Doppler Chapter 19 full chapter Flashcards Preview

Physics > Doppler Chapter 19 full chapter > Flashcards

Flashcards in Doppler Chapter 19 full chapter Deck (110):

what is demodulation?

The process of extracting the low Doppler frequency from the tranducer's carrier frequency

*(Doppler shift is a low frequency that "rides" on top of a much higher transducer frequency)


What is the equation for doppler shift?

doppler shift = reflected frequency - transmitted frequency

doppler shift = 2 x velocity of blood x transducer freq x cos angle/

propagation speed


Doppler shifts are created when transmitted sound waves strike moving _______

red blood cells


When blood cells move toward the transducer, the doppler shift is ______


the reflected frequency is HIGHER than the transmitted frequency


blood cells moving toward the transducer reflect sound with a higher frequency....true or false



When blood cells move away from the transducer, the Doppler shift is _______


the reflected frequency is LOWER than the transmitted frequency


blood cells moving away from the transducer reflect sound with a lower frequency...true or false



Do doppler frequencies indicate velocity or speed?



what is speed?

purely a magnitude

indicated the distance that a red blood cell moves in 1 second

units of cm/s or any distance divided by time


what is velocity?

a magnitude AND a direction


What is the relationship between doppler shift and velocity?

directly related to the velocity of blood cells

the faster the velocity the greater the doppler shift


Why is there a 2 in the doppler equation?

there are actually 2 doppler shifts during a clinical ultrasound exam

first: when the sound wave from the transducer strikes moving blood vessels

second: after the sound wave strikes blood cells, reflections return to the transducer


The x-axis of a doppler spectrum represents ________. The y-axis represents _______ or ________


doppler shift, velocity


what unit is frequency measured when it comes to doppler?



the actual data is ________, with units of _______. the derived information is ___________ with units of ________






What is the relationship between transmitted frequency and the Doppler shift?

doppler shift is directly related to the frequency of the transmitted sound

if one is doubled, the other is doubled


Doppler shift measured in Hz has what relationship to velocity and transducer frequency?

directly related to velocity

directly related to transducer frequency


What does the measurement of the doppler frequency depends on the relationship between what?

the direction of blood flow and the direction in which the sound wave propagates


What happens when the blood cells are moving PARALLEL to the sound beam?

the ENTIRE velocity is measured

when the blood cells move either directly toward or directly away from the transducer the velocity measurement is 100% accurate


What happens when an angle exists between the direction of flow and the sound beam?

the measured velocity is less than the true velocity


If the sound beam and flow direction are not parallel, what determines how much of the velocity is measured?

the cosine of the angle between the sound beam and the direction of motion.

**measured velocity = true velocity x cos angle**


What is the relationship between the cosine angle and the doppler shift?

the doppler shift is directly related to the cosine of the angle between the direction of flow and the direction of sound

if one is reduced by half so is the other


Doppler shift is ________related to ________. when the cosine doubles, the doppler _______doubles. The cosine ________ as angles approach _______degrees


cosine angle





What is the relationship between actual velocity and measured velocity when blood moves parallel to the sound beam?

When the angle between the direction of motion and sound is 0 degrees or 180 degrees, flow is parallel

the measured and true velocities are equal

cosine 0 = 1, flow toward the transducer

cosine 180 = -1 flow is away from the transducer


What is the relationship between actual and measured velocities when blood moves perpendicular to the sound beam?

when the direction of flow is perpendicular (90 degrees to the sound beam), the measured velocity is zero because the cosine of 90 degrees is 0

doppler shifts and velocities cannot be measured with perpendicular incidence


What is the relationship between actual velocity and measured velocity when blood moves at a 60 degree angle to the beam?

since the cosine of 60 degrees is 0.5, the measured velocity at 60 degrees is one-half the actual velocity

When blood travels at 2 m/s at a 60 degree angle it is reported as 1m/s because it is halved


What does a positive doppler shift indicate?

flow TOWARD the transducer


what does a negative doppler shift indicate?

flow AWAY from the transducer


In relation to the baseline, how is flow toward the transducer displayed? away from the transducer??

toward: above the baseline

away: below the base line


What is phase quadrature or quadrature detection?

commonly used signal processing technique for bidirectional doppler

(displays flow above the base line AND below the baseline)


arterial velocities are ______than venous velocities



How many crystals are required in the continuous wave doppler?


one to constantly transmit ultrasonic energy, while the other constantly receives reflections from blood cells


what is the greatest advantage of continuous wave doppler?

its ability to accurately measure very high velocities


what is the primary disadvantage of continuous wave doppler?

that the exact location of the moving blood cells cannot be determined.


With a continuous wave doppler, what is range ambiguity?

signals arise from all blood cells in the region of overlap between the transmit and receive beams, these are the velocities that are measured

it is a disadvantage and limitation


Why is continuous wave doppler's lack of time gain compensation (TCG) a disadvantage?

reflections created from blood cells located DEEPER will have a LOWER amplitude than reflections that are at SHALLOWER depths

as a result doppler shifts produced by DEEPER blood cells may be incorrectly interpreted as having been created by FEWER blood cells.


What can a dedicated continuous wave transducer NOT do?

create anatomic images

Without the ability to create images of anatomy, short duration pulses are unnecessary


What is the shape of the two elements in the continuous wave transducer?



What is duplex imaging?

simultaneous anatomic imaging and doppler


Dedicated continuous wave transducers do not use backing material, what does that result in?

undampened transmitted signal

narrow bandwidth

high quality factor (Q-factor)

higher sensitivity


What kind of transducers have a matching layer?

both duplex and dedicated continuous wave transducers

(matching layers increase the efficiency of sound transmitted in and out of the body - an advantage to both)


What kind of sensitivity do continuous wave transducer have?

increased sensitivity

they detect low amplitude reflections and small doppler shifts


In a pulsed wave doppler how many crystals are necessary?

only one PZT crystal is necessary

the crystal alternates between sending and receiving sound pulses


What is the greatest advantage of pulsed doppler?

being able to select the exact location where velocities are measured

aka: RANGE RESOLUTION, RANGE SPECIFICITY or freedom from range ambiguity artifact


What is range resolution?

the ability to select the exact location where velocities are measured


What is range specificity?

the ability to select the exact location where velocities are measured


What is the sample volume or gate?

It is a marker placed where the sonographer wants to measure the velocity.

the system then calculates the time of flight for a sound pulse traveling to and from the gate


What is the primary disadvantage of pulsed doppler?

inaccurate measurement of high velocity signals


What is aliasing?

it means false identity, an artifact

high velocity flow in one direction is incorrectly displayed as traveling in the opposite direction

on spectral display the tip of the wave wraps around the bottom


What are the characteristics of pulsed doppler transducers?

like imaging transducers they contain backing material

low quality factor

lower sensitivity

wide bandwidth pulses


What happens with high flow when you try to take a sample of it?

the flow is above the baseline indicating flow toward the transducer.

The velocity of these blood cells reaches to the very top of the spectral display and then incorrectly wraps around and reappears at the very bottom of the display -

the very top is called the Nyquist limit, the bottom is aliasing


What is the Nyquist Limit?

high velocity of blood cells that reach the top of the spectral display before wrapping around...this upper limit is the Nyquist limit.


Can the Nyquist limit be underneath the baseline?


when the spectrum reaches the very bottom and then incorrectly wraps around and reappears at the very top....the very bottom is the Nyquist limit


What conditions create aliasing?

occurs only with pulsed doppler (never continuous wave)

occurs when the doppler sampling rate is too low in comparison to the measured blood velocities


What is the limit at which aliasing appears and how is it determined?

acts like a speed limit

aliasing appears when the doppler shift exceeds this speed limit

the Nyquist limit is one-half of the pulse repetition frequency (PRF)


What is the equation for the Nyquist limit?

Nyquist limit (Hz) = PRF (Hz) / 2


What is the Nyquist frequency?

aka: nyquist limit

the highest doppler frequency or velocity that can be measured without the appearance of aliasing


What is PRF?

pulse repetition frequency

The sampling rate of pulsed doppler

is the number of pulses of a repeating signal in a specific time unit, normally measured in pulses per second


What are the two ways to avoid aliasing?

raise the Nyquist limit

reduce the doppler shift


What is the PRF for doppler

velocities are sampled many times per second...the sampling rate is the PRF


When the sample volume is ______ the PRF is _____ and the Nyquist limit is _____





How is the presence of aliasing affected by deep sample volumes?

Velocity is sampled fewer times each second and the system is unable to accurately measure high velocities.

Deeper sample volume have lower PRF and create aliasing


When the sample volume is shallow, the PRF is ______ and velocity is sampled many times per second so the Nyquist limit is ______



the system accurately measures high velocities without aliasing


higher frequency transducers create more or less aliasing?


doppler shift is directly related to the transducers frequency


Smaller Doppler shifts from lower frequency transducers are less likely to exceed _______

they Nyquist limit

therefore aliasing artifacts are less common with lower frequency transducers


What creates less aliasing?

slower blood velocity

lower frequency transducer

shallow gate (high PRF)


what creates more aliasing?

faster blood velocity

higher frequency transducer

deep gate (low PRF)


What 5 technique may be used to avoid aliasing artifacts?

1. adjust the scale to its maximum

2. select a new ultrasonic view with a shallower sample volume a lower frequency transducer

4. use baseline shift

5. use continuous wave doppler


What are the advantages and disadvantages of to adjusting the scale to its maximum?

advantages: maxing PRF raises the Nyquist limit and aliasing is less likely to appear

disadvantages: Higher PRF reduces sensitivity to low velocities


What are the advantages and disadvantages to selecting a new ultrasonic view with a shallower sample volume?


a new view with a shallower sample volume eliminates aliasing.

the transducer can be moved to a new location with the same region of interest at a shallower depth...which increases PRF

Disadvantages: None


What are the advantages and disadvantages of selecting a lower frequency transducer?


doppler shift is directly related to frequency of transducer...

the lower frequency will reduce the height of the doppler spectrum because there will be lower doppler shift since they are directly related

lower shifts less likely to alias there fore not exceed Nyquist limit

disadvantages: no significant disadvantages


What are the advantages and disadvantages of using the baseline shift?


baseline shift simply slides the display baseline down so that the entire velocity scale is devoted to one direction,

info remains the same just looks nicer, measurements remain accurate


it only changes the visual appearance.

this will be ineffective when doppler shift is so high it wraps around itself


what are the advantages and disadvantages of using continuous wave doppler?


aliasing never appears with continuous wave doppler...only with pulsed doppler

disadvantages: Range ambiguity


What are the gray shades of a spectrum related to?

amplitude of the reflected signal

number of blood cells creating the reflection


What is color flow doppler?

color flow is a form of two-dimensional doppler

velocity info is coded into colors and superimposed on a 2d gray scale

is a pulsed US technique

has range resolution (range specificity)

subject to aliasing

provides some info on direction of flow


what kind of velocity does color doppler report?

average or mean velocities

continuous wave doppler report peak velocities

spectral doppler measures peak velocities


What is a color map?

it's like a dictionary displayed as a vertical bar of various colors

black region in the center

has two modes: velocity and variance mode


what do the colors in a velocity mode map mean?

the colors provide information on flow DIRECTION and VELOCITY

displays 2 different velocity mode maps

the black regions indicates no doppler shift

above the black = toward transducer, positive doppler shift

colors closest to the black line indicate lower velocity

color change is always up and down, never side to side


What does variance mode do?

distinguishes laminar flow from turbulent flow


What do the colors in a variance mode map mean?

indicate direction of flow

top = toward
bottom =away

different colors side to side = lt side, laminar flow; rt side turbulent flow

colors change up and down and side to side

turblent flow associated with pathology


What is a doppler packet?

with color doppler, multiple ultrasound pulses are used to accurately determine blood velocities.

the multiple pulses are called a packet or ENSEMBLE


Packets composed of a larger number of pulses (larger packets or longer ensemble lengths) have two advantages:

more accurate velocity measurement

increased sensitivity to low flow


packets with more pulses have three disadvantages:

more time needed to acquire data

reduced frame rate

decreased temporal resolution


packet size should be carefully selected to balance what two competing interests of color doppler?

accurate velocity measurements

adequate temporal resolution


what is power doppler

only identifies the presence of a doppler shift - NON DIRECTIONAL COLOR DOPPLER

does NOT evaluate speed or direction


What is power doppler also known as?

energy mode

color angio

the amplitude of the reflection is directly related to the number of moving blood cells


What are the three advantages of power mode?

increased sensitivity to low flow or velocity, such as venous flow or flow in small vessels

unaffected by doppler angles, unless the angel is exactly 90 degrees

no aliasing since the velocity information is ignored


what are the three disadvantages of power mode?

no measurement of velocity or direction

lower frame rates (reduced temporal resolution) than conventional color flow doppler

susceptible to motion of the transducer, patient, or soft tissues which may result in a burst of color or flash artifact


What is ghosting and clutter?

doppler artifacts

on spectral displays, low frequency Doppler shift artifacts are called clutter with color doppler they are called ghosting artifact


how is clutter and ghosting artifact eliminated?

a WALL FILTER is used to eliminate low frequency doppler shifts from moving anatomy rather than from moving blood cells

wall filters: eliminate low frequency doppler shifts around the baseline of a spectral effect on high frequency

wall filters; eliminate color arising from slow velocity reflectors

wall filters: aka high pass filters


What is crosstalk?

a special form on "mirror image" artifact

only with spectral doppler

appears an identical doppler spectrum both above and below the baseline

the true flow is unidirectional, but it appears bidirectional


Why does crosstalk appear?

doppler gain set too high

the incident angle is near 90 degrees between the sound beam and the flow direction, when the flow is at the beam's focus


What is spectral analysis?

a tool that breaks the complex signal into its basic building blocks

and identifies the individual velocities that make up the reflected doppler signal


what two methods of spectral analysis are currently used?

Fast Fourier Transformation (FFT)



What is FFT?

fast fourier transformation

a digital technique used to process both pulsed and continuous wave doppler signals


what are the two advantages of FFT?

exceedingly accurate

displays all individual velocity components that make up the complex reflected signal


what is the significance of displaying all component frequencies with fast Fourier transforms?

the spectral display of an FFT distinguishes laminar flow from turbulent flow


What is laminar flow?

most blood cells within the sample volume travel at similar velocities.

the spectral trace is a narrow, well defined line

the region between the baseline and the spectrum is called the spectral window (its clear)


What is turbulent flow?

blood flow is chaotic

many different directions and speeds are within the sample volume

the pulsed doppler spectral window is filled in (spectral broading)

Reynolds number >2000 = turbulent flow


what is spectral broading?

a wider range of velocities and doppler shifts within a sample volume


What is autocorrelation?

aka: correlation function

the digital technique used to analyze COLOR FLOW DOPPLER

used with color doppler because of the enormous amount of data that is processed

somewhat less accurate but way faster to perform than FFT


Summary of continuous wave:

identifies highest velocity jets anywhere along the length of the US beam

range ambiguity

most sensitive

very good temporal resolution

no aliasing

peak velocity measurements


summary of pulsed wave:

accurately identifies the location of flow

range resolution

very good temporal resolution

subject to aliasing

peak velocity measurements


Summary of color flow:

provides 2D flow info directly on anatomic image

range resolution

moderately sensitive

size of color jet is most affected by color doppler gain settings

reduced temporal resolution due to multiple packets

based on pulsed US, subject to aliasing

mean velocity measurements


Summary of Power mode:

used with low velocity or small volume blood flow

range resolution

greater sensitivity than color flow

lowest temporal resolution

subject to flash artifact, not to aliasing

no velocity measurements


What is Doppler Shift?

Change in frequency

When the sound source and receiver move closer together or farther apart

*principal used to measure the velocity of blood in circulation*


Modern ultrasound systems actually measure the __________ ________ between the received and transmitted sound waves

frequency difference


Compare and Contrast Pulsed Doppler and CW Doppler

Pulsed: CW:

range resolution ---------------------range ambiguity

sample volume -----------------------region of overlap

Nyquist Limit (limited max velocity) ---unlimited max velocity

aliasing ------------------------------no aliasing


Compare and contrast Pulsed Doppler Transducer and CW Doppler Transducer

Pulsed: CW:

At least one crystal ------------------ at least 2 crystals

dampened PZT ---------------------- undampened PZT

low Q-factor --------------------------high Q factor

wide bandwidth --------------------- narrow bandwidth

lower sensitivity ---------------------higher sensitvity


Compare and contrast imaging vs Doppler

Imaging: Doppler:

Normal incidence 90* ------------0* or 180*

^frequency improves resolution ---lower freq avoids aliasing

pulsed wave only ------------------pulsed or CW

Min of 1 crystal -----------------min 1 (pulsed) or 2(CW) crystals