In power mode the amplitude (power) of color Doppler signals is displayed, regardless of the velocity. Power does not have negative values and is independent of sampling frequency. An aliasing artifact does not occur in power mode images. Caused by plotting the quantity enhanced by echo contrast agents in a power map, power mode is often used in contrast Doppler ultrasound examinations.
Also known as energy mode.

(PI) The pulsatility index quantifies the shape of the blood velocity waveform by the ratio of the flow volume amplitude and mean flow volume.

See also Pulsatile Flow.

The refraction is the change of the sound direction on passing from one medium to another. In ultrasound, refraction is due to sound velocity mismatches combined with oblique angles of incidence, most commonly with convex scanheads. When the ultrasound wave crosses at an oblique angle the interface of two materials, through which the waves propagate at different velocities, refraction occurs, caused by bending of the wave beam.

See also Refraction Artifact, Acoustic Shadowing, Acoustic Mismatch, and Duplication Artifact.

Different sound velocities in tissue are causing refraction artifacts. With convex elastomer lens transducers, sound beam refraction at the skin interface can alter the transducer's focusing characteristics and beam profile, cause element to element nonuniformity, and cause phase changes in the acoustic wave. These cumulative refraction induced errors degrade the image quality through distortion and loss of resolution. Because the amount of refraction is proportional to the velocity mismatch, the greater the mismatch, the greater the refraction.

Spectral Doppler refers to the combination of either continuous wave Doppler or pulsed Doppler with a spectral display. Spectral Doppler provides a quantitative analysis of the velocity and direction of blood flow.
The Fourier spectrum analyzer performs a fast Fourier transformation on the Doppler signal. The amplitudes of the resulting spectra are encoded as brightness. In the 2D spectral display, the frequency shift is depicted in the vertical and the time in the horizontal axis. The range of blood velocities in the volume produces a corresponding range of frequency shifts.

See also Acceleration Index and Triplex Exam.