Principles of Spectral Doppler

Abstract

Spectral-Doppler ultrasound provides a quantitative visual presentation of blood-flow information. The spectral display is a real-time presentation of Doppler shift vs. time on the vertical and horizontal display axes, respectively. The vertical axis is normally calibrated in flow-speed units by solving the Doppler equation with Doppler angle incorporation (by the instrument operator). The spectral display provides flow information (presence, direction, speed, character) at the site of observation (sample volume location). Information regarding proximal and distal flow conditions can also be derived from the spectral display. Pulsed wave Doppler systems provide the ability to select the depth from which Doppler information is received. Spectral analysis provides visual information on the distribution (spectrum) of Doppler-shift frequencies resulting from the distribution of scatterer velocities (speeds and directions) encountered within the sample volume. The spectrum is derived electronically using the fast Fourier transform and is presented on the display as Doppler shift versus time, with brightness or color indicating the strength of the Doppler shifts. Flow conditions at the site of measurement are indicated by the (vertical) width of the spectrum, with spectral broadening indicative of disturbed and turbulent flow. Flow conditions downstream, especially distal flow impedance, are indicated by the relationship between peak systolic and end-diastolic flow speeds. Proximal stenosis is indicated by the tardusparvus waveform. Aliasing displaces systolic Doppler shifts to the wrong side of the baseline when the Nyquist limit is exceeded. Aliasing is corrected by baseline shifting and, in extreme cases, scale change, which increases the sampling rate (PRF).