The DOUBLE LINE' SIGN

 These images were obtained from a dialysis patient during routine screening for renal cell carcinoma. In addition to kidneys being small with thin parenchyma, note the hypoechoic, sharply demarcated area in the perirenal area bilaterally mimicking intraperitoneal free fluid. 

However, the ‘double line’ sign is suggestive of perirenal fat pad and not fluid. It essentially means that the perirenal hypoechoic structure is surrounded by two hyperechoic bright lines caused by fascial planes surrounding the kidney. Moreover, the fat pads typically contain low-level echoes unlike fluid and move with the kidney as it changes position during respiration. It may be difficult to distinguish this structure from post kidney biopsy subcapsular hematoma, but visualization of double line sign favors fat pad, especially when bilateral. Of note, recent epidemiological studies have shown that perirenal fat is a risk predictor for cardiovascular disease.