Contrast radiography consists of the placement of a radioopaque substance (barium or organic iodinated contrast) within a known organ or organ system. The patient is then exposed to a beam of radiation which is selectively absorbed by tissues. Only those X-rays with enough energy to pass through the patient cause varying changes in opacity on the film. Radiographic contrast agents absorb all of the energy from the X-rays passing through them causing the X-ray film to appear white in the area corresponding to the location of the contrast media. This allows detection of changes in the size, shape, location and margination of the system being evaluated.

A lot of information can be gained from survey radiographs. Therefore contrast procedures should never be done without previous evaluation of those survey films. Contrast procedures are done to supplement or confirm information obtained from the survey radiographs.

The indications for contrast examinations include:

 Information concerning mucosal surface and luminal contents of a viscus (looking for a radiolucent foreign body within the stomach or for a radiolucent cystic calculus).

 Evaluation of organ function (oesophogram can be used to evaluate oesophageal motility, or contrast iodinated product can be used for evaluation of renal function).

 Evaluation of anatomy, location and integrity of an organ or structure (cystography can be used to evaluate the urinary bladder for rupture).

There are several questions one must ask about the patient prior to doing a contrast examination and these include:

 Will the examination provide information not available on survey radiographs that will assist in management of the case and development of a diagnosis, without harming the patient?

 If anaesthesia is needed, is the patient an acceptable anaesthetic risk?

 Can the patient tolerate having food withheld?

 The patient's temperament: is it frightened, is the animal vicious, is it highly strung?

Contrast studies are a benefit-risk procedure. The information to be gained should be significant relative to the stress to the patient and the expense to the client.

Patient preparation for contrast studies will differ depending on the study to be done. Animals must be kept off any drugs especially those that are contraindicated for the procedure to be done. Selection of a contrast agent is important. We must consider the study to be performed and consider what our presumptive diagnosis will be. This is important because if we suspect a gastric perforation we will want to stay away from barium which may cause severe problems if it leaks into the peritoneal cavity. Choice of product depends on what is available to you or your clinic and the status of the patient.

There are two basic groups of contrast agents: negative contrast media and positive contrast media. Negative contrast media is radiolucent and includes room air and carbon dioxide. The more soluble in blood the product is, the safer it is. Room air is cheap but fatal air embolism may occur secondarily and has been documented. Carbon dioxide is absorbed rapidly and is very soluble in blood. Some examples of examinations using a negative contrast medium are pneumogastrogram, pneumocolon and pneumocystography. Negative contrast medium can be used associated with positive contrast media in a double-contrast procedure such as double-contrast cystography. Positive contrast media include two major groups: organic iodinated compounds and barium. Barium is an inert non-absorbable heavy metal which can be used to evaluate the gastrointestinal (GI) tract. The other positive contrast medium, organic iodine, is a water-soluble contrast material. It can be used in evaluation of the bladder, kidney, fistulous tracts and the vascular system as well as the spine with myelography (not all iodinated contrast products can be used for myelograms).

Stomach

Indications for the use of contrast in evaluation of the stomach include:

 Suspicion of luminal or mural gastric masses

 Radiolucent gastric foreign bodies

 Haematemesis

 Recurrent or non-responsive vomiting

 Gastric localisation, identification, size, shape and margination

 To evaluate motility

Preparation of the patient includes survey radiographs which should always precede contrast studies. Survey radiographs allow evaluation for subsequent adjustment of technical exposure settings for the contrast study. The animal should be fasted 12-24 hours before radiography. Contrast procedures should always be individualised. Many drugs affect motility and these drugs should be discontinued for an appropriate interval before any contrast study is done. Contrast agents include negative (room air) and positive (barium sulphate suspension which is micropulverised). If perforation is suspected an organic iodinated solution like iohexol should be used. Other equipment includes mouth gag and an orogastric tube. Gastrograms are most often used as part of an otherwise standard upper GI series (the small bowel evaluation follows the introduction of a positive contrast medium). Technique includes dosage of barium of approximately 10 ml/kg (room air 20 ml/ kg), administered via a gastric tube. Radiographs are routinely taken in right lateral and ventrodorsal views but for complete and accurate evaluation of the stomach dorsoventral and left lateral films may be taken. Films are taken immediately to evaluate the complete stomach, before it starts to empty.

Small Intestine

Indications for contrast evaluation of the small intestine include:

 Severe or protracted vomiting or diarrhoea

 Bloody vomitus or diarrhoea

 Further evaluation of an abdominal mass

 Suspected obstruction

 Uncertainty of organ location

 Abdominal pain

 Fever of unknown origin

 Confirmation of various types of abdominal hernias

Contraindication to evaluation of the small bowel with contrast is radiographic survey changes consistent with obstruction. Contrast media used are similar to those for a gastrography. Organic iodinated compounds such as iohexol should be given at a dosage of approximately 10 ml/kg. This may be diluted 1:2 or 1:3 with water. Indications for the use of iodinated contrast medium rather than barium include a suspected perforation, quick determination of small intestinal patency or quick identification of small intestinal location. Preparation for an upper GI study consists of a 24-hour fast and an enema the night before and 2-3 hours before the procedure. Each study needs to be individualised.

Cystography

Indications for evaluation of the urinary bladder with contrast include clinical signs associated with lower urinary tract disease. Indications also include those changes in the urinalysis which may be consistent with lower urinary tract disease. Equipment necessary for cystography include contrast medium, which may be negative (room air or carbon dioxide) or positive (soluble iodinated contrast medium), catheters, sterile lubricant gel and three-way stop cock. There are two different types of contrast studies that can be done. The first is positive contrast cystography, which is the technique of choice for identifying the urinary bladder location and integrity (for tears or rupture). The second type of contrast study is called a double-contrast cystogram, which utilises both negative and positive contrast material. This technique is superior for demonstrating lesions involving the urinary bladder wall and intraluminal filling defects.

Patient preparation includes withholding food for 12-24 hours and warm cleansing enemas the night before and 2-3 hours prior to the procedure. Sedation or tranquillisation may be necessary especially in cats or female dogs for catheterisation. A small amount of lidocaine (1 ml mixed with 2 ml of sterile saline) may be infused into the urinary bladder before the contrast and may help to reduce urinary bladder spasm. The catheter tip should be placed within the bladder neck. The urinary bladder should be empty. For double-contrast cystography, 1-7 ml of positive contrast medium (depending upon the animal's size) may be used after infusion of negative (room air/CO₂) contrast. The patient should be rotated and the bladder massaged to distribute this contrast medium and coat the entire mucosal surface. The negative contrast should be slowly infused into the bladder. Complete distention should be achieved and recognised via palpation, back pressure felt upon the syringe plunger or reflux of gas around the catheter. This will be best noted if a Foley or balloon-type catheter is not used. The survey films which are taken prior to contrast study will provide adequate technique for the double-contrast cystogram. An increase of 4-6 kVp is needed for a positive contrast study. Take at least two views (lateral and lateral oblique). Ideally three views with a lateral and two lateral obliques can be done. Contrast should be removed from the bladder after the study.

Complications associated with a cystography include fatal air embolism due to embolisation of a gas within the vascular system. This occurs immediately after administration of negative contrast medium. There is increased risk of this occurring with a severe ulcerative or erosive cystitis. The author recommends use of carbon dioxide, which is much more soluble in blood, in an animal that has a large accumulation of blood within the urine.

Urethrography

Equipment necessary includes catheter, sterile gel, lidocaine and a water-soluble iodinated contrast medium. The technique for patient preparation is similar to that for cystography. Survey films should always be taken prior to the study to evaluate technique and patient preparation. A urinary catheter should be placed. In a female dog or cat the catheter should be placed in the distal urethra and just proximal to the os penis in the male dog. Infusion of 2-5 ml of lidocaine before injection of contrast may reduce urethral spasm. Male dogs should be positioned with their legs drawn cranially. The contrast medium should be injected as a bolus: 10-20 ml of contrast is needed for male dogs, 5-10 ml for female dogs and cats. Radiographs should be taken during injection of the last few millilitres. A lateral radiograph may be sufficient but repeat lateral and ventrodorsal obliques may be helpful. Complications include iatrogenic trauma and bacterial contamination.