Spectral Doppler (ultrasound)

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. 

Terminology

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. 

Radiographic features

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

Continuous wave Doppler (CWD) 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. 

Pulsed wave Doppler

In pulsed wave Doppler (PWD), 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 recorded from that area are recorded. While avoiding the range ambiguity of continuous wave Doppler, the intermittent sampling of PWD, especially at targets that are further away from the transducer, renders the modality vulnerable to aliasing at higher velocities ⁴.

Practical points

• utilization of continuous wave Doppler
  • accurately records high flow velocities
  • unable to determine the exact origin of any velocity it records
  • common uses in sonography include the analysis of:
    • aortic stenosis
    • mitral stenosis
• utilization of pulsed wave Doppler
  • accurately records velocities at a user-defined origin
  • vulnerable to aliasing at higher velocities and with deeper targets
  • common uses in sonography include the analysis of:
    • diastolic dysfunction
    • left ventricular outflow tract obstruction
    • cardiac output
    • hypertrophic cardiomyopathy
    • ventricular septal defects
    • transcranial Doppler