David S. Biller, DVM, DACVR
Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University
Normal Radiographic Anatomy of the Urinary Tract
The kidney is shaped like a bean in a dog and is much more rounded in appearance in a cat. The length which is measured on a ventrodorsal radiograph in a dog is 2.5-3.5 times the length of the 2nd lumbar vertebra. The length of the normal feline kidney on ventrodorsal radiograph is 2-3 times the length of the 2nd lumbar vertebra. The long axis of the kidneys on the ventrodorsal radiograph is parallel to that of the spine approximately 1/3 of the way between the spine and body wall. Kidneys are retroperitoneal structures and are generally surrounded by fat (this is what gives us the kidney is less mobile than the left kidney. The right kidney in the dog is usually cranial to that of the left and located at approximately T13-L2 (left is located at approximately L1-L3). In the cat the right kidney maybe cranial to the left or at approximately the same level. The right kidney in the cat is approximately located at L1-L3 and the left side at L1-L3 (sometimes at L2-L4). The kidneys are smooth and curved in margination. Their density is normally that of soft tissue (fluid) opacity, usually visualized because of the difference in opacity with the surrounding retroperitoneal fat.
The urinary bladder is divided into three anatomical parts including: the vertex, body and neck. The trigone (the dorsal neck area) is where the ureters insert into the bladder. In the female dog and in the cat the bladder normally has a longer, more gradual tapering neck than in the male dog. Therefore, the bladder is farther from the pelvic brim than in the male dog.
Survey radiographs yield minimally diagnostic information (the urethra is not visible in the normal animal on survey radiographs), except when radiopaque calculi are present, but should always be done prior to contrast urethrography. Contrast urethrography is indicated in all cases of suspected urethral disease.
Normal Appearance of the Kidneys with Excretory Urography
The kidneys will appear more opaque within 10-20 seconds following injection. This opacification should fade over the next 1-3 hours. The kidneys should demonstrate homogenous opacification. During the vascular phase (immediate phase post injection) the cortical area will appear more opaque than the medullary portion of the kidneys.
The renal pelvis and diverticula (collecting system) may be seen within a few minutes of injection of contrast material intravenously. The pelvis is thin, curved and funnel shaped. The diverticuli will appear as thin projections from the pelvis towards the periphery of the kidneys. There will be a central lucency within the diverticuli which will represent the interlobar vessels that travel in the same direction as the diverticuli from the central part of the kidney peripherally. The diverticuli of the cranial and caudal poles of the kidney may appear somewhat irregular in the normal animal. The pelvis and diverticuli should appear symmetrical within each kidney. They should appear smooth and sharply marginated.
Ureter is seen as a 2-3 mm tubular structure proximally with the remainder of the ureter slightly narrower. It courses from the renal pelvis to the bladder trigone. It is thin, retroperitoneal and curves in a cranioventral direction at the ureterovesicular junction on the lateral radiograph (trigone of the bladder). It will appear radiopaque. Usually the entire ureter on the single radiograph it not visualized due to normal peristalsis (which is responsible for getting urine from the collecting system of the kidney to the urinary bladder).
Abnormal Radiographic Appearance of the Urinary Tract
The abnormal appearance radiographically of the kidneys can be divided when considering shape and size into increased size normal shape, and abnormal shape. The other category which should be considered is decreased size abnormal shape.
The differentials for increased size normal shape include acute renal disease, hydronephrosis, infiltrative disease such as lymphosarcoma (neoplasia), feline infectious peritonitis/granulomatous nephritis, or amyloidosis. Compensatory hypertrophy and subcapsular fluid such as urine, hemorrhage, or perinephricpseudocysts should also be considered in this group. Abnormal shape increased size differentials should include: neoplasia, cysts (solitary or polycystic), abscess/granuloma, or hematoma. There may very well be overlap between groups because of difficulty evaluating margins.
Renal masses remain dorsal in the abdomen because of a tough retroperitoneal fascia preventing ventral migration. Masses associated with the right kidney usually cause medial and ventral displacement of the descending duodenum and ascending colon. Ventral and left displacement of the adjacent portion of the small intestine may occur. Left renal masses cause ventral and medial displacement of the descending colon and adjacent small intestine.
Decreased renal size and abnormal shape differentials usually include: chronic renal disease of many etiologies including infection. Other causes would be any type of disease which may cause infarct in the kidney, renal dysplasia or other congenital diseases such as hypoplasia. The change in the normal position of the kidney may change or be affected by adjacent organs or masses. Ectopic kidney may also be a cause for abnormal location of the kidney. There are numerous causes for irregular margination of the kidney including infarcts or scars, mass lesions, chronic infection/infarct, renal dysplasia/hypoplasia or all causes of end stage renal disease. Increased opacity of the kidney maybe caused by: renal calculi, nephrocalcinosis (mineralization of the renal parenchyma usually dystrophic, and toxic/ethylene glycol). Decreased opacities of the kidneys may occur secondary to gas within the collecting system (this is usually secondary to a reflux air from the lower urinary tract).
With introduction of positive contrast the kidneys normally demonstrate good initial opacification but with time this opacification decreases in intensity. With diseases such as poor renal function/decreased GFR the initial opacification is usually reduced. With diseases such as contrast induced renal failure or hypotension however the opacification may be progressive (increased) or be persistent. You may also notice that urine will not be produced nor will contrast be present within the urinary bladder.
With use of a contrast study of the kidney (excretory urography) the renal pelvis and diverticula can be well evaluated. They may be increased in size (hydronephrosis, pyelonephritis) with causes of obstruction or infection. The position of the collecting system may be deviated or distorted by mass lesions (neoplasia, cyst, abscess) or infiltrative disease in the kidney. Contrast excretory urography may also be excellent in the evaluation of the integrity of the collecting system (easily demonstrated ruptured kidney associated with trauma).
The ureter may become increased in size diffusely or focally. Differentials for increased size of the ureter or hydroureter may occur secondary to obstruction or infection. With chronicity the ureter may be not only dilated but tortuous. Focal dilatation of the ureter at the level of the bladder (ureterocele) may occur with obstruction at the ureterovesicular junction or ectopic ureter. The ureter is a retroperitoneal structure whose position may be affected by surrounding structures and masses. It may also be found to terminate abnormally in cases of ectopic ureter. With contrast its patency may be evaluated for obstruction and its integrity for rupture which may occur in trauma.
Abnormal Survey Radiographic Findings of the Urinary Bladder
The urinary bladder may subjectively appear enlarged and this is most likely due to obstruction (calculi, neoplasia, or congenital anomaly). Other causes of an enlarged urinary bladder should include neurologic, or conditional.
The inability to visualize urinary bladder may occur secondary to post voiding, displaced urinary bladder such as one that may be in a perineal hernia, rupture of the urinary bladder, peritoneal fluid, emaciated animal, or animals that are younger in age than 6 months. The normal bladder opacity is that of fluid or soft tissue. Increased urinary bladder opacity may occur secondary to calculi. The most opaque calculi in the urinary bladder are silicates followed by phosphates, oxalates, cystine and the least radiopaque is that of a urate (radiolucent). Another cause of increased bladder opacity may be calcification of the bladder wall itself (always make sure an area of mineralization can move in the lumen to differentiate calculi from mineralization of the wall). Causes of decreased bladder opacity include gas within the lumen which may be iatrogenic or gas within the wall which may be iatrogenic or the results of infection such as emphysematous cystitis.
Abnormal Radiographic Signs of the Urinary Bladder with Contrast Radiography
The normally distended bladder wall will appear approximately 1 mm thick. Lesions associated with the wall or filling defects that are free within the urinary bladder are best demonstrated with a double contrast cystogram. Intramural lesions which may increase the wall thickness may cause loss of distensibility. Filling defects that are free appear radiolucent when surrounded with positive contrast material. This is because the contrast material is more opaque than the filling defect. There are three free luminal filling defects present within the urinary bladder including: air bubbles, calculi, and blood clots. Air bubbles appear round, smooth, and are found at the periphery of the contrast pool. Calculi are round to slightly irregular, have indistinct borders, and are in the center of the contrast pool. Blood clot have an irregular shape, indistinct border, and can be found anywhere in the urinary bladder lumen.
Normal Urinary Tract Ultrasound Renal
The normal renal cortex appears homogeneously echogenic, similar or less than the liver in echogenicity. The renal medulla is hypoechoic (relative to the renal cortex) and in young animals may appear anechoic. Arcuate vessels at the cortical medullary junction serve as a marker to help separate the cortex from medulla. The renal medulla may appear as rounded hypoechoic structures (in coronal plane) separated by linear hyperechoic radiating (from central to peripheral) structures representing intralobar vessels and renal diverticuli. The renal sinus contains a collecting system (pelvis, diverticuli), renal vessels, lymphatics, fat, and fibrous tissue. The renal sinus is hyperechoic due to fat and fibrous tissue contained within. The renal pelvis is not normally appreciated unless dilated.
Abnormal Renal Ultrasound
Hydronephrosis is a dilatation of the renal collecting system (pelvic and diverticuli), associated with progressive atrophy of the kidney due to obstruction of urine outflow. There are numerous causes to hydronephrosis. Normally, there is no separation of the dense echo collection in the region of the renal sinus or collecting system. Minimal dilatation (approximately 1-3 mm) in both the dog and cat may be visualized with diuresis. The amount of collecting system dilation or hydronephrosis depends on parameters such as: duration of obstruction, and renal output. Hydronephrosis may be graded as to severity including mild, moderate, and severe.
Changes within the parenchyma of the kidney are usually nonspecific. Acutely no changes may be visualized. With chronicity increased echogenicity cortex with preservation of the corticomedullary junction may be noted. Increased echogenicity of the cortex and medulla may also occur. Other changes which may be noted include a kidney that becomes irregular with the increased echogenicity of the cortex and medulla and a loss of cortical medullary definition. Differentials for this diffuse change are numerous and include acute or chronic glomerulonephritis, acute tubular necrosis, ethylene glycol toxicity, pyelonephritis, renal dysplasia, and nephrosclerosis.
Abnormal Urinary Bladder Ultrasound
Cystic calculi are highly echogenic structures that demonstrate posterior acoustic shadowing. They tend to fall to the most dependent portion of the bladder when the patient is repositioned. Inflammatory disease of the urinary bladder will cause a diffuse or focal bladder wall thickening. With chronic inflammatory disease the bladder wall may become irregular. With chronic disease inflammatory polyps may also develop and appear as echogenic mass lesions. Echogenic structures protruding into the bladder lumen differentials should include not only inflammatory polyps but neoplasia. The accuracy of detection of neoplasia depends on the size and location of the mass lesion. Three different patterns have been demonstrated for neoplasia in the urinary bladder including: echogenic or complex focal filling defect arising from the inner surface of the bladder, massive tumor obliterating the bladder lumen, or infiltrating wall lesions producing thickening of the bladder wall.