Blood Flow Measurement Due to

Doppler EffectEE01083064EE01083293

Introduction

Beam of ultrasound with a transmitted frequency fT is transmitted and reflected back from the red blood cells.

fD = fT(2v/c) cosθ

fT= Transmitted frequencyfd=Doppler shift frequency

θ = angle of incidenceC= the velcity of ultrasound

Doppler shift frequency normally is in audible range.

According to the EquationfD = fT(2v/c) cosθ

The signal reflected back contain the information about the velocity of the blood and direction of

blood flow.

HOW CAN WE EXTRACT THESE INFORMATION FROM THE SIGNAL??????

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.

Mixing

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.

AT = A cosωT tAR = B cos(ωT+ ωD) tAT Transmitted SignalAR Received Signal

V= AT x AR

V= A cosωT t x B cos(ωT+ ωD) t

V=AB/2 cos(2ωT+ ωD)+ AB/2 cos(ωD t)

Mathematical Calculation

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.

Limitations of this Simple System

Have two separate demodulator channels, one for forward and other for reverse flow.

Use zero crossing detectors to decide the direction of blood flow.

Solution for this Limitation(Zero crossing Detector)

Received signal is multiplied by Ecosωt and by a phase shifted version of transmitted signal E cos(ωt+φ)

If received signal is same asAR=Bcos(ωt+ωD)

then VB= BE/2cos(ωDt) ……….(1) so, VA = BE/2cos(ωDt- φ) ……….(2)

So, signal is two channels A & B are identical apart from a phase shift φ

∠ VB - ∠ VA = φ

When φ = 90 & ωD > 0 ( forward flow)

When φ = 90 & ωD < 0 ( Reverse flow)

True Directional Doppler Demodulation System

Preserves all the directional information in the signal

Combination of previous circuit and some new components.

Has limitation that It does not do frequency analysis of the reflected signal.