Color Flow Imaging: An Overview

Color Flow Imaging Basics

• Provides an overall view of flow in a region, though with limited flow details.
• Has poor temporal resolution and flow dynamics due to potentially low frame rates when scanning deep.
• Utilizes different color maps and provides direction and velocity information, including turbulent flows.
• Employs color-coded maps of Doppler shifts superimposed on a B-mode ultrasound image.
• Uses pulsed wave ultrasound.
• Processing differs from that used for Doppler sonograms.
• May produce thousands of color points of flow information for each frame on the B-mode image.
• Uses fewer, shorter pulses along each color scan line, yielding a mean frequency shift and variance in small measurement areas.
• Frequency shift displayed as a color pixel.
• The scanner repeats this process to build a color image superimposed on the B-mode image.
• Achieves the impression of a combined simultaneous image by rapidly switching transducer elements between B-mode and color flow imaging.
• The pulses for color flow imaging are typically three to four times longer than those for B-mode, resulting in some loss of axial resolution.
• Color assignment to frequency shifts is usually based on flow direction (e.g., red towards, blue away) and magnitude via different color hues or saturation levels.
• Image quality relies on general Doppler factors, mainly a good beam/flow angle.
• Experienced operators adjust scanning approaches to achieve good insonation angles for quality flow images.
• Employs pulse-echo technique.
• Echoes from stationary tissue in grayscale show depth and brightness.
• In each scan line, differing returning frequency (Doppler shift detection) results in echoes represented with color Doppler (pixels display color).
• The color shown relies on whether the Doppler shift was positive or negative
• Colors further from baseline show a greater Doppler shift

Components of Color

• Hue is the color perceived by the viewer, based on frequency, with red being the lowest and violet being the highest.
• Saturation indicates the amount of a hue mixed with white; highly saturated colors are more vivid.
• Luminance indicates the brightness of the hue and saturation.

Saturation Details

• Color saturation shows the vividness of a color, or the gradation of hue from unsaturated to fully saturated (100% of a given color).
• For example, red has a more saturated color than pink because pink, consists of red and white.
• Saturation is zero (no hue) in 2D gray scale anatomic imaging.

Color Flow Doppler ImagingSpecifics

• Velocity information is coded into colors, which are superimposed on a 2D grayscale anatomic image.
• Displays average velocities, not peak velocities.
• Color flow maps use a table to convert measured velocities into distinct colors.
• Vertical color bars on one side of the image represent this table or map.
• The two main color flow map modes are velocity mode and variance mode

Velocity and Variance Modes

• Velocity mode colors inform about flow direction and speed
• Variance mode provides more information than Velocity Mode
• Variance mode can distinguish laminar flow from turbulent flow, this is in addition to direction and speed information contained in Velocity Mode

Velocity Mode Details

• The black region shows no Doppler shift.
• Colors above indicate movement toward the transducer (positive Doppler shift).
• Colors below indicate movement away from the transducer (negative Doppler shift).
• The black region in the center is the “0” baseline which means no flow, or no Doppler shift
• Colors above the black region indicate the color a reflector shows as it moves toward the transducer which shows a positive Doppler shift
• The top colors are a greater positive Doppler shift
• Colors below, mean movement away from the transducer, showing a reflector color as a negative Doppler shift
• Bottom colors, show a greater negative Doppler shift
• Numbers at the top/bottom, show the mean velocity in which aliasing begins
• Low velocity reflectors toward or from the Doppler signal are shown by more saturated colors
• High velocity reflectors are shown by less saturated colors
• Further stripe colors from theblack, indicates higher velocities
• Two color map
• Map 1 shows Flow Direction
• Map 2 shows changes in velocity

Velocity Mode Characteristics

• Color change runs vertical, not horizontal
• Closer color to black, indicates lower velocity

Variance Mode Characteristics

• Laminar flow appears as colors down the left (red or dark blue).
• Turbulent flow is shown as colors down the right (yellow, green, or light blue).
• Variance mode maps shift side-to-side, but shifts vertically as well
• It shows laminar and turbulent flows
• Colors along the display show the colors which displays laminar flow
• Colors on the right, show colors that display turbulent flow
• A pathology can be observed when turbulent flow is present

Color Doppler Angle and Flow Determination

• An angle is needed to show direction of flow, but is less important than PW or CW
• A 90 degree angle will result in a non-Doppler shift
• Use phasing to steer the beam to resolve this issue
• Imaginary lines determine flow in vessels
• To find flow, create an imaginary “home” line
• Extend a "home" line downwards from the upper corner
• The Home Line does not run into the color
• Moving towards the "home"line, is the vessels flow
• First, examine the color map, which colors are "towards and away"
• Then you slide your finger from “towards”, to “away”, to determine the vessel flow

Determining Direction of Flow

• Steered color box
• Draw a “transducer” line extending straight downward, from the top corner, which does not intersect the color
• Put your finger the blue color, and move it away from the transducer line
• With this process, you will find the vessels flow direction

Flow Direction and Doppler Shifts

• Right side flow is progressing
• Uniform positive Doppler shifts are observed steer color, towards the left
• Uniform negative Doppler shifts are observed steer color, towards the right
• No Doppler Shift if the Doppler is at a 90 degree angle

Aliasing

• Blood cells traveling at high speeds show the wrong color
• Color Doppler detects, using pulse echo, and can display as aliasing

Doppler Shift Detection

How?

• Using autocorrelation the signal processor detects Doppler shifted echoes

Autocorrelation Characteristics

• Applies digital math approach, to examine color Doppler flow, to analyze
• Determines mean, and signs of shift signals
• Pulses sent, scan per lines (ensemble/packet length)
• 10-20 Pulses, form a scan line
• It is less accurate, but faster

Ensemble Packet

• It needs a bunch of pulsed ultrasound
• Large ensemble lengths are a benefit
• More accurate velocity
• Low flow sensitivity
• The draw back is increased time and a slower framerate

Ensemble Length

• Increased Ensemble length means better doppler sensitivity
• Decreased frame rate

Color Controls

• Removes Low Frequency Doppler Shifts
• Helps filter clutter that’s caused by tissue

Wall Filter

• Helps filter settings
• Color flows well when it’s a low wall setting
• Color is affected by moving anatomy
• Reflectors in Low Velocity will not show with increased velocity

Color Box

• Use of the Color Box, must be small as possible
• Increased Time, improves resolution
• Decreased time, worsens resolution
• Steering the color box means your changing the insonation
• Small steering has better image quality, and bigger is unusable

Adjusting Color Velocity

High, Absent Flow

• Medium, Normal Flow
• Low, Color Aliasing
• High, Absent Flow, Apparent Absence in PV
• Normal Scale (med), normal with patent PV
• Low Scale, Color Aliasing in PV

Adjust Color Gain

• Has to have high settings - in order to see speckle
• High setting on flow, mean’s it’s “bleeding out”

Flow Rates

• Its a noise when it’s sensitive to the flows
• It has no directions
• Low resolution
• Susceptible to noise

About Power Doppler

• Power Doppler = ultrasound angio/ Doppler Energy
• Encodes w/ the color
• Strengths show power
• Amplitude and reflection is directly, related to cells moving
• Better vessel, less aliasing
• No velocity direction, motion sensitive, resolution, has less aliasing

Benefits and Draw Backs

Non Directional - Good, detects stenosis

• Great for Tortuous vessels
• Does not show speed, direction
• Motion Issues and resolution

Not dependent on any shifting

• Power Shift only
• All different power brightness
• Not quantified

Factors to Affect

• power given power to tissues
• Gains sensitivity to the flow
• Frequency trades penetration
• Repetition can change the scales
• Aliasing at high speeds
• Focus on Color Imagery

Power and Gain Details

• Uses power better the beams
• Important to pay attention for good signal/flow

Other Factors

• Important use proper gain - because minimizing help minimize
• Frequency gives a good special reolution or a low signal
• Low is less likely to get a aliasing issue
• Scale should be at low to examine
• When you see high the issues will be on velocity flow
• Focus should at the level