Transcranial Doppler (TCD)

TCD systems employ 2-MHz ultrasound probe since at this low-frequency (2 MHz) attenuation by soft tissues and bone is low compared with high frequencies. When a small amount of electric current is passed through the piezoelectric crystal in the 2 MHz transducer, it vibrates to generate pulses of ultrasound waves of a particular frequency. The frequencies of echoes emitted after striking moving red blood cells in various intracranial arteries are different from those emitted by the source. This difference between the transmitted and the reflected sound frequencies is called a ‘Doppler shift’, and this provides information about the direction and speed of blood flow. The basic principle of TCD is based that the cerebral blood flow velocity (CBFV) in a cerebral artery is inversely proportional to the diameter of the and hence TCD provides information about the velocity in the cerebral vessel as well as indirect assessment of the diameter through calculating the Doppler shift. Blood flow towards the probe is seen as a red signal on PMD screen and above the baseline on flow spectra and flow away from the probe is seen as a blue signal on PMD screen and below the baseline on flow spectra.