Measurements of the relative Doppler power scattered by an embolus and the surrounding blood are widely used to infer the composition of the embolus. For a given embolus, measured embolus-to-blood ratio (MEBR) is affected by the Doppler sample volume shape, the geometrical relationship between the sample volume and the vessel, and the embolus trajectory through the blood vessel. The likely magnitudes of such effects were quantified using a model that allowed calculation of theoretical values of MEBR as a function of the geometrical relationship between a blood vessel and a defined sample volume. Overall, the effects of embolus trajectory, likely insonation angles, and plausible vessel misalignments introduced uncertainties in MEBR values of 10 to 12 dB for a given vessel size. In practice, the only operator-controlled factors are the position and orientation of the transcranial Doppler probe on the patients’ heads. Probe positioning can significantly affect MEBR and suboptimal positioning may result in the reduced detection of emboli
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