Renal calculi (kidney stones) appear on ultrasound as hyperechoic foci within the renal collecting system, typically producing clean posterior acoustic shadowing. Stones in the renal pelvis or calyces are usually well seen; ureteral stones are more difficult to visualize due to overlying bowel gas. Twinkling artifact on color Doppler — a rapidly alternating color signal behind a stone — improves detection sensitivity. Associated findings may include ipsilateral hydronephrosis when the stone causes obstruction.