Doppler Ultrasound in Hepatic Hemodynamics

In recent years duplex Doppler equipment, combining real-time imaging with pulsed Doppler ultrasound (US), has been introduced into clinical practice, thus providing an additional facility in the diagnostic evaluation of the abdominal vessels. The Doppler principle, on which these instruments are based, observes that when an ultrasound beam is reflected from a moving object, for example, red blood cells, the returning beam has a different frequency than the outgoing beam; the frequency is higher if the reflector is moving towards the beam and lower if it is moving away. This frequency variation can be expressed by the following equation: $${F_{d}} = (2F\nu \cos \alpha )/C$$ where F d = Doppler frequency, F= emitted frequency, v = velocity of moving target (red cells), a = the incident angle between the ultrasound beam and the direction of blood flow, and C = the velocity of ultrasound in the biological tissues (approximately 1500 m/s). The frequencies are expressed in hertz.