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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.
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
Just like with radiographs, it is imperative to always scan in at least 2 planes. Adjusting the near gain is important to visualize the ventral most parts of the bladder. The normal bladder has a body, neck, and trigone. The trigone is the region where the ureters enter, called the ureterovesicular junction, in the dorsal caudal most bladder. The urethra exits the trigone. The normal urinary bladder is filled with anechoic urine with no floating debris. The bladder wall should be uniformly thin and smooth. Bladder wall thickness is dependent on the amount of luminal distention. The bladder wall thins with increasing amounts of urine. The normal bladder wall even when the lumen is empty should not exceed 5mm. When distended it is usually 1-2mm. Both the mucosal and serosal surfaces should be smooth. The bladder wall has 4 sonographically distinct layers, however all 4 are only seen when the bladder is not distended. The outside layer is the serosal layer and it is very hyperechoic. A slightly hypoechoic smooth muscle layer is under the serosa. Next is a slightly hyperechoic lamina propria layer and finally on the inside of the lumen is the hypoechocic mucosal surface. When the bladder is fully distended, only 3 layers can be seen. Very hyperechoic serosal layer (outside), hypoechoic submucosal layer (middle), and combined hyperechoic lamina propria/mucosal surface (inner). Normal ureters are not seen. Ureteral papillae (ureterovesicular junction) can be seen in some dogs and cats where the ureters enter the bladder in the dorsal aspect of the trigone. These will appear as focal tuft of tissue protruding into the lumen of the bladder. Careful inspection over this area will often yield "fountain bubbles" as urine is ejected from the ureter into the bladder lumen. These are most commonly seen when the patient is on lasix or have been administered contrast agents such as renograffin. Rarely, the proximal most urethra is seen just before it "hides" under the brim of the pubis.Pathology of the bladder can be divided into benign and malignant categories. Benign diseases include cystic calculi, gas in the bladder wall or lumen, blood clots, cathethers, sand like crystalluria, cystitis, bladder wall rupture, and ectopic ureters. Calculi are easily identified with ultrasound. US is nearly 100% accurate at detecting cystic calculi and is more accurate than radiographs. Serial US examinations can be used to monitor dissolution of stones. All calculi despite make-up cause posterior shadowing (even radiolucent stones will shadow). Reasons that calculi may be difficult to visualize with ultrasound include very small size, thick viscous debris in the bladder, and if the stone is not in the focal zone.
Though you cannot determine the type of stone with ultrasound, silicate calculi are often spiculated and adhere to the bladder wall. Sonographically stones appear as curved, intensely hyperechoic foci with clean distal shadowing that are usually in the dependent portion of the bladder. They are usually freely moveable. Gas can occur in the bladder lumen (usually iatrogenic due to cystocentesis) or in the wall of the bladder. Gas is intensely hypererchoic in the non-dependent portion of the bladder with dirty distal shadowing. Gas in the bladder wall (emphysematous cystitis) can be associated with diabetes mellitus. Blood clots are commonly seen as sequellae to chronic UTI. They are irregularly shaped, sharply marginated, variously sized heteroechoic masses within the urinary bladder that can be either free floating or attached to the bladder wall (causing them to appear similar to a tumor). An indwelling urinary catheter will appear as hyperechoic parallel lines within the bladder ("railroad tracts"). Crystalluria (aka sand) is really very small calculi that can sometimes appear as free floating debris within the anechoic urine. Sand will settle to the most dependent portion of the bladder where it can appear as a solid stone, until shaken up, and then it is similar to "snow falling". Cystitis is more common in females. Ultrasonographically is seen as a focal cranioventral or diffuse thickening of the bladder wall. The thickening initially is hypoechoic, undulating with an irregular mucosal margin but the wall can become more hyperechoic as the disease becomes chronic due to fibrosis. There is usually concurrent floating cellular debris within the lumen. Urinary bladder rupture can be missed on ultrasound as the bladder can appear normal and even distended with urine. However one can also see a focal bladder wall thickening. Free fluid in the peritoneal space can be seen. Perform a creatinine on it to determine if its urine. Alternatively you could perform a bubble study by injecting agitated saline through a urinary catheter to see if the bubbles end up in the peritoneal space.
Ectopic ureters may be detected by carefully looking at both sides of the trigone area for the "pulsating ureteral fountains", yet this is very dependent on the sonographer's ability and the dog's cooperation. Color flow doppler may increase accuracy. An IVP probably is still the diagnostic test of choice. Common neoplastic diseases in the urinary system include transitional cell carcinoma and botryoid rhabdomyosarcoma. Transitional cell carcinoma is typically located in the trigone. The masses are well marginated, mixed echogenic with an irregular shape. The normal parallel pattern of the bladder wall cannot be discerned. Transitional cell carcinoma can occur anywhere along the bladder wall or in the urethra. The kidneys should always be examined for evidence of pyelectasia due to ureteral obstruction. Tumors less than 5mm in size are more difficult to see, especially when the bladder is not distended, or if the lesion is in the caudal most part of the bladder (proximal urethra). If need be, distend the bladder with saline, via a catheter, A less common tumor in the bladder is botryoid rhabdomyosarcoma. This appears as a very complex, hypoechoic mass which has been described as a "bunch of grapes". It is more common in young animals (<1 yr.)