Spectral broadening widens the bandwidth of the returning Doppler shifted signal due to its interaction with scatterers traveling at different velocities. This can be seen with turbulent or disturbed flow distal to an arterial stenosis.

Variance is the modification of Doppler frequencies within each pixel during a pulse packet with color Doppler flow imaging. Variance is mapped to the color green and is used to detect turbulence. At low variance (laminar flow) pixels within each range gate are encoded in red or blue according to flow direction. At high variance (turbulent flow) a percentage of pixels within each range gate are encoded with green according to the degree of spectral broadening.