Principles of colour and Spectral doppler Flashcards
What is the doppler effect?
Change in frequency of sound, light or other waves caused by the motion of the source or the observer
In terms of ultrasound, the doppler effect is what?
The change in frequency of sound caused by the motion of red blood cells
What is the basic formula for doppler for ultrasound?
Transmitted frequency - received frequency
What is antegrade flow?
If the red blood cells are moving towards the transducer, the echo frequency will be larger than the transducer frequency
What is retrograde flow?
If the cells are moving from the transducer, the echo frequency will be smaller
What is beat frequency?
When the reflected wave will vary slightly from the original transmitted wave the algebraic sum gives off this frequency
The method used to measure doppler shift is based on what?
Wave interference
RBC move at different rates during what?
The cardiac cycle
The RBCs speed up and slow down with ____ and _______ and will move at ________ ________ within a cross section of the ________. Cells closest to ____ ________ of the vessel move ________ than the ones near _______ ________.
- Systole
- Diastole
- different speeds
- Vessel
- The center
- Faster
- The walls
What is the detailed Doppler equation?
Velocity is ______ to the doppler shift
Proportional
Velocity in doppler is dependent on what?
Angle
Doppler shift frequency is dependent on what?
Angle
Doppler shift frequency is dependent on what?
Transducer frequency
What is the Nyquist limitation?
- The upper limit to the doppler shift that can be detected by pulsed instruments
- Pulsed ultrasound must wait fro the first pulse to return before it can send out the second and so it is limited in how fast it can sample the RBC
What is the formula for the Nyquist limitation? And what is the range?
1/2 PRF and is in the range of 5-30 kHz
When the Nyquist limitation is exceeded what happens?
Aliasing occurs
Why does aliasing happens?
1.The PRF is too slow for the true Doppler shift and is sampling the flow too late and only capturing parts of it.
2. The wave form is incomplete and appears to wrap around the baseline
Aliasing is seen on colour doppler when what happens?
The colour “wraps” around the colour bar showing both colours in a vessel with no separation
What can we do to compensate for aliasing?
- Move the baseline
- Increase the PRF (scale)
- Increase the doppler angle
- Lower the operating frequency
- Change to CW
What does the high PRF mode allows us to do?
Allows the machine to send out subsequent scan lines before the first is returned
Having a high PRF mode might lead to what disadvantage?
Range ambiguity where the machine does not know what depth the echoes are being returned and could show misrepresented echoes on the display
We can avoid the range ambiguity issue for high PRF mode if what happens?
If the Phantom SV does not lie in an artery or vein
Doppler information can be represented in what 3 ways?
- Spectral
- Colour
- Sound
What is spectral doppler?
- Graphed information show on on the display
- Quantitive information in that you can precisely measure the speed of the RBCs at a given time by a process called FFT
What is FFT?
- Taking the complex beat frequency and dissecting it into the component waves.
- Each component wave is the result of a reflection moving blood cells within the sample volume
What is colour doppler?
When we can visualize flow in a vessel by first sending out scan lines to obtain 2D information and then another set of scan lines to obtain Doppler information
Doppler information is overlaid on top of what?
The 2D scan
What is autocorrelation?
When colour doppler is obtained and represents the mean or average of the flow velocity
What is sound doppler?
Doppler shift information that can be heard
What is a continuous wave doppler?
Sound waves emitted continuously from one crystal and reflected sound is continuously received by a second crystal
What is the zone of sensitivity?
Velocities that come from a large overlap zone in CW
What is an advantage and disadvantage of CW doppler?
No limit to the velocities that can be detected but the downside is that there is no range resolution or 2D image
What is PW doppler?
- When one crystal is used to fire and then listens for returning echoes.
- Reception of the echoes is gated in time
What is a benefit of PW doppler?
We know how fast the sound is travelling so we can determine at what depth we would like to sample.
What does the range gate do for PW doppler? And what does the operator do?
Range gate determines where the sample volume will be positioned and the operator determines both the size of the gate and the depth.
What is width of the sample volume equal to?
The width of the sample volume is equal to the beam width and its other dimensions are determined by the receiver gate length and the emitted pulse length
How do we get a effective sample volume length, what is an example?
- 1/2 of the pulse length is added to the gate length to yield the sample volume length
- The pulse length shortens as the gate size is reduced
What is colour doppler similar to?
Similar to colour doppler in that it is the overlay of colour onto the 2D image
How do we obtain power doppler?
Instead of analyzing the doppler shift information, power doppler uses the amplitude of the returning doppler signals to detect flow.
Because of how Power doppler is obtained, what does it result in?
It makes it very sensitive to low flow states and is independent of angle
