Ch 10 CD + Power Doppler Flashcards
Is CD a pulse-echo imaging technique?
Yes!
What does CD present?
CD presents real-time blood flow or tissue motion information along the grayscale anatomic image
Signal processor uses a mathematical technique called what?
Autocorrelation
(this determines the mean + variance of the doppler-shifted signal for each scan line)
What is used for each scan line to avoid perpendicular angles?
Phasing
CD + 2D B-mode pulses are to the right or left of the vertical scan line?
CD: to the left
2D: to the right
What is the ensemble length (packet size)?
-# 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
List 3 things FR is dependent on?
-Color box width (more scan lines create longer FT)
-Color box depth
-Packet size/ensemble length (more pulses per scan line creates slower FR)
List 4 ways FR + CD can degrade our temporal resolution?
-Greater # of scan lines in the CD box
-Wide CD box
-High packet size
-Increased depth of CD box
List advantages + limitations to using CD?
Advantages:
-demonstrates blood flow
-determines direction of flow
Limitations:
-angle dependent
-lower FR
-lack of detailed spectral info (only mean velocities are displayed)
Explain the nyquist limit + what aliasing is?
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
The color hues on the scale indicate what?
Which direction flow is moving (either positive/towards probe or negative/away from probe)
How can we adjust the saturation?
By moving up or down the color map
Differentiate saturation vs brightness?
Saturation:
-magnitude of the doppler shift
-represents different velocities (ex. light hues have higher velocity than darker hues)
Brightness:
-amplitude of the color signal
What is CD?
A pulsed doppler system that acquires sampling of RBCs
Is aliasing always considered bad?
No, can give us a better understanding of flow characteristics
List situations that are more likely to cause aliasing?
-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
What are CD wall filters?
-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)
When would we shift our CD baseline?
-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
Can we invert the color map/scale?
Yes (be careful)
How is CD gain different from 2D gain?
-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
What is color bleeding or color blossoming?
When our color gains is turned up too high + the color flow signals are overlapping the non-flow regions
What is color persistence?
-Frame averaging to smooth out the color appearance by reducing speckle (noise)
-M/c used when color signal is weak
What is a downside to using color persistence?
It can distort the temporal characteristics of the flow signal
(not ideal for cardiac scanning)
What is color priority?
-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
