Chap. 7 Level 1 Flashcards
What is the Doppler effect
An apparent change in frequency caused by relative motion (Police sirens driving away) that results from a change in wavelength
Equation for determining Doppler shift change in frequency
Fdop = F detected - F transmitted = change in frequency
What is formula for Doppler shift
Fdop = 2(Fo)V/C
What is the Doppler effect
An apparent change in frequency which results in a change of wavelength
Doppler shift equation solving for velocity
V = C x Fdop/2(Fo) x cos
What angle is the flow when coming towards the transducer and away from the transducer
- 0 degrees
- 180 degrees
Formula for Fdop with angle
Fdop = 2(Fo)(V)(cos)/c
Increasing “ “ increases reflectivity and attenuation
Frequency
What is the optimal frequency for Doppler
Less than 2Mhz, you can see a depth > 5cm
What is RBC aggregation
RBC cannot be measured individually. RBC combine as a group creating constructive interference and create increased signal strength
Define the Doppler insonification angle
The angle that is formed between the transducer and direction of the target
What Doppler angles represent a positive and negative shift
- Doppler angle < 90 = pos shift & pos velocity
- Doppler angle > 90 = negative shift & neg velocity
What is wall filtering
Attenuates the lower frequency velocity signals
What is the human visible dynamic range
Less than 36 decibels (fewer than 64 shades of gray)
Equation for dynamic range of Doppler
20 x Log (A biggest/A smallest)
Wall filter range for adult echo
200hz - 600 hz
What happens if wall filters are set to low
Doppler circuit saturates creating large white spikes on Doppler
What happens if wall filters are set too high?
Doppler signal may be completely eliminated
What is fast Fourier transform
A technique for separating a spectrum into its individual frequency components called bins
What are the bins in FFT
- Each bin corresponds to a narrow range of frequency shifts that can be converted to velocity using the Doppler equation
- lowest frequency = lowest velocity
What post processing element is used to make Doppler signals have a greater dynamic range
Compression
What quantities does Doppler show on 2D display
Velocity, amplitude, time
Which modality does aliasing occur?
Pw because of the listening period
How to determine max detectable frequency shift in pw
Nyquist variation: Fdop = PRF/2
