Twinkling artifact appears as a rapidly alternating red-blue color Doppler signal immediately posterior to strongly reflective surfaces, most notably renal and ureteral calculi. It is caused by phase jitter in the Doppler signal at rough or irregular reflective surfaces. Twinkling does not indicate true flow but serves as a useful marker to detect and confirm the presence of calculi, particularly small stones that may be difficult to identify by shadowing alone. It is enhanced by higher Doppler gain settings.