Doppler in radiology refers to a specialized ultrasound technique that evaluates blood flow in vessels. It uses the Doppler effect to measure the speed and direction of blood flow, helping diagnose conditions like blood clots, arterial blockages, and varicose veins. Common types include Color Doppler, Spectral Doppler, and Power Doppler. It's widely used in vascular studies, fetal monitoring, and cardiac assessments.