Spectral Doppler (ultrasound)

Last revised by Raymond Chieng on 25 Jul 2023

Utilizing automated Fourier analysis to convert returning sound waves into a series of individual frequencies, spectral Doppler refers to ultrasound modalities which yield graphical representations of flow velocity over time. 

The frequency of the sound waves returned to an ultrasound transducer when interrogating blood flow represents a composite of the heterogenous Doppler shifts yielded by each red blood cell in motion, each of which is in motion at a unique velocity and direction. Spectral Doppler modalities utilize Fourier analysis (fast Fourier transformations) to average the frequencies over a circumscribed time period, usually 5 milliseconds, convert frequencies to velocities using the Doppler equation, and display a "spectrum" of these frequencies as Doppler waveforms (hence the term "spectral") 3

These velocity-time spectral recordings may be obtained with two conventional modalities, both of which are typically used in conjunction with B-mode ultrasonography; pulsed wave and continuous wave Doppler.

Continuous wave Doppler simultaneously transmits and receives sound waves with separate piezoelectric crystals, recording every velocity received along a path defined by the operator. It is capable of recording the direction and velocity of flow even at high velocities but is unable to localize from where individual velocity elements originate 1

In pulsed wave Doppler, the user defines a small area (the sample "volume" or "gate") within the B-mode image, and (based on pulse repetition frequency, or the time required for returning sound waves) only the Doppler shifts from that area are recorded. While avoiding the range ambiguity of continuous wave Doppler, the intermittent sampling of pulsed wave Doppler, especially at targets that are further away from the transducer, renders the modality vulnerable to aliasing at higher velocities 4.

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