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.

Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.

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.

The autocorrelation is a mathematical procedure used to quantify periodicity in a Doppler signal and forms the basis of most color Doppler velocity estimators. The autocorrelation multiplies waveforms by successively time-shifted sections of itself.

Ultrasound physics is based on the fact that periodic motion emitted of a vibrating object causes pressure waves. Ultrasonic waves are made of high pressure and low pressure (rarefactional pressure) pulses traveling through a medium.

Properties of sound waves:

The speed of ultrasound depends on the mass and spacing of the tissue molecules and the attracting force between the particles of the medium. Ultrasonic waves travels faster in dense materials and slower in compressible materials. Ultrasound is reflected at interfaces between tissues of different acoustic impedance e.g., soft tissue - air, bone - air, or soft tissue - bone.
The sound waves are produced and received by the piezoelectric crystal of the transducer. The fast Fourier transformation converts the signal into a gray scale ultrasound picture.

The ultrasonic transmission and absorption is dependend on:
See also Sonographic Features, Doppler Effect and Thermal Effect.