affect the detection of gastro-oesophageal reflux or hiatus hernia.

5. The patient is asked to roll onto the right side and then quickly over in a complete circle, to finish in an RAO position. This roll is performed to coat the gastric mucosa with barium. Good coating has been achieved if the areae gastricae in the antrum are visible.

Images

There is a great variation in views recommended. One scheme is:

1. Spot exposures of the stomach (lying):

From the left lateral position the patient returns to a supine position and then rolls onto the left side and over into a prone position. This sequence of movements is required to avoid barium flooding into the duodenal loop, which would occur if the patient were to roll onto the right side to achieve a prone position.

2. Spot image of the duodenal loop (lying):

An additional view to demonstrate the anterior wall of the duodenal loop may be taken in an RAO position.

3. Spot images of the duodenal cap (lying):

4. Additional views of the fundus in an erect position may be taken at this stage, if there is suspicion of a fundal lesion.

5. Spot images of the oesophagus are taken, while barium is being swallowed, to complete the examination.

Modification of technique for young children

The main indication will be to identify a cause for vomiting. The examination is modified to identify the three major causes of vomiting – gastro-oesophageal reflux, pyloric obstruction and malrotation, and it is essential that the position of the duodeno-jejunal flexure is demonstrated:

1. Single-contrast technique using 30% w/v barium sulphate and no paralytic agent.

2. A relatively small volume of barium – enough to just fill the fundus – is given to the infant in the supine position. An image of the distended oesophagus is exposed.

3. The child is turned semi-prone into a LPO or RAO position. An image is taken as barium passes through the pylorus. The pylorus is shown to even better advantage if 20–40° caudocranial angulation can be employed with an overhead screening unit. Gastric emptying is prolonged if the child is upset. A dummy coated with glycerine is a useful pacifier.

4. Once barium enters the duodenum, the infant is returned to the supine position, and with the child perfectly straight a second image is taken as barium passes around the duodenojejunal flexure. This image should just include the lower chest to verify that the child is straight.

5. Once malrotation has been diagnosed or excluded, a further volume of barium is administered until the stomach is reasonably full and barium lies against the gastro-oesophageal junction. The child is gently rotated through 180° in an attempt to elicit gastro-oesophageal reflux.

In newborn infants with upper intestinal obstruction, e.g. duodenal atresia, the diagnosis may be confirmed if 20 ml of air is injected down the nasogastric tube (which will almost certainly have already been introduced by the medical staff). If the diagnosis remains in doubt, it can be replaced by a positive contrast agent (dilute barium or LOCM if the risk of aspiration is high).

Complications

N.B. It must be emphasized that there are many variations in technique, according to individual preference, and that the best way of becoming familiar with the sequence of positioning is actually to perform the procedure oneself.

Small bowel follow-through

Contrast medium

E-Z Paque 100% w/v 300 ml usually given divided in 10–15-min increments, although some radiologists give the full 300 ml at once. The transit time through the small bowel has been shown to be reduced by the addition of 10 ml of Gastrografin to the barium. In children, 3–4 ml kg−1 is a suitable volume.

In general water-soluble small bowel contrast studies are avoided as contrast becomes diluted in small bowel fluid resulting in poor mucosal detail compared with barium. An exception is in adhesional small bowel obstruction where conservative investigation and ‘treatment’ with water-soluble contrast agents, frequently Gastrografin, may reduce the need for surgical intervention.1 In this case limited images are usually acquired at 1, 4 and 24 h, stopping once contrast is seen in the colon.

Small-bowel enema

Technique

1. The patient sits on the edge of the X-ray table. If a per-nasal approach is planned, the patency of the nasal passages is checked by asking the patient to sniff with one nostril occluded. The pharynx is anaesthetized with lidocaine spray or Xylocaine gel instilled into a nostril. The Silk tube should be passed with the guidewire pre-lubricated and fully within the tube, whereas for the Bilbao-Dotter tube the guidewire is introduced after the tube tip is in the stomach.

2. The tube is then passed through the nose or the mouth, and brief lateral screening of the neck may be helpful in negotiating the epiglottic region. The patient is asked to swallow with the neck flexed, as the tube is passed through the pharynx. The tube is then advanced into the gastric antrum.

3. The patient then lies down and the tube is passed into the duodenum. Various manoeuvres may be used alone, or in combination, to help this part of the procedure, which may be difficult:

4. When the tip of the tube has been passed through the pylorus, the guidewire tip is maintained at the pylorus as the tube is passed over it along the duodenum to the level of the ligament of Treitz. The tube is ideally passed beyond the duodenojejunal flexure to diminish the risk of aspiration due to reflux of barium into the stomach.

5. Barium is then run in, ideally with a controllable mechanical pump, or by gravity. Initially start at 50 ml min−1 and, with regular initial screening, aim to ‘chase’ the leading edge of the barium distally to maintain an unbroken column of contrast within the small bowel. The infusion can usually be increased rapidly to 100 ml min−1 depending on the progress of the barium through the bowel. If methylcellulose is used, it is infused continuously, after an initial bolus of 500 ml of barium, until the barium has reached the colon.

6. The tube is then withdrawn, aspirating any residual fluid in the stomach. This is to decrease the risk of aspiration.

7. If the terminal ileum is obscured at the end of the examination it can be helpful to further re-screen the patient after an interval when barium has emptied into the colon as better views may be then obtained. A pneumocolon, as per small bowel follow-through, may also help.

Barium enema

Technique

The double-contrast method:

1. The patient lies on their left side, and the catheter is inserted gently into the rectum. It is taped firmly in position. Connections are made to the barium reservoir and the hand pump for injecting air.

2. An i.v. injection of Buscopan (20 mg) or glucagon (1 mg) is given. Some radiologists choose to give the muscle relaxant half way through the procedure at the end of step (3).

3. The infusion of barium is commenced. Intermittent screening is required to check the progress of the barium. The barium is run to splenic flexure in the left lateral position and then the patient is turned prone. Contrast is then run to the hepatic flexure and is stopped when it tips into the right colon. Gentle puffs of air may be needed to encourage the barium to flow. The patient rolls onto their right and quickly onto their back. An adequate amount of barium in the right colon is confirmed with fluoroscopy. The column of barium within the distal colon is run back out by either lowering the infusion bag to the floor or tilting the table to the erect position.

4. The catheter tube is occluded and air is gently pumped into the bowel to produce the double-contrast effect. CO2 gas has been shown to reduce the incidence of severe, post enema pain.

Exposures

There is a great variation in views recommended. Fewer films may be taken to reduce the radiation dose. The sequence of positioning enables the barium to flow proximally to reach the caecal pole. Air is pumped in as required to distend the colon.

A suggested sequence of positioning and films in a standard over couch image intensifier include:

Enema variants

These variants are less used with the wider availability of CT but may provide extra information in selected cases.

The ‘instant’ enema

Reduction of an intussusception

This procedure should only be attempted in full consultation with the surgeon in charge of the case and when an anaesthetist with appropriate paediatric training and experience, and with paediatric anaesthetic equipment, is available.1,2

Preliminary investigations

1. Plain abdominal film – to assess bowel distension and to exclude perforation. A right-side-up decubitus film is often helpful in confirming the diagnosis by showing a failure of caecal filling with bowel gas because of the presence of the soft-tissue mass of the intussusception. Normal plain films do not exclude the diagnosis, and the clinical findings and/or history should be sufficient indications.

2. US – should confirm or exclude the diagnosis. Absence of blood flow within the intussusceptum on colour-flow Doppler should lead to cautious reduction.6 US may identify a lead point; if so, even attempted reduction (to facilitate surgery) should be followed by surgery. If US identifies fluid trapped between the intussusceptum and intussuscipiens, the success rate is significantly reduced.7

Technique

A 16–22-F balloon catheter is inserted into the rectum and the buttocks taped tightly together to provide a seal. It may be necessary to inflate the balloon but if this is done it should be performed under fluoroscopic control so that the rectum is not over distended.

Pneumatic reduction

1. The child is placed in the prone position so that it is easier to maintain the catheter in the rectum and the child is disturbed as little as possible during the procedure.

2. Air is instilled by a hand or mechanical pump and the intussusception is pushed back by a sustained pressure of up to 80 mmHg. Pressure should be monitored at all times and there should be a fail-safe pressure-release valve in the system to ensure that excessive pressures are not delivered.

3. Reduction is successful when there is free flow of air into the distal ileum.

4. If the intussusception does not move after 3 min of sustained pressure, the pressure is reduced and the child rested for 3 minutes. Two further attempts are made increasing the pressure to 120 mmHg for 3 minutes with a 3-minute rest. If the intussusception is still immovable it is considered irreducible and arrangements are made for surgery.

5. The intussusception is only considered completely reduced when the terminal ileum is filled with air. However, it is not uncommon for there to be a persisting filling defect in the caecum at the end of the procedure, with or without reflux of air into the terminal ileum. This is often due to an oedematous ileocaecal valve. In the presence of a soft-tissue caecal mass, a clinically well and stable child should be returned to the ward to await a further attempt at reduction after a period of 2–8 h rather than proceed to surgery. A second enema is often successful at complete reduction or showing resolution of the oedematous ileocaecal valve.8

6. When air (or barium) dissects between the two layers of the intussusception – the dissection sign – reduction is less likely.9

References

1. The Royal College of Anaesthetists. Guidelines for the Provision of Anaesthetic Services. Guidance on the Provision of Paediatric Anaesthesia. London: Royal College of Anaesthetists; 1999.

2. The British Association of Paediatric Surgeons. A Guide for Purchasers and Providers of Paediatric Surgeons. London: British Association of Paediatric Surgeons; 1994.

3. Kirks, DR. Air intussusception reduction: ‘The winds of change’. Pediatric Radiology. 1995; 25:89–91.

4. Stringer, DA, Ein, SH. Pneumatic reduction: advantages, risks and indications. Pediatr Radiol. 1990; 20:475–477.

5. Daneman, A, Navarro, O. Intussusception Part 2: An update on the evolution of management. Paediatr Radiol. 2004; 34:97–108.

6. Lim, HK, Bae, SH, Lee, KH, et al. Assessment of reducibility of ileocolic intussusception in children: usefulness of color Doppler sonography. Radiology. 1994; 191:781–785.

7. Britton, I, Wilkinson, AG. Ultrasound features of intussusception predicting outcome of air enema. Pediatr Radiol. 1999; 29:705–710.

8. Gorenstein, A, Raucher, A, Serour, F, et al. Intussusception in children: reduction with repeated, delayed air enema. Radiology. 1998; 206:721–724.

9. Barr, LL, Stansberry, SD, Swischuk, LE. Significance of age, duration, obstruction and the dissection sign in intussusception. Pediatr Radiol. 1990; 20:454–456.

10. Khong, PL, Peh, WC, Lam, CH, et al. Ultrasound-guided hydrostatic reduction of childhood intussusception: technique and demonstration. RadioGraphics. 2000; 20:E1.

Contrast enema in neonatal low intestinal obstruction

The differential diagnosis of low intestinal obstruction in the newborn consists of five conditions which comprise nearly all possible causes. Three involve the colon: Hirschsprung’s disease, functional immaturity of the colon (small left colon syndrome, meconium plug syndrome) and colonic atresia. Two involve the distal ileum: meconium ileus and ileal atresia. All infants with low intestinal obstruction require a contrast enema.

Contrast medium

Dilute ionic contrast medium as is used for cystography, e.g. Urografin 150. This has the advantage of not provoking large fluid shifts and being dense enough to provide satisfactory images. Non-ionic contrast media and barium offer no advantages (and the latter is contraindicated with perforation as a possibility). Infants with meconium ileus or functional immaturity will benefit from a water-soluble contrast enema and so their therapeutic enema commences with the diagnostic study. The non-operative treatment of meconium ileus was first described using the hypertonic agent Gastrografin, which dislodged the sticky meconium by drawing water into the bowel lumen. However, most paediatric radiologists now believe that a hypertonic agent is not necessary for successful treatment.1

Technique

If the enema demonstrates that the entire colon is small (a microcolon; <1 cm in diameter) then the diagnosis is likely to be meconium ileus or ileal atresia. (The microcolon of prematurity and total colonic Hirschsprung’s disease are alternative rare diagnoses.) For the treatment of meconium ileus, the aim is to run the water-soluble contrast medium into the small bowel to surround the meconium. An attempt should be made to get the contrast medium back into dilated small bowel. If successful, meconium should be passed in the next hour. If no result is seen and the infant’s condition deteriorates then surgical intervention will be necessary. If the passage of meconium is incomplete and the clinical condition remains stable, multiple enemas over the succeeding few days will be necessary to ensure complete resolution of the obstruction.

Overall success rate is approximately 50–60%, with a perforation rate of 2%.1

Herniogram

Images

The patient is asked to cough and perform the Valsalva manoeuvre while these are taken. The tube may be angled 25° caudally.

Evacuating proctogram

Coeliac axis, superior mesenteric and inferior mesenteric arteriography

Technique

Ultrasound of the gastrointestinal tract

Endoluminal examination of the oesophagus and stomach

Hypertrophic pyloric stenosis

The typical patient is a 6-week-old male infant presenting with non-bilious projectile vomiting.

Technique

1. The right upper quadrant is scanned with the patient supine. If the stomach is very distended, the pylorus will be displaced posteriorly and the stomach should be decompressed with a nasogastric tube. If the stomach is collapsed, the introduction of some dextrose, by mouth or via a nasogastric tube, will distend the antrum and differentiate it from the pylorus.

2. The pylorus is scanned in its longitudinal and transverse planes and images will resemble an olive and a doughnut, respectively. The poorly echogenic muscle is easily differentiated from the bright mucosa. Antral peristalsis can be seen and the volume of fluid passing through the pylorus with each antral wave can be assessed.

3. A number of measurements can be made. These include muscle thickness, canal length, pyloric volume and muscle thickness/wall diameter ratio, but there is no universal agreement as to which is the most discriminating parameter.

Small bowel

Further Reading

CT of the gastrointestinal tract

Intraluminal contrast agents

Luminal contrast agents may be positive (oral contrast density) or negative (water density). Positive oral contrast (e.g. 25 ml Omnipaque 300 made up to 1 l with water, low in density to avoid artefacts) is universally used in abdominal and pelvic CT to delineate the bowel from pathology which is especially useful in the pelvis. Oral contrast administration is recommended to help assess tumour response using the internationally recognized RECIST criteria. Multislice CT scanners have enabled the acquisition of thin slices (<3 mm) allowing multiplanar reformats and some have suggested that the routine use of intraluminal contrast for all abdominal scanning (especially trauma and follow-up cancer imaging) may not be always feasible or necessary. Nevertheless the detection and characterization of GI pathology is greatly helped by luminal distension produced by luminal contrast agents.

Colonic pathology is best demonstrated on CT by a combination of catharsis, distension with gas and intraluminal contrast (see CT colonography). For those unable to tolerate full catharsis oral contrast given at least 24 h before the examination (e.g. 30 ml Omnipaque 300 given the evening before), ‘tags’ the faeces and enables obvious colonic pathology to be more easily identified. Divided doses given over 3 days achieve more uniform colon tagging.

‘Negative’ oral contrast is recommended for the evaluation of oesophageal and stomach pathology. The simplest negative contrast is water and this is adequate for assessing upper GI pathology (i.e. staging oesophageal and gastric tumours with up to 1 l of water as tolerated starting 5–10 minutes before the scan).

Positive oral contrast is contraindicated if the question is whether GI bleeding is present, as it will obscure any bleeding points.

In the small bowel, positive oral contrast generally enables extraluminal pathology (e.g. abscess) and fistulae to be more easily visualized and in general is useful in the post-surgical abdomen. However, negative oral contrast allows mucosal enhancement (important in Crohn’s disease) to be more easily seen and not obscured by adjacent positive contrast. As water gets rapidly resorbed, it is not as good for distending the distal small bowel which limits its usefulness. Agents with increased osmolality to avoid resorption are therefore advised. Water–methylcellulose mixtures, polyethylene glycol (the osmotic laxative Klean-Prep®), mannitol (e.g 250 ml mannitol 10% and 750 ml water), locust bean gum (a food additive), Volumen® (a low-density barium-based agent not available in the UK at present) and even milk have been used. In general, better small bowel distension is obtained with a higher volume of fluid and approximately image l drunk gradually over 30 minutes to fill small bowel is usually required, with a larger volume required in some and less in others. Patient encouragement and supervision are important to achieve this. ‘Enterography’ is the term used to describe this protocol but the same contrast may also be administered via an NJ tube when it is termed ‘enteroclysis’. In high-grade intestinal obstruction oral contrast is not necessary as fluid within dilated bowel will act as a negative contrast agent and allow definition of the transition point.

Computed tomographic colonography

CTC, also known as ‘virtual colonoscopy’, is the radiological test of choice for the detection of colonic neoplasia of the large bowel due to superior sensitivity and better patient experience compared to barium enema.

Bowel preparation

Full bowel preparation with standard laxatives is used in many centres but the benefits of faecal tagging are increasingly accepted. Three types of preparation may be considered:

Technique

1. Patient to go to toilet immediately before procedure.

2. 20 mg i.v. Buscopan is given (glucagon is not recommended).

3. Patient positioned on their left side and a thin (e.g. Foley) catheter is placed in rectum and gas insufflated. The gas may be air but CO2 is better tolerated by patients. Gas is best administered by a dedicated pump. If a pump is available then 1.5–2 l of CO2 is initially administered, the patient is turned supine and further gas (typically up to 4–6 l) administered. Pumps limit the administered pressure to 25 psi and deliver further gas to maintain this pressure. Manual inflation with a bulb-sized hand pump or alternatively an empty enema bag filled with air/CO2 and gentle pressure on the enema bag are other options.

4. CT scout performed to check satisfactory gaseous distension of large bowel.

5. CT parameters will depend upon the type of CT scanner available but collimation thickness should be between 1–3 mm. Symptomatic patients typically receive i.v. contrast but asymptomatic patients from cancer screening programmes should not receive i.v. contrast. CT scans may be performed using a low-dose technique (e.g. 80 mA) without i.v. contrast, or using usual diagnostic CT parameters with i.v. iodinated contrast performed in portal phase (70 s delay).

6. Patient is turned prone; further insufflation is usually needed (e.g. 2 l CO2) and a further low-dose scan performed. If the patient is unable to turn prone they can be scanned in the left lateral position. The supine and prone scans should be reviewed to ensure all areas of the colon are distended on at least one of the acquisitions and if not then consideration of a further low-dose scan after repositioning in a lateral position and extra gas should be made.

Further Reading

Contrast agents

Domjan, J, Blaquiere, R, Odurny, A. Is minimal preparation CT comparable with barium enema in elderly patients with colonic symptom. Clin Radiol. 1998; 53(12):894–898.

Geffroy, Y, Rodallec, MH, Boulay-Coletta, I, et al. Multidetector CT angiography in acute gastrointestinal bleeding: why, when, and how. RadioGraphics. 2011; 31(3):E35–E46.

Graça, BM, Freire, PA, Brito, JB, et al. Gastroenterologic and radiologic approach to obscure gastrointestinal bleeding: how, why, and when? RadioGraphics. 2010; 30(1):235–252.

Ilangovan, R, Burling, D, George, A, et al. CT enterography: review of technique and practical tips. Br J Radiol. 2012; 85(1015):876–886.

Silva, AC, Pimenta, M, Guimarães, LS. Small bowel obstruction: what to look for. RadioGraphics. 2009; 29(2):423–439.

Slater, A, Planner, A, Bungay, HK, et al. Three-day regimen improves faecal tagging for minimal preparation CT examination of the colon. Br J Radiol. 2009; 82(979):545–548.

Wittenberg, J, Harisinghani, MG, Jhaveri, K, et al. Algorithmic approach to CT diagnosis of the abnormal bowel wall. RadioGraphics. 2002; 22(5):1093–1107.

Magnetic resonance imaging of the gastrointestinal tract

Techniques

Small-bowel magnetic resonance enteroclysis

1. Pass Bilbao Dotter tube to DJ flexure using fluoroscopy.

2. Transfer patient to MR scanner and obtain venous access.

3. Scan prone (though some centres scan supine) and obtain ‘scout’ localizer.

4. Connect Bilbao Dotter tube to enteroclysis pump (situated in control room if not MR compatible). Infuse (80–100 ml min−1) oral contrast under MR fluoroscopy (coronal thick-slab single-shot sequence, e.g. half Fourier acquisition single-shot turbo spin echo (HASTE)) to monitor filling of small bowel to ileocaecal valve. Check for reflux to stomach and slow/stop if significant. Stop infusion when contrast reaches colon.

5. Give Buscopan 20 mg intravenously.

6. Obtain sequences in coronal and transverse axial planes using HASTE, FISP sequences to include one fat-saturated sequence.

7. 3D T1W fat suppressed, e.g. volumetric interpolated breath-hold examination (VIBE) sequences pre-and post-intravenous gadolinium.

Radionuclide gastro-oesophageal reflux study

Technique

Radionuclide gastric-emptying study

Radiopharmaceuticals

Many radiolabelled meals have been designed for gastric-emptying studies, but as yet no standard has emerged. Emptying rate measured by radiolabelling is influenced by many factors, for example meal bulk, fat content, calorie content, patient position during imaging and labelling stability in vivo. For this reason, so-called ‘normal’ emptying times need to be taken in the context of the particular meal and protocol used to generate them. It is important that the meal used is physiological and reproducible. For centres new to the technique it is better to use a meal for which published data exist, rather than create yet another formulation with inherently different behaviour.

Both liquid and solid studies may be performed, separately or simultaneously, as a dual isotope study. Liquids have generally shorter emptying times than solids, and tend to follow an exponential emptying pattern. Solids tend to empty linearly after a lag phase. Prolonged solid emptying is highly correlated with prolonged liquid emptying, and there is debate, therefore, as to whether both studies are routinely necessary.1 Examples of meals used are:

Technique

Imaging from a single projection can cause significant errors due to movement of the meal anteriorly as it transfers to the antrum, thereby altering the amount of tissue attenuation of gamma-photons. The problem is likely to be exacerbated in obese patients. This can largely be overcome by taking pairs of opposing views and calculating the geometric, mean stomach activity in each pair:

Radionuclide meckel’s diverticulum scan

Radionuclide imaging of gastrointestinal bleeding

Radiopharmaceuticals

1. 99mTc-labelled red blood cells, 400 MBq max (4 mSv ED). Red cells are pre-treated with a stannous agent. 99mTc-pertechnetate is added and is reduced by the stannous ions, causing it to be retained intracellularly. Labelling efficiency is important, as false-positive scans can result from accumulations of free pertechnetate. In vitro preparation gives the best labelling efficiency, but is complex and time-consuming. However, commercial kits are available which can reduce the preparation time to around 30 min. In vivo labelling is least efficient, and there is also a compromise in vivo/vitro method where the labelling occurs in the syringe as blood is withdrawn from the patient.1

2. 99mTc-colloid, 400 MBq max (4 mSv ED). This used to be a commonly used alternative to labelled red cells, but studies showed it to be a less sensitive tracer for detecting bleeding sites,2 hence it is not recommended. Colloids are rapidly extracted from the circulation, so bleeding occurring only within 10 min or so of injection can be detected. It also localizes intensely in liver and spleen, masking upper gastrointestinal bleeding sites.

Technique

1. The patient lies supine.

2. The camera is positioned over the anterior abdomen with the symphysis pubis at the bottom of the field of view.

3. 99mTc-pertechnetate (in vivo method) or 99mTc-labelled red cells (in vitro or in vivo/vitro methods) are injected i.v.

4. A 128 × 128 dynamic acquisition is begun immediately with 2-s images for 1 min to help to demonstrate vascular blood pool anatomy, followed by 1-min images up to 45 min. Dynamic imaging permits cinematic viewing of images to detect bleed sites and movement through the bowel.3

5. Further 15 × 1-min dynamic image sets are acquired at 1, 2, 4, 6, 8 and 24 h or until bleeding site is detected (imaging much beyond 24 h is limited by radioactive decay).

6. Oblique and lateral views may help to localize any abnormal collections of activity.