Contrast studies: How useful are they in diagnosing gastrointestinal disease in vomiting patients?

Q: Are contrast studies still needed as diagnostics in vomiting patients?

A: Dr. Gabriela Seiler gave an excellent lecture on Old Techniques Revisited in Digital: Contrast Radiography of Vomiting Patients, at the 2008 American College of Veterinary Internal Medicine Forum in San Antonio. Here are some relevant points:

Questions

• Do contrast studies still have a place in the diagnostic work-up of a vomiting patient with gastrointestinal (GI) disease?

• Have they been replaced by more advanced imaging techniques, such as ultrasound, endoscopy, Computed Tomography (CT) and Magnetic Resonance Imaging?

Answers

• There still are some indications where contrast studies using barium or iodinated contrast medium are performed. However, the indications have become fewer and occur less frequently.

Contrast media

Barium sulfate is the most commonly used contrast medium for gastrointestinal contrast radiography of the vomiting patient. Oral barium sulfate is commercially available in powder form, which has to be reconstituted with water, or in liquid form, where addition of water usually is necessary to obtain the desired concentration.

Barium sulfate liquid preparations are recommended over the powder form and have a wide range of ingredients; their exact composition often remains a secret of the manufacturer. Additives include deflocculation, suspending, dispersing, wetting, anti-drying, anti-foaming and flavoring agents and magnesium.

They are important in providing one of the most important characteristics of a barium sulfate contrast agent: good adherence and coating of the GI mucosa. The pH of the barium sulfate suspension often is mentioned as an important factor for good mucosal coating and a tendency to fall out or "flocculate." However the pH is not just determined by the product on the shelf, but by the pH of the tap water added. Therefore, this can be quite variable.

Advantages and disadvantages of barium sulfate

Advantages

Excellent mucosal coating

No absorption

No mucosal irritation

High contrast

Low cost

Disadvantages

Causes irritation if peritoneal leakage

Aspiration pneumonia if inhaled

Iodinated contrast media are water-soluble and can be divided into two main groups: ionic and non-ionic. Ionic contrast media are cheaper, but have the disadvantage of a high osmolality. High osmolality leads to interstitial fluid being drawn into the GI tract; ionic contrast media should therefore not be administered orally to hypovolemic patients. Similarly, aspiration of ionic contrast media leads to pulmonary edema. In addition to the potential complications, the contrast is diluted, compromising diagnostic quality of the study.

Advantages and disadvantages of iodinated contrast media

Advantages

Rapid transit time

No irritation if peritoneal leakage

Disadvantages

Poor mucosal coating

Absorption

Hyperosmolality (ionic)

Expensive

Bitter taste

Gastric evaluation

Investigation of gastric disease is the most common indication for contrast studies in the vomiting patient. Different contrast methods for evaluation of the stomach in a vomiting patient are available. Placement of a gastric tube or nasoesophageal tube is indicated in patients with swallowing difficulties. If necessary, sedation with acepromazine or a light and short anesthesia with ketamine may be used.

Positive-contrast gastrography using barium sulfate usually is the first choice, based on good mucosal adherence. A simple technique that often is forgotten but may give excellent results, especially in patients with gastric masses or foreign bodies, is negative-contrast gastrography, using air from a carbonated beverage.

Double-contrast gastrography is the best method to evaluate the gastric mucosa, but is not often used in veterinary medicine compared with human medicine. It is more time-consuming and technically complicated than positive- or negative-contrast gastrography. The patient has to be anesthetized using an anesthetic agent which paralyzes the gastric wall (barbiturates, gas anesthetics).

Alternatively, glucagon at a dose of (0.10 to 0.35 mg IV) may be used to induce paralysis of the gastric wall. Administration of glucagon is contraindicated in patients with pheochromo-cytoma and uncontrolled diabetes mellitus. because glucagon can cause catecholamine release and hyperglycemia. Contrast studies of the small intestines often are low-yield procedures and should be reserved for select patients, such as those with persistent vomiting, negative findings on survey radiographs or unusual abnormalities seen on survey films.

Intestinal evaluation

Barium sulfate suspension is used routinely for evaluation of the small intestines (upper GI series). Non-ionic, water-soluble contrast agents with low osmolality have been shown to provide adequate opacification of the GI tract and may be used, especially in smaller patients such as cats and pediatric patients.

Contrast studies and digital imaging

Digital imaging is being more commonly used in veterinary medicine. Early versions of digital radiography were limited in spatial resolution; however, newer versions have overcome this problem. The advantages of digital radiography for contrast radiography are much more rapid image acquisition, lower patient dose and high-contrast resolution. To maximize diagnostic accuracy and efficiency of digital images, dedicated viewing work stations should be used with good monitor resolution and brightness.

Gastric diseases, outflow obstructions

Mechanical or functional (pyloric spasm) disorders often contribute to chronic vomiting. Differential diagnoses to consider are hypertrophic pyloric stenosis, chronic hypertrophic gastropathy and inflammatory conditions, including inflammatory polyps and neoplastic infiltrates. If available, digital fluoroscopy is an excellent tool to observe gastric motility after barium sulfate administration. Gastric contractions against a spastic or narrowed pylorus result in typical radiographic appearance of a "beak-sign" or "teat-sign" with a narrow contrast column entering the pyloric canal, whereas the antrum is distended. Positioning will greatly influence gastric motility and pyloric outflow, right lateral recumbency facilitates emptying.

Delay of gastric emptying may be diagnosed with liquid barium. Normal emptying should begin within 15 minutes after barium administration and should be complete within one to four hours in dogs and one to two hours in cats.

Because liquid barium better delineates lesions of the gastric wall, it usually is administered first. In some instances, a barium meal (barium sulphate mixed with food) has to be administered as well, to reflect more closely the physiologic conditions, followed by observation of gastric motility under fluoroscopy or by taking serial radiographs.

Gastric emptying time is difficult to evaluate, because addition of food significantly prolongs emptying and a wide normal range exists (seven to 15 hours). Radiographic signs to look for are filling defects and narrowing of the pyloric lumen. Intraluminal filling defects are caused by gastric foreign bodies, which can be moved to different gastric compartments by repositioning the patient. Intramural filling defects result in focal wall thickening and are consistent with inflammatory lesions, severe mucosal hypertrophy and, in some cases, neoplastic infiltrates. Diffuse, circumferential filling defects are seen with hypertrophic pyloric stenosis, pyloric spasm, inflammatory or scar lesions of the pylorus and some neoplastic infiltrates.

Pedunculated gastric masses that protrude into the lumen can be delineated with negative contrast gastrography. The sensitivity of double-contrast studies is much higher than simple contrast studies for mass recognition.

A small-sized mass often is completely obliterated by the contrast medium on simple positive contrast studies and often becomes visible only on double-contrast studies. Nevertheless, the diagnosis is not always straightforward even on double-contrast studies because artifacts caused by rugal folds or peristaltic activity can mimic gastric masses. A lesion should always be confirmed on different projections.

Contrast studies (simple contrast gastrography or, preferably, double contrast gastrography) may be necessary to detect non-radiopaque gastric foreign bodies, because they cannot always be differentiated from food on plain radiographs or abdominal ultrasound.

An upper GI study is a good choice of diagnostic work-up in a suspected gastric foreign body. Imaging findings include a filling defect initially if the foreign body is large enough. Different patient positioning will determine in most cases if a filling defect is freely moveable within the stomach (foreign body) or if it represents a rugal fold, wave of peristalsis or pedunculated intramural gastric mass.

Smaller foreign bodies usually are obscured initially. A delay in gastric emptying may be observed. Since most foreign bodies absorb some barium sulfate, a late post-contrast radiograph (after 12 hours) is often the diagnostic view because it shows persistent barium within the stomach.

Gastric and duodenal ulcers and perforation

Their radiographic identification is not easy, and necessitates high-quality images. Ulcerations are seen as out-pouching of contrast on positive-contrast studies, because the contrast accumulates in the crater of the ulcer, if the ulcer is tangential to the X-ray beam. When the X-ray beam strikes the ulceration perpendicularly, the lesion is seen "en face" and appears as a focal pool of contrast medium surrounded by a lucent line representing a rim of wall thickening. In this region, there is effacement of the normal rugal folds and an overall thickening of the gastric or duodenal wall. If fluoroscopy is used, decreased motility of the affected wall segment may be observed.

If GI perforation is suspected, typical ultrasonographic signs of perforation, such as free fluid, hyperechoic mesentery and free gas are relatively easily detected. It becomes more difficult in postoperative patients, where a certain degree of mesenteric inflammation, fluid and gas in the abdomen are already present.

Patients with gastric peg-tubes and patients who are more painful, vomiting or generally not recovering as expected after peg-tube placement or enterotomies often are evaluated with contrast radiography for GI tract leakage.

Traditionally, barium is not recommended for investigation of GI perforations because it has been shown to irritate the serosal layers and peritoneum, causing chemical peritonitis. A water-soluble contrast agent should be administered initially to detect a leak. However, small leaks can be difficult to detect because iodinated contrast is less opaque than barium.

If no leak is detected, barium should be added to rule out a small perforation, followed by laparotomy and peritoneal lavage if leakage is present. Barium sulfate is a known safe method to diag-nose esophageal perforations.

Small intestinal obstructions

Interpretation of survey radiographs in patients with suspected mechanical obstruction can be difficult. Findings can be subtle, and conditions such as enteritis can lead to a similar radiographic appearance. A simple contrast study to clearly localize the colon and differentiate it from dilated small bowel loops is a pneumocolon.

The small intestines are more easily accessible with ultrasound than is the stomach, and upper GI studies are not routinely performed anymore, but they are a good alternative to ultrasound to rule out foreign-body obstruction.

Surgeons often are concerned about complications after enterotomies in patients with intraluminal barium. However, barium's effects on transmural wound healing of the GI tract appear minimal. Foreign bodies create a filling defect within the barium column and, depending on the size of the foreign object (partial vs. complete obstruction), barium accumulation can be seen oral to the obstruction.

Linear foreign bodies are visible only as filling defects if they have a certain thickness; nevertheless, the contrast agent (barium or, in cats, also iodine) is helpful to better delineate the abnormal shape and contour of the bowel loops containing the linear foreign body.

Infiltrative bowel disease

Contrast evaluation of patients with suspected infiltrative bowel disease are rarely performed anymore. Ultrasound provides more information about extent of disease, integrity of wall layers and additional findings such as lymphadenopathy. In upper GI series, neoplastic and non-neoplastic infiltrates result in a similar appearance. Main changes include irregular mucosal surface, thickened wall, change of luminal diameter, corrugated appearance of the intestines and "thumbprinting" — irregularly arranged indentations into the contrast column. These changes have to be differentiated from peristalsis, and have to be verified on more than one radiographic view.

by Johnny D. Hoskins DVM, PhD, Dipl. ACVIM Dr. Hoskins is owner of Docu-Tech Services. He is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail: jdhoskins@mindspring.com.