Three types:
1. Continuous wave
2. Pulsed wave
3. Colour flow
Signal processing controls:
1. Doppler reciever gain
2. Wall filter
3. Velocity scale
4. Baseline shift
5. Sample volume size
6. Color threshold
7. Steering angle
8. Color box size and position
9. Variance
10. Color inversion
11. Color maps
Doppler effect:
The apparent change in the frequency of waves due to relative motion of the source and observer. Christian Johann Doppler described this in 1842. The apparent frequency increases if source is moving towards (red-shift) and decreases if moving away (blue-shift). The difference between the transmitted frequency and the received frequency is known as Doppler Shift. This can be calculated from Doppler equation.
fD = f0[v/c-v]
fD - Doppler frequency
f0 - Transmitted frequency
v - Velocity of the source
c - Speed of propagation of the wave in the medium.
In echocardiograpgy doppler shift occurs twice. First when the wave travels from transmitter to the scatter and second when it reflects and travels back to the receiver.
So,
fD = f0[2v/c-v]
As c>>>v c-v ≈ c
Then,
fD = 2f0v/c
Then the velocity of the scatter can be derived from
v = fD c / 2 f0
If the scatter is moving at an angle θ, then
v = fD c / 2 f0 cos θ
This angle is known as the crossing angle. The error is negligible if this angle is less than 20 degrees.
