Introduction
Contrast procedures have been developed to increase the native contrast of organs in order to separate them from surrounding tissues. There are many situations where they can be a useful supplement to the information gained from routine radiographic examination. They can provide information on the size, shape and position of structures. They might help outline the internal structure of an organ, including its mucosal pattern, and sometimes help assess its function.
In small animal practice, contrast radiography is mainly used for the diagnosis of gastrointestinal and urogenital disorders, although it is also used in myelography, angiography and in the assessment of joints and sinus tracts.
In large animal practice, it is mainly used in the assessment of joints and tendon sheaths.
The two main categories of contrast agent are positive and negative agents. Positive contrast agents have a high atomic number, either barium sulphate or iodine, and appear more radiopaque than the surrounding tissue. Negative contrast agents are gases of low density (air, oxygen, carbon dioxide) which appear radiolucent.
The agents can also be combined to produce a double-contrast study which is often the best way to give optimal mucosal detail.
Gastrointestinal Studies
Barium or iodine-based solutions can be used for positive contrast studies. Barium is contra-indicated if there is suspected perforation anywhere in the gastrointestinal tract as leakage can result in a granulomatous reaction and adhesions.
Air is naturally present in the GI tract and provides some natural contrast, but air can also be introduced and provide negative contrast images.
Patient Preparation
If the condition is chronic, there is usually adequate time to ensure a good preparation of the patient.
If the oesophagus is the area of interest, little preparation is necessary.
For any other area, food should be withheld for 24 hours to ensure an empty stomach and small intestine.
If the colon is being examined, an enema should be performed 2 to 4 hours prior to the contrast study. Enemas might also be useful in minimising superimposition of colonic contents if a small intestinal study is being performed.
Plain radiography should be performed first in all cases.
Sedation is usually necessary, and consideration should be given to the effect of sedatives on transit time.
Studies
Oesophagus: positive contrast study using barium or barium and food.
Stomach/small intestine: positive contrast study using barium or iodine solution, pneumogastrogram, double-contrast gastrogram.
Views are taken at regular intervals to assess gastric emptying and intestinal motility.
Large intestine: pneumocolon or barium enema
Urogenital Studies
Iodine-containing agents are usually used, ionic or non-ionic. Air can also be used although is contraindicated if rupture is suspected due to the potential for a fatal air embolism.
Patient preparation
Ideally, the patient should be starved for 24 hours and have enemas to empty its colon. Control radiographs should always be performed.
Cystocentesis and urinalysis should be performed before catheterisation and contrast agent introduction.
Studies
Kidneys and ureters: intravenous urogram (IVU) study using a non-ionic iodinated solution; antegrade pyelography using ultrasonographic guidance
Bladder: usually retrograde cystrography (pneumocystogram, positive contrast cystrogram, double contrast cystogram)
Urethra: retrograde urethrogram in males, retrograde vaginourethrogram in females. It should not be performed in females in oestrus unless essential due to the patency of the cervix and increased sensitivity of the vaginal mucosa.
Myelography
Non-ionic iodine containing media should be used such as iohexol.
Contrast should be injected under aseptic conditions into the subarachnoid space either at the cerebellomedullary cistern or at the lumbar cistern.
Vascular Studies
Portal venography can be used to diagnose a porto-systemic shunt.
Articular Studies
Shoulder arthrography is useful in cases of suspected biceps tendon injury, for evaluation of shoulder Osteochondrosis Dissecans and for any other joint pathology such as tumours.
Dacryocystorhinography
Contrast Studies in Horses
Contrast studies are mainly used to investigate orthopaedic conditions such as OCD in the shoulder, stifle and tarsus, and degenerative joint disease. Contrast medium is injected aseptically into the joint and orthogonal radiographs are taken.
Contrast studies are also extremely useful for the assessment of joint and tendon sheath integrity following a traumatic incident. Contrast can either be injected into the penetrating wound tract to assess dispersal of the medium. Alternatively, contrast can be injected into a joint or tendon sheath in an area away from the puncture, and leakage through the wound can be assessed.
References
Kahn, C. (2005) Merck Veterinary Manual Merck and Co
Latham, C. (2005) Practical contrast radiography: 1. Contrast agents In Practice 27; 348-352
Easton, S. (2006) Veterinary radiography: a workbook for students Elsevier Health Sciences