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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
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Color Doppler Instruments
Utilize doppler shift information, time of flight, and amplitude information to illustrate blood flow in color
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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?
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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
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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
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How is the color image created?
Autocorrelation
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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
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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
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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
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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
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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
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HUE
Represents the frequency of the light that can be detected by the human eye
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Velocity Map
- Colors are arranged vertically
- Slowest flow closest to the baseline
- Faster flow at ends of scale
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Variance Map
- 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)
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Angle
- 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
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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
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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
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Frame Rate
- 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
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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
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Color Gain
- Controls the strength of teh signal displayed
- Too little diminishes the sensitivity to smaller slow disturbances
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Wall Filter
Different flor venouse or arterial flow
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Baseline
Change the zero baseline of color assignments
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Smoothing/Persistence
Average of consecutive images to improve signal to noise, creating a smoother color
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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
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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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