Module 3 Doppler review Flashcards
What does Doppler tell the sonographer? 6
- The presence of flow
- The direction of flow
- The quality of flow
- The velocity of the RBC
- Resistances/ pulsatility
- Timing
What is doppler?
A change in frequency of a sound wave in a moving source
How can we relate doppler to RBC’s?
We can use doppler to determine the direction RBCs are moving
When RBCs are moving towards the transducer what happens?
Received frequency will be higher than the transmitted frequency
antegrade to the transducer or positive shift
When RBCs are moving away from the transducer what happens?
Received frequency will be less than the transmitted frequency
retrograde flow or negative shift
How is doppler shift calculated?
We can calculate D/S by subtracting the transmitted frequency from the received frequency
D/S = Fr - Ft
What is the more detailed formula for Doppler shift?
D/S = ((2)(Fo)(v)(cos))/c
If the operating frequency or RBC velocity increase, what happens to D/S?
It increases?
If the angle of insoation increases, what happens to doppler?
It decreases
Doppler shift frequencies are processed by what?
Fast fourier transform (FFT)
Doppler shifts or velocities are plotted on what?
The Y axis over time
Stationary reflectors make up what?
Gray scale
Moving RBCs create the doppler shift and are color coded to mean what?
Color coded relative to their direction and mean velocity
Each scan line is sampled how many times to do what?
Sampled hundreds of times to create an image of color
Autocorrelation is what?
The technique that analyzes pulses in a scan line
Autocorrelation is similar to what?
FFT
How does RBCs create a doppler shift and how doe we use this?
Moving RBCs creates a doppler shift and are colour coded relative to their direction and mean velocity
Each scan line is sampled how many times to do what?
Each scan line is sampled hundreds of times to create an image of color
Stationary reflectors make up what?
Gray scale
What is autocorrelation?
The technique that analyzes pulses in a scan line
How is autocorrelation and FFT similar?
It is a mathematical means of obtaining the sign and mean velocity of the blood flow
Since mean frequencies are obtained, the information is what?
Qualitative
With multiple pulses required to obtain the mean, there is a reduction of what?
PRF effecting the temporal resolution
Framerate determines what kind of resolution?
Temporal resolution
In terms of flow we have only considered arterial flow. Flow in the venous system compare how?
It will be quite different since the mechanical for flow are different and varied
Venous flow needs to demonstrate 4 characteristics to be normal?
- Spontaneous
- Unidirectional
- Respirophasic
- Augmentable
What are some controls we use to optimize the image when performing doppler? 6
- Gain
- Scale (PRF)
- Baseline
- Angle
- Gate size
- Wall filter
Why should gain be used to assist in doppler?
Used to overcome the attenuation of small RBCs and scatter
What happens if gains are set to low?
- poor visualization of the spectral trace
- Poor color fill
- Make for weak signals
When we set gain to high what will happen?
We create artifacts
What is scale also known as?
Velocity range
How should we adjust scale to assist in doppler?
Adjust so that peak and minimum velocities are seen
A Scale that is set to low will result in what (for doppler)?
Aliasing
A scale that is set to high will do what (in terms of doppler)?
- Make the spectral trace to small to interpret
- Make it difficult to see low velocity shifts in the color
How can we use baseline to assist in doppler?
Adjusted to allow entire spectral signal to be seen (forward and reverse components)
Typically where do we have the baseline?
1/4 of the space above the baseline and 3/4 of the space below for venous
this is the opposite for arterial
For most studies the color baseline will not be what?
Adjusted The exception is in echo
How can the angle/ box angle help doppler?
Since doppler requires a non-perpendicular angle to detect doppler shifts, an angle is crucial to scanning
With venous flow, we do not need what?
To angle correct since velocities are not important
How should we adjust gate size help for doppler?
Gate size in a pulsed system should be approximately 1/3 the size of the vessel if possible
some vessels are very small and this becomes impossible
When the gate size is to big, what may happen?
Spectral broadening occurs
When the gate is too small, what might happen?
It might become difficult to interrogate the true middle of the vessel
For doppler what should we do with box size/ position?
The color box should be sized to maximize the FOV without compromising the framerate
this will vary with the exam being performed
How should we position the box? for doppler?
Should be positioned so that the vessels fit in it completely
How will wall filter assist in doppler?
- Eliminates low frequency noise
- Adjust so that low velocities are not missed in arterial or venous flow states
What is the priority control?
A control that can give more emphasis to the color when overwriting the grayscale
If the priority is high what happens?
The color will be displayed
If the priority to low what happens?
The grayscale shows instead of the color
Is priority something that we typically adjust on modern day equipment?
Nope
When scanning vessels, we typically do what? 2
- We keep the veins blue and the arteries red regardless of the direction of flow relative to the probe
- We show venous flow below the baseline and arterial flow above it
What are some doppler artifacts we see? 4
- Noise or blooming
- Flash or clutter
- Aliasing
- Mirror image
What is noise/blooming?
This occurs when the doppler gains are set too high causing the color to blossom outside the vessel or appear in anechoic structures that how no flow
With spectral, the over gaining can artificially do what?
Artificially increase the peak systolic velocities which can impact the diagnosis
How can we get a halo effect in terms of doppler?
Over gaining the spectral trace
What is doppler flash or clutter?
This appears as low level echoes in the color flow seen in the tissue as the result of tissue movement like the heart wall, valve or vessels wall motion.
How can we control doppler flash or clutter?
There are filters present within the machine to help eliminate this motion as their velocities are usually lower then the flowing of blood
if we set them too high there is a risk of eliminating real low flow info
What is aliasing? How do we get it?
An artifact that occurs if the nyquist limit is exceeded
What is the nyquist limit?
1/2 the PRF
When the Nyquist limit is exceeded, what happens?
The blood cells are not being sampled fast enough so a false reading occurs and parts of the signal are wrapped around the baseline
1/2 PRF usually falls between what frequencies?
5-30 kHZ
How can we correct for nyquist limitation? 5
- Move the baseline
- Increase the PRF
- Increase the doppler angle (heel or toe probe)
- Lower the operating frequency
- Change the continuous wave
In the LE venous study, aliasing is expected when?
Expected in the normal limb when we augment the flow
What is a doppler mirror image?
This is an artifact that can affect both color and spectral tracing
Spectral mirroring has doppler flow where?
on both sides of the baseline and is caused by over gaining or having a angle of incidence too close to 90 degrees
With color doppler a false vessel will appear how?
Deep to the real one due to a deeper strong specular reflector
What is power doppler?
Power or intensity of signal is measured rather than doppler shift
What is power doppler based on?
Density of RBCs, not on speed
What is Power doppler used to assess? 3
- Small vessel
- Tissue perfusion
- Slow flow
Power doppler has how much aliasing?
No aliasing and is more sensitive to low flow states
How dependent is power doppler on doppler angle?
Less dependent
The sensitivity to low flow during power doppler is more subject to what?
Flash from other slow moving structures, like bowel
Pulsed vs continuous wave. Compare the element use?
PW has one and CW has two
Pulsed vs continuous wave. Compare the images
PW produces an image CW does not
Pulsed vs continuous wave. Compare the depth
PW has a specific depth, CW samples in zones of sensitivity
Pulsed vs continuous wave. Compare Aliasing?
PW: Aliasing is possible; CW: No aliasing