Diseases of the urinary tract are common in dogs and cats. Many diagnostic imaging modalities are available to investigate the urinary tract, and diagnostic ultrasound has replaced most contrast studies of the urinary tract. There are, however, a few indications that are still best investigated with contrast radiography. In the following, some practical tips to achieve a successful contrast procedure are discussed with case examples.

Excretory Urography

This procedure has largely been replaced by ultrasound particularly for the purpose of assessing renal size, shape and location, and presence of mass lesions or pelvic dilation. Excretory urography (EU) still remains the method of choice to determine presence of rupture of the excretory pathway particularly in trauma cases. Other indications include ectopic ureters and ureteral obstructions, both are often investigated with ultrasound as well with a similar sensitivity and specificity. Contraindications include dehydration, anuria, severe proteinuria, co-exiting congestive heart failure and known hypersensitivity to iodine.

In trauma patients, the most important preparation for an EU is to make sure the patient is well hydrated prior to and during the procedure to avoid contrast medium-induced renal failure. For all other elective procedures, fasting the patient for at least 24 hours is essential to ensure evacuation of the large bowel and small intestine. Anesthesia is recommended for this procedure.

Survey radiographs are always the first step in performing an EU. Reasons for the survey radiographs are to establish the normal radiographic features of the abdomen of this particular patient, ensure appropriate radiographic technique and make sure that there is no fecal material left in the colon that would obscure the urinary tract.

An intravenous catheter is placed preferably into the cephalic vein, and the largest possible catheter (18–20 g) is recommended for ease of contrast medium administration and to avoid extravasation. Sterile placement of a Foley catheter into the urinary bladder, followed by drainage of the bladder is recommended for most procedures. A tightly fitted urethral catheter may also be used to prevent leakage from the lower urinary tract. If the goal of the study is to determine the location of the vesicoureteral junctions then it may be advantageous not to have a Foley bulb in the bladder trigone. On the other hand, the Foley bulb sometimes helps delineating the trigone and the beginning of the proximal urethra. Air is introduced into the urinary bladder (palpate the bladder to avoid over-distension) prior to the EU to achieve a double contrast cystogram as well. A three-way stopcock attached to the catheter is always a good plan, it will aid in retaining the gas and will facilitate further drainage of urine, administration of contrast medium etc.

Warming the contrast medium to body temperature reduces its viscosity and improves ease of injection. Non-ionic iodinated contrast medium is recommended as it has been shown to reduce the incidence of adverse effects. The dose is 880 mg of iodine per kg body weight, injected as a bolus. Be prepared to take a ventrodorsal radiograph immediately (when the contrast medium arrives in the renal vasculature, about 10 seconds after injection), followed by a lateral projection at which time the contrast medium should be in the renal parenchyma during the nephrogram phase. Repeat lateral and ventrodorsal radiographs every 5 minutes to document arrival of contrast medium in the renal pelvis ( pyelogram phase), ureters and urinary bladder. Ureteral peristalsis leads to only partial visibility of the ureters at any given time unless they are abnormally dilated, and several sets of radiographs have to be taken to see contrast medium in all ureteral segments. Oblique lateral projections can be very helpful to offset the left and right ureters.

Ultrasound-guided pyelography results in the best delineation of the ureters, and each ureters can be investigated separately. Renal pelvic distension larger then 5 mm in diameter facilitates performing this study. A 22-gauge or thinner needle is placed into the renal pelvis under ultrasound guidance. Urine is aspirated from the pelvis (samples should be kept for culture and cytology) and half of the aspirated volume is replaced by contrast through a 3-way stopcock and extension set system. Lateral and ventrodorsal radiographs are acquired immediately after the procedure.

Cystography

Preparation for cystography includes fasting of the patient and obtaining survey radiographs, for the same reasons outlined above.

For negative cystography, simply catheterize the urinary bladder in a sterile way, drain all urine and refill the bladder with air or gas (CO₂) until it palpates slightly turgid. Take lateral and ventrodorsal radiographs, and oblique projections if needed. This technique can be used to localize the bladder, or outline the mucosa for example to delineate a bladder wall mass. If there is evidence of hematuria room air should not be used to avoid the risk of air embolism.

Contrast medium instilled into the bladder for a positive contrast cystogram is typically diluted to 20–50% with sterile saline, this will result in adequate opacity but still outline wall lesions and intraluminal filling defects. The volume of contrast medium/saline solution instilled is on average 5 ml/kg in a dog and a total of 25 ml in a cat. The bladder has to be palpated during instillation of contrast medium though and if overly turgid or if there is resistance to injection, the injection has to be halted. A diseased urinary bladder may only be able to hold a volume of 1 ml/kg and rupture may occur if too much contrast medium is injected. VD and lateral radiographs are taken immediately after contrast medium injection.

The third contrast procedure is to then drain the positive contrast from the bladder and re-instill air until the bladder is again turgid, resulting in a double contrast cystogram. The positive contrast which adheres to the mucosa of the bladder will usually do so because of inflammatory bladder wall lesions. This study will also help to identify small calculi which were previously not visible on the negative or positive contrast studies. Polyps, bladder wall infiltrating masses, urachal diverticuli and inflammatory bladder wall thickenings can all be identified using this sequence of bladder studies.

Urethrography

Survey radiography of the urethra is limited, but radiopaque calculi may be seen. In order to evaluate the entire urethra in male dogs, an additional lateral caudal abdominal radiograph with the pelvic limbs flexed cranially has to be obtained. Since the majority of the urethra is also not accessible with ultrasound, urethrography is indicated in almost all cases of suspected urethral disease.

Urethrography is performed using water soluble iodinated contrast media diluted with sterile saline to approximately 20% of the original concentration. Air should not be used because of the risk of urethrocavernous reflux and air embolization. The bladder should be moderately full to give some resistance to urethral contrast flow and allow good filling of the urethra. Urethrography can be performed either antegrade or retrograde. For retrograde urethrography a balloon-tipped catheter is placed into the distal urethra and 10–15 ml contrast medium is injected in dogs (5–10 ml in cats), and lateral radiographs are obtained. For antegrade urethrography the catheter is first inserted into the urinary bladder and then retracted while injecting contrast, a radiograph is obtained towards the end of the injection. In female dogs urethrography is sometimes replaced by vaginourethrography with the balloon placed in the vestibule of the vagina. Rupture of the vagina is a relatively common complication of vaginourethrography. It mostly happens when the bulb of the Foley catheter is placed proximal to the urethral opening and pressure buildup cannot be relieved by contrast medium escaping into the urinary bladder.

References

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4.  Schwarz PD, Willer RL. Urinary bladder neoplasia in the dog and cat. Probl Vet Med. 1989;1(1):128–140.

5.  Essman SC. Contrast cystography. Clin Tech Small Anim Pract. 2005;20(1):46–51.