Limitations of dual frequency measurements for embolus classification

Transcranial Doppler (TCD) ultrasound is a sensitive technique for detection of micro-emboli in the cerebral circulation. Such emboli may be solid, liquid or gaseous, and their backscatter properties, and likely clinical significance, depend both on their size and especially their composition. It is often difficult to distinguish between signals from small and innocuous gaseous emboli and those from large and potentially dangerous solid emboli. A useful technique in this respect is one in which the ´measured embolus to blood ratios´ (MEBRs) are found at two different frequencies. A potential problem of this technique is that it is not possible to match exactly the ultrasonic beam shapes at the two frequencies, so that differences in MEBRs are not purely a result of the acoustical properties of the embolus. A commercial ultrasound transducer (DWL) designed for dual frequency measurements at 2 MHz and 2.5 MHz was studied. Experimental measurements were made of beam shapes at the two frequencies after they had passed through samples of human temporal bone. A simple mathematical model was developed to determine the uncertainties in measurement of relative MEBRs in the middle cerebral artery at the two frequencies, assuming a realistic range of vessel-beam configurations. Even if all other aspects of the measurement process were perfect, the results predict that errors due to slight differences in beam shapes at 2.0 MHz and 2.5 MHz would cause 15-30% of solid emboli to be classed as gaseous.