(AI) This index is the ratio between the acceleration of the Doppler spectral waveform and the relative peak systolic velocity. The systolic acceleration is determined by the change in distance between the begin of systolic flow and the peak systolic velocity (cm/sec), divided by the acceleration time (AT - time interval from the onset of flow to the initial peak).
The acceleration index is reported in frequency units as KHz/sec or velocity units as cm/sec2.

(AT) The acceleration time is the duration of upstroke from end-diastole to peak systole.

See Acceleration Index.

The velocity of flowing blood is usually measured in cm/s. It is always zero at the vessel wall and the velocity profile across a vessel can have various shapes depending upon the type of flow being observed (see also pulsatile flow). Laminar flow giving rise to a laminar velocity profile, plug flow giving rise to a flat velocity profile and disturbed flow can be distinguished.
Normal peak systolic velocities in the femoral and popliteal arteries vary from 90 to 110 cm/s in the femoral artery and from 40 to 70 cm/s in the popliteal artery.

See also Bi-directional Flow, Pulsatility Index, and Acceleration Index.

Spectral Doppler refers to the combination of either continuous wave Doppler or pulsed Doppler with a spectral display. Spectral Doppler provides a quantitative analysis of the velocity and direction of blood flow.
The Fourier spectrum analyzer performs a fast Fourier transformation on the Doppler signal. The amplitudes of the resulting spectra are encoded as brightness. In the 2D spectral display, the frequency shift is depicted in the vertical and the time in the horizontal axis. The range of blood velocities in the volume produces a corresponding range of frequency shifts.

See also Acceleration Index and Triplex Exam.

Sound waves must have a medium to pass through. The velocity or propagation speed is the speed at which sound waves travel through a particular medium measured in meters per second (m/s) or millimeters per microsecond (mm/μs). Because the velocity of ultrasound waves is constant, the time taken for the wave to return to the probe can be used to determine the depth of the object causing the reflection.
The velocity is equal to the frequency x wavelength.
V = f x l
The velocity of ultrasound will differ with different media. In general, the propagation speed of sound through gases is low, liquids higher and solids highest. The speed of sound depends strongly on temperature as well as the medium through which sound waves are propagating. At 0 °C (32 °F) the speed of sound in air is about 331 m/s (1,086 ft/s; 1,192 km/h; 740 mph; 643 kn), at 20 °C (68 °F) about 343 metres per second (1,125 ft/s; 1,235 km/h; 767 mph; 667 kn)

Velocity (m/s)

Doppler ultrasound visualizes blood flow-velocity information. The peak systolic velocity and the end diastolic velocity are major Doppler parameters, which are determined from the spectrum obtained at the point of maximal vessel narrowing. Peak systolic velocity ratios are calculated by dividing the peak-systolic velocity measured at the site of flow disturbance by that measured proximal of the narrowing (stenosis, graft, etc.).

See Acceleration Index, Acceleration Time, Modal Velocity, Run-time Artifact and Maximum Velocity.