Color Flow Imaging in Ultrasound

Color Flow Imaging Overview

• It is an ultrasound technique using color-coded maps to show Doppler shifts
• The maps are superimposed on standard B-mode ultrasound images
• Pulsed wave ultrasound is used
• Color points of flow are produced for each frame on the B-mode
• Fewer, shorter pulses are used on color scan lines to find the mean frequency shift and variance

Color Flow Capabilities and Limitations

• Produces an overall view of flow in a region
• Has limited flow information
• Suffers from poor temporal resolution and flow dynamics
• The frame rate can be low when scanning deep

Constructing a Color Image

• The frequency shift is displayed as a color pixel
• The scanner builds the color image over several lines
• The color image is laid over the B-mode image
• Transducer elements switch rapidly between B-mode and color flow imaging

Pulses

• Pulses are typically three to four times longer
• This causes a loss of axial resolution in the color flow image

Color Assignment

• Color assignment is based on direction and magnitude of frequency shifts
• Red shows Doppler shifts toward the ultrasound beam
• Blue shows Doppler shifts away from the ultrasound beam
• Different color hues or lighter saturation indicate magnitude

Dependence on Doppler factors

• Color Doppler images depend on general Doppler factors
• Good beam/flow angle are needed
• Operators adjust scanning for good insonation angles

Color Doppler Technique

• Employs pulse-echo technique
• Echoes from stationary tissue are gray scale, showing depth and brightness
• In each scan line, if a different frequency returns, a Doppler shift is detected
• Echoes are represented with color Doppler

Color Representation

• Colors indicate whether the Doppler shift was positive or negative
• Colors further from the baseline indicate increased Doppler shift

Components of Color

• Hue: the color perceived by the viewer
• Saturation: the amount of hue mixed with white; describes color vividness
• Luminance: the brightness of the hue and saturation

Hue

• Eye sees red, orange, yellow, green, blue, and violet
• Red has the lowest frequency
• Violet has the highest frequency

Saturation Details

• It describes color vividness from unsaturated (white) to fully saturated (100% of the given color)
• Red has a higher saturation than pink, because pink is red mixed with white
• Saturation is zero in a 2D gray scale anatomic image

Luminance Details

• Luminance is the brightness of the hue and saturation

Color Flow Doppler Display

• Velocity information is coded into colors and superimposed on a 2D grayscale anatomic image
• Represents average velocities instead of peak velocities

Color Flow Maps

• Color flow maps convert measured velocities into specific colors
• The "table" or map has vertical color bars on the image
• Common modes are Velocity mode or Variance mode

Color Map Modes

• Velocity: Colors indicate flow direction and velocity
• Variance: Provides more information than velocity mode by distinguishing laminar from turbulent flow

Velocity Mode Details

• The black region indicates "no Doppler shift"
• Colors above the black strip indicate movement toward the transducer and positive Doppler shift
• Colors below the strip indicate movement away from the transducer and negative Doppler shift

Velocity Mode Maps Details

• Black region in the center indicates no flow or Doppler shift
• Colors above the black region indicate the color of a reflector moving toward the transducer with positive Doppler shift
• Colors at the top indicate greater positive Doppler shift
• Colors below the black region indicate the color of a reflector moving away from the transducer and negative Doppler shift
• Colors at the bottom indicate a greater negative Doppler shift
• Numbers at the top and bottom of the color bar is where aliasing will begin

Aliasing Explanation

• Aliasing flow is faster than can be measured
• It is displayed on the wrong side of the baseline

Velocity Mode Maps Traits

• Low velocity reflectors are more saturated
• High-velocity reflectors are less saturated
• Colors farther from the black stripe represent higher velocities
• Change is always up and down, not side to side

Variance Mode Details

• Laminar flow appears as red and dark blue on the map's left side
• Turbulent flow appears as yellow, green, and light blue on the map's right side
• Variance mode maps change side-to-side and up and down

Laminar and Turbulent Flows

• Laminar flow indicates normal flow patterns
• Turbulent flow indicates a pathology

Color Doppler Angle

• An angle to flow is needed to have color
• The Doppler angle is less important than with pulsed wave or continuous wave spectral Doppler
• Linear transducers have problems, as a 90° angle results in no Doppler shift
• This can be solved by using phasing to steer the beam

Determining Flow Direction: Rectangular Image

• Visualize an imaginary "home" line
• The "home" line extends downward from the upper corner on the exterior of the steered box
• The line must not run into the color
• Moving toward the home line indicates the direction of flow in the vessel

Determining Flow Direction: Sector Image

• First, determine which color on the color map is "toward" and which is "away"
• Place your finger on the toward color and slide to the away color
• Direction of flow

Aliasing in Color Doppler

• Blood cells traveling at high velocities display as the wrong color
• Color Doppler uses pulsed ultrasound, making it subject to aliasing

Doppler Shift Detection Process

• A signal processor receives echoes from a beam former
• Doppler-shifted echoes are detected in the signal processor using autocorrelation

Autocorrelation Technique

• It is a digital mathematical technique used to analyze color Doppler flow
• Determines mean, variance, and sign of the Doppler shift signal
• Requires several pulses per scan line and creates one scan line of color

Number of Pulses

• Multiple pulses, typically 10-20, create one scan line of color information
• Autocorrelation has a low frame rate and decreased temporal resolution
• Less accurate, but faster than FFT

Ensemble/Packet Details

• Accurate color Doppler flow representation requires multiple ultrasound pulses
• Larger ensemble lengths lead to a more accurate velocity representation and sensitivity to low flow
• Larger ensemble lengths need more time to acquire data
• Reduced frame rate and decreased temporal resolution result

Color Controls: Wall Filter

• It removes clutter and ghosting artifacts caused by tissue or vessel wall motion
• It removes low-frequency Doppler shifts, which can filter out low-velocity flow

Adjusting the Wall Filter

• Filter setting is displayed on a color scale with a horizontal arrow
• Color "bleeds" into moving anatomy with a low wall filter setting
• Raise it to prevent low velocity reflectors, like tissue, from showing color

Color Flow Inversion

• Can falsely suggest the flow of reversal

Color Box

• The box should be as small as possible to optimize temporal resolution

Steering to Correct Artifact

• Adjusting color velocity scale is imperative

Optimal Color Gain

• Gain should be set as high as possible without displaying random color speckle

Power/Energy/Amplitude Flow

• Sensitive to low flows, lacks directional information in some modes
• Very poor temporal resolution
• Susceptible to noise

Power Doppler Details

• Power Doppler is also known as color power Doppler, ultrasound angio, color Doppler energy, energy mode, color power angio, and color angio
• It is a non-directional color Doppler
• Vessels are encoded with the same color
• It processes the strength of the reflected signal without regard as to direction or speed
• The amplitude of the reflection is directly related to the number of moving blood cells

Power Doppler: Advantages

• More sensitive to low flow or velocity and smaller vessels
• Unaffected by Doppler angles
• No aliasing because velocity information is ignored

Power Doppler: Disadvantages

• Provides no measurement of velocity or flow direction
• Low frame rates compared to conventional color Doppler
• Reduced temporal resolution
• Susceptible to motion of the transducer, patient, or soft tissue
• May result in a color burst/flash artifact

Benefits of Power Doppler

• Angle independent and increases sensitivity to detect low-velocity flow
• Useful in imaging tortuous vessels and increased accuracy of grading stenosis

Power Doppler Properties

• Not dependent on Doppler frequency shift, velocity, or angle
• Dependent on Doppler power shift only
• Assigns color brightness to echoes of different power and is not quantitative

Factors Affecting Color Flow Image

• Power: Transmitted power into tissue
• Gain: Overall sensitivity to flow signals
• Frequency: Trades penetration for sensitivity and resolution
• Pulse Repetition Frequency: Low to look at low velocities and high to reduce aliasing
• Focus: Color flow image is optimized at the focal zone

Power and Gain Settings

• Color flow uses higher-intensity power than B-mode
• Attention should be paid to safety indices and set to obtain a good signal for flow
• Minimizes the signals from surrounding tissue

Frequency Selection Guidelines

• Many transducer combinations permit the changing of frequency
• High frequencies give better sensitivity to low flow and have better spatial resolution
• Low frequencies have better penetration and are less susceptible to aliasing at high velocities

Velocity Scale Optimization

• Examine low velocities with low pulse repetition frequencies
• Aliasing may occur if high velocities are encountered

Focus Adjustment

• Should be at the level of the area of interest
• This can impact the appearance and accuracy of the image