How does spectral Doppler Ultrasound detect vascular stenosis

How does spectral Doppler Ultrasound detect vascular stenosis?

Velocity measurements in a vessel help to detect stenosis. As a vessel narrows, the velocity in the vessel increases at the point of stenosis in order to maintain the same rate of blood flow through the stenosed region. The degree of vessel stenosis correlates best with the PSV measurement when the stenosis is >50%, such that the greater the degree of stenosis the higher the PSV in the waveform. Another measure of stenosis or resistance in a vascular bed is the resistive index (RI): RI = (S-D)/S, where S is the PSV of blood flow in a vessel and D is the EDV. PSV and EDV can both be used to evaluate blood flow in a vessel when the direction of flow can be clearly identified (i.e., in the carotid artery), whereas RI is more commonly used when the direction of flow cannot be identified (e.g., with renal transplant vascularity). 

The table below outlines the characteristic changes of an arterial waveform in relation to a stenosis.

Arterial Stenosis Spectral Doppler Waveform Changes

LOCATIONPSVDIASTOLIC FLOWOTHER
Proximal to stenosisDecreasedDecreased
At stenosisIncreasedIncreasedTurbulence with spectral broadening
Distal to stenosisDecreasedDecreasedDecreased acceleration time; tardus parvus
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