C7: Doppler Flashcards
define doppler effect
a perceived change in pitch due motion of the source of sound or the observer
when a source of sound is moving towards you, will the pitch be higher or lower
higher
when a source of sound is moving away from you, will the pitch be higher or lower
lower
what 3 pieces of information can spectral doppler tell you
presence of flow
direction of flow
quality of flow
when you are using duplex scanning, how many pulses does the machine send out
2….. 1 for 2D image and 1 for spectral doppler
the amount of doppler shift depends on what 2 qualities of blood flow
direction and velocity of RBCs
is the intensity of Rayleigh scatter proportional to the # of RBCs passing through the SV
yes (so it indicates the quantity of blood in a sample)
the doppler shift is based on what principle
wave interference
what causes a beat frequency
when the reflected sound wave varies slightly in frequency from the transmitted frequency
what is the most basic doppler formula
Doppler shift = received frequency - transmitted frequency
describe fast Fourier transformation
mathematical technique that separates individual doppler shifts from the beat frequency and displays them in a spectral waveform
what is the detailed doppler formula
Change in frequency = (2 (operating F) x velocity x cos theda) / c
as the angle of insonation increases, what happens to the doppler shift
it gets smaller
is a lower or higher frequency probe better for doppler and why
lower
lower is better because the RBCs return weak echos (they attenuate a lot)
when using duplex scanning, how will the frequencies used diff b/w the 2D image and the doppler signal
2D: higher frequency for better resolution
doppler: lower to reduce attenuation
why is the angle of insonation so important for doppler
it effects the velocity reading so will make calculations incorrect if its inaccurate (greater than 60)… and the angle increases so does the % error for velocity
whats the most accurate angle for doppler
zero
what is the nyquist limit
what happens if its exceeded
the limit reached when the RBC velocities are faster than the machines ability to sample
it is equal to 1/2 the PRF… if half the PRF is EXCEEDED than aliasing will occur b/c RBCs arent being sampled fast enough
what is another word for PRF
scale or velocity range
what are the 5 ways to correct for aliasing
which are acceptable to use?
- move the baseline
- increase the PRF
- increase the doppler angle
- lower the operating frequency
- change to CW
1 and 2
where should the SV be placed in a vessel for doppler
in the middle of the vessel or at the area of highest velocity
what is multi-gating
allows you to sample multiple depths at the same time
what are the 3 things that determine the SV
beam width
gate length
length of the emitted pulse (e.g. # of cycle in a pulse)
true of false…. the SV is equal to 1/2 the pulse length + the gate length
true
for doppler, are longer or shorter pulse better
why?
longer pulses…. it results in a more accurate beat frequency/doppler shift
minimum number of cycles-per-pulse required for spectral doppler
4 cycles per pulse
is range gating possible w/ CW?
what about angle correct?
no
no
where is the CW doppler SV found? what is it called
at the intersection of the transmitted and received beam… called the zone of sensitivity
with CW, how do we compensate for the fact that angle correct is not possible
you must get parallel to flow
does CW have a nyquist limit
can you get aliasing?
no b/c youre always sampling
no
do you have an image w/ CW
no
whats the problem with the large SV that CW has
theres the possibility of picking up the waveform of more than 1 vessel
what does the doppler spectral tracing represent
the range of doppler shift frequencies produced from each RBC that passed through the SV
what are the 3 axes shown in a doppler spectral tracing
time (x)
frequency or velocity (y) - can be either
power (z)
how is the z axis/power represented in a spectral tracing
what does it depend on
represented by the brightness of pixels
it is proportional to the number of RBCs moving at one specific velocity at a given time
what does a bright spectral tracing indicate
theres a large number of RBCs moving through the SV at that velocity
what are the 5 parts of a spectral wave form?
- peak systolic velocity
- envelope (thickness of the tracing, showing range of velocities)
- window (area under the tracing)
- dicrotic notch
- end diastolic velocity
which small blood vessels help determine the shape of the spectral tracing
arterioles
is the y axis of the spectral tracing usually expressed as frequency or velocity
velocity
why is velocity preferred over frequency for the y axis
it compensates for variation is vessel alignment and its more meaningful to the technologist
do pulsatility and resistance mean the same thing
yes
what are the characteristics of high pulsatility/resitance waveforms
- tall, narrow, sharp systolic peak
- reversal or absent diastolic flow
in which vessels do we commonly see high resistance waveforms?
arteries of the extremities…. body parts that are not vital to life
e.g. CFA
what are the characteristics of low pulsatility/resistance waveforms
- broad systolic peak
- forward flow through diastole
in which vessels do we commonly see low resistance waveforms?
in vessels that feed vital organs
e.g. ICA, renal arteries
are low resistance waveforms typical removed from the baseline?
yes, b/c there is always constant forward flow
what are the characteristics of moderate pulsatility/resistance waveforms
- tall, narrow, sharp systolic peaks
- forward flow through diastole
in which vessels do we commonly see moderate resistance waveforms?
vessels that feed both high and low resistance vascular beds
e.g. CCA (splits into ICA and ECA)
what is the pulsatility index
what is the formula
formula used to quantify the impedance to flow (like in the case of a stenosis)
PI = A - B / Mean
A = peak systolic velocity B = furthest point from peak systole
what is the resistive index
what is the formula
formula used to evaluate the resistance to flow (like in the case of a transplanted organ)
RI = A - B / A
A = peak systolic velocity B = endiastolic velocity
what is the systolic/diastolic ratio
what is the formula
formula to quantify the change in pulsatility
S/D ratio = A / B
what does it say about the resistance of a vessel if the S/D ratio is low?
high?
the vessel is low resistance
the vessel is high resistance
formula for acceleration
A = delta V / delta T
what is spectral broadening
what does it indicate
the vertical thickening of the spectral tracing or envelope
indicates that there is a greater range of velocities w/in the SV
if the velocities of the RBC are all the same, how will the spectral tracing appear
thin
what can cause spectral broadening
disturbances in flow: bifurcations, anastomoses, curves in vessels
pathology in arteries = plaque
what is false spectral broadening
why does it occur
an artifact that can occur when doppler gains are too high or if the SV gate length is too large relative to the vessels
what is autocorrelation
mathematical technique that assess the doppler shift signal to determine the mean, luminance and variance
what is the rouleau effect
when RBCs clump together, common in low flow vessels.
looks like smoke on the US monitor
what is the purpose of the zero crossing detector
it counts how many times the voltage shifts from + to – …. more shifts means higher frequency and greater doppler shift
