Where is spectral broadening and elevated velocities typically seen in a stenosis?

Spectral broadening and elevated velocities are typically seen at the entrance of a stenosis due to the turbulence and disturbed flow caused by the narrowing of the vessel. When blood flow encounters a stenosis (a narrowing of the vascular lumen), the fluid dynamics change significantly.

As blood rushes into the area of stenosis, it accelerates due to the conservation of mass principle (continuity equation), which states that as the cross-sectional area decreases, the velocity must increase to maintain the same flow rate. This high-velocity flow entering the stenosis generates a disturbed flow pattern, leading to increased turbulence. The turbulence results in spectral broadening on the Doppler waveform, which is characterized by a wider range of velocities detected within the sample volume.

In contrast, other areas such as distal or proximal to the stenosis may exhibit different flow characteristics. Distal to the stenosis, there may be post-stenotic turbulence and changes in flow patterns, but that often does not show the same degree of spectral broadening as the entrance does. Proximal to the stenosis, the flow generally remains more laminar and does not exhibit the same elevated velocities because it has not yet been affected by the narrowing.