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Physics, Techniques and Procedures

Low pass filter

a filter of electrical signals that removes frequencies above a certain threshold. In demodulation of signals, low pass filtering is used to extract the low-frequency informative part of the signal from the high-frequency carrier wave (e.g. extraction of the Doppler frequency shift from the ultrasound frequency in Doppler ultrasound and isolation of the low-frequency part of the magnetic resonance signal that contains the frequency encoded positional information). In Doppler ultrasound, low pass filtering is also used in the sample and hold unit to isolate frequencies below the Nyquist limit. In the processing of digital images in general, low-pass spatial filtering (see Fourier filtering) is used to remove high-frequency noise (noise smoothing).

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