Special radiographic procedures using contrast media are used in certain cases to delineate internal structures. A contrast medium is a substance which is either highly radiolucent or highly radiopaque and can be administered to an animal to increase radiographic contrast within an organ or system. This technique allows the visualisation of soft tissue structures and evaluation of size, shape and position. It may be possible to evaluate organ function or to assess the physiological condition. Contrast studies are used to supplement or confirm information gained from routine survey radiographs.
There are two categories of contrast media: positive and negative.
Positive contrast agents, such as barium or iodine compounds, contain elements of high atomic number. These absorb more x-rays than do soft tissues or bones. Positive contrast media are radiopaque and appear white on radiographs. These compounds can be used to fill or outline a hollow organ (e.g. alimentary tract  Fig. 1 , urinary bladder), or can be injected into a blood vessel for immediate visualisation of the vascular supply or for subsequent excretion evaluation. (Fig. 2)
Negative contrast agents are gases with low specific gravity; air, oxygen and carbon dioxide are most frequently used. These appear black on a radiograph (Fig. 3). Fizzy drinks are sometimes given to animals as a form of negative contrast to produce an outline image of the stomach (Fig. 4).
Some special procedures call for the use of both positive and negative contrast agents, or double contrast. A double-contrast study gives optimal mucosal detail and avoids the masking of small anomalies by large volumes of positive contrast media.
Barium sulphate is a positive contrast suspension, and is the medium of choice for radiographic studies of the gastrointestinal tract. It may be administered in liquid or paste form, or mixed with food. If it leaks into the thoracic or abdominal cavities, however, it may cause a granulomatous response as it is not absorbed or eliminated. Therefore it should not be used if there is a possibility of perforation of the gastrointestinal tract.

Patient preparation is important. Food should be withheld for 12-24 hours, and an enema may also be administered, to evacuate the GIT.

Oesophogram: Barium study of the oesophagus and pharynx to assess function or evaluate disease. Contrast medium is administered orally and radiographs are immediately taken. The radiographs will indicate the longitudinal folds of the mucous membrane in the dog. In the cat, the proximal ¾ of the oesophagus has longitudinal folds and the distal ¼  has oblique mucosal folds, giving a herringbone pattern.

Stomach: Stomach rugae are visible with contrast studies.

Descending duodenum: Multiple views must be seen on contrast studies to make a definitive diagnosis of duodenal problems.

Lymphatic craters: Normal depressions on the antimesenteric border of the duodenum that are seen in contrast studies. They are often mistaken for ulcers and have been called pseudo-ulcers.

Heart: Contrast studies of the heart (angiocardiography) are radiographs of the heart taken while a radiopaque contrast medium circulates through it. A cannula is inserted into the external jugular vein and contrast medium is injected quickly as a bolus and the radiograph taken. This enhances contrast on the right side of the heart. The right ventricle occupies the cranial part of the cardiac silhouette. The pulmonic valve is visible as small indentations at the origin of the pulmonary trunk (MPA). The pulmonary arteries are seen extending off into the lung tissue.
Left side of the heart: The aorta and left ventricle are seen (Fig. 5).

Myelography: Radiography following the injection of a positive contrast medium (I₂) into the spinal subarachnoid space.  The contrast medium is non-ionic and of low osmolarity.  It is used to visualise lesions which may not be seen on survey radiography. The subarachnoid space becomes visible as two white lines separated by a space (the invisible spinal cord). These contrast lines should be smooth, reflecting the smoothness of the spinal canal. The spinal cord has a cervical and a lumbar enlargement that are normal at the brachial plexus and lumbar intumescence. Look for any deviations in the contrast lines, therefore in the cord. A break or thinning in the subarachnoid space may indicate swelling of the spinal cord. A break appearing to be pushed inward could be due to a mass outside the meninges (herniation of a disc).  Contrast medium is heavier than CSF, so gravity can be used to move it up or down the subarachnoid space.

Cervical myelogram: Technique performed by injecting contrast medium into the dorsal subarachnoid space. Flex the head ventrally and palpate the wings of the atlas, spine of the axis, and the occipital protuberance. Draw a line between the wings and a line from the occipital protuberance to the spine of the axis. Place the needle on the midline ½ inch in front of the line between the wings. Go roughly parallel to the caudal wall of the skull and feel for the “pop” of resistance as the needle passes through the dorsal atlanto-occipital ligament. Stop when through the ligament. DO NOT pith (needle through the brain-stem) the dog! Pull out the stylet and check for CSF flow to indicate if the site is correct. Collect CSF for analysis and then inject the contrast medium. Never aspirate CSF; this can cause the spinal cord to be drawn into the needle or cause a change of pressure in the subarachnoid space resulting in herniation of the brain stem.
Lumbar myelography is also performed but is technically more difficult.

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