INTRODUCTION
Contrast radiography allows better visualisation of soft tissue structures than plain film radiographs. Studies with which you should be familiar are:

1.  
   • The gastrointestinal tract and the use of barium as a contrast medium.
   • The urogenital system and the use of air and water soluble iodine contrast media.
   • The spinal cord and the use of low osmolar, non-ionic water soluble contrast media.

In all cases, plain radiographs should be taken to check the positioning of the patient and exposures before starting the contrast examination.

1. BARIUM
   Barium is available as a ready-made paste or suspension, or as a powder, which is mixed with water to achieve the desired consistency. It is a positive contrast agent; and will thus appear more opaque (whiter) than the surrounding tissues. It is excellent in that it provides very fine mucosal detail and is therefore ideal for use in studies of the oesophagus, stomach and intestines. A barium series is a sequence of radiographs taken every 15-60 minutes following the passage of the contrast media from the oesophagus through to the colon. Barium may be given orally alone, mixed with a small portion of food or as an enema and is therefore quite versatile. It does have some limitations though, and these are listed below:
   • Heavy sedation or anaesthesia are contraindicated if oral barium is to be given.
   • Risk of aspiration pneumonia with orally administered barium.
   • Food should be withheld for 24 hours prior to the procedure.
   • Contraindicated if a perforation is suspected; use water soluble iodine instead.

   
   Dosage guide:
   Enemata = 10mls/kg.
   Per os = 8-12mls/kg for cats and small dogs; 5-7mls/kg for larger dogs.
   Via stomach tube = 15-100mls depending on patient size and rate of gastric emptying (Glucagon may be given to slow the rate of gastric emptying).
   
   

2. AIR AND WATER SOLUBLE IODINE CONTRAST MEDIA
   Air is a NEGATIVE CONTRAST agent, which means that it will appear less opaque on a radiograph (blacker) than the surrounding tissues. Room air is most commonly used, although oxygen, carbon dioxide or nitrous oxide are also suitable. Air is used to inflate hollow organs such as the stomach or bladder in order provide effective visualisation. Water soluble iodine preparations are POSITIVE contrast agents and will appear more opaque than the surrounding tissues. Commonly used are Conray and Urografin. These may be administered intravenously for examination of the kidneys and ureters or via a urinary catheter for examination of the lower urinary tract. In addition, they may be given orally to visualise the stomach, but are less effective than barium due to their hypertonicity, which causes dilution by the gastrointestinal secretions. A DOUBLE CONTRAST radiograph of a hollow organ utilises both negative and positive agents in order to provide good mucosal detail without obscuring small details within the organ itself. Important terminology to remember:
   • The prefix "pneumo" denotes a negative contrast radiograph.
   • Gastrogram = contrast radiograph of the stomach.
   • Urogram = contrast radiograph of the kidneys and ureters (2 methods: bolus or infusion).
   • Cystogram = contrast radiograph of the stomach.
   • Retrograde urethrogram = contrast radiograph of the male urethra.
   • Retrograde vaginourethrogram = contrast radiograph of the female urethra.

   Dosage guide:
   Gastrogram - 15-100 mls Gastro-conray via stomach tube (followed by 20mls/kg air for double contrast gastrogram).
   Bolus Urogram - up to 850mg of iodine/kg (@ 50mls for a 25kg dog) intravenously as a rapid injection.
   Infusion Urogram - up to 1200mg of iodine/kg (@ 200mls for a 25kg dog) intravenously as a slow infusion diluted with saline.
   Cystogram - 50-300mls iodine for positive contrast, 30-300mls air for negative contrast, 2-15mls iodine followed by air until bladder palpates taut; all given via urinary catheter.
   Retrograde urethrogram - 5-15mls iodine via urinary catheter.
   Retrograde vaginourethrogram - up to 1ml/kg via Foley catheter taken utmost care to avoid vaginal rupture
   
   

3. LOW OSMOLAR, NON-IONIC WATER SOLUBLE CONTRAST AGENTS
   Low osmolar, non-ionic media are used in myelography and for some intravenous studies. Iopamidole and Iohexol are preferred since these are the least irritant to nervous tissue. The agent may be introduced by cisternal puncture (via insertion of the needle into the cisternal magna situated at the cranial end of the subarachnoid space just behind the skull), or by lumbar puncture (via insertion of the needle between the 4th and 5th or 5th and 6th lumbar vertebrae). General anaesthesia is essential in both cases to prevent movement during needle placement. A myelogram shows opacification of the cerebrospinal fluid and subsequently demonstrates the spinal cord. Tumours or prolapsed intervertebral discs which may not be evident on plain film radiographs may be identified.
   
   Dosage guide:
   0.3ml/kg of 330mg/ml Iopamidole or Iohexol; maximum 9mls.