Detection of small vessel occlusion using Doppler ultrasound; an in vitro study

'Small vessel knock' is a recently reported Doppler ultrasound finding detected in patients with acute ischemic stroke. Knock signals are periodic, of short-duration (<100ms), and localised within the head. Typically, signals have a unidirectional component in systole and a reflected component during diastole. Currently it is not known whether this signal arises due to occluded blood-flow, wall motion, or some other mechanism. The potential uses of Doppler ultrasound for location of small-vessel occlusion in stroke patients therefore remains controversial.

The aim of this study was to determine the origins of knock-type signals by reproducing conditions for small-vessel occlusion in vitro. Synthetic bifurcations were moulded from latex, artificially occluded, and placed in a closed-loop flow-rig mimicking physiologically realistic blood-flow conditions. Doppler signals associated with blood-flow and wall-motion were examined under controlled conditions using Doppler, B-mode and Colour Doppler Duplex imaging.

Wall motion produced periodic bidirectional signals of longer duration than typically associated with knock signals. The temporal response of the wall motion signal followed the blood-flow velocity profile within the vessel. Introduction of scatterers to the circulating fluid immediately revealed an asymmetric (alternating) signal, which was similar to knock-type signals observed clinically. Further imaging of the flow using Colour Doppler revealed helical forward and reflected components.

We conclude that the clinically observed small-vessel knock signals are consistent with Doppler signals associated with blood-flow in and out of an occluded side vessel. Further work is required to clarify criteria for detection and classification of knock signals in clinical practice.

Fig. 1: Example of a ‘small vessel knock’ signal