class 9

reflector moving towards the xdcr

freq inc

reflector moving away from the xdcr

freq dec

Factors Affecting The Doppler Shift

blood flow Velocity, Transmit freq, Angle and C

the 2 factor in the equation comes from the

result of two Doppler shifts. 1. Doppler shift with a moving receiver encounters a wave 2. Doppler shift with a moving emitter re radiates a wave

receiver moving toward the source experiences a

higher freq

a moving source produces a

shorter wavelength (inc freq) in front of it

A moving reflector is a combination of both a

moving receiver and source

Fd is directly proportional to

speed

Two Types of Doppler Instruments

PW (driven by pulses) and CW (continuously oscillating voltage)

smaller angle

higher shifts

0deg (parallel)

aligned with the direction of motion

if perpendicular

the machine detects nothing

cw uses

two elements to continuously transmit (active) and receive (passive)

cw has no

time/depth info since its always transmitting (on)

cw duty factor

100% duty cycle (running all the time/higher exposure)

cw measures within the

sample volume/active region

different freq have

different angles

cw demodulator

Detects freq changes from the reflected signals (uses the transmitted freq as a reference)

Amplitude Modulation

Info from amp variations (AM radio and US imaging)

Frequency Modulation

Info from freq variations (FM radio and Doppler US)

Fd is the

difference between freq after the multiplier

low pass filter

keeps the difference but rejects the sum (high freq)

multiplier mixes the

input and output waves (sum and difference)

after the low pass filter we get the

doppler shift

transmitted (carrier) signal is modulated by

scatterer motion

Frequency Demodulation two-step process

Multiplication (mixing) and Low pass filtering

Wall Filter

Removes low freq (high amp) signals associated with tissue motion

Tissue velocity

low compared to blood velocity (tissue motion also has high backscatter)

Doppler shifts sound range

200Hz - 15000Hz audible sound

delta f is extracted by

demodulation

doppler equation

Fd = Fr - Ft