Doppler Physics Flashcards
what is a doppler shift?
Change in reflected frequency compared to frequency sent out when there is motion between source and reflector.
aka the difference between the reflected frequency and the transmitted frequency.
Doppler Shift (MHz) = Reflected Fr – Transmitted Fr
what happens due to dopper effect when you move away from low freq waves? how about when you move toward high frequency waves?
less waves/sec (bottom)
more waves/sec (top)
what is the difference b/t a positive and negative dopple shift?
Positive Doppler shift if reflected frequency is greater than transmitted. Happens with movement toward the transducer
Negative Doppler shift if reflected frequency is less than transmitted. Happens with movement away from transducer
what are + and - shifts dependent upon?
directio nof reflector relative to transducer
What do these 3 doppler shifts tell you?
top lt - little dop shift b/c 90 degrees
top rt - + dop shift b/c flow toward Td
bottom lt - neg dop shift b/c flow away from Td
what is the amnt of freq change dependent on?
velocity of reflector, transmitted frequency and angle between source and reflector.
how do you increase doppler shift?
Increase velocity of reflector
Increase frequency of transducer
Make angle closer to 0 degrees (parallel)
what doppler shift do you get w/ a 90 degree angle?
none
where is the doppler angel?
between the flow direction (vessel) and beam.
what happens to the doppler shift as the doppler angle increases?
the amnt of shift decreases.
doppler shift vs angle
lt pic - Doppler angle is 90 degrees, there is very little Doppler shift created. The measured velocity is .16 m/s. (This velocity is not accurate because of the large angle)
rt pic - Doppler angle is 45 degrees, there is more Doppler shift created. The measured velocity is .4 m/s. (This velocity is not accurate because we have not told the instrument the Doppler angle - however it works for demonstration)
is doppler shift related to velocity?
no.
what does the instrument do when the doppler angle is placed on the image?
calculate a velocity based on Doppler shift and angle correction (if no angle correction true velocity is not displayed).
what is this equation Fd=2FoV(Cos Angle)/C?
how does it do it?
when does it do it?
its the doppler equation for velocity.
it uses the doppler shift to calculate.
when you tell it the doppler angle
What can you say about these angles? what does the velocity?
lt pic - No angle correction. The instrument calculates a velocity of .4 m/s
rt pic - Same vessel with angle correction of 58 degrees.
The instrument calculates a velocity of 1 m/s.
how do you improve the doppler angle?
by steering linear Td beam heel-toe
what does teh doppler angle need to be?
what happens if its greater than this?
<65 degrees
After 65 degrees the possible error in the calculate of velocity is unacceptably high at approximately 15%.
Do not measure the velocity if the angle is greater than 65 degrees.
advantages of CW doppler/ disadvantages (pencil probe 5-10 MHZ, not duplex, no image)
+/- of CW duplex (below)
doppler (below): advantage: no aliasing, inexpensive.
duplex: Can sample large Doppler shift frequencies without aliasing. Can see the vessel
doppler: disadvantage: no range gating, no image.
duplex: Contamination: receives DS frequencies from a large area, nearly the length of the beam. Expensive equipment
what exam is:
most commonly used with duplex instruments, 5 MHz
advantage: range gating
disadvantage: aliasing
pulsed wave doppler
___ must be adequate to show doppler shift
PRF – if it’s too low this is what you will see (below)
what types of Td were used in either of these?
1st image – Duplex using a curved linear
2nd image - Duplex using a linear. I’ve added the green line to help demonstrate the Doppler beam. It is steered to create a better angle.
which has proper beam steering
60 degree.
what does the following describe…
Multiple gates along each line to detect Doppler shift information
Multiple lines to create a 2D image with the color Doppler information
Keep the color area small to increase frame rate
color doppler scan lines
what is the difference between these vessels?
first - Correct steering of the color Doppler box to be more “parallel” to the vessel and flow. Positive Doppler shift, flow is toward the beam and displayed in red.
second - Wrong steering of the color Doppler box to be more “perpendicular” to the vessel and flow. This also caused the Doppler shift to become negative (away from the beam) and display in blue.
what can you say the direction?
IVC in blue with flow away from the beam, negative Doppler shift, displayed in blue.
Aorta in red with flow toward the beam, positive Doppler shift, displayed in red.
what does this show you below?
describe it.
AKA: Color Doppler Energy (CDE), Color Power Angio (CPA), etc.
Displays the power of the DS that are detected in each gate, rather than displaying Doppler shift itself:
——The displayed data is related to the # of RBCs moving within the sample volume.——-Not related to the velocity.
what do you see here?
what was used?
This is an image of a tortuous Internal Carotid Artery. Notice it is displayed in red and blue because the direction of flow changes relative to the transducer. Using Color Doppler Energy would display the vessel in one color b/c directional information is not provided.
Higher sensitivity
Doppler angle effects ignored
making nondirectional display easier to interpret
No aliasing
what is the difference between these images?
1st color doppler of ivc – Flow is identified as away
from the beam by negative Doppler shift in blue.
2nd power doppler of ivc - Flow is identified but no directional information is available.
what’s going on here?
1st pic - Doppler beam is at 90 degrees, some flow is detected both toward and away from the transducer because of natural beam divergence. No flow in the center at exactly 90 degrees.
2d pic – Doppler beam is at 90 degrees, Power Doppler is more sensitive and displays better flow even at 90 degrees. No information about direction is available.
what is this an example of?
what does it do?
image is Displaying power/amplitude but with direction. Notice no velocity information on the color map.
Directional Power Doppler
Newer technology, not available on all machines
Displays power/amplitude of Doppler shift, so it is sensitive like Color Doppler Energy
Detects positive vs. negative to provide direction
No velocity information
what type of imaging are each of these images?
how can you tell?
CDI – Displaying Doppler shift frequency information with velocity and direction on the color map.
Power Doppler – Displaying power/amplitude without direction. Notice no velocity or direction information on the color map.
Directional Power Doppler – Displaying power/amplitude but with direction. Notice no velocity information on the color map.
what type of imaging is this? what do the colors tell you?
CDI – Direction and velocity is displayed. Scale has been set to show aliasing. The fast red flow aliases to yellow, then green.
what is the difference b/t these images?
Power Doppler – Flow is demonstrated without velocity or direction information. No aliasing will occur.
Directional Power Doppler – Flow is demonstrated with direction information but without velocity information. No aliasing will occur.
