MULTIFREQUENCY DOPPLER DISCRIMINATES BETWEEN GASEOUS AND SOLID MICROEMBOLI
Introduction: Although cerebral microemboli may be detected using transcranial Doppler, it is at present extremely difficult to determine if they are composed of gaseous or solid elements. Theoretically this is possible by insonating an embolus simultaneously with two different ultrasound frequencies since the reflected ultrasound power will differ for each frequency depending on embolus type. Methods: The Doppler instrumentation used in this study discriminates between solid and gaseous microemboli by insonating them simultaneously with 2.0 and 2.5 MHz frequencies. In the experimental part of the study 61 solid (50–80 μm plastic) and 45 gaseous (15–30 μm) microemboli were introduced into a pulsatile closed-loop system. Five prosthetic heart valve patients and 10 carotid artery stenosis patients took part in the clinical studies. Results : In the experimental study analysis of the reflected Doppler power for the two different ultrasound frequencies showed correct differentiation between solid (N = 61) and gaseous (N = 45) microemboli for 95% of the microembolic events. Four hundred microemboli in the 5 prosthetic heart valve patients and 20 microemboli in the 10 carotid stenosis patients were analysed. In the prosthetic heart valve patients 344 (86%) were gaseous and 56 (14%) solid whereas all 20 (100%) microemboli in the carotid stenosis patients were solid. Conclusions: These studies have shown that it is now possible to discriminate between solid and gaseous cerebral microemboli using multifrequency transcranial Doppler monitoring.