Ultrasound Physics - Color and Power Doppler

uses for color doppler

presence, direction, speed, and character of flow

when was color doppler first introduced to echo and general?

echo - 1984; general - 1987

what is color doppler a combination of? how is this combo achieved?

2D gray scale and color mapping - depicts motion by superimposing color onto gray scale image to provide visual representation of blood flow direction and velocity.

what does color encoding depend on?

velocity of the reflector

frame rate in color

~10 frames/ sec (lower than gray scale)

frame rate in gray scale

4-32 frames/sec

what sacrifice might need to be made to obtain a good color image?

speed of image formation (degraded temporal resolution)

determinations made by color

echo distance, echo strength (brightness), presence of doppler shift

amplitude of doppler shift, flow direction

what is doppler shift proportional to

blood flow velopcity and doppler angle

how are different frequency shifts represented in color doppler

different colors

high frequency

positive shift (towards)

low frequency

negative shift (away)

flow direction in color is similar to which other doppler

spectral

how does the system determine an object is stationary or moving

evaluates amplitude, phase, and frequency — moving structures cause a phase shift (indicates presence and flow of direction: doppler shift)

how does laminar flow appear on color doppler

flow is faster in the middle, therefore the colors are lighter/brighter in the middle

what do different hues represent in color doppler

different ranges of velocities

light colors on color doppler represent

fast velocities

dark colors on color doppler represent

slow velocities

why is color doppler more sensitive than

more pulses are needed to collect info from multiple echoes (from the same reflector)

ensemble length, pulse packets, dwell time

data acquisition using multiple pulses sequences along each scan line

what does multiple pulses lead to

better velocity estimate (homogenous color display) and slower frame rate (slower acquisition time)

how is color doppler displayed

autocorrelation — determines degree of motion; results in real time value for mean flow speed and the info displayed is color coded

who controls the velocity display

sonographer

velocity range of arteries

high

velocity range of veins

low

hue

perceived color

luminance

intensity

what does the machine do because scatterers move at diff speeds and directions

determines mean frequency shift at each specific location

what is variance

indicator of spread around the mean — indicates turbulence

how does variance relate to turbulence

inc variance = inc turbulence

what does variance mode add to color doppler

greens and yellows to help identify turbulence

plug flow

little to no variation — velocities are almost uniform

what is the color box

area of the image in which the color doppler is displayed - angle (scan lines) can be steered

what does widening the color box do to frame rate

decreases fr

what is the color bar divided by

zero baseline (black area) — represents no doppler shift and the wall filter

as you approach 90 degrees doppler angle, what happens to frequency

decrease

what do the sides of the color bar represent

top (towards), bottom (away), left (laminar/normal flow), right (turbulent/abnormal flow)

power doppler aka

energy doppler, color amplitude imaging, color angia

what is the purpose of power doppler

evaluate amplitude — does not look at doppler shift

what is amplitude proportional to

number of moving rbcs

what display mode is used for power doppler

autocorrelation amplitude info

how is noise treated in power doppler

assigned specific color by manufacturers (less intrusive and displayed as low power) — signal amplitude is always stronger than noise

why is power doppler more sensitive than color

detects lower velocities and has improved signal/noise ratio

advantages of power doppler

• angle independent

• no aliasing

• increased sensitivity and better signal/noice noise

• detect lower velocities

• can be amplified more —use higher gain w/o compromising image quality

  • good vessel oulines and lumen definition

what happens to weak signals in power doppler

summed to a pos number

higher frequency does what to amplitude

increase

power doppler limitations

• no speed or direction info

• more subject to motion/flash artifact (bc higher frame averaging)

• slow frame rate

how to increase color sensitivity

• smaller box

• better angle

• adjust focus

• decrease scale (PRF)

• decrease wall filter (inc threshold)

• adjust sv

• accumulation

• change frequency (TD)

to eliminate aliasing

• inc scale (PRF)

• decrease baseline

to increase frame rate

• dec box width

• dec b mode sector width

• dec depth

• inc prf (scale)

• dec line density

• dec frame averaging (inc TR; dec LR)