THE EFFECT OF DIFFERENT SETTINGS OF ULTRASOUND PULSE AMPLITUDE, GAIN AND SAMPLE VOLUME ON THE APPEARANCE OF EMBOLI STUDIED IN A TRANSCRANIAL DOPPLER MODEL

Circulating cerebral emboli can be detected by transcranial Doppler (TCD) sonography. However, there has been little systematic study of parameters necessary for optimal embolus detection. Both the intensity increase associated with an embolic signal, compared to the background, and the duration of an embolic signal have been suggested as indicators of embolus size and material. We investigated the effect of different TCD machine settings on the appearance of 145 embolic signals associated with microspheres of known size in a tube model of the middle cerebral artery. The results demonstrated that optimal separation of emboli from the background signal will occur if recordings utilise the lower part of the dynamic range. This is due to the fact that the dynamic range of the displayed intensity is logarithmically compressed. The intensity of the embolic signal relative to the intraspectral background was inversely related to both gain (p < 0.0001) and sample volume (p < 0.0001). There was only a weak relationship between relative intensity increase and emitted ultrasound pulse amplitude. These findings will also need to be taken into consideration if signal analysis is used to derive information on the size of emboli.