Ultrasonography and Doppler Techniques

Acute kidney injury (AKI) is characterized by a rapid loss of kidney excretory function. Ultrasound (US) helps nephrologists in AKI diagnosis, in order to describe and follow kidney alterations and find possible causes of AKI. US is a safe, non-invasive, and repeatable imaging technique. Approximately 70% of community-acquired cases of AKI are due to pre-renal causes. Acute tubular necrosis (ATN) is the most common type of AKI related to parenchymal damage. Conventional US shows enlarged kidneys with hypoechoic pyramids. Although the role of color Doppler in AKI is still debated, many studies demonstrate that renal resistive indexes (RI) vary on the basis of primary disease. Moreover, several studies assessed that higher RI values are predictive of persistent AKI. Acute interstitial nephritis is also a frequent cause of AKI and conventional US does not allow a definitive diagnosis. Kidneys appear enlarged and widely hyperechoic. Also in this condition, hemodynamic changes are closely correlated to the severity and the progression of the anatomical damage. Renal artery and vein thrombosis may also cause parenchymal AKI. Acute renal infarction is a rare pathological condition due to clots or cholesterol aggregates that occlude renal artery or its branches. Renal vein thrombosis may be primary or secondary to retroperitoneal neoplasm or inflammatory diseases. Color Doppler US can detect thrombus within the lumen and document the absence of venous flow. The incidence of AKI related to urinary tract obstruction is low. It occurs in bilateral renal or lower urinary tract obstruction or in ureter obstruction in patients with a single functioning kidney or with pre-existing chronic kidney disease. Conventional US has a high sensitivity (>95%), but low specificity (<70%) in the diagnosis of urinary tract obstruction. Nevertheless, the use of color Doppler, through the evaluation of RI, ureteral jet, and twinkling artifact, may also have a very useful prognostic value.