class 12 color doppler

autocorrelation

repeats pulses several times (8-20) along the same scan line to compare echo phases for each depth

stationary reflectors will be

in phase

autocorrelation estimates

MEAN velocity and variance at each depth

Color Doppler frame rate vs b mode

multiple pulses dec the FR (lower than b mode)

To improve time resolution

small color box/ensemble length

color doppler assigns a

color to a mean Doppler shift (gives direction, speed and character of flow)

how is phase measured

look at the RF peaks to see if the phase is constant or changing

Autocorrelation rapidly calculates

direction, mean and variance of the Doppler shifts

Autocorrelation requires a minimum of

3 pulses to Separate motion from bulk tissue shifts

Quadrature-sampled signals

90deg phase shift between samples

motion will have a

change in phase between the delayed and undelayed signals from that depth

phase difference indicates

velocity and direction (toward or away)

B-Mode vs color

B mode has 1 pulse per scan line while color has a minimum of 3 pulses per scan line (usually ends up being 8-20)

PRF

pulses/sec (pulses/line is the ensemble length)

lines/frame depends on

width/line density of the color box

Color Doppler Frame rate

PRF/(Nn) (N = # of scan lines/frame and n = ensemble length or pulses/line)

ensemble length control

Sensitivity

Longer ensemble (more pulses per line)

Better velocity estimate but Worse time resolution

dec line density

inc frame rate but dec spatial resolution

dec color box size

fewer lines but dec FOV

Hue

perceived color based on the freq of light (color changes with velocity)

Saturation

amount of color mixed with white (less white = greater saturation)

Value

brightness of a color

more white

higher velocity

Variance

Range of velocities within an ensemble

Variance is similar to

spectral broadening

High variance

Disturbed flow

spectral waveform displays

velocities within the sample volume over a period of 4sec to show the velocities in systole and diastole

PRF spectral vs color

PRF is higher for spectral

Color vs. Spectral Processing

Color uses Autocorrelation while spectral uses Fourier Analysis/FFT

Power Doppler Displays

Doppler signal strength (No velocity or direction info)

Power Doppler is sensitive to

low flow velocities and tissue motion (includes xdcr movement)

Power Doppler uses the

total power in the Doppler signal to produce a real time color image of blood flow

colors in the power Doppler image indicate that

blood flow is present (no info on flow velocity)

power Doppler uses

Integrals (Signal magnitude instead of phase changes)

total power of the Doppler signal from blood is independent of

blood flow velocity, direction and Doppler angle

Power Doppler is not subject to

aliasing (allows you to use a lower prf)

Color Doppler noise appears as

signals with random phase shifts

random noise has

uniform low power so its displayed with uniform dark color in Power Doppler (separated from high power Doppler signals from blood flow)

Noise is less of a problem in

power doppler

power doppler tends to apply more

averaging giving better vessel wall definition (from temporal avg and Sensitivity to low flow near walls)

Smoothing button controls

spatial avg and Persistence controls temporal avg

inc PRF (inc scale) minimizes

aliasing and color artifacts

if Wall Filter is too low it displays

tissue motion

Limitations of Color Doppler

Mean velocities only, No angle correction, Aliasing and tissue motion artifacts

Color Doppler has limited

spatial resolution and accuracy of velocity estimates

Blood flow Doppler signals

high velocities and low amp

Myocardial wall Doppler signals

low velocities and high amp

Conventional Doppler ( vs tissue doppler)

the high pass filter prevents detection of high amp signals (myocardium) while in TDI wall filter is bypassed

in Tissue Doppler Imaging (TDI):

Blood flow signals are eliminated by an amp threshold

Tissue Doppler Modes

color (2d/m mode) and PW