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40 IMAGING ESSENTIALS PEER REVIEWED Caution should be used when interpreting the medullary rim sign; it should not be considered an accurate indicator of renal disease, although it could represent sentinel signs of early renal disease or past renal insult. 27 It has also been detected in clinically normal dogs and cats. 27,28 Chronic Interstitial Nephritis Kidneys become small, irregular, and diffusely hyperechoic from fibrosis. 16 When kidneys are diffusely hyperechoic, the corticomedullary demarcation becomes difficult to delineate. Capsular or Pericapsular Diseases Perirenal or Subcapsular Fluid The differential diagnosis for perirenal or subcapsular fluid includes: • Urine leakage 29 • Hemorrhage 30 • Abscess 31 • Acute renal failure 29 • Ureteral obstruction 29 • Ethylene glycol toxicosis 29 • Leptospirosis 29 • Amyloidosis • Cryptococcosis Perinephric (Perirenal) Pseudocysts Ultrasonographically, perirenal pseudocysts appear as an accumulation of anechoic fluid around one or both kidneys, most commonly between the capsule and the renal cortex ( Figure 10 ). 32 Subscapular perirenal pseudocysts are formed by accumulation of a transudate between the capsule and parenchyma of the kidney due to underlying parenchymal disease. 32 This is more common in cats. Abnormalities of the Renal Pelvis and Proximal Ureter Pyelectasia Pyelectasia is a dilation of the renal pelvis ( Figure 11 ). 33 Pyelectasia may be caused by intravenous fluid administration, diuretic administration, increased diuresis from renal insufficiency, distended urinary bladder or lower urinary tract obstruction, pyelonephritis, ureteritis, ectopic ureter, or another congenital malformation. Pyelonephritis Pyelonephritis can affect one or both kidneys and is usually a result of ascending ureteral infection TABLE 1 Differential Diagnoses of Diffuse Renal Cortical and Medullary Hyperechogenicity DIFFERENTIAL DIAGNOSIS IN DOGS DIFFERENTIAL DIAGNOSIS IN CATS Cortical hyperechogenicity • Interstitial or glomerular nephritis • Pyelonephritis 24 • Leptospirosis • Acute tubular necrosis • Ethylene glycol toxicosis 25 • End-stage renal disease • Nephrocalcinosis • Interstitial or glomerular nephritis • Ethylene glycol toxicosis 25 • Diffuse renal lymphoma ( Figure 8 ) • Feline infectious peritonitis • Cryptococcosis • Amyloidosis Cortical and medullary hyperechogenicity (reduced corticomedullary distinction) • Congenital renal dysplasia • Juvenile boxer nephropathy 26 • Chronic renal disease • End-stage kidney disease • Congenital renal dysplasia • Chronic renal disease • End-stage kidney disease FIGURE 10. Long axis sagittal image of the right kidney in a domestic shorthaired cat. The kidney is abnormally shaped, lobulated, and contains multiple hyperechoic, wedge-shaped defects within its cortical margin. The diverticuli contains multiple ovoid, hyperechoic foci (black arrowheads) with distal acoustic shadowing. There is a large amount of anechoic fluid surrounding the kidney, consistent with a perinephric pseudocyst (white arrows).