Color Flow Imaging in Ultrasound

Color Flow Imaging Overview

• Color flow imaging provides an overall view of flow in a region
• Flow information is limited
• Temporal resolution and flow dynamics are poor due to low frame rates when scanning deep
• Different color maps can be used
• Provides direction and velocity information
• Can detect turbulent flows

Color Flow Imaging Process

• Color flow Doppler ultrasound creates a color-coded map of Doppler shifts
• Doppler shifts are superimposed onto a B-mode ultrasound image
• Pulsed wave ultrasound is used
• Processing differs from that of the Doppler sonogram
• Color flow imaging may produce thousands of color points of flow information per frame, superimposed on the B-mode image
• Uses fewer and shorter pulses along each color scan line to provide a mean frequency shift and variance

Image Creation

• Frequency shift is displayed as a color pixel
• The scanner repeats this process for several lines to build up the color image, which is superimposed on the B-mode image
• Transducer elements are rapidly switched between B-mode and color flow imaging
• This gives the impression of a combined simultaneous image
• Pulses for color flow imaging are three to four times longer than those for B-mode images, resulting in a loss of axial resolution

Color Assignment

• Color assignment to frequency shifts is based on direction
• Red indicates Doppler shifts toward the ultrasound beam
• Blue indicates shifts away from the ultrasound beam
• Color assignment also depends on magnitude, with different color hues or lighter saturation for higher frequency shifts
• Color Doppler image quality depends on general Doppler factors, especially a good beam/flow angle
• Experienced operators adjust the scanning approach to get good insonation angles which will achieve good quality flow images

Color Doppler Technique

• Employs the pulse-echo technique
• Echoes from stationary tissue are displayed in gray scale, representing depth and brightness
• If a returning frequency differs in a scan line (Doppler shift detected), echoes show color Doppler (pixels will show color)
• Color indicates whether the Doppler shift was positive or negative
• Colors further from the baseline indicate greater Doppler shift

Components of Color

• Hue is a component of color
• Saturation is a component of color
• Luminance is a component of color

Hue

• Hue refers to the color perceived by the viewer like "what color is it?"
• Hue is based on frequency
• Red has the lowest frequency
• Violet has the highest frequency

Saturation

• Saturation involves the amount of hue present mixed with white
• Describes the vividness of a color, or the gradation of hue from unsaturated (white) to fully saturated, 100% of the given color
• Red is more saturated than pink because pink is red mixed with white
• Saturation is zero with no hue in 2D gray scale anatomic imaging

Luminance

• Luminance refers to the brightness of the hue and saturation

Color Flow Doppler Imaging Specifics

• Velocity information is coded into colors and superimposed on a two-dimensional grayscale, anatomic image
• Represents average (mean) velocities, not peak velocities

Color Doppler Maps

• Color flow maps use a table to convert measured velocities into specific colors
• This table or map is represented by vertical color bars on one side of the image
• Velocity mode is a common color flow map mode
• Variance mode is a common color flow map mode

Color Maps: Velocity Mode

• Velocity mode colors provide information about flow direction and velocity

Color Maps: Variance Mode

• Variance mode provides more information than velocity mode
• In addition to direction and speed, variance mode distinguishes laminar flow from turbulent flow

Velocity Mode Display

• The black region in the middle of the color map indicates "no Doppler shift"
• Colors above the black stripe indicate movement toward the transducer, or positive Doppler shift
• Colors below the black stripe indicate blood cells moving away from the transducer, or negative Doppler shift

Velocity Mode Maps Information

• Provides information on flow direction
• A black region in the center of the map indicates the "0" baseline, where there is no flow or no Doppler shift
• Colors above the black region means a reflector is moving toward the transducer, which is a positive Doppler shift
• Colors at the top indicates greater positive Doppler shift moving toward the transducer
• Colors below the black region means a reflector is moving away from the transducer, which is a negative Doppler shift
• Colors at the bottom indicate greater negative Doppler shift moving away form the transducer
• Numbers at the top and bottom of the color bar indicates the mean velocity at which aliasing will begin
• Two Color maps exist in this mode
• Low velocity reflectors, toward or away from the Doppler signal, are represented by more highly saturated colors
• High velocity reflectors are shown by less saturated colors
• Colors further from the black stripe means higher velocities
• Map 1 displays flow direction information
• Map 2 displays changes in velocity
• Color change is always vertical (up and down) and never side to side in this mode

Determining Direction of Flow

• Create an imaginary "home" line
• The "home" line extends downward from the upper corner of the steered box
• This line should not run into the color
• Movement toward the home line indicates the direction of flow in the vessel

Determining Flow in Sector Image

• Identify which color represents flow "toward" and which represents "away"
• Place a finger on the "toward" color, then slide it toward the "away" color to determine flow direction

Variance Mode

• Two different variance mode maps exist
• Laminar flow show in the colors on the left side of the maps in red and dark blue
• Turbulent flow is shown as colors on the right side in yellow, green and light blue
• Variance mode maps can switch side-to-side as well as up and down
• Indicates flow away and towards as well as laminar and turbulent flow

Analyzing Variance Mode Display

• Note which colors located along the left side of the display which will represent laminar flow
• Note which colors located along the right side of the display indicate the colors which will represent turbulent flow
• Laminar flow is normal flow patterns; turbulent flow can mean pathology

Color Doppler Angle

• An angle to flow is needed to have color, but the Doppler angle is less important than with PW or CW spectral Doppler
• Linear transducers may result in no Doppler shift because of the 90° angle
• Phasing the beam can be used to solve this problem

Determining flow direction in steered color box

• Draw a transducer line extending straight down from the top corner of steered color box
• the line should not intersect the color
• Use the finger to move away from the blue color, thats the direction of the flow

Analyzing flow direction

• To analyze which way flow is progressing, determine if the flow is moving from right to left or left ot right

Doppler Angles

• A Doppler angle of 90 degrees causes no Doppler shift

Color Doppler flow direction observation

• Uniform positive Doppler shifts are obesrved when steered to the left
• Uniform negative doplper shifts are observed when steered to the right

Aliasing

• Blood cells traveling at high velocities are displayed as the wrong color
• Color Doppler is subject to aliasing as it uses a pulsed ultrasound technique

Aliasing observations

• Positive (blue) Doppler shifts are shown in the arterial flow in the image
• The negative doppler shifts have exceeded the lower Nyquist limit and are convered into the equivalent flow speed and wrapped towards the positive side of the color bar

Color doppler aliasing

• The difference in color scale ranges in Color Doppler images can mean aliasing

Doppler Shift Detection process

• A signal processor gets echoes from the beam former
• Doppler shifted echoes are then detected in the signal processor using a technique known as autocorrelation

Autocorrelation

• Mathematical technique used to analyze color Doppler flow
• Determines mean (average), variance (size) and sign (positive or negative) of the Doppler shift signal
• Requires that several pulses be sent out per scan line (ensemble/packet length)
• Multiple pulses, between 10-20 (minimum of 3 pulses) create one scan line of color information
• Low frame rate
• Decreased temporal resolution
• Results in (less accurate/less detailed information, but faster than FFT)

Ensemble/Packet

• Accurate color Doppler flow representation requires multiple ultrasound pulses
• Larger ensemble lengths provide more accurate velocity representation and are more sensitive to low flow
• Disadvantages of larger ensemble lengths:
• More time needed to acquire data
• Reduced frame rate
• Decreased temporal resolution

Color Controls: Wall Filter

• Wall filter removes clutter/ghosting, which are artifacts, caused by tissue motion in the heart or vessel wall
• Removes low frequency Doppler shifts, which may filter out low velocity flow

Adjusting Color Wall Filter

• Filter settings are displayed on the color scale, using a horizontal arrow
• Using a low wall filter setting, color will "bleed" into moving anatomy
• When the color is bleeding, raise the filter so that low velocity, like tissue, does not show color

Color Flow Inversion indication

• Appearance of PV flow in blue falsely suggests flow reversal
• With correct settings appropriate direction flow noted

Color Box Size and impact to imaging

• Color Box should be as small as possible
• Small means will be High Frame Rate, High Temporal resolution
• Large meands will be Low Frame Rate, Low Temporal Resolution

Color Box Steering observations

• Changing the angle of insonation
• Good sensitivity comes with smaller steering
• Sensitivity decreases as beam is steered

Factors Affecting Color Flow Image

• Power is a factor
• Gain is a factor
• Frequency is a factor
• Pulse repetition frequency (also called scale) is a factor
• Focus is a factor

Power and Gain in B-Mode Scanning

• Color flow uses higher intensity power than B-mode
• Attention should be paid to safety standards
• Set Power and gain such that good signal for flow is present while minimizing the signals from surrounding tissue

Frequency Selection

• Many transducer combinations permit changes of frequency
• Setting Higher frequencies provide better sensitivity to low flow with better spatial resolution
• Lower frequencies have better penetration and are less susceptible to aliasing at high velocities

Velocity Scale and Pulse Repetition observations and adjustments

• Low pulse repetition frequencies should be used to examine low velocities but aliasing may occur if high velocities are encountered
• If repetition is set low, color image will show ambiguity, increase the scale to show clear image

Focus observations

• Focus should be set at the level of the area of interest
• This results in a significant difference to the appearance and accuracy of the image

Power/Energy/Amplitude Flow observations and limitations

• Sensitive to low flows
• No directional information in some modes
• Very poor temporal resolution
• Susceptible to noise

Power Doppler Displays

• AKA: Color power Doppler, ultrasound angio, color Doppler energy, energy mode, color power angio, color angio
• Non-directional color Doppler
• All vessels are encoded with the same color
• Processes the strength of the reflected signal, with no regard as to direction or speed
• Amplitude of the reflection is directly related to the number of moving blood cells (scatterers)

Power Doppler characteristics

• Power Doppler is more sensitive to low flow velocity, and smaller vessels
• Power Doppler is unaffected by Doppler angles
• There is no aliasing in Power Doppler, so information of velocity is ignored
• There is no measurement of velocity flow in any direction
• The frame rates are slower and the temporal velocity is reduces
• Power Doppler is susceptible to any movement of the transducer, this results in a flash aritfact