Ultrasonography or contrast radiography?

Both ultrasonography and contrast radiography can be used to image the urinary tract. Contrast radiography has the advantages of enabling visualisation of the ureters (which are visible ultrasonographically only if dilated) and the urethra (which is largely inaccessible to ultrasound). Ultrasonography has the advantages of being quick and convenient, requiring no patient preparation, enabling examination of structures (e.g., regional lymph nodes) that are not normally visible radiographically, and guiding cystocentesis or fine needle aspiration of lesions.

There are various indications for imaging the urinary bladder:

1.  To identify the bladder, e.g., in a patient with caudal abdominal mass or perineal rupture

2.  Haematuria

3.  Suspected urinary calculi

4.  Urethral or vaginal discharge

5.  Urinary incontinence

6.  Dysuria

7.  Anuria after trauma, i.e., suspected ruptured bladder

Ultrasonography is well suited to indications 1-3; indications 4-6 are likely to be associated with urethral lesions so are best suited to contrast radiography, although ultrasonography might be useful if the bladder or distal ureters are also involved. Suspected ruptured bladder remains an excellent indication for contrast radiography because extravasation of urine can be confirmed and the site of leakage identified.

Ultrasonography is a useful method for guiding cystocentesis. If the bladder is not palpable or if there is peritoneal fluid, ultrasound guidance ensures that urine is collected. Performing ultrasound-guided cystocentesis is also a good way of practising the hand-eye co-ordination needed for biopsy.

Don't forget to examine the kidneys

Animals with disease affecting the urinary bladder frequently have secondary lesions in the kidneys. For example, animals with cystitis or ectopic ureters can have pyelonephritis; animals with urinary obstruction or ectopic ureters can have hydronephrosis. This makes it important to examine the kidneys whenever performing an ultrasound scan of the bladder.

Normal anatomy

The urinary bladder may be imaged from a ventral or lateral approach. My preference is to use a lateral approach with the patient in lateral recumbency and the transducer upright. This means that any sediment or calculi will be found in a predictable location--at the bottom of the bladder as seen on the screen. Knowing exactly where to look for the calculi aids identification of tiny calculi.

The urinary bladder normally appears ultrasonographically as a rounded anechoic space bounded by a thin wall. It is well suited to ultrasonography because of its consistent superficial location and optimal ultrasound transmission through urine. It may act as an acoustic window to deeper structures in the abdomen, such as the uterus and medial iliac lymph nodes. The bladder wall normally is 1-2mm thick and may have a faint layered appearance. When almost empty, the bladder wall is thicker and slightly irregular on its mucosal surface. The proximal part of the urethra may be visible ultrasonographically if it is dilated, e.g., in dogs with prostatomegaly or urethral obstruction.

The ureterovesicular junctions are normally visible ultrasonographically as small convex structures on the dorsal aspect of the bladder mucosa. These can be identified consistently in dogs with normally positioned bladders; however, if the bladder neck is located in the pelvis it may not be possible to do an adequate examination by transabdominal ultrasonography.

Peristalsis of the distal ureters and flow of urine into the bladder ("ureteral jets") may be observed in two-dimensional, grey-scale ultrasound images as a series of small echogenic foci streaming periodically from the ureterovesicular junction. Ureteral jets are visible when there is a difference in the specific gravity of ureteral and bladder urine. For diagnostic purposes, the visibility of ureteral jets may be enhanced by filling the bladder with sterile water or saline of lower specific gravity than the ureteral urine. An alternative strategy that avoids the need for catheterisation involves allowing the dog to urinate, withholding water for several hours to ensure that bladder urine is concentrated, and then allowing access to water so that more dilute ureteral urine is produced. Administering a diuretic has a similar effect. Use of colour-flow Doppler ultrasonography is not necessary for visualising ureteral jets.

Cystitis or bladder neoplasia?

In animals with persistent signs of bladder disease, one of the key questions that must be answered is, "Are these signs associated with cystitis or bladder neoplasia?" The criteria used to answer this question are similar for ultrasonography and contrast radiography.

Animals with cystitis usually have

 Shallow thickening of the bladder mucosa

 May be diffuse or more marked at the cranial aspect

 Bladder may be small

 Calculi frequently present. Note that calculi always appear hyperechoic in ultrasound images and may cast an acoustic shadow depending on their size and composition.

Animals with bladder neoplasia often have

 Focal thickening of the bladder wall that protrudes into lumen

 Thickening in various locations, often near the bladder neck

 Signs of urinary obstruction, e.g., big bladder, dilated ureters

 Enlarged regional lymph nodes

Remember that other causes for thickening of the bladder wall include physiological thickening in an animal with nearly empty bladder, previous obstruction, cystotomy scar.

Ectopic ureters

Ultrasonographic diagnosis of ectopic ureter is possible based on absence of a ureteral jet and presence of a dilated ureter passing caudal to its normal site of insertion on the bladder. Although it is difficult to visualise normal ureters, approximately 50% ectopic ureters are dilated, which makes them more easily seen. Ureters exhibit peristalsis, which is visible in real-time images, and also aids their identification.

References

1.  Biller DS, et al. Diagnostic ultrasound of the urinary bladder. J Am Anim Hosp Assoc 1990;26:397-402.

2.  Lamb CR. Ultrasonography of the ureters. Vet Clin North Am: Small Anim Pract 1998;28:823-848.