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Imaging of Calculi of the Urinary System

Daniel VanderHart, DVM, DACVR

Imaging

|February 2017|Peer Reviewed|Web-Exclusive

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Urinary tract calculi are well documented in companion animals, and diagnostic imaging is important for detection. Radiography has long been the mainstay for detection of mineral opaque urinary calculi, but ultrasonography has gained in usefulness for both detection and better tissue characterization of the disease.1,2

Positive and negative contrast studies can provide additional, often valuable, information to aid in diagnostic evaluation of the urinary tract.

Nephrolithiasis and Ureterolithiasis

Identification of the normal appearance of the kidney on radiography and ultrasonography is critical prior to evaluation for abnormalities (Figure 1). In addition, several potential artifacts—or “pitfalls”—must be taken into account when radiographically or ultrasonographically evaluating the upper urinary system in dogs and cats (Figure 2). 

Calculi within the kidney or ureter present diagnostic and management challenges. It is important to recognize renal calculi and differentiate them from dystrophic mineralization of the renal diverticula. While both dystrophic mineralization and calculi can be incidental findings, the presence of calculi can result in further complications. Calculi can obstruct the renal pelvis or ureter, predispose to pyelonephritis, or result in compressive injury of the renal parenchyma leading to progressive chronic kidney disease.3

Renal calculi can vary markedly in size, number, shape, and opacity (Figure 3). Dystrophic mineralization of the renal parenchyma, often associated with the collecting system, is another differential for mineralization localized to the kidney (Figure 4). A combination of radiography, positive contrast radiography (excretory urography), and ultrasonography has been shown to have an increased sensitivity for the diagnosis of ureteral calculi when compared with ultrasonography alone (Figure 5).4 

Ureteral calculi are a common cause of ureteral obstruction; localization of ureteral calculi is imperative prior to choosing appropriate management. While ureteral calculi have been reported as the most common cause of ureteral obstructions, other causes such as iatrogenic ligation, blood clots, tumor, strictures (congenital and acquired), solidified blood stones, and a circumcaval ureter have been reported.4-7 

The obstruction can be located at any point of the ureter and can vary in severity. Normal ureters are typically not seen on ultrasonography due to their small size. The easiest way to locate a dilated ureter is to trace the ureter from the renal pelvis. In most cases, the ureter is dilated proximal to the site of an obstruction and tapers to a more normal appearance distal to the site of obstruction. Imaging can also reveal retroperitoneal effusion which can result from ureteritis and possible urine leakage.

Potential indications for removal of renal calculi in dogs include obstruction, recurrent infection, progressive calculi enlargement, presence of clinical signs (renal pain), and patients with calculi in a solitary functional kidney.8 The most common indication for removal of calculi in cats is obstruction caused by ureteral calculi.9 

Nonobstructive renal calculi in cats is not typically treated unless the obstruction moves into the ureter and causes ureteral obstruction. Nonobstructive renal calculi usually have minimal impact on progression of chronic renal disease in cats, so the presence of calculi alone is not justification for treatment in cats.10

Clinical signs associated with ureteral calculi may range from chronic non-specific signs to acute or chronic renal failure. The presence of hydronephrosis can be highly suggestive of a ureteral obstruction (Figure 6). 

Differentiation between a complete vs partial ureteral obstruction can be difficult with survey radiography and ultrasonography alone. Antegrade pyelography (nephropyelocentesis with renal pelvic injection of iodinated positive contrast medium using ultrasound guidance) may be useful for documenting a complete vs partial obstruction (Figure 7).11 

Antegrade pyelography is beneficial when compared with standard IV urography, as it lowers the risk of potential contrast-induced renal damage and provides excellent filling of the renal collecting system, regardless of renal function.11

Calculi of the Urinary Bladder and Urethra

Calculi formation within the urinary bladder occurs from a variety of inherited and environmental conditions, and management is often imperative.7 Radiographic detection of urinary bladder calculi has up to 87% sensitivity with improvement to 95% when used in combination with positive-, negative-, or double-contrast cystography.15

The lower detection rates for survey radiography are largely due to the variations in the chemical composition of different calculi. The most common types of calculi in small animal practice are struvite and calcium oxalate; both are mineral opaque (Figure 8).13,14,16,17

Cystine and urate calculi are less common overall, but more common in bulldog and dalmatian breeds, respectively. These are often non-mineral opaque and are unable to be visualized with survey radiography alone.18 One helpful mnemonic for remembering the non-mineral opaque calculi is “I can’t C U.” Cysteine and urate calculi cannot be visualized as they often do not mineralize. For non-mineral opaque calculi, contrast cystography or ultrasonography will aid in detection (Figure 9). 

Urinary bladder calculi can also descend into the urethra, which can potentially lead to urethral obstruction. In male dogs, it is imperative that the entire urethra is included within an additional radiographic image using the appropriate collimation (Figure 10). 

A separate center of ossification associated with the os penis may mimic a urethral calculus (Figure 11 A and B). A separate center of ossification can be seen at either end of the os penis and will be in line with the os penis. A calculus within the penile urethra would be seen ventral to the os penis in the location of the urethra. Contrast urethrography can be a helpful way to differentiate between a separate center of ossification and urethral calculus (Figure 11C). 

Conclusion

Radiography and ultrasonography can provide information related to anatomic changes that are present within the kidneys. This includes changes in renal size, shape, and margination. Ultrasonography can help identify and differentiate between the presence of renal dystrophic mineralization and renal calculi. Contrast radiography can also supplement these imaging modalities for visualization of obstructions of the ureters or urethra or the presence of non-radiopaque (non-mineralized) urocystoliths.

References and Author Information

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