Ultrasound of the kidney: obstruction and medical diseases

Ultrasound has emerged as the primary imaging modality in conditions where either renal obstruction or renal medical disease is suspected on the basis of clinical and laboratory findings. In urinary tract obstruction, pathoph ysiologic changes affecting the pressure in the collecting system and kidne y perfusion are well understood and form the basis for the correct interpre tation of real-time US and color Doppler duplex sonography (CDDS). Ultrasou nd is very sensitive for the detection of collecting system dilatation ("hy dronephrosis"); however, obstruction is not synonymous with dilatation, as either obstructive or nonobstructive dilatation may be present. To differen tiate these conditions, CDDS with measurement of the resistive index (RI) i n the intrarenal arteries is extremely helpful, as obstruction (except in t he peracute stage) leads to intrarenal vasoconstriction with a consecutive increase of the RI above the upper limit of 0.7, whereas nonobstructive dil atation does not. Diuretic challenge to the kidney may further enhance thes e differences in RI between obstruction and dilatation. Based on these find ings, the present value of US and CDDS in the assessment of the patient wit h flank pain or renal colic is suggested, especially with respect to promis ing, results for spiral CT and based on cost analysis. In renal medical dis ease, distinguishing different pathologic conditions using gray-scale US an d CDDS (RI) criteria is still very difficult. Nevertheless, US is the fist- line imaging modality in the patient with renal insufficiency.