Ultrasonic detection of cerebral emboli

It is now known that the majority of strokes are caused by emboli from distal sites blocking vessels in the brain, rather than by local haemorrhagic or occlusive processes. 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 both in research and clinical settings. 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 transitory 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 detectability 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 characterising composition, and whilst in general considerable progress has been made in this direction there are still many challenges in distinguishing between large particulate emboli and small gaseous emboli. Unfortunately the sizing of emboli remains a distant goal.