C7: Doppler Flashcards by M B

define doppler effect

a perceived change in pitch due motion of the source of sound or the observer

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when a source of sound is moving towards you, will the pitch be higher or lower

higher

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when a source of sound is moving away from you, will the pitch be higher or lower

lower

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what 3 pieces of information can spectral doppler tell you

presence of flow
direction of flow
quality of flow

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when you are using duplex scanning, how many pulses does the machine send out

2….. 1 for 2D image and 1 for spectral doppler

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the amount of doppler shift depends on what 2 qualities of blood flow

direction and velocity of RBCs

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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)

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the doppler shift is based on what principle

wave interference

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what causes a beat frequency

when the reflected sound wave varies slightly in frequency from the transmitted frequency

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what is the most basic doppler formula

Doppler shift = received frequency - transmitted frequency

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describe fast Fourier transformation

mathematical technique that separates individual doppler shifts from the beat frequency and displays them in a spectral waveform

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what is the detailed doppler formula

Change in frequency = (2 (operating F) x velocity x cos theda) / c

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as the angle of insonation increases, what happens to the doppler shift

it gets smaller

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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)

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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

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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

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whats the most accurate angle for doppler

zero

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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

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what is another word for PRF

scale or velocity range

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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

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where should the SV be placed in a vessel for doppler

in the middle of the vessel or at the area of highest velocity

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what is multi-gating

allows you to sample multiple depths at the same time

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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)

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true of false…. the SV is equal to 1/2 the pulse length + the gate length

true

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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

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?

1. peak systolic velocity 2. envelope (thickness of the tracing, showing range of velocities) 3. window (area under the tracing) 4. dicrotic notch 5. 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