Technique

Patient preparation is a key to obtaining a diagnostic excretory urogram. Food, but not water, should be withheld for at least 12 to 24 hours before the study. A cleansing enema can be used to evacuate the colon, preferably several hours before the study to allow fecal material and gas to pass. The patient should be hydrated and have adequate renal function. Because iodinated contrast medium leads to increased urine-specific gravity and may also inhibit bacterial growth for at least 24 hours, urinalysis should be performed before excretory urography.¹² Survey radiographs are always obtained before an excretory urogram to ensure correct radiographic technique and assess colon contents, but most important, small radiopaque calculi may be obscured by the contrast medium. Sedation or general anesthesia is recommended, and an intravenous catheter should be in place throughout the study to provide venous access if complications arise. Iodinated contrast medium is administered intravenously as a bolus injection at a dose of 600 to 700 mg iodine (I) per kg body weight.^13–14 Ventrodorsal and right lateral radiographs are acquired immediately after injection and are repeated typically after 5, 20, and 40 minutes or until a diagnosis is reached. Oblique projections for better visualization of the ureters can be added at 5 minutes and later time points. To evaluate the renal arteries, a ventrodorsal radiograph should be acquired 5 to 7 seconds after bolus injection of contrast medium.

Normal Imaging Findings

The renal arteries become opacified approximately 5 to 7 seconds after injection of a bolus of contrast medium. The excretory urogram can be divided into two phases: the nephrogram phase and pyelogram phase. In the nephrogram phase, contrast-medium arrival in the glomerular vessels and filtration into the nephron leads to uniform opacification of the renal parenchyma (Fig. 38-3, B). Initially, the cortex can be more opaque than the medulla. The nephrogram begins after approximately 10 seconds and lasts up to 2 minutes until the pyelogram phase starts and the nephrogram begins to fade. Nephrogram opacity should continue to decrease progressively; only approximately 25% of normal dogs still have a detectable nephrogram 2 hours after contrast medium injection.¹⁵ In the pyelogram phase, contrast medium is concentrated in renal tubules as a result of reabsorption of water and excreted into the renal pelvis with its diverticula, and the ureters (Fig. 38-3, C). If renal function is normal, the collecting system is consistently more opaque than the renal parenchyma. The normal renal pelvis is curvilinear and less than 2 mm in width. The pelvic diverticula may be seen in some dogs and most cats and appear as thin (1 mm) spikes radiating from the pelvis toward the periphery. Neither the renal pelvis nor diverticula should have blunted, rounded edges. The ureters tend not to be filled with contrast medium uniformly throughout their length because of ureteral peristalsis. Radiographs, therefore, may have to be repeated to assess the entire length of the ureters. Normal ureters are not more than 2 to 3 mm in width. They terminate at the dorsal aspect of the bladder neck, where they curve cranially for a short distance before entering the wall. Duration and intensity of the opacification of the kidney during an excretory urogram depends on the dose of contrast medium administered; renal perfusion; glomerular filtration; tubular reabsorption of water; the patient’s hydration status, which is important to maintain adequate renal perfusion; and patency of the renal outflow tract.¹⁴ Duration and degree of kidney opacification during an excretory urogram is an indication, although not very precise, of renal function and has to be monitored closely throughout the study, as described later under renal function.

Technique

Sedation or general anesthesia is necessary. The skin is prepared aseptically, and a 25-gauge needle connected with an extension set and three-way stopcock is introduced into the renal pelvis under ultrasound or fluoroscopic guidance. Depending on the size of the renal pelvis, 1 to 2 mL of urine is removed for laboratory testing. Ionic or nonionic iodinated contrast medium equivalent to half of the volume of urine removed is then introduced slowly, and the size of the renal pelvis is monitored with ultrasound or fluoroscopy.¹⁶ Occasionally, replacement of the entire volume of urine removed is needed. Overdistention of the renal pelvis should be avoided because rupture may occur. Immediately after contrast-medium injection, ventrodorsal, oblique, and lateral abdominal radiographs are acquired. If bilateral abnormalities are suspected, the procedure can be repeated on the contralateral side.

Normal Imaging Findings

The renal pelvis becomes distended with mild blunting of the pelvic diverticula, as contrast medium is introduced under mild pressure. If the ureter is patent, contrast medium should appear in the urinary bladder almost immediately (Fig. 38-4). Narrowing of the ureter may be observed because of peristalsis.

Technique

The highest frequency transducer that manages to penetrate adequately should be chosen to image the kidneys. In smaller dogs and cats, 7.5 to 15 MHz is appropriate; 5 MHz or lower frequency transducers should be used only in large dogs. Subcostal or intercostal windows with the patient in sternal or dorsal recumbency allow access to the kidneys, depending on patient size and thoracic conformation. Standard image planes include dorsal (Fig. 38-5, A), sagittal (Fig. 38-5, B), and transverse (Fig. 38-5, C) views, always followed by fanning the transducer through the entire kidney. The renal pelvis is visible best in dorsal and transverse image planes. The ureters can be followed caudally from the renal pelvis if dilated. To verify patency and normal location of the ureterovesical junction, the bladder should be evaluated for presence of two urine jets from the dorsolateral bladder wall. If not seen with conventional grayscale imaging, Doppler ultrasound can be used to better delineate the jet. In some animals, administration of a diuretic is necessary to increase urine production and alter the specific gravity of the urine produced compared with the urine already present in the bladder; this will increase the frequency and visibility of a ureteral jet.

Normal Imaging Findings

Normal canine and feline kidneys are outlined smoothly by a thin hyperechoic capsule and have a clear distinction between cortex and medulla (see Fig. 38-5). The renal cortex in most patients is hypo- to isoechoic to the liver and spleen, although dogs and cats without evidence of renal disease may have a renal cortex that is hyperechoic to liver.²⁷ In cats, cortical hyperechogenicity can be seen in association with fat deposition.²⁸

The normal medulla is very hypoechoic, creating a well-defined transition to the cortex. The interlobar vessels and pelvic diverticula give the renal medulla a lobulated appearance. The arcuate vessels are seen at the corticomedullary junction as short, hyperechoic parallel lines that may produce a distal shadow, not to be confused with renal mineralization. The renal pelvis is collapsed normally, but a smoothly margined, thin rim of fluid of up to 2 mm in diameter can be seen in normal animals, especially when using high-resolution transducers.²⁹ Increased diuresis, caused by exogenous fluid administration or polyuria/polydipsia, results in mild pyelectasia.²⁹ The pelvic diverticula are not visible normally. The renal pelvis is surrounded by the hyperechoic sinus, which contains dense fibrous tissue and fat, particularly in overweight cats.

Normal kidney size in cats ranges between 3.0 and 4.3 cm.30–31 In dogs, size is much more variable depending on body weight and conformation. As a rule of thumb, 10 mm per 10 pounds can be added within the normal range of 3 to 10 cm.³² A ratio of 5.5 to 9.1 of the maximal renal length to the luminal diameter of the aorta at the level of the kidney is proposed to take patient size into account.³³ Even using this method, the large range of normal values makes detection of subtle size changes challenging. Renal length, therefore, is not very helpful as an isolated criterion, unless markedly abnormal and all other ultrasonographic findings have to be taken into account to determine if a kidney is normal or not.

With Doppler ultrasound, the renal artery and vein, interlobar, arcuate, and interlobular vessels can be identified (Fig. 38-5, D). The resistive index should be less than 0.72 in dogs and 0.70 in cats, and the pulsatility index less than 1.52 in dogs and 1.29 in cats.³⁴ Normal ureters are visible only with high-resolution imaging systems, and ideal patient-related conditions. Ureters are best followed from a transverse view of the renal pelvis, as they turn medially and can be tortuous until they extend caudally, lateral to the aorta and caudal vena cava. Thin hyperechoic walls and occasional waves of peristalsis with a small bolus of fluid passing through can be seen in a normal ureter.

Technique

Imaging protocols always include a precontrast image series to identify presence of mineralization in the renal parenchyma or collecting system because mineralization cannot be differentiated from the hyperattenuating contrast medium. In most patients, intravenous contrast medium has to be administered (400 to 800 mg I/kg body weight) to outline renal parenchymal lesions and filling defects in the collecting system. The renal parenchyma is enhanced immediately after contrast-medium administration. Optimal ureteral opacification occurs 3 minutes post injection³⁵; however, image series may have to repeated several times to visualize the entire course of both ureters because of ureteral peristalsis. Ureteral opacification persists for about 1 hour.³⁵ Patient positioning in sternal recumbency with the pelvis elevated is helpful in outlining the ureterovesical junctions because the contrast medium will accumulate in the bladder apex away from the neck, facilitating the distinction between the contrast-medium–filled ureters and the bladder neck (Fig. 38-6). Filling the bladder with carbon dioxide increases the contrast between the bladder and ureters further; however, the balloon of the Foley catheter may distort anatomy and impair visibility of the ureteral openings. Dual-phase renal angiography can be performed to assess the arterial and venous blood supply of the kidneys separately. This is used mainly for presurgical assessment of feline kidney donors.^38–39

Normal Imaging Findings

Normal kidneys are soft tissue attenuating, except for a small amount of fat surrounding the renal pelvis. Uniform contrast enhancement is expected, followed by concentration of the contrast medium in the renal pelvis and diverticula, ureters, and urinary bladder (see Fig. 38-6).