LMNO pee: the ABCs of urinary ultrasonography (Proceedings)
Let's begin with the upper urinary tract – the kidneys and ureters. Knowing normal anatomy is of course initially necessary to perform an adequate ultrasound examination. You should always scan in two planes (sagittal and transverse). The right kidney is harder to visualize as it is located at the level of T13 and is located in the caudate fossa of the liver. The cortex is on the outside and the medulla is on the inside. The renal cortex should be homogeneous, moderately coarse with slightly less echogenicity than both the liver and spleen. The echogenicity is due to the abundance of collagen, fat and interstitial adventitia of the glomeruli. The renal cortex should always be hyperechoic to the medulla. The width of the cortex should be uniform, and occupy approximately 1/2 of the kidney diameter (cortex to medulla ratio is 1:1), and is <5mm in cats. The renal medulla is hypoechoic to anechoic. This is because there are only thin walled loops of Henle which contain urinary filtrate in this region. The medullary size can increase during periods of intense diuresis. The other clinically significant structures visible via ultrasound are the renal pelvis, diverticuli and proximal ureter. Vessels that are identifiable in the kidney and surrounding areas include aorta, caudal vena cava, renal arteries, arcuate arteries, and renal vein. 1/4 of dogs will have multiple vessels entering the left kidney. In the renal hilus can differentiate the ureter from renal artery and vein using Doppler (it has no flow). Arcuate arteries are seen as small hyperechoic, parallel lines at the corticomedullary junctions. The renal pelvis is located at the renal hilus, at the junction of the proximal ureter and is normally a potential space and if seen, is only a very thin, black line. The proximal most ureter can often be seen on transverse sections as it exits from the renal pelvis. The rest of the ureter is not seen normally. Feline kidneys are fairly uniform in size and measure 3.8 to 4.5 cm long and 2.7 to 3.1 cm wide. There are no normal sizes for canine kidneys as they vary greatly with breed and weight. Once we can identify these normal structures, it is easier to detect pathology by the change in apprearance of the kidneys. Calculi appear as focal, very hyperechoic structures in the renal pelvis or diverticuli and all (radiolucent and radiopaque stones) have posterior shadowing. Ureteroliths can be seen via US when hydroureter is present. Ureteroliths and ureteral stricture can both resulting in hydroureter. Ureteral strictures are not visible ultrasonographically. Renal cortical infarcts occur secondary to vascular occlusion. Initially (hours after vascular occlusion) they appear as hypoechoic foci in outer renal cortex. Once they become chronic, in weeks to months, they appear as well defined, triangular shaped (with the base of the triangle near the capsule), hyperechoic foci causing indentation of the renal capsular margin. Renal cysts are thin walled, round, variously sized anechoic foci with posterior enhancement. They are focal or multifocal throughout the cortex, usually do not cause capsular bulging, and are considered incidental findings in most dogs. However, inherited polycystic renal disease has been reported in Cairn Terriers as well as long haired cats. Perinephric pseudocysts are accumulations of anechoic fluid around one or both kidneys, most often between the renal capsule and the renal cortex. These are more common in cats. Though ultrasound guided centesis can be performed, the fluid reaccumulates in days. Urinomas appear similar to perinephric pseudocysts but the fluid around the kidney is urine which is felt to be due to traumatic extravasation from trauma to the kidney or ureter. Abscesses in the kideys are rare and their appearance changes with time. Initially they have an anechoic center then later on the material in the central portion of the abscess becomes more echogenic. Hematomas can occur in the kidneys and are usually secondary to trauma. Similar to abscess their appearance changes with time. They are initially complex and hypoechoic, become smaller and hyperechoic over time. Nephrocalcinosis (aka hypercalcemic nephropathy) is seen with paraneoplastic syndromes (such as lymphosarcoma and apocrine gland adenocarcinoma) and other conditions that may result in metastatic mineralization. In nephrocalcinosis, calcium deposits in the more metabolically active outer medullary region and appears as a <3mm thick, hyperechoic, band just inside and paralleling the corticomedullary junction. Despite it being mineral it typically does not have posterior shadowing. Ethylene Glycol Toxicity (antifreeze toxicity) has nearly pathognomonic ultrasonographic changes. The renal cortices are mildly to markedly, diffuse, and hyperechoic (can become almost totally white). The appearance is caused by the ethylene glycol crystals themselves. They reflect sound. In severe cases the medulla also may be affected. A hypoechoic rim at the corticomedullary junction ("halo sign") is considered a very poor prognostic indicator. Contrast media induced renal failure is usually identified initially radiographically as a failure to clear the nephrogram phase of an IVP. On ultrasound there is diffuse increased echogenicity of the renal medulla, beginning at the corticomedullary junction and progressing towards the renal crest. Pyelonephritis is commonly diagnosed. In this disease there is dilation of the renal pelvis (pyelectasia). In transverse view there is increased space between the renal crest and proximal ureter (widening of the renal pelvis) > 3mm. (Diuresis usually does not cause the degree of dilation that is seen with pyelonephritis.) The differentiation between hydronephrosis and pyelectasia is with hydronephrosis there is visible cortical atrophy (thinning). Hydronephrosis is more often caused by obstruction. Hydroureter is a continuation of the dilated renal pelvis into an anechoic, thin walled, torturous, tubular structure. Color flow Doppler can help distinguish between ureter and a blood vessel. If the hydroureter is severe, it can sometimes be followed caudally to the trigone/ ureterovesicular region. Chronic renal disease can be caused by glomerulonephritis, interstitial nephritis, FIP, diffuse infiltrative lymphosarcoma, amyloidosis, or "end stage" kidneys. Unfortunately ultrasound cannot determine the etiology of chronic renal disease. All of these conditions cause diffuse, increased echogenicity of the renal cortex with bilaterally small, misshapen kidneys. Neoplasia is uncommon in the kidney. However both primary and metastatic carcinomas (most common) as well as sarcomas can be seen. 50% of sarcomas are hyperechoic, while 50% are hypoechoic. The majority of carcinomas are hypoechoic. Most primary tumors appear as complex, mixed echogenic masses that are expansile (it may be hard to recognize it's a kidney). Most are poorly delineated from the adjacent normal renal parenchyma and they may have foci of mineralization. The most common neoplasia seen in feline kidneys is lymphosarcoma. This usually appears as bilaterally, enlarged, mixed echogenic to hypoechoic kidneys. Total derangement of the normal internal architecture is common. Sonographic accuracy of identifying a tumor increases with size of the nodule. Nodules need to be > 0.5 cm to be confidently seen. Biopsy is needed for confirmation of neoplasia. But ALWAYS remember to evaluate other abdominal organs for the presence of metastasis!
Next is the lower urinary tract - the urinary bladder. Because the urinary bladder is normal filled with anechoic urine, it is optimally designed for imaging with US. Best evaluation of the bladder is when it is moderately distended and with animal in dorsal recumbency. Ultrasound is used to evaluate size and shape of the urinary bladder, as well as the bladder contents and bladder wall