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Physics, Techniques and Procedures

Demodulation

the extraction of low-frequency modulated information (e.g. sound and image in television broadcasting) from a high-frequency carrier wave.

Doppler ultrasound

In Doppler ultrasound, the low-frequency Doppler shifts (in the kHz range) are extracted from the high-frequency ultrasound echoes (in the MHz range) through demodulation (Fig.1). In a part of the instrument called the multiplier (or mixer), the high-frequency electrical signals induced in the transducer crystal by the received echoes (frequency fe) are multiplied with another high-frequency electrical signal having the transmit frequency of the Doppler transducer (frequency f0). The result of this multiplication is two electrical signals, one with a high (MHz) frequency equal to the sum of the two high frequencies (fe + f0), and the other with a low (kHz) frequency equal to the difference between the two high frequencies (fe - f0), which is the Doppler frequency shift, fD. The high-frequency signal is blocked by a low pass filter. Note that the simple demodulation described above, gives no information on the flow direction, i.e. it cannot discriminate between a negative and positive fD. To obtain directional information, a more complex demodulation called quadrature detection may be used.

Magnetic resonance imaging

In MRI, the high-frequency (MHz) magnetic resonance signal is demodulated in a part of the receiver (the detector or demodulator) and converted to a low frequency (kHz) signal that contains the "modulated information", i.e. the frequency range across the field of view encoded by the frequency encoding gradient.

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Fig.1

Simple demodulation of Doppler signal. f: transmit frequency, f
Demodulation, Fig.1