Renal ultrasonography: kidneys big, small, and in-between (Proceedings)
Clipping the hair over the last 2-3 intercostal spaces and extending the area dorsally is important for complete visualization of the right kidney in the dog. The left kidney in a dog and both kidneys in a cat are located caudal to the rib cage and are generally easily identified with the animal in dorsal recumbency (lateral recumbency can also be used). A transducer with a small footprint may be necessary for intercostal imaging, while a linear or curvilinear can be used for subcostal imaging. Generally a 5-10 MHz transducer is adequate for most dogs. Higher frequency may be used for cats and small dogs for best image resolution. Scanning the kidneys completely in the longitudinal and transverse plane is important so lesions are not missed.
Ultrasound of the normal kidneyOverall kidney size in a dog can be assessed on ventrodorsal radiographs in comparison to the L2 vertebral body length (normal 2.5-3.5 times the length of L2). Because of the variable size of dogs a method using kidney length to aortic diameter may be used. The ratio of the length of the kidney compared to the aortic diameter at the level of the kidney (K/Ao ratio) is considered normal between 5.5 and 9.1. Normal measurements for the cat kidney range from 3-4.3 cm in length.
The kidneys should be symmetrical in size and shape (oval to bean-shaped).
Margins of the kidney should be smooth and there should be good corticomedullary definition. The renal cortex is more echogenic than the medulla. The renal cortex is less echoic than the spleen and similar to the liver in a dog. In cats the renal cortex can be more hyperechoic (similar to the spleen and greater than the liver). Linear echoic lines (interlobar vessels and diverticuli) divide the medulla to give it a lobulated appearance. At the corticomedullary junction the arcuate arteries may be seen as short, paired, hyperechoic lines.
The renal arteries and veins can be followed to the aorta and caudal vena cava, respectively. The renal pelvis can sometimes be seen in the normal animal measuring less than 2 mm. The ureter should not be seen in the normal animal. The renal sinus surrounding the pelvis contains fat, which gives it a hyperechoic appearance.
Ultrasound of big kidneys
Renomegaly can have numerous causes, many of which can be determined with ultrasound.
Neoplasia may be focal or diffuse. Focal lesions in dogs and cats may vary in echogenicity and would most commonly include renal carcinoma and lymphoma. Lymphoma often results in bilateral diffuse disease, particularly in cats. Multifocal nodules can also be seen with lymphoma. A hypoechoic rim of tissue at the periphery of the cortex is also commonly seen with lymphoma. Depending on the location of the disease there may or may not be changes to the margin.
Renal cysts are characterized by their round to oval shape. Cysts have anechoic contents, a thin wall, and exhibit distal acoustic enhancement. Large numbers of renal cysts, such as with polycystic kidney disease in cats, results in bilateral renomegaly with irregular margins. Focal cysts in dogs often are small; therefore, they do not cause enlargement or shape changes. Large cysts can cause enlargement.
Granulomas and abscesses in the kidneys are uncommon, but could cause focal or diffuse enlargement. Granulomatous disease, as with feline infectious peritonitis, commonly results in bilateral renomegaly in cats.
Ultrasound of small kidneys
Chronic renal disease of various etiologies will result in small, irregular kidneys with increased echogenicity and decreased corticomedullary definition. Often renal infarcts, characterized by hyperechoic and wedge-shaped areas in the renal cortex, will be present in chronic disease. Additionally it is not uncommon to see mineralization (nephrocalcinosis or nephroliths). The inciting cause of the disease usually cannot be determined in cases of chronic disease.
Congenital renal dysplasia results in small, irregular kidneys with increased echogenicity and decreased corticomedullary definition. Pylectasia may also be present.