Ultrasonography of the kidneys is part of a complete examination of the abdomen. Renal ultrasound is often indicated, when abnormal size and shape of the kidneys are palpated or abnormal shape, size, and/or opacity are seen on radiographs. In addition, abnormalities in the renal parameters in serum chemistry and urinalysis call for renal ultrasound.
The kidneys are scanned on longitudinal, transverse, and dorsal planes. The left kidney is located caudal to the stomach and dorsal and/or medial to the spleen. Sometimes, the spleen is used as an ultrasound window to visualize the left kidney. The right kidney is best found with a subcostal approach. When an ultrasonographer is moving a transducer, there is a tendency to angle the beam medially. It is important to keep a right angle to the table so that you can scan the lateral and dorsal areas of the abdomen. You may need to move the transducer laterally or angle the transducer laterally to visualize the area of kidneys. When small and large intestinal luminal gas prevents visualization of the right kidney, an intercostal approach should be used. Then, clipping of hair should be extended more dorsally.
Cortex and medulla of the kidneys should be appreciated. The medulla has hypoechoic round appearance, and the cortex surrounds the medulla. Between medulla, slightly hyperechoic bands are seen. At the center, the renal sinus or pelvis is seen as hyperechoic bands due to fat accumulation. The renal borders should be smooth. A normal range of the size of canine kidneys is summarized below. The normal range of feline kidney length is 30 to 43 mm.
Normal Renal Size in Dogs
5–9 kg body weight
Ultrasound abnormalities of the kidneys can be divided into changes in the renal collecting system, the cortex, and the perinephric area.
Renal collecting system. A common abnormality in the renal collecting system is the presence of renal calculi. However, differentiation between renal calculi and nephrocalcinosis may be difficult. Thus, radiographic assessment of the changes is helpful. The renal sinus contains fat. It may show acoustic shadowing when you use a high frequency (7.5 or 10 MHz) transducer. Again, radiographs should be evaluated for presence of mineralized opacities.
Another problem in the collecting system is hydronephrosis. It is caused by chronic partial obstruction of the ureter. Due to its guarded prognosis, transitional carcinoma of the trigone area is a significant cause of partial obstruction. Unfortunately, ultrasound cannot evaluate function of the kidney, and excretory urography may be indicated. A recent report proved that the renal pelvis would dilate with fluid therapy. In this case, the pelvis width should be less than 2 mm in my experience. Dogs with PU/PD may show a similar finding.
Pyelonephritis can cause dilatation of the renal pelvis. In an acute phase, the kidney may be enlarged. In chronic cases, the corticomedullary junction becomes indistinct. In addition, the kidney may be small and irregular.
Cortical changes. Cortical changes can be divided into diffuse or focal/multifocal. A diffuse change is most likely more hyperechoic than normal. You compare echogenecity of the cortex to the spleen and liver. Sometimes, you subjectively call it a hyperechoic cortex based on comparison to the medulla. In either case, the interpretation is very subjective. In addition, you need to consider abnormalities in the liver and spleen.
Differential diagnoses for hyperechoic renal cortex include amyloidosis, feline infectious peritonitis, glomerulonephritis, renal dysplasia (usually the architecture is not recognizable), ethylene glycol toxicity, lymphosarcoma, and end-stage kidney (again, the architecture is not recognizable).
Renal cysts are probably not clinically significant in dogs. In cats, when multiple cysts are noted, polycystic kidney disease should be considered. Familial transmission is proven in Persian cats.
Renal infarct is probably not clinically significant. It is usually seen as a wedge-shaped area in the cortex. In an acute phase, it is hypoechoic to the rest of the cortex, while in a chronic stage, it becomes hyperechoic. With chronicity, the area shows indentation (scar formation).
Neoplasia of the kidney is uncommon. When image quality is not optimal, the medulla of the cranial and caudal poles may look like a hypoechoic mass. In my experience, renal neoplasia shows mixed echogenecity and usually bulges out.
Perinephric area. Perinephric changes are occasionally seen. Usually they appear as a hypoechoic to anechoic area surrounding the cortex. It can be caused by hemorrhage, urine accumulation, or cellular infiltrates by neoplasia, such as lymphoma. Fine needle aspirates of abnormal areas may yield a cytological diagnosis.
Ureters. Ureters are not normally visible. However, when the ureter is distended, you may be able to follow the ureter from its proximal end.
The Urinary Bladder
Although ultrasonography provides us with excellent tomographic images of the urinary bladder and the sublumbar area, it should be considered as a supplemental study to radiography. Advantages of ultrasonography include evaluation of kidneys and urinary bladder in the same study. Indications include hematuria, dysuria, and abnormal urine cytology (e.g., transitional cell carcinoma). In addition, sublumbar lymphadenopathy is readily detected with ultrasonography.
Since the urinary bladder is located against the ventral abdominal wall, the ventral wall of the urinary bladder may be difficult to see due to the near field artifacts. Use of a stand-off pad will be helpful. Longitudinal and transverse views are evaluated. On the transverse view, bifurcation of abdominal aorta and caudal vena cava is in the area of sublumbar lymph nodes. Prostate and uterus are also evaluated in two planes.
The urinary bladder is an anechoic sac. It has a smooth mucosal layer. The wall thickness should be less than 3 mm. However, when the urinary bladder is not well distended, the results of measurement should be cautiously interpreted. Sometimes, the vesicoureteral junction is visible.
Cystic calculi are seen as hyperechoic structures with strong acoustic shadowing. Radiopaque as well as non-radiopaque calculi are detected with ultrasound. Gas in the colon may cause a hyperechoic zone with acoustic shadowing in the dorsal border of the urinary bladder. Changing the position of the animal to see if echogenic structures move, is helpful to distinguish gas from calculi.
Cystitis is a relatively common problem in dogs. Bacterial infection should be considered. The wall of the urinary bladder is thickened. Since the most thickened area is in the cranioventral wall, you may need to use a stand-off pad to better visualize the area. Polypoid cystitis is a rare condition. The mucosal layer is severely affected. The layer is irregular and cobblestone-like. Unfortunately, this appearance can also be caused by transitional cell carcinoma.
Transitional cell carcinoma is the most common urinary bladder neoplasm. Usually, it affects the dorsocaudal area (neck). If it is severe, the distal ureter may be partially occluded. Then, hydronephrosis is seen. There is no “typical” appearance. However, visualization of sublumbar lymphadenopathy is helpful in determining aggressiveness. On radiographs, you may find vertebral involvement with metastases. Fine needle aspiration is sometimes performed. However, a chance of seeding may be high. We now use a catheter placed in the urinary bladder to obtain cytology samples. By moving the catheter while applying suction, it is possible to cut a small piece of mucosa.
The Prostate Gland
Ultrasound study of the prostate is indicated when irregular, asymmetric prostatomegaly is palpated. In addition, a radiographically enlarged prostate should be examined by ultrasound to determine its internal architecture.
The prostate gland is evaluated in at least two planes: longitudinal and transverse. Echo texture and contour (symmetrical lobes) are evaluated. A transducer needs to be directed toward the tail and pushed in front of the pubis to better image the prostate. The prostate is a bi-lobed organ. Its size is subjectively evaluated. It should be slightly hyperechoic to the surrounding fat. The urethra may be seen as a slightly hypoechoic area at the midline on transverse views.
An abscess usually shows asymmetrical enlargement of the prostate gland. It is anechoic to hypoechoic with irregular borders. However, the appearance is similar to neoplasia. Fine needle aspirate of a content of the abscess may be useful.
Multiple cavitary areas are often seen with prostatitis. The prostate gland is enlarged. The cavitary areas may appear tubular, suggesting dilation of the prostatic ducts.
Benign Prostatic Hyperplasia
Benign hyperplasia is usually seen as a smooth, symmetrical enlarged gland. Aged intact males commonly show the change. The testes should be evaluated for presence of a hypoechoic or hyperechoic mass. Testicular neoplasia is often found in cases of prostatomegaly.
Usually, neoplasia is seen in an enlarged gland. However, it may occur in a castrated male, and the gland may not be enlarged. It is usually seen as a focal hypoechoic nodule. It is difficult to distinguish neoplasia from other prostatic abnormalities. Fine needle aspirate may be indicated, but clients should be well informed that seeding of the neoplasia might occur with the procedure.
Paraprostatic cysts can be seen as an additional fluid filled sac, commonly next to the urinary bladder. The prostate gland may not show severe morphological changes.
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