Doppler ultrasound assesses blood flow and tissue movements. The basic principle of Doppler—the Doppler effect—depends on the shift in frequency between pulse-echo signals. In other words, as a particle moves, it reflects sound beams at a different frequency than it receives them. A higher-frequency sound (compared to the emitted wave) is produced when the object is moving toward the detector, and vice versa, a lower frequency sound is detected when the object is moving away from the detector. Doppler is especially helpful in characterizing the motion of flow in vascular structures.
Blood flow can be laminar, which is the regular pattern of blood flow. Laminar flow is characterized by flow of blood in different velocities, with velocity higher in the center of the vessel, and gradually decreasing toward the periphery. Turbulent flow occurs when the range of velocities in a vessel significantly increases. Laminar and turbulent flow can be distinguished also by the audio of the Doppler instrument, as well as by spectral analysis. Spectral analysis is the process of breaking down the multi-frequency signals into their individual components, which are then expressed as a function of time. For example, if blood cells are moving at the same velocity, a single narrow band will appear on the spectral display. When the flow becomes more turbulent, the spectrum widens (spectral broadening). A very high-velocity lesion, e.g., a stenotic arterial wall, will cause complete filling of the spectral window due to high turbulence.
As its name implies, continuous wave (CW) Doppler transmits sound continuously rather than in short pulses and uses separate elements for receiving and sending. Continuous wave is used to record high-velocity flow patterns, usually above 2 m/s, and is especially useful in cardiology. In contrast, pulsed wave Doppler is used for low-velocity flow and uses the same crystal for receiving and sending the sound. Color Doppler is used to analyze the phase information, frequency, and amplitude of returning echoes, and colors are used to mark different velocity frequency changes. Color maps may be adjusted to obtain different color assignments for velocity levels. Also, the direction of phase shift (relative to the transducer) can be identified by different colors.
Instead of evaluating frequency shifts, power Doppler estimates the power or strength of the Doppler signal. Once the Doppler shift has been detected, the frequency components are ignored, and only the total energy of the Doppler signal is considered. The resultant color pattern represents the volume of moving blood. Compared to color Doppler imaging, power Doppler will have low-level noise at all gains. The disadvantage of power Doppler is that it does not provide information about the velocity or direction of blood.