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 Beam of ultrasound with a transmitted frequency f T is transmitted and reflected back from the red blood cells. f D = f T (2v/c) cosθ f T = Transmitted frequency f d =Doppler shift frequency θ = angle of incidence C= the velcity of ultrasound

 Doppler shift frequency normally is in audible range.

 Process to find these terms.  This is done by simple possible system contain oscillator.  This drives the trasmitting crystal at its resonant frequency.  Resonant frequency would be 10MHZ for superficial blood vessel and 5MHZ or less for deeper vessels such as aorta or the aliace arteries.

 The Received crystal converts the reflected ultrasound energy into an electrical signal.  Then it is amplified by RF amplifier.  Then multiplied by transmitted signal.  Further there is a low pass filter to take just Doppler frequency.

A T = A cosω T t A R = B cos(ω T + ω D ) t A T Transmitted Signal A R Received Signal V= A T x A R V= A cosω T t x B cos(ω T + ω D ) t V=AB/2 cos(2ω T + ω D )+ AB/2 cos(ω D t)

 The Direction of Blood Flow is Lost.  Due to these limitations, It is only used for simple test such as detection of deep vein thrombosis.

 Have two separate demodulator channels, one for forward and other for reverse flow.  Use zero crossing detectors to decide the direction of blood flow.

 Received signal is multiplied by Ecosωt and by a phase shifted version of transmitted signal E cos(ωt+φ)  If received signal is same as A R =Bcos(ωt+ω D ) then V B = BE/2cos(ω D t) ……….(1) so, V A = BE/2cos(ω D t- φ) ……….(2) So, signal is two channels A & B are identical apart from a phase shift φ ∠ V B - ∠ V A = φ