Imaging of the Urinary System
Martha Moon, DVM, MS, Dipl.ACVR
The kidneys are fairly easily visualized on survey abdominal radiographs as long as there is sufficient retroperitoneal fat for contrast. Renal size, location, and occasionally shape can often be determined without other imaging procedures. Renal size in the cat (measured on the VD radiograph) varies from 2.4-3.0 times the length of L-2 (second lumbar vertebral body). However, it is fairly common for older cats (over 10 years of age) to have kidneys that range from 2.0-2.4 times the length of L-2, without any clinical signs, or hematological changes that might indicate renal disease. In the dog, renal size should be 2.5-3.5 times the length of L2. Mineralized foci in the renal pelvis usually indicates the presence of calculi. Occasionally linear calcifications in the area of the diverticuli are seen, especially in older animals. Nephrocalcinosis ( diffuse mineralization of the renal parenchyma) can be seen with calcium and phosphorus disorders such as hyperparathyroidism, or dietary abnormalities. Additional imaging techniques such as excretory urography or ultrasound are usually needed to achieve a more definitive diagnosis, and in many cases, a biopsy eventually is necessary.
Several intravenous organic iodinated contrast agents are available. They are composed of an anion (usually iothalamate or diatrizoate) and a cation, either sodium, meglumine, or a mixture of the two. The contrast agents differ also in their concentration of iodine. Despite the large number of products on the market, there is little practical difference once the iodine concentration has been decided on. All of these contrast agents are hypertonic, and while reactions are rare and usually mild, they may occasionally be fatal. The most common reaction is vomiting, usually seen during a rapid bolus injection of contrast material. More severe reactions, including urticaria (hives) and bronchospasm may occur. Hypotension, cardiac, and respiratory arrest are rare, but have been reported. The kidneys can also have adverse reactions to the contrast agent, both directly, and indirectly as a result of hypotension. Acute oliguria/anuria may occur following intravenous contrast administration. This is seen more commonly with dehydration, therefore all patients should be well hydrated prior to the study. Azotemia is not a contraindication for intravenous urography (IVU) if the patient is well hydrated. In fact, there is no correlation between the BUN and the quality of the study, unless urine specific gravity is low. In people, IVU is contraindicated in cases of diabetes mellitus, multiple myeloma, and combined renal and hepatic failure. Although not substantiated in the veterinary literature, it may be wise to avoid intravenous contrast agents in patients with these conditions. Nonionic intravenous contrast agents (such as those used for myelography) are available, and may cause fewer reactions. However, they are not used routinely for intravenous studies in veterinary medicine because of the greater cost.
The nephrogram phase occurs when contrast is present in the renal tubules and vasculature, in the first 3 minutes post-injection. This phase allows good visualization of renal size, shape, and location, and is an indication of renal perfusion. The kidneys should have a dense, uniform opacification, which starts to fade as contrast filters into the collecting system of the kidney. This initiates the pyelogram phase, where contrast is visible in the renal pelvis, diverticuli, and ureters. The pyelogram phase starts within 3 minutes, and may persist for more than 2 hours in the normal dog and cat. Normally, the renal pelvis is visualized as a thin (less than 2mm diameter) curvilinear structure with well defined, paired diverticuli branching away into the parenchyma. The diverticuli may not be visualized in some normal dogs without abdominal compression; they are usually better visualized in the cat. The ureters are seen as narrow (less than 2mm diameter) linear structures extending through the retroperitoneal space to enter the trigone of the bladder. They may curve cranially as they enter the bladder, but are otherwise fairly straight. The entire ureter is rarely visualized on a single film as peristaltic contractions will decrease visualization of some portions.
Abnormalities can occur in all phases of the IVU. Abnormalities in the nephrogram phase include increased or decreased opacity, as well as persistent opacity of the kidney. A faint nephrogram may indicate an inadequate contrast dose, or chronic renal disease (inability to concentrate). A persistently dense nephrogram without a subsequent pyelogram is seen with acute renal failure, hypotension, or acute obstruction. An irregular or mottled pattern of opacification may indicate avascular structures such as cysts, or some tumors. The nephrogram is also useful for visualization of changes in renal size and shape. Infarcts, tumors and congenital anomalies may be visualized during this phase.
Abnormalities in the renal pelvis, diverticuli, and ureters are best visualized during the pyelogram phase. Dilation of the pelvis and diverticuli (hydronephrosis) can occur with obstruction in the ureter or at the bladder trigone. Rupture of the ureter can cause a functional obstruction, with subsequent proximal dilatation. Chronic pyelonephritis usually results in a slightly dilated pelvis, with blunted, distorted diverticuli. The proximal ureter is often dilated, and the kidney may appear small and irregular. Filling defects within the contrast in the renal pelvis can be caused by calculi, tumors, and cellular debris. Air bubbles within the pelvis are seen occasionally if they have passed up the ureters from an air filled bladder.
Ultrasound examination of the kidney
The renal cortex is uniformly echoic, usually hypo- or isoechoic compared to the liver (the right kidney is easily compared to the caudate liver lobe). In some normal cats, and dogs, the renal cortex may be hyperechoic to the liver due to fat deposition. The echogenicity of the renal cortex is enhanced when using high frequency (7 MHz or higher) transducers, so the normal kidney may be hyperechoic to the liver. The renal cortex should be hypoechoic to the spleen (the left kidney is used for comparison), however, and there should be a clear demarcation between cortex and medulla. The renal medulla is anechoic to hypoechoic, and is separated into compartments by linear hyperechoic interlobar vessels and diverticulawhich radiate from the renal sinus. The renal sinus is hyperechoic and centrally located at the renal hilus (medial). The hyperechogenicity is due to peri-pelvic fat. High intensity echoes can be seen at the corticomedullary interface, and in the renal sinus, and are produced by reflective fibrous or fat interfaces in the pelvic diverticula, and vasculature. In cats, 3.0--4.4cm is considered the normal renal length. In dogs, because of breed variation, renal measurements vary considerably. Diuresis may cause slight dilation of the renal pelvis. Ureters are not normally visualized.
Abnormal renal ultrasound
Diffuse renal disease; if severe, will have increased cortical and possibly medullary echogenicity. Differentials include; glomerulonephritis/interstitial nephritis, pyelonephritis, endstage renal disease, congenital renal dysplasia, and nephrocalcinosis. FIP, LSA, and ethylene glycol toxicity may cause enlargement as well as hyperechogenicity
Renal neoplasia may occur as diffuse enlargement of the kidney, or as a focal mass lesion.
Acute renal infarcts are typically hypoechoic, although they increase in echogenicity with age. They may have a typical wedge shape, with the base towards the capsule, and apex pointing towards the medulla.
Hydronephrosis results in a separation of the normally hyperechoic central sinus echoes by an anechoic space. With severe hydronephrosis, the renal parenchyma becomes compressed into a thin layer around an anechoic, dilated renal pelvis.
Pyelonephritis causes a mild-to-moderate pelvic dilation (pyelectasia), not as marked as in hydronephrosis. There may also be increased cortical echogenicity , and loss of the cortico-medullary definition if the disease is chronic. Acute pyelonephritis may cause no changes
Renal calculican usually be easily visualized, even if the calculi are radiolucent radiographically. The calculi are highly echogenic, often with acoustic shadowing. Occasionally, pelvic dilation will be associated with the calculus if obstruction is present.
Imaging techniques of the bladder
The urinary bladder is usually well visualized on survey abdominal radiographs unless the bladder is empty, or the patient lacks abdominal contrast. Because of its distensible nature, bladder size is variable. The bladder is soft tissue opacity in the normal animal. Radiopaque calculi can be visualized in the dependent portions (middle) of the bladder. Dystrophic calcification of the bladder wall may be secondary to chronic inflammation or neoplastic disease. Air is not normally present within the bladder, although air bubbles occasionally are seen after bladder catheterization or cystocentesis. Emphysematous cystitis results in gas within the bladder wall. This is reported most commonly with diabetic patients.
The bladder wall should be thin and uniform, approximately 1-2mm thick. The mucosal surface should also be smooth and uniform. On a double contrast study, the positive contrast puddle should be well defined, with uniform margins and density. Occasionally, reflux of contrast, either positive or negative can be seen extending up the ureters and into the renal pelvis. This can occur in young patients, or with overdistension, and is not considered pathologic. However, in patients with cystitis, this may predispose to pyelonephritis. It should be remembered that bladder wall changes have to be fairly advanced before changes will be visible on a cystogram. Therefore, a normal cystogram does not rule out bladder disease.
Contrast material is visualized free in the peritoneum after bladder rupture. If only a small bladder leak is present, the bladder must be fully distended with contrast material before the site of the rupture is visible. If a rupture is suspected, but not visible on a positive contrast study, a ureteral or urethral rupture should be considered. Occasionally, contrast can be seen within the bladder wall and subserosa, appearing to extend into the peritoneum adjacent to the bladder. This is has been reported after maximum bladder distension, but will also occur in cats with minimal bladder distension, and does not usually result in a clinical problem.
Cystitis, especially chronic or severe, causes thickening of the bladder wall, primarily the cranio-ventral aspect. In some cases the entire bladder wall is affected. Neoplasia can have a similar appearance, and may require a biopsy to differentiate from severe cystitis. In most cases however, bladder tumors are seen as focal (single or multiple) mass lesions involving the bladder wall. If the mass involves the trigone area, an intravenous urogram is helpful for detection of ureteral involvement and obstruction.
For intraluminal filling defects, double contrast cystography is best. Calculi are visible as focal, slightly irregular filling defects in the middle of the positive contrast puddle. Air bubbles have smooth margins and tend to accumulate at the periphery of the contrast puddle. Blood clots are seen as irregular filling defects of variable size, usually mobile, and falling into the middle of the contrast puddle.
Ultrasound examination of the urinary bladder
The bladder lumen is usually anechoic with acoustic enhancement, but occasionally contains sediment which creates intraluminal echoes. Normal bladder wall thickness is no more than 3 mm when distended, but may appear thickened with less complete bladder distension. The colon may impinge on the bladder causing distortion, and can mimic wall masses. The urethra is inconsistently evaluated.
Abnormal bladder ultrasound
Calculi are highly echogenic, usually with acoustic shadowing, even if radiolucent. They tend to be mobile, and will fall to the dependent side of the bladder. Occasionally crystalline urine sediment will accumulate in the dependent bladder, and cause shadowing. This sediment is usually easily resuspended by bladder agitation.
Blood clots are usually irregular, hyperechoic masses which may be mobile or adhered to the bladder wall. When adhered, blood clots are difficult to differentiate from bladder tumors on ultrasound exam alone.
Cystitis may cause thickening or irregularity of the mucosa, mainly involving the cranioventral bladder wall. Blood clots may be associated with cystitis, as are calculi and echogenic urine sediment.
Neoplasia can have a variable appearance. Tumor masses may project into the lumen, or cause diffuse irregular thickening of the bladder wall. Tumor masses are usually a complex (mixed) echogenicity. Invasive tumors may have little demarcation between normal and abnormal areas. Neoplasia must be differentiated from severe cystitis and blood clots. Biopsies are usually required. It is important in cases of suspected bladder neoplasia to check the sublumbar area for enlarged lymph nodes.
Bladder rupture is not reliably diagnosed on ultrasound exam alone, although the presence of free abdominal fluid is easily seen. A positive contrast cystogram is more reliable for the diagnosis.