chapter 19: Doppler

doppler shift AKA

doppler frequency

a change or difference in frequency of sound as a result of motion between the sound source and the receiver is known as?

doppler shift

is a lower frequency that rides on top of the much higher transducer frequency. this is?

doppler shift

doppler shift is created when?

transmitted sound strikes moving RBCs

doppler shift is used to measure?

the velocity of blood in the circulation

doppler shift ranges from…

20Hz-20,000Hz

extraction of the Doppler frequency (doppler shift) from the transducer’s frequency is known as?

demodulation

what is the equation used to calculate the positive to negative shift?

doppler shift (Hz) = reflected frequency - transmitted frequency

when the doppler shift is positive the blood cells are traveling in what direction?

toward the transducer

During a doppler examination, sound with a frequency of 5,000,000 Hz is transmitted from the transducer and reflects off of moving red blood cells. the frequency of the reflected signal is 5,003,000Hz. what was the doppler shift?

the doppler shift was +3,000 Hz the shift is positive because the reflected sound has a higher frequency than the transmitted sound

when blood cells move toward the transducer, the doppler shift is?

positive

in a positive doppler shift, the reflected frequency is…than the transmitted frequency

higher

when blood moves away from the transducer, the doppler shift is?

negative

in a negative doppler shift the reflected frequency is…than the transmitted frequency

lower

what do doppler frequencies indicate? what do they NOT indicate?

• velocity

• does not indicate speed

• indicates the distance that a RBC moves in 1 second

• magnitude

• distance/time

speed

defined by a magnitude (speed) and a direction, known as?

velocity

negative doppler shifts have…wavelength

longer

negative doppler shifts are color coded?

blue

positive doppler shifts have…wavelength

shorter

postive doppler shift is color coded?

red

what is the relationship between doppler shift, velocity and frequency of transmitted sound?

direct

what is the doppler shift equation?

doppler shift = 2 x velocity of blood x transducer frequency x cos (theta 0) / propagation speed

what does the number 2 mean in the doppler shift equation? pg 306

2 doppler shifts are occurring

1. first shift is when sound strikes moving blood

2. second shift is the reception of sound wave from moving RBC

if velocity is 1/2, the doppler shift is…

halved

is velocity is 2x, the doppler shift is…

2x

transducer frequency doubled, the Doppler shift is…

doubled

transducer frequency is halved, the doppler shift is…

1/2

blood cells moving parallel (directly toward or away) to the sound beam provide a…accurate velocity

100%

what happens to velocity if there is an angle in the way blood cells are moving?

the velocity will be less than the true velocity

• frequency is measured in?

• velocity is measured in?

• Hz

• m/s

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

the percentage of true velocity that is measured depends on the cosine of the angle between the sound beam and the direction of motion

measured velocity = true velocity x cos 0 (pg 309)

what is the relationship between doppler shift and cosine of theta, the angle between the direction of flow and the direction sound?

direct

if cosine is halved, the doppler shift is…

1/2

if cosine is 2x, doppler shift is…

2x

what is the relationship between the actual velocity and measured velocity when blood moves parallel to the sound beam? pg 310

• when the angle between is 0 degrees or 180 degrees, flow is parallel to the sound beam

• measured and true velocities are equal and flow toward the transducer when cosine 0 degrees = 1.0 cosine

• cosine 180 is -1, indicating flow away

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

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

T/F doppler shifts and velocities can be measured with perpendicular incidence

false. doppler shifts and velocities cannot be measured when 90 because cosine is zero

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

cosine for 60 is o.5, the measured velocity at 60 is ½ the actual velocity. when blood travels at 2m/s at a 60 degree angle to the sound beam, the velocity is 1 m/s

what are the types of doppler?

• bidirectional

• continuous wave

• pulsed wave doppler

• color doppler

• power doppler

which doppler distinguishes the direction of flow? and is most common?

bidirectional

is a signal processing technique for bidirectional doppler

phase quadrature (aka quadrature detection)

distinguishes positive shifts from negative shifts. what kind of doppler?

bidirectional

• requires two crystals in the transducer

• one crystal constantly transmits ultrasonic energy

• second crystal continuously receives reflections from blood cells

what kind of doppler?

continuous wave doppler

what are the advantages of continuous wave doppler?

accurately measures very high velocities

what are the disadvantages of continuous wave doppler?

• exact location of moving blood cells can not be determined

• range ambiguity limitation

• lack of TGC

T/F continuous wave transducer creates anatomic images

False

the sensitivity of continuous wave transducer is?

increased, does not use backing material.

simultaneous atomic imaging and doppler is called?

duplex imaging

continuous wave transducers are able to detect…amplitude reflections and…doppler shifts

low, smaller

• only one PZT crystal is necessary

• the crystal alternates between sending and receiving sound pulses

what type of doppler is this?

pulse wave doppler

advantage of PW doppler?

able to select exact location where velocities are measured (same volume range gate) called range resolution, range specificity freedom from range ambiguity artifact

disadvantage of PW doppler

inaccurate measurement of high velocity signals posed as traveling in an opposite direction (aliasing)

most common error associated with pulsed wave doppler

aliasing

aliasing is due to?

the sampling rate, aka PRF, being too low in comparison to the measured blood velocities

the very top of the display on spectral tracing is called?

nyquist limit. the top cannot be measured because it goes beyond what can be displayed

what is the equation for nyquist limit (Hz)?

PRF(Hz)/2

what are 2 ways to reduce aliasing?

• rise the nyquist limit

  • increase PRF

  • decrease depth

• reduce the doppler shift

  • lower the transducer frequency

what are the 5 ways to avoid aliasing?

1. using a lower frequency transducer

2. shallow gate

3. adjust the PRF scale to its max

4. shift the baseline up or down

5. use continuous wave doppler

T/F aliasing only occurs with pulsed doppler, never with continuous wave

TRUE

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

nyquist level

higher transducer frequency create more?

aliasing

• slower blood velocity

• lower frequency transducer

• shallow gate (high prf)

how does this affect aliasing?

less aliasing

• faster blood velocity

• higher frequency transducer

• deep gate (low prf)

how does this affect aliasing

more aliasing

spectral dopplers shades of gray are related to?

• amplitude of selected signal

• number of blood cells creating the reflection at each velocity

spectral window

spectral width

spectral intensity

the filling of the spectral window is known as?

spectral broadening

form of two dimensional doppler. velocity information coded into color and superimposed on a gray scale image. what kind of doppler?

color flow doppler

color doppler reports the…blood flow velocities

average

• pulsed ultrasound technique

• has range resolution

• subject to aliasing

• tell direction of flow

what kind of doppler?

color flow doppler

color doppler measures…velocity. spectral doppler (pulsed and cont) measures…velocity

mean, peak

measured velocities converted into color and displayed on a vertical bar?

called color map

what are the two types of color map?

• velocity mode map

• Variance mode map

what kind of color map?

velocity

what kind of color map?

variance

shows flow direction and velocity. what kind of map?

velocity

shows laminar and turbulent flow indicated from side to side. what kind of map?

variance

color above the black bar indicates?

• toward flow

• + doppler shift

color below the black bar meant?

• away flow

• doppler shift

color change is always up and down and not side to side

which map?

velocity

in velocity mode map, lower flow velocities are near?

the black line

in velocity map, the higher flow velocities are near the?

top or bottom

laminar and turbulent flow is indicated from side to side

left side = laminar flow

right side = turbulent flow

what kind of map?

variance mode map

with color Doppler multiple ultrasound pulses are used to accurately determine blood velocities. what is this called?

Doppler packets

advantages of larger packets (doppler packets) or longer ensemble length

• more accurate velocity measurement

• increased sensitivity to low flow

disadvantages of large packets (doppler packets)

• decrease frame rate and decreased temporal resolution

• more time needed to acquire new data

only identifies the presence of the doppler shift?

power mode

• does not eval for speed or direction

• non-directional speed

power doppler

advantages in power doppler?

• increases sensitivity to low flow or velocity

• angle doesnt matter unless 90 degrees

• no aliasing

disadvantage of power mode

• no measurement of velocity direction

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

• susceptible to motion of the transducer, patient or soft tissue, resulting in a burst of color or flash artifact

• aka spectral mirroring

• minor artifact seen in spectral doppler that appears as identical doppler spectrum both above and below the baseline

• appears bidirectional but is truly unidirectional

crosstalk

a tool that identifies individual velocities that make up a doppler signal

spectral analysis

what are the 2 methods of spectral analysis?

• fast Fourier transformation

• autocorrelation

• a digital technique, used to process both pulsed and continuous wave doppler waves

• distinguishes laminar flow from turbulent flow

  • clear spectral window (laminar flow) or spectral broadening (turbulent flow)

fast Fourier transform

• digital technique used to analyze color flow

• less accurate but faster to perform than FFT

autocorrelation or correlation function

power doppler is also called?

energy mode or color angio

on spectral displays, low frequency doppler shift artifacts are called?

clutter