Instrumentation for the investigation of the extracranial carotid circulation

The original ultrasound Doppler shift flowmeter was a non-directional velocity sensing device. Various methods of signal processing, including phase-quadrature detection and heterodyne detection were developed in order that flow direction could be resolved. In the past ten years the advent of the real-time audio spectrum analyser allowed both the flow direction and the Doppler shift spectrum to be displayed. It is this latter development which has been the major contribution to the clinical acceptability of Doppler flowmeters. The combination of a pulsed Doppler flowmeter with a real-time ultrasound imaging system has meant that velocity information can be gained from specific blood vessels, non-invasively. The development of the colour flow mapping systems has demonstrated that haemodynamic information can now be incorporated into the ultrasound image in real-time. These sophisticated systems are capable of displaying functional and anatomical information simultaneously.