The Doppler spectrum indicates how the echo power is distributed according to the Doppler shift frequency. The Doppler shift frequency is directly related to the radial velocity of the scatterer.

See also Modal Velocity, Doppler Effect and Doppler Ultrasound.

The diastole is the period of relaxation in the cardiac cycle which alternates with systole. During diastole, the ventricles fill and the aortic and pulmonary valves are closed. On a Doppler spectrum analysis, diastole can be identified as beginning at the dicrotic notch (a small abrupt upswing in the deceleration phase of systole) and ending with the systolic upstroke.

See also Echocardiography and Cardiac Ultrasound.

The modal velocity is the frequency component which contains the most energy. In the display of the Doppler spectrum, the mode corresponds to the brightest parts of the individual spectra.

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Spectral analysis is the quantitative analysis method to display the distribution of frequencies. A difficult Doppler signal is separated into the frequency components so that the range of frequencies in a Doppler shifted signal can be analyzed. This allows measurement of blood flow velocity by positioning of a probing cursor in the artery (on the screen), and the signal representing blood flow velocity is generated. The peaks and ebbs create the spectrum, corresponding to systolic and diastolic blood flow. The signal is both visual and auditory.