The advantage of contrast studies is that they highlight and allow assessment of the tissue-contrast interface, and allow assessment of the size, shape, location and patency of various viscera. Contrast can be used to locate structures not apparent on survey films, such as masses, obstructions, and foreign object.
These are the most common radiographic contrast studies performed on camelids at Oregon State University:
1. Nasal passages (Choanal atresia)
B. Vascular Ring Anomaly
D. Gastrogram – differentiate viscera
E. GI Series
F. Enema for stricture
5. Ear canals
6. Urinary calculi
The advantage of contrast studies is that they highlight and allow assessment of the tissue-contrast interface, and allow assessment of the size, shape, location and patency of various viscera. Contrast can be used to locate structures not apparent on survey films, such as masses, obstructions, and foreign object. Contrast may also give a "functional" assessment of a system, demonstrating whether the contrast material moves through in a smooth and timely fashion. However, abnormal motility may also alter the passage of contrast, and hence can interfere with assessments of patency and function.
Similar to the interpretation of other radiographic studies, the interpretation of contrast radiographs can be affected by the view. The general rule is that a lesion should be seen on at least 3 separate films to considered real:
"Once, an artifact. Twice, a coincidence. Three times, real!"
In many ways, contrast radiography has been replaced by ultrasonography or endoscopy, but equipment for those other studies may not be available, and in some cases, radiography is still superior. It is hard to see much past the esophagus endoscopically in camelids, and the narrow nasal passage (and sharp teeth!) mean it takes a long, narrow scope to perform most studies, Nasal passages are especially narrow in crias, and ear canals are narrow in all ages of camelid.
Ultrasonography is limited by its inability to penetrate gas or bone, thus limiting studies of the thorax or through the first compartment. It is also sometimes difficult to discern what large viscera are, such as differentiating a gastric compartment from an enlarged colon.
The contrast used for GI studies include Barium sulfate and Iodinated media. They are radiopaque and attenuate the x-ray beam, producing an opaque (lighter) area on the radiograph wherever they accumulate. Barium is available as paste or liquid suspension. Both are useful. The paste stays around long, where the liquid gives better mucosal coating. Leakage into tissues causes granulomatous inflammation, so barium should be avoided if a perforation is suspected. Iodinated media can be ionic (Conray - iothalamate), which dilute in the GI tract making them difficult to see, and cause pulmonary edema if aspirated, or nonionic (Omnipaque -iohexol), which provides inferior mucosal coating to barium.
For all contrast studies, survey films should be taken first to establish the appropriate technique.
The most common nasal study is to diagnose choanal atresia in neonates. Candidates include crias in respiratory distress with open-mouthed breathing, or camelids or any age with a definite lack or reduction of air flow through one of both nostrils. A non-ionic iodine compound is preferable to barium if there is doubt about lack of patency, as the chance of aspiration is high. The camelid is positioned in sternal recumbency with the nose elevated perpendicular to the ground. Lateral, obliques, and a VD or DV may be taken from this position. Liquid contrast is introduced into one or both nostril using a soft, flexible tube, and radiographs may be taken as soon as the contrast is in. Choanal atresia results in pooling of contrast around the level of the orbits. Dorsal pharyngeal masses due to bot fly larvae have also been defined this way.
Another common study on the head is to examine ear canals in camelids with evidence of otitis. The ear canal in normal adults llamas is 4-6 mm in diameter and sigmoidal in shape. It is about half as big in alpacas. In adults Ear 5 to 10 ml of omnipaque is introduced as well into the canal as possible. Some degree of stenosis or deformation may be seen in as few as 3 days. A passage of contrast into the middle ear confirms tympanic membrane rupture.
The single most common contrast study performed on camelids at OSU is the esophagram. Usually only lateral films are taken, with the camelid either standing or in sternal recumbency. Camelids are prone to a number of esophageal disorders, including megaesophagus, choke, stricture, esophagitis, and vascular ring anomalies. These disorders cause similar signs, including reflux of saliva and feed material, cervical swelling and fluid waves, weight loss, and possible respiratory signs. Although survey films may provide some evidence, and feed material and saliva may act as a form of contrast in some cases, the use of contrast agents is helpful in differentiating the disorders, Because assessment of motility and patency as well as mucosal appearance each are important, we usually use a mix of liquid, paste, and paste mixed with pellets. Unless a perforation is suspected (rare), barium is used. In patients with enough wherewithal to have an intact gag reflex, the compounds are tubed, syringed and manually introduced into the mouth or cranial esophagus. Usually, 60cc dose syringes in the mouth are used for the liquid and paste, then the pellet-paste mixture is placed in the mouth. Films may be taken as soon as the majority has been swallowed. Esophageal motility is usually good enough in unaffected camelids, that almost all the barium is in the stomach by the time the first film is taken. Thin lines of contrast left in the esophagus are permissible. Pooling evokes the 3-view rule. Megaesophagus usually results in large amounts of pooling, usually through out the thoracic esophagus, and often in the lower cervical esophagus as well. Impacted feed may affect the distribution of contrast. In less affected camelids, the pooling will be more subjectively abnormal, but it must be remembered that in normal animals, the contrast moves very quickly through the esophagus.
With choke, a vascular ring, or a stricture, there is abrupt cessation or narrowing of the column of contrast, with pooling. With vascular rings, the narrowing is near the base of the heart, often with pooling evident on both sides, Strictures are relatively rare, and are more variable in location, but might cause a similar pattern. Contrast may also highlight the obstruction in choked camelids.
Esophagitis is usually caused by ingestion of rhodendron, laural, or azalea leaves, or a caustic medication. There is usually greater evidence of luminal patency than with the other disorders, although poor motility and pooling are common. The mucosa is usually roughened as well. Fluoroscopy with contrast is also helpful in assessing esophageal pooling and patency.
GI series are relatively rarely preformed, because gastric retention is so long (days) after the neonatal period. However, we have found orally administered contrast to be helpful in differentiation enlarged viscera, especially in younger animals. Ultrasound is often unable to help in differentiation the first compartment from an enlarged ascending colon, especially when the large structure fills most of the scannable abdomen. Oral contrast will enter and highlight C1, regardless of the location of the lesion, and hence can be used to differentiate structures. Once again, Barium sulfate suspension (20-30% w/v) is usually used, but organic iodine solution is preferred if there is any suspicion of perforation. Contrast may be swallowed or administered by tube. Make sure the tube is in the right place first.
Contrast has also been used to describe fistulous tracts and look for sequestra. For such internal applications, non-ionic iodides are superior.