M7: Doppler Flashcards
block diagram for doppler instrumentation
TX > beam former > signal processor > image processor and sound production
2 parts of the signal processor for doppler
doppler shift detector
spectrum analyzer
how does frequency change when we scan in 2D vs doppler
frequency is lower for doppler than for 2D due to more attenuation from the RBCs (Rayleigh scatter)
how many cycles per pulse does the pulser generate for doppler
why
5-30 cycle pulses
to improve accuracy and interact w/ the cardiac cycle longer
doppler shift detection is the result of which process
how does it work
quadrature phase detector
the transmitted signal (reference signal) is duplicated and stored… the duplicated reference signals are the same except they are 1/4 out of phase w/ each other…
… the returning signal is compared to the reference signal to determine a + or – shift, which determines the resultant signal
when you see quadrature phase detector what do you think
think 1/4 out of phase, and trying to detect + or – changes
where is the resultant signal sent
to a speaker or spectral analyzer for image processing
what does the overall doppler shift signal contain
contains all the shifts that occur in the sample gate
how does the spectrum analyzer work
it separates the individual doppler shifts from the complex beat frequency… (this is where fast Fourier transformation occurs)
the phenomenon of fast Fourier transformation is synonymous to what
a prism separating the different hues that comprise white light
how is the doppler signal separated into the doppler shifted frequencies (e.g. how is the spectral doppler tracing produced)
fast Fourier transformation
describe how fast Fourier transformation works
it analyzes the complex doppler shift signal and breaks the signal into the component or separate frequencies displaying both magnitude and amplitude
which axis displays magnitude
which displays time
mag: Y
time: X
what is the magnitude of the doppler shift
the amount of + or – shift
is a + 4 kHz doppler shift the same magnitude as a – 4 kHz shift
yes… refers to ‘distance’ from the baseline only… magnitudes are the same, but the sign is opposite
factors affecting magnitude
RBC velocity, angle of insonation, operating frequency
e.g. everything in the doppler shift formula except C which is constant
relationship b/w doppler shift and magnitude
greater doppler shift = greater magnitude
directly related
what is the amplitude of the doppler shift
represents the number of RBCs that are contained w/in the SV
other names for amplitude
brightness, power, intensity
will two individual doppler shifts of 4 kHz have the same amplitude
no. they have the same magnitude but may not have the same # of RBCs moving through the SV… depends on RBC density
how is amplitude displayed on the spectral tracing
Z-axis or brightness of the tracing
factors affecting amplitude
RBC density
attenuation
power
gain
what is the SV comprised of
beam width, gate length (SV size) and pulse length
as well as size in the Z axis
is the SV range gated?
what does that mean
yes
we know how long it takes the sound to go out the sampled depth and come back because we are able to specify a desired depth
is the SV of PW doppler three dimensional
yes
does the SV size change relative to depth
what happens when its placed in the far field
yes…
it becomes very large due to the divergence in 3 dimensions…. there is the potential for spectral broadening and picking up signals from other vessels
why does aliasing occur
the velocity of the RBCs are exceeding the sampling limit of the machine w/ PW… we are exceeding the nyquist limit which is 1/2 the PRF
MUST BE OVER HALF THE PRF
what limits the amount of sampling
depth (PRF)
formula for nyquist limit
NL = 1/2 PRF
5 ways to correct for aliasing
which ones do we do
change the baseline increase the PRF (scale) lower the frequency increase the doppler angle switch to CW
the first 2
why cant we lower the frequency, increase the doppler angle or switch to CW to correct for aliasing
- usually defaulted to lowest frequency already
- increase % error
- dont usually have correct probe, but good in the echo lab
describe high PRF mode
can be used when the velocities exceed the nyquist limit… allows the machine to fire the next pulse before the echos from the first pulse have come back
purpose of high PRF mode
possible consequence
can help raise the nyquist limit
range ambiguity
how can range ambiguity affect the spectral tracing
can cause you to pick up flow from multiple depths resulting in a complex spectral tracing that displays multiple flow profiles of vessels
with what type of measurements should you use a fast sweep speed
why
intracycle measurements (AR, PR) due to easier caliper placement and increased accuracy for systolic events since the waveform is more spread out
with what type of measurements should you use a slow sweep speed
why
intercycle measurements like rate and rhythm
describe how CW works
CW has an oscillator that produces a continuous AC voltage for motion detection.. the echo signal can be sent to a speaker to processed into an image/spectra tracing just like PW
newer development w/ CW probes
fire half of the crystals with pulsed to get a 2D image and the other half of the crystals w/ CW to get a CW signal… allows for 2D and CW simultaneously
BUT CW STILL DOESN’T PRODUCE AN IMAGE
describe wall filter
why is it needed
used to reject low frequencies from wall motion of pulsatile vessels…. e.g. gets rid of noise but
needed b/c doppler is sensitive to ANY motion, not just moving RBCs
risk w/ a high wall filter
could reject real, low level echos in the lower frequency range if set to high
what type of info does colour doppler provide
what does it include in its assessment
complete qualitative blood flow analysis over a wide field of view…
combines anatomical detail from the 2d image and physiological hemodynamics w/ the colour overlay
other names for colour doppler (CD)
colour doppler imaging (CDI)
colour flow imaging (CFI)
colour flow mapping (CFM)
two processes required to display colour doppler
what does mean for the # of pulses per scan line, whats the effect of this
first, the standard 2D process - 1 pulse per scan line
second, the doppler processing - at least 3 pulses per scan line
we need several pulses per scan line which has a huge affect on FR
what processing technique is used to produce colour doppler
autocorrelation
describe autocorrelation
mathematical means of rapidly determining the sign, mean, power and variance of the returning signal
what does the term “sign” refer to w/ colour doppler
direction of flow
how many doppler samples are displayed on the monitor per scan line
how does this affect FR and why
100-400
dramatically reduces it b/c autocorrelation must occur at each depth along the scan line
typical FRs w/ colour doppler
what does it depend on
5-50 Hz
depends on depth and size of the field of view
what is ensemble length
of pulses used for each scan line for colour doppler
another name for ensemble length
packet size
another name for scan line
A-line
what does the colour doppler that we see represent
mean or average of the velocities present
how many pulses per scan line are commonly used to calculate the mean velocity for colour doppler
10-20…. need a minimum of 3 pulses
relationship b/w ensemble length and FR
longer ensemble length = slower frame rate
what 4 pieces of information does autocorrelation provide about blood flow
sign (Hue)
magnitude (saturation)
power (luminance)
variance
what is hue
the colour that you see that represents the sign or direction of flow
If you arent getting reversal in arteries, how should you change the colour scale
lower it
describe saturation
is it a mean
saturation is the amount of colour in a mix of white and represents the magnitude of the doppler shift or the velocity of blood
yes
what does more white mean w/ saturation
more white = less saturated and faster flow/velocity
e.g. red is more saturated than pink
are people more sensitive to changes in hue or changes in saturation
hue
describe luminance
the brightness of the hue and saturation present… it represents the power of the flow and tells us the RBC density in the field of view
relationship b/w # of RBCs and luminance
more RBCs travelling at the same velocity in a sample will look brighter on the display
what is variance
how does it appear on CD
the # of velocities that diverge from the mean velocity
will show all difference colours/hues based on the actual velocity, not the mean
advantage of a variance map
can give greater detail than the averaged velocities in a hue or saturation map (can help pin point disease)
what is priority
another CD control that determines whether or not the colour overlay will overwrite the low level echos
what happens if priority is set low
high?
low: precedence is given to non-doppler shifted weak echos
high: allows the colour to overwrite the low 2D echos
risk if priority is set too high
colour could overwrite plaque and you miss pathology
describe power doppler
does it display magnitude or amplitude
represents the strength of the doppler shift rather than velocity
amplitude ONLY
advantages to power doppler
no aliasing, less dependant on angle, very sensitive to low flow states
disadvantages to power doppler
no direction flow speed or character information
susceptible to blooming artifact
other names for power doppler
energy mode
power-angio
w/ what type of exam might power doppler be very useful
EV, looking for stalk connecting to poly, cant get an angle
Another name for scale
Velocity range
