The physical principles of Doppler and spectral analysis

The Doppler effect provides an ultrasonic method for the detection of echoes from moving structures, particularly flowing blood. In its most simple form, the continuous wave Doppler offers velocity information without depth resolution and is therefore used mainly for the examination of superficial structures. The pulsed Doppler, in combination with real-time imaging, provides a more flexible tool for the interrogation of selected sites in an ultrasound image for motion and flow. The recent development of Doppler flow imaging, in which limited Doppler information is displayed over an entire ultrasound image, usually in color and in real-time, promises to secure the association between Doppler and conventional ultrasound imaging techniques. Spectral analysis permits features of the Doppler signal to be identified which are associated with hemodynamic phenomena, such as flow disturbance and wave reflection. In addition, it allows the quantitative application of Doppler to the estimation of such physiological variables as velocity, flow rate, and pressure difference.