Background and purpose: The detection of circulating particulate cerebral emboli using transcranial Doppler ultrasonography has been recently reported. It has been suggested that this method might allow discrimination between different embolic materials; this would be very useful for selecting specific pharmacological treatment in individual patients. This study was designed to identify those parameters of the Doppler signal that might prove useful in discriminating between different types and sizes of particulate cerebral emboli.
Methods: An extracorporeal circuit filled with a saline/Tween solution and driven by a peristaltic pump was used. The tubing was placed in a skull in the position of the middle cerebral artery. Using transcranial Doppler ultrasound, flow was insonated via the transtemporal window. The following embolic materials of measured sizes (range of maximum dimension, 0.5-5.0 mm) were introduced into the circuit: thrombus (n = 20), platelet-rich aggregates (n = 15), atheromatous material (n = 20), and fat (n = 20). The Doppler signal was recorded during the passage of each embolus. Off-line analysis was performed to measure the maximum amplitude and duration of the signal.
Results: For all embolic materials there was a highly significant relation between embolus size and maximum amplitude of the Doppler signal. The closest correlation was obtained when the logarithm of maximum amplitude was used (for thrombi, r = 0.74; for platelet, r = 0.87; for atheroma, r = 0.46; and for fat, r = 0.68). The slope of the regression line differed for the different embolic materials and was significantly steeper for platelets than for atheroma (p < 0.01). Platelet emboli of maximum dimension < or = 1.5 mm resulted in a significantly lower maximum amplitude than similarly sized atheroma emboli (157 dB versus 206.7 dB, p < 0.01). For larger emboli (> 2 mm) there was little further increase in maximum amplitude with increases in embolus size. For all embolic materials there was a highly significant linear relation between embolus size and duration of the high-amplitude (> 150 dB) signal (for thrombi, r = 0.75; for platelet, r = 0.90; for atheroma, r = 0.77; and for fat, r = 0.86).
Conclusions: Platelet emboli result in lower-amplitude signals, and therefore analysis of maximum amplitude may provide information on the type of embolic material. However, it may be difficult to determine whether a given signal is associated with a large platelet embolus or a small atheroma embolus. Duration of the high-amplitude signal will allow accurate estimation of the size of emboli, particularly where the emboli are all of the same material.