Ch 10 CD + Power Doppler

Question 1

Is CD a pulse-echo imaging technique?

Answer 1

Yes!

Question 2

What does CD present?

Answer 2

CD presents real-time blood flow or tissue motion information along the grayscale anatomic image

Question 3

Signal processor uses a mathematical technique called what?

Answer 3

Autocorrelation

(this determines the mean + variance of the doppler-shifted signal for each scan line)

Question 4

What is used for each scan line to avoid perpendicular angles?

Answer 4

Phasing

Question 5

CD + 2D B-mode pulses are to the right or left of the vertical scan line?

Answer 5

CD: to the left
2D: to the right

Question 6

What is the ensemble length (packet size)?

Answer 6

-# of pulses per color scan line
-Min 3 pulses, but m/c 10-20 pulses
-Greater packet sizes allow for improved accuracy of velocity, color representation + slow blood flow
-Trade off is lower FRs

Question 7

List 3 things FR is dependent on?

Answer 7

-Color box width (more scan lines create longer FT)
-Color box depth
-Packet size/ensemble length (more pulses per scan line creates slower FR)

Question 8

List 4 ways FR + CD can degrade our temporal resolution?

Answer 8

-Greater # of scan lines in the CD box
-Wide CD box
-High packet size
-Increased depth of CD box

Question 9

List advantages + limitations to using CD?

Answer 9

Advantages:
-demonstrates blood flow
-determines direction of flow

Limitations:
-angle dependent
-lower FR
-lack of detailed spectral info (only mean velocities are displayed)

Question 10

Explain the nyquist limit + what aliasing is?

Answer 10

Nyquist limit:
-refers to the highest detectable doppler frequency shift w/o aliasing
(NL = 1/2 PRF, PRF = 2x NL)

Aliasing:
-occurs when the highest doppler shifted frequency exceeds 1/2 the PRF
-“wrapping around” of color

Question 11

The color hues on the scale indicate what?

Answer 11

Which direction flow is moving (either positive/towards probe or negative/away from probe)

Question 12

How can we adjust the saturation?

Answer 12

By moving up or down the color map

Question 13

Differentiate saturation vs brightness?

Answer 13

Saturation:
-magnitude of the doppler shift
-represents different velocities (ex. light hues have higher velocity than darker hues)

Brightness:
-amplitude of the color signal

Question 14

What is CD?

Answer 14

A pulsed doppler system that acquires sampling of RBCs

Question 15

Is aliasing always considered bad?

Answer 15

No, can give us a better understanding of flow characteristics

Question 16

List situations that are more likely to cause aliasing?

Answer 16

-Low PRF (color scale)
-High velocity flow
-Areas of acceleration (stenosis, etc)
-Angles resulting in a greater doppler shift (0 or 180 degrees)
-High frequency probes

Question 17

What are CD wall filters?

Answer 17

-They filter out very low doppler shifted frequencies
-It is the black part in the middle of the CD scale (we can adjust the amount of filtering)
-As we change our CD scale, the CD wall filter will automatically change as well

(ex. if we increase our scale, the filter will increase as well to eliminate slower blood flow signals)

Question 18

When would we shift our CD baseline?

Answer 18

-Shift it up or down to correct aliasing
-The baseline wall filter is the black part in the CD scale which represents no doppler shift OR a low DSF that is not displayed

Question 19

Can we invert the color map/scale?

Answer 19

Yes (be careful)

Question 20

How is CD gain different from 2D gain?

Answer 20

-Every pixel in CD gain is displayed with the same level of brightness or amplitude (unlike 2D gain)
-For CD, a color pixel is either displayed or not displayed depending on whether the color signal exceeds a threshold display

Question 21

What is color bleeding or color blossoming?

Answer 21

When our color gains is turned up too high + the color flow signals are overlapping the non-flow regions

Question 22

What is color persistence?

Answer 22

-Frame averaging to smooth out the color appearance by reducing speckle (noise)
-M/c used when color signal is weak

Question 23

What is a downside to using color persistence?

Answer 23

It can distort the temporal characteristics of the flow signal

(not ideal for cardiac scanning)

Question 24

What is color priority?

Answer 24

-Determines whether the B-mode or color doppler signal, that are both detected at the same location, will be displayed on the final pixel

-This occurs when a pixel has both a grey scale signal + color signal
-Color priority decides if the pixel will be displayed as color or grayscale

Question 25

What is power doppler?

Answer 25

-Presents 2D doppler info by color encoding the signal strength/amplitude of the doppler shifts

PW vs CD: PW
-Is based on amplitude (not frequency shift info)
-Velocity + direction is NOT shown
-Can NOT determine flow characteristics (ex. laminar vs turbulent flow)
-Uses frame averaging to improve signal sensitivity, but lowers temporal resolution

Question 26

The power of the doppler shift is determined by what?

Answer 26

The concentration of the moving scatterers

Question 27

Is it possible to see forward + reverse flow with power doppler?

Answer 27

Only on some u/s systems

-There is no PRF/scale since the mean velocity can not be detected
-Has the ability to image at close to 90 degree angles

Question 28

List advantages + disadvantages to using power doppler?

Answer 28

Advantages:
-angle independent
-no aliasing
-improved sensitivity
-more sensitive to low flow velocities

Disadvantages:
-no directional info
-no flow speed or flow character info
-reduced temporal resolution due to frame averaging

Question 29

What is flash artifact?

Answer 29

-Sudden appearance of color over the whole power doppler window
-Is due to relative motion b/w the probe + the pt

Question 30

When would we use power doppler?

Answer 30

-For detecting low velocity flow OR flow in very small, deep vessels (ex. testicles, kidney, liver, spleen)
-To confirm thrombosis or occlusion of vessels
-For transcranial imaging + u/s contrast imaging
-Improves visualization of tumors + inflammation in the prostate, testicular or ovarian torsion + renal parenchymal flow