INTRODUCTION

Contrast radiography in veterinary practice implies special radiographic procedures that include the use of contrast media and vertical beam radiography, without the use of fluoroscopy. Purpose of contrast examinations is to gather more information in addition to non-contrast survey radiography ("plain radiography").

Requisites for successful contrast radiography include knowledge of the possibilities and limitations of non-contrast radiography, enthusiasm for this field of veterinary diagnostic imaging including concern for radiation safety, and understanding of the different techniques of contrast radiography.

For interpretation of non-contrast radiographs the principles of radiographic imaging and normal radiographic anatomy should be understood. Visualization of structures on a radiograph depends on the identification of organ profiles, and the contrast that results from structures of different densities that lie in close contact with one another.

A non-contrast radiograph is a composition of four basic radiographic densities: gas--fat--fluid (soft tissue)--bone.

When fat is insufficiently present (emaciation, puppy abdomen) or fluid density abundantly present (ascites, mass lesions), especially with abdominal radiography the administration of contrast media can be of great help to visualize intraabdominal viscera and organ systems that are otherwise not visible on non-contrast films in this certain situation.

Technically, contrast radiography requires good X-ray machines with diaphragm and collimation, in combination with a grid or buckey attached to the radiographic table.

Contrast catheters and sondes of different size and length must be at hand.

The contrast media of choice must be inert and harmless (as much as possible) to the animal under investigation and harmless to the veterinarian. They must be easily to use, sterile if necessary (i .v. injections), and not too expensive. Contrast media are based on gas for negative contrast studies, for which most of the time room air is very suitable, or on barium and iodine for positive contrast studies.

Contrast examinations use single-contrast techniques or double-contrast techniques.

With single-contrast techniques, an organ or organ system is filled or surrounded by either a positive or a negative contrast medium. This provides the possibility of examination the visceral contour and lumen of the organ. The wall of the organ is visualized "en profile".

With double-contrast techniques, a hollow viscus is first filled with a positive contrast medium in order to coat the internal wall of that organ with a thin layer of the contrast material, thereafter emptied, and then refilled with a negative contrast medium. The thin layer of adhered positive contrast medium against the blackness of the hollow viscus now provides the possibility to examine the mucosal surface of the hollow viscus on more detail. This is called "en face" visualization.

The majority of contrast examinations in veterinary practice will include single-contrast techniques.

Means for sedation and general anaesthesia (intubation), and for stabilisation in the rare case of anaphylactic reactions (I.V. drip, antihistaminics) must be at hand.

General rules for contrast radiography include preparation, standardization, and the use of investigation protocols and good indications.

Preparation

This includes the removal of all intraluminal contents and external, superimposing densities from the organ system under investigation. For example, preparatory measures prior to abdominal contrast radiography may include:

1.  Withholding of food for 24 hrs

2.  Mild rectal cleansing laxative

3.  Emptying of urinary bladder

Next, the application of sedation or general anaesthesia is essential.

And, preliminary non-contrast radiographs are always made to verify the effect of preparation and to examine the present status of the animal.

Standardization

Standardization of examination techniques is the only way that justifies comparison of individual results. For a practitioner with lack of regular experience, this may be the only way to understand the results of his examinations.

Standardization of techniques results in investigation protocols.

Protocols and indications

Protocols are used for investigation technique and for interpretation. Technical protocols inform what to do, when to do it, and how to do it. Protocols for interpretation are used to extract the maximum information out of the examination results.

Contrast radiography increases the cost of a diagnostic work-up considerably.

Therefore, the veterinarian must always judge the cost versus the expected return in diagnostic information. Any procedure performed should have an acceptable probability of changing and upgrading the management of the case. In no event should an animal be subjected to an unnecessary radiographic examination either out of embarrassment with an unsolved diagnostic problem ("maybe it helps") or for no benefit other than the clinician's curiosity.

Unnecessary examinations can be avoided when

1.  Definite and appropriate indications are established, and

2.  Clinical investigation protocols are followed.

For example, the efficacy of gastrointestinal radiography largely depends upon the reason(s) for which it is performed, based on the patient's history, clinical examination, and previous laboratory findings.

Radiographic examinations based on nonspecific complaints such as suspected gastrointestinal bleeding or abdominal pain, acute vomiting and/or diarrhoea in the absence of obstructive signs, unexplained weight loss, or suspected abdominal mass will have a very low yield of positive findings.

While, examinations based on more specific criteria such as signs of obstruction, abnormally painful abdominal palpation, haematemesis and melena, palpable mass, or known history of foreign body ingestion or earlier abdominal surgery with complications may result in a high yield of positive findings.

Next to the selection of proper criteria and clinical investigation protocols, care must be taken to ensure examination techniques of high diagnostic quality and accuracy, not only in terms of sensitivity (accuracy of positive findings), but also in terms of specificity (accuracy of negative findings). It should be kept in mind that "negative results", as long as they are reliable, may help to rule out one or more diseases that had been considered as probable causes of the diagnostic signs. And, although negative examination results may not have proven the definite diagnosis, they will greatly assist in the management of the case and are probably well worth the costs to the client and the endeavours to the veterinarian.

However, this is only true for an examination technique that guarantees a high diagnostic accuracy both for sensitivity as for specificity.

The gastrointestinal and urological tract may be imaged by a variety of modalities including conventional radiography (non-contrast and contrast), fluoroscopy, ultrasonography, nuclear scintigraphy, and computed tomography.

The choice of imaging modalities depends both on the information required (related either to organ location and morphology, or to function) and the imaging facilities available. Because of practical circumstances, contrast radiography (with or without additional image-intensified fluoroscopy) still remains the principle additional imaging method used to evaluate gastrointestinal or urological disease in small animals.

Or, in other words, contrast examinations are still necessary!

An example of how things can work out:

Contrast radiography of the gastrointestinal tract: barium or air?

Since its introduction in 1910 "barium" has always been the contrast medium of choice for radiographic investigation of the gastrointestinal tract in small animals.

In 1974 the large-volume upper gastrointestinal contrast study (UGI-study) that is still routinely used today was introduced. This single-contrast study concentrates on examination of contour and lumen of the stomach and small bowel loops when filled with barium. One is usually content to identify a structural abnormality such as a foreign body or a stenotic and space-occupying lesion. Advantages of this practical examination technique include that it is easily performed and not expensive, and that fluoroscopic monitoring is not necessary. However, the use of a time protocol for interval radiography in combination with the irregular filling of the small intestinal loops and absence of full distension results in an examination technique of poor diagnostic quality and long examination time.

In the 1980's, improvements in fluoroscopic and radiographic equipment, the image intensifier with television monitoring and video-recording, and the introduction of more specific and safer contrast media have resulted in the development of refined double-contrast techniques such as double-contrast gastrography, enteroclysis and double-contrast enema studies with high rates of sensitivity and specificity for the detection or negation of abnormalities in small animals with gastrointestinal complaints. However, these techniques also require sophisticated and expensive X-ray devices, the application of sedation or anaesthesia, and more time and experience of the examiner.

Recently, with the development and accessibility of gastrointestinal endoscopy and ultrasonography a call for more practical and fast-speed examination techniques has been raised. In this respect, air-contrast studies as pneumogastrography, pneumo-colon and, in a lesser degree, pneumoenterography can replace many of the earlier barium studies, and still with a high yield of sensitivity and specificity.

For the veterinary practitioner, non-contrast survey radiography and contrast radiography are the principal imaging methods available for evaluation of gastrointestinal disease in small animals. And here, the air-contrast studies as pneumogastrography and pneumocolon must be the first choice. Barium investigations of stomach or colon are most of the time not necessary and only seldom indicated.

Pneumogastrography and pneumocolon are easily-performed screening procedures that don't take much time, preparation or special equipment. Both techniques are simple but very rewarding procedures that in many instances solve problems instantly, making costly and time-consuming barium examinations unnecessary. Occasionally, animals must be sedated, but general anaesthesia is never applied. Longstanding preparation to completely evacuate and empty the gastrointestinal tract is not necessary. With holding of food on the morning of the investigation and physiologic evacuation of the colon by outdoor walking following the introduction of a non-irritating clysma are sufficient. Small amounts of gastric or colonic contents will not hamper these air-contrast investigations. Necessary equipment includes a gastric tube and mouth protector for pneumogastrography, and a standard rectal sonde without inflatable cuff for the pneumocolon examination. The contrast medium that will be used is room air. This can be used in any quantity that is desirable, with a minimum of 10 cc per kg of bodyweight. Too much air introduced into the stomach will be regurgitated, and an excessive amount of air introduced into the rectum and colon will be evacuated by the animal.

Pneumogastrography

Indications for pneumogastrography include:

1.  Identification and localization of the stomach

2.  Identification and localization of a foreign body

3.  Identification and localization of a mass lesion of the gastric wall and pyloric orifice

4.  Evaluation of abnormal size and shape of the stomach

5.  Evaluation of gastric wall compliance and rigidity

6.  Evaluation of extrinsic compression and dislocation of the stomach

Increased mucosal thickness around or in the direct vicinity of the pylorus due to chronic hypertrophic pyloric gastropathy or chronic hypertrophic gastritis or a mass lesion due to antral polyposis or neoplasm is most of the time well defined by pneumogastrography.

Positive-contrast and double-contrast gastrography are indicated to more precisely define and characterize mucosal abnormalities in cases of neoplastic or inflammatory disease (in the absence of endoscopy or ultrasonography) or to differentiate between (infiltrative) intrinsic or extrinsic disease. The barium meal investigation is indicated when based on clinical findings and on non-contrast abdominal radiographs a functional pyloric outflow obstruction is suspected, without the presence of an obvious "mass" lesion or foreign body at the pyloric orifice.

Pneumocolon

Indications for pneumocolon include:

1.  Identification and localization of colon and caecum

2.  Evaluation of the ileocolic sphincter region

3.  Evaluation of large bowel compliance

4.  Suspicion of a large bowel mass lesion

5.  Determination of extrinsic large bowel compression

Pneumocolon will very well outline a ileocolic intussusception or cecal inversion or a large neoplastic mass lesion of the colonic wall. Minor large bowel wall irregularities or ulcerations will not be visualized with the pneumocolon technique. For evaluation of these abnormalities, the more refined double-contrast enema procedure is required. Other more specific indications for a positive or double-contrast enema study include evaluation of the oral (cranial) extension of a rectal tumor, the evaluation and characterization of type and severity of invasive large bowel wall lesions, and the functional evaluation of ileocolic sphincter competence.

As a conclusion, the reintroduction of air-contrast techniques makes gastrointestinal radiography more accessible to veterinary practitioners. Hopefully, this will improve confidence in radiology as an accurate diagnostic tool for the investigation of small animals with gastrointestinal complaints.