Objectives: To provide an overview of the current state-of-the-art of detection of cerebral emboli with transcranial Doppler, including methods of distinguishing embolic signals from artifacts, of distinguishing between particulate and gaseous emboli, and of sizing emboli.

Abstract: Many strokes are caused by emboli from distal sites blocking vessels in the brain. The discovery that emboli of various types can be detected using Doppler ultrasound as they are carried through the major cerebral arteries has led to a new field of study, which has considerable potential. The basic principle of detection is extremely simple: if an embolus backscatters more power than the surrounding blood in which it is moving, then the transient increase in power can be detected and measured. Questions that arise from this principle surround the circumstances under which such power increases can be detected, and whether the size and composition of the embolus can be inferred from such measurements.
The delectability of an embolus is determined by many factors including its size and composition, the ultrasound frequency, the size of the Doppler sample volume, the embolus trajectory and its interaction with the ultrasound beam. In general even relatively small gas bubbles will be detected, but some larger solid emboli may be missed. With regard to size and composition, several techniques have been suggested as being useful for characterizing composition, and whilst considerable progress has been made in this direction there are still many challenges in distinguishing between large particulate emboli and small gaseous emboli.