A filter in electronics is a circuit that only passes certain signals. In ultrasound is a filter a device to suppress acoustic or electromagnetic waves of certain frequencies, letting other frequencies pass, e.g. high pass filter, low pass filter. The high pass filter is the wall filter (also called thump filter) used in Doppler devices to eliminate low frequency Doppler shifts caused by clutter. For blood flow measurement, a low pass filter is often used to strip out high frequency noise, leaving only the biological components of interest. Frame averaging is a form of a low pass filter.

The motion discrimination detector is used for signal processing in color Doppler systems to distinguish between Doppler shifts from moving blood and moving tissue.

See also Zero Crossing Detector.

(PWD) Pulsed wave (PW) Doppler is a Doppler ultrasound mode that evaluates blood flow velocities in a range specific area along the length of the sound beam. Measured are changes in received frequency due to relative motion (flow) between a sound source (transducer) and sound receiver (transducer).
PW Doppler produces an audible signal as well as a graphical representation of flow. The Doppler shift produced by moving blood flow is calculated by the ultrasound system.

See also Amplitude Indicator, Pulsed Ultrasound.

Quadrature detection is used in Doppler ultrasound as well as in magnetic resonance imaging and is also called quadrature demodulation or phase quadrature technique. Quadrature detection is the acquisition of Mx and My simultaneously as a function of time by using two separate detector channels. This signal processing method is used for directional Doppler in which the signal reference frequency for the two channels has a phase shift of 1/4 period. The output Doppler signal phase for both channels also depends on the Doppler shift whether positive or negative.
The fast Fourier transform analyzer performs spectral Doppler analysis in ultrasound machines and displays different quadrature Doppler frequencies, when a sample volume cursor is used along time.

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.