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
What parameters effect aliasing on Doppler shift
- higher velocity of blood
- insonification angle close to 0 or 180
- high transmit frequency
What is aliasing
The Doppler signal peaks peaking into the bottom of the scale
What is happening when adjusting scales
- PRF changes; low PRF = big scale
- As you decrease scales, you decrease dead time of pw
Alternative options if aliasing continues
- lower freq transducer
- different view that represents same flow
- use cw
- use hprf doppler
What is the spectral window
The open area between the baseline and the spectrum
What is modal flow
- Refers to a visual spectral envelope
- the mode of all velocity measured
How to determine if range ambiguity is present
- if changing Doppler scale, the envelope shouldn’t change
What is HPRF Doppler
- a hybrid of both CW and PW
- uses timing of shallower gate depth w/ focusing on deeper depth
- allows for higher detectable velocity that pw can’t pick up
What degree does inson angle need to be to receive 100% Doppler shift
0 degrees or 180 degrees
If Doppler shift angle is other than 0 or 180, what can occur
The calculated velocity would be less than the actual velocity
What is color Doppler
A color image that is generated by repeatedly generating lines of data from the same direction to create a color packet
Why does color Doppler typically have Lower resolution
Larger packet sizes improve color sensitivity but require more time, lowering the resolution of the image
