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What is Color Doppler?
- Color doppler imaging presents two dimensional, cross sectional, real time blood flow or tissue motion information along with two dimensional, corss sectional, real time gray scale anatomic imaging.
- Color flow information is qualitive not quantitative
Color Doppler Instruments
Utilize doppler shift information, time of flight, and amplitude information to illustrate blood flow in color
For each color image the equipment makes teh following determinations:
- How long has it taken for the sound beam to travel to and from the site of the echo?
- How strong is the echo?
- Is a doppler frequency shift present?
- What is the magnitude of the doppler frequency shift?
- What is teh direction of the doppler shift?
How is the color image created?
- Color flow represents the MEAN velocity from teh sample volumes and the angel between the ultrasound veam and blood flow
- It is a form of pulsed doppler
Packet or Ensemble Length
- Many pulsed scan lines and gates (sample volumes) over the area of interest
- Typically have between 3-30 pulses per line of color
- The number of pulses used to produce each scan line of color
How is the color image created?
What is autocorrelation?
- A mathematical technique that detects the doppler shift
- It rapidly determines the mean and variance of the doppler shift information at each location along the scan line
- Faster than the FFT used in spectral
Autocorrelation: Phase Shift Approach
- Compares 2 consecutive pulses returning from a given sample volume
- Uses time delay to send out pules
- -1 pair of pulses = a phase difference
- -2 phase determinations = 4 pulses
- The phase shift between 4 or more pulses are used to estimate the mean velocity
Signal Processing Part I
- Returning echoes filtered by the clutter filter
- Removes high amplitude signals returning from stationary tissue and preserves low amplitude blood signals
- Doppler statistic estimator remaining signal is analyzed to obtain the MEAN relative velocity
Signal Processing Part II
- Post processing smoothes teh data by combining data from consecutive images frame averaging
- Blood tissue discrimnator selects teh imaging signal intensity assigns either a shade of gray or color
- -operator controlled
- -also called the color priority
Display of Doppler Information
- Color Maps
- Used by the examiner interpreting teh study to determine the flow direction
- Darkest colors are closest to teh baseline-they are slower
Represents the frequency of the light that can be detected by the human eye
- Colors are arranged vertically
- Slowest flow closest to the baseline
- Faster flow at ends of scale
- Addional colors added on side
- Side to side changes relate to laminar/turbulent flow
- Green or yellow used to TAG turbulent flow (right to left)
- Since color is a form of pulsed dopper it is angle dependent
- Angle of insonation is between teh blood flow and the color dooppler beam
- Linear array transducer - the direction of the color beam runs parallel to the sides of the color box, - can steer teh beam to improve doppler angle
Pulse Repetition Frequency
- The range of velocities displayed by the color scale is governed by the PRF used to obtain the blood flow information
- The maximum frequency that can be detected with color flow is limited by teh samply frequency
- 1/2 the PRF highest frequency that can be displayed without aliasing
High Pass Clutter Filter
- Affects the lowest frequency that can be displayed
- Only allows frequencies greater than the cutoff frequency to be displayed
- Eliminates low flow during diastole
- Linked to the PRF
- Increase PRF = Increased Filter
- The number of new images produced per second
- Need multiple pulses to produce each line of color
- TIme required to produce a color image is much longer
- Consequently color flow has a slower frame rate
Frame rate dependent on:
- Size of color box, especially with width
- Position of the color box-deepr the box= longer it takes to create a scan line
- Line density-higher the number of scan lines = better resolution=slower frame rate
- Low PRF- longer for the pulse ensemble to be transmitted= slower frame rate
- Controls the strength of teh signal displayed
- Too little diminishes the sensitivity to smaller slow disturbances
Different flor venouse or arterial flow
Change the zero baseline of color assignments
Average of consecutive images to improve signal to noise, creating a smoother color
Advantages of Color Imaging
- Can visualize vessels to small to be seen by gray scale
- Aids in determining whre teh peak systolic velocities are in order to place sample volume
- Documentation of pathologies such as occlusions, severe stenosis, turbulent flow
- Can see flow in entire vessel lumen
Disadvantages of Color Imaging
- Qualitative information
- Low PRF adn Frame rate (degrades gray scale and color images, loss of spatial resolution, greater tendency for aliasing, diminished temporal resolution)
- Blood flow is angle dependent
- Flow direction is arbitrary
- Color may obscure pathology within a vessel
- Color flash
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