Doppler Physics

Question 1

what is a doppler shift?

Answer 1

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

Question 2

what happens due to dopper effect when you move away from low freq waves? how about when you move toward high frequency waves?

Answer 2

less waves/sec (bottom)

more waves/sec (top)

Question 3

what is the difference b/t a positive and negative dopple shift?

Answer 3

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

Question 4

what are + and - shifts dependent upon?

Answer 4

directio nof reflector relative to transducer

Question 5

What do these 3 doppler shifts tell you?

Answer 5

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

Question 6

what is the amnt of freq change dependent on?

Answer 6

velocity of reflector, transmitted frequency and angle between source and reflector.

Question 7

how do you increase doppler shift?

Answer 7

Increase velocity of reflector
Increase frequency of transducer
Make angle closer to 0 degrees (parallel)

Question 8

what doppler shift do you get w/ a 90 degree angle?

Answer 8

none

Question 9

where is the doppler angel?

Answer 9

between the flow direction (vessel) and beam.

Question 10

what happens to the doppler shift as the doppler angle increases?

Answer 10

the amnt of shift decreases.

Question 11

doppler shift vs angle

Answer 11

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)

Question 12

is doppler shift related to velocity?

Answer 12

no.

Question 13

what does the instrument do when the doppler angle is placed on the image?

Answer 13

calculate a velocity based on Doppler shift and angle correction (if no angle correction true velocity is not displayed).

Question 14

what is this equation Fd=2FoV(Cos Angle)/C?

how does it do it?

when does it do it?

Answer 14

its the doppler equation for velocity.

it uses the doppler shift to calculate.

when you tell it the doppler angle

Question 15

What can you say about these angles? what does the velocity?

Answer 15

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.

Question 16

how do you improve the doppler angle?

Answer 16

by steering linear Td beam heel-toe

Question 17

what does teh doppler angle need to be?

what happens if its greater than this?

Answer 17

<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.

Question 18

What is the difference b/t doppler CW and PW?

Answer 18

Continuous wave Doppler instruments detect Doppler shifts along entire path of beam.

Can be “pencil probe” type or duplex.

Contamination is of concern, esp. with cardiac imaging.

Question 19

advantages of CW doppler/ disadvantages (pencil probe 5-10 MHZ, not duplex, no image)

+/- of CW duplex (below)

Answer 19

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

Question 20

what exam is:

most commonly used with duplex instruments, 5 MHz

advantage: range gating
disadvantage: aliasing

Answer 20

pulsed wave doppler

Question 21

What is range gating? what type of exam?

Answer 21

acquiring Doppler information from a specific depth based on arrive time of the reflections. PW Doppler

Question 22

what is aliasing? it is deteted by the _____ limit?

Answer 22

artifact which occurs when the rate of pulsing (pulse repetition frequency) is not high enough to detect a large Doppler shift.

Nyquist limit - maximum Doppler shift which can be detected w/o aliasing.

Nyquist limit = 1/2 PRF

Question 23

___ must be adequate to show doppler shift

Answer 23

PRF – if it’s too low this is what you will see (below)

Question 24

What is duplex?

Answer 24

Provides a 2D image with Doppler
Time-sharing provides for “apparent” real-time of both
Need an electronic transducer
Advantage: angle correction to calculate velocity
Disadvantage: expensive, large
Triplex: 2D, color and spectral Doppler “simultaneously”

Question 25

what types of Td were used in either of these?

Answer 25

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.

Question 26

which has proper beam steering

Answer 26

60 degree.

Question 27

what is color doppler imaging?

Answer 27

CDI All the same principles about Doppler that we discussed in spectral imaging apply to color. Such as angle, gain, PRF/scale, filter. The image is created using DS information, displaying it in a 2D format.

Question 28

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

Answer 28

color doppler scan lines

Question 29

what is the difference between these vessels?

Answer 29

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.

Question 30

what can you say the direction?

Answer 30

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.

Question 31

what does this show you below? describe it.

Answer 31

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.

Question 32

what do you see here? what was used?

Answer 32

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

Question 33

what are the disadvantages of power doppler compared to CDI?

Answer 33

Slower image rates. More flash artifact. Does not display direction.

Question 34

what is the difference between these images?

Answer 34

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.

Question 35

what's going on here?

Answer 35

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.

Question 36

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.

Answer 36

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

Question 37

what type of imaging are each of these images? how can you tell?

Answer 37

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.

Question 38

what type of imaging is this? what do the colors tell you?

Answer 38

CDI -- Direction and velocity is displayed. Scale has been set to show aliasing. The fast red flow aliases to yellow, then green.

Question 39

what is the difference b/t these images?

Answer 39

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.

Question 40

SUMMARY Doppler shift occurs with motion of the reflector. Movement toward the transducer is positive Doppler shift, movement away is negative Doppler shift. Amount of Doppler shift is dependent on the velocity of reflector, frequency of the transducer and Doppler angle. No Doppler shift at 90 degrees, maximum at 0 degrees. To calculate velocity must know angle, must be less than 65 degrees.

Answer 40

Continuous wave has no range gating, but also no aliasing. Pulsed wave has range gating but will have aliasing. Duplex provides 2D image with Doppler information for proper placement of the beam and angle correction. Color Doppler imaging displays the Doppler shift, providing velocity and direction information. Power Doppler (energy, amplitude, angio) displays the amplitude of the Doppler shift. No velocity or direction, no aliasing. Directional Power Doppler (energy, amplitude, angio) displays the amplitude of the Doppler shift with positive/negative directional information.