Physics Exam #2 Flashcards
Doppler
Name some random things that Doppler can be used for
Weather forecasting
Aviation safety
Police surveillance
Automatic door openers
Home burglar alarms
Motion detector lights
Doppler effect founded by
Johann Christian Doppler in 1842
Define the Doppler effect
it is an apparent shift or change in frequency of an interrogating wave caused by relative motion between an observer and an object
shift is result of changes which occur to the wavelength because of the relative motion
The source (transducer) determines the ______ of the waves.
frequency
The medium determines the ________.
propagation speed
the frequency and speed of propagation determine the
wavelength
λ = c / f
wavelength = speed divided by frequency
Stationary receiver sees the ________ number of pressure waves as they are emitted by a stationary source.
same
with the doppler effect, either the source or the receiver is
moving either toward or away from the other
if a wave with a given wavelength reflects from an object which is moving, the wavelength of the reflected wave will be _______ than the original wavelength
different
in a positive doppler shift, the reflector is moving toward the transducer, the reflected frequency is _______ than the transmitted frequency
higher
in a negative doppler shift, the reflector is moving toward the transducer, the reflected frequency is _______ than the transmitted frequency
lower
Doppler change in frequency is in what range?
audible range
doppler shift is between 200 Hz and 15,000 Hz
Audible range is between 20 - 20,000 Hz
Doppler shift frequency examples:
Ultrasound observer A detected an absolute frequency of 1.98 MHZ while observer B detected an absolute frequency of 2.02 MHZ
What are the Doppler shifts for observer A and observer B
Doppler shift for observer A is:
1.98 MHZ - 2 MHZ = -0.02 MHZ = -20 kHz (negative shift)
Doppler shift for observer B:
2.02 MHZ - 2MHZ = .02 MHZ = +20 kHz (positive shift)
What is the doppler shift equation?
fDop = 2f0V / c
where:
fDop = frequency of Doppler
V = velocity of RBC’s
f0 = frequency of transducer
c = speed of sound in tissue (1540 m/s)
Think of “2” as roundtrip of the sound
Does how fast the sound source or receiver (or both) affect the Doppler shift?
yes… an increase in the velocity between the sound source and receiver causes a greater change in the received frequency
fDop = 2f0V / c
Doppler frequency is __________ proportional to the reflector velocity
directly
*if the reflector velocity doubles, the Doppler frequency doubles etc etc
Higher transmit frequency =
Lower transmit frequency =
higher frequency shift
lower frequency shift
compression leads to a _____ frequency and decompression leads to a ______ frequency
higher, lower
A faster velocity will result in a greater frequency shift but it will not tell you any other information, such as
direction (if it is a positive or negative shift)
in real life US, the flow direction we measure in the body is _______ directly or towards the transducer
rarely
a flow direction DIRECTLY toward the transducer (impossible in real life) would have a Doppler flow angle of 0 degrees
The ______ between the observer and the motion affects the Doppler effect
angle
If you know the angle (or cosine) you can add it to the Doppler equation…
fDop = 2f0V(cos) / c
the cosine in the Doppler equation represents what exactly
the angle of incidence relative to direction of blood flow
Doppler angle is defined as
the angle that is formed between the observers line of sight and the direction of the target object
The US angle is often referred to as
the insonification angle, insonation angle
measured between the beam steering direction and the direction of the flow
When blood cells are moving _____ to the sound beam (0 degrees), the entire velocity is measured
parallel
when an angel exists between the direction of flow and the sound beam, the measured velocity is
less than the true velocity
What does a +1 and -1 cosine represent? What angles do they correlate to? A cosine of 0 is what angle?
+1.0 cosine is an angle at zero degrees and represents a positive Doppler shift. It is optimal but literally impossible in the real world. A zero cosine represents an angle at 90 degrees. A -1.0 cosine is an angle at 180 degrees and represents a negative Doppler shift.
increments of 30 degrees
(ex. 0, 30, 60, 90, 120, 150, 180)
180 degree angle would be blood flow going directly _____ from the transducer
away
If the cosine of 30 is 0.87, how much of the true Doppler shift is detected?
87%
When the direction of flow is perpendicular to the sound beam (90 degrees) the MEASURED VELOCITY is what
zero
Can Doppler shifts and velocities be measured with perpendicular incidence?
NO
What’s 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 is one half the actual velocity.
Example: when blood travels at 2 m/s at a 60 degrees angle to the sound beam Doppler reports the velocity at 1 m/s
correctly identifying the insonification angle is critical for 4 big reasons
- determine flow direction
- assess Doppler measurement accuracy
- minimized Doppler error sources
- assess likelihood of artifact related issues such as spectral broadening
Angles of ______ degrees or less are generally achievable through steering and angulation.
60 degrees
________ transducers are electronically steered
linear array
To convert the Doppler frequency to a reflector velocity, what is needed?
angle correct
Doppler Angle: if the angle increases does the frequency shift increase or decrease?
decrease
Spectral Doppler displays what
blood flow measurements graphically, displaying flow velocities recorded over time
Spectral display - blood flow direction
towards or away
Spectral display - blood flow characteristics
waveform will indicate whether flow has a high resistance or low (pulsatile or phasic)
Spectral display - quality of flow
laminar (narrow band of frequencies, “clean” window) OR turbulent flow (window filling or spectral broadening)
Spectral display - quantification of blood flow
peak or mean velocities
In Spectral Doppler, the “y axis” represents what and the “x axis”represents what?
y axis is velocity
x axis is time
What can you see here?
The min velocity and max velocity
Flow types.. identify
Top is high resistance flow
Middle is low resistance flow
Bottom is pulsatile flow
The Doppler signal spectral displays depict what two things in the reflected signal?
- frequency bandwidth
- range of amplitude in reflected signal
In Spectral Doppler, the amplitude or signal power depends on the
relative number of red blood cells
Spectral broadening is usually caused by
turbulence; results in a “fill in” of the area between a curve and the baseline due to varying velocity of reflectors in the sampling area
This is a hallmark feature of poststenotic flow
spectral broadening
Identify the blanks
Line A Line B and Line C represent what in terms of stenosis
Line A is normal
Line B is mild stenosis
Line C is severe stenosis
Spectral broadening is related to a wider range of Doppler frequencies seen with:
Large sample gates (detect more info)
Turbulence (most common reason)
Small diameter vessels
Stenotic or tortuous vessels
Wide beam widths
Increased overall gain
Gate not aligned with center of vessel
POOR ANGLES
Spectral broadening can happen from how you place the sample gate… what are two ways that are no no’s
- sampling area is too wide
- gate is too close to the vessel wall
Spectral broadening can also be a sonographer error due to what basic machine function?
gain!!! overgaining can make the waveform look like there is spectral broadening
The process of extracting the individual component frequencies of the complex signal is
spectral analysis
Spectral analysis is done in two ways:
- Spectral Doppler - FFT (Fast Fourier Transform)
- For Color Doppler - Autocorrelation
The FFT is what? Used to process what?
is a mathematical technique for separating a spectrum into its individual frequency components for display (computer based). Generates a waveform that allows the display of flow velocities over time.
Used to process PW and CW
Two advantages of FFT are
Exceedingly accurate! (can tell laminar from turbulent flow)
Displays all individual velocity components that make up the complex reflected signal
In _________ Doppler, only ONE PZT crystal is used
Pulsed wave
The crystal alternates between SENDING and RECEIVING US pulses
Has backing material
What is the time of flight for pulsed wave Doppler again?
13 microseconds per centimeter (ms/cm)
In pulsed wave Doppler, the transducer emits a sound pulse and then waits that exact time before listening for a reflection. If the transducer receives an echo during this short listening time… the echo
was created at the gate
in pulsed wave, the sample volume is also called the
receive gate
In pulsed wave the sample length is usu between (length)
1 and 10 mm (typically 3 to 5mm)
In pulsed wave, why keep the cursor in the center of the vessel?
the center represents the fastest flow for clear window
If the sample gate is too large, we know it will cause spectral broadening. However, what if it is too small?
If too small you may not pick up velocities
In pulsed wave, the ideal gate size is what percentage of the lumen
half the lumen size
the ability to detect signals from a specific area is called
range resolution
range resolution has a few different synonyms
range specificity, range discrimination, freedom from range resolution
PRP and PRF recall: PRP determines the ________. When PRP is short the PRF is _______. (PRP = 1) When PRP is long the PRF is _____ and it takes the system __________.
maximum depth
high
low
longer to listen
The number of ultrasound pulses sent per second is the
PRF
PRF is determined by (2)
the speed of sound and the distance it travels
*the PRF is how often the pulses can be sent out
To measure reflectors moving with high velocity and producing large Doppler shifts, is a high or low PRF necessary.
A high PRF. A high PRF though limits the depth that can be sampled. Why? Because a certain amount of time is required to collect the echoes arising from that depth before the next pulse is sent out.
PRF = C / 2R(D)
The PRF limit where aliasing occurs is called
the nyquist limit, where very high velocities in one direction are incorrectly displayed as going in the opposite direction
Flow information that is GREATER THAN HALF THE PRF (nyquist limit)
cannot be displayed correctly
Two ways to avoid aliasing
- raise the speed limit (nyquist) (increase scale)
- reduce the doppler shift (speed limit) (not always possible) (because depth being a problem)
This is the “speed limit” of Doppler shift
Nyquist limit
Nyquist is 1/2 PRF (# of pulses per sec) what is the limit on the scale?
0.5 m/s
5 methods to reduce aliasing
- increase scale / PRF
- get closer, adjust depth
- lower transducer frequency
- shift baseline - if reverse flow not a concern, baseline can be adjusted to incorporate entire range of velocity in sample to forward flow
- use a CW device, aliasing does not happen with CW (no range specificity though)
If you needed to detect high velocities at large depths, the US instrument can be placed in a mode where the Doppler PRF is
higher than that allowed to avoid range ambiguity. Some instruments provide a “high PRF” option
When aliasing cannot be eliminated… ______ mode happens.
High PRF mode
Higher velocities can be displayed but you don’t know where that velocity is coming from
Pulsed wave has lower or higher sensitivity overall?
lower `
CW requires how many crystals in transducer?
2! one constantly transmits while the second continuously receives reflections from RBC’s.
In CW, how are the 2 crystals arranged?
usu angled slightly toward each other to produce an overlapping region of max transducer frequency where the beams cross (overlap-zone of sensitivity)
“Zone of sensitivity”
region where the two beams overlap in CW … is the region where flow is detected
CW is always on producing a duty factor of
1 or 100%
The CW Doppler shift consists of all the Doppler shifts generated by moving interfaces within
the Doppler sample volume, along the entire sound beam
What is phase quadrature or quadrature detection?
Is commonly used signal processing technique for bidirectional Doppler (forward flow from reverse flow)
Pedoff probes have no ______ material.
backing
Pedoff probe characteristics
non imaging
no backing material = longer ring = high Q factor
more pure ring
leads to low bandwidth (narrow) because there’s only one frequency
higher sensitivity
a higher sweep speed displays ________ waveforms and a lower sweep speed displays ________ waveforms.
higher, lower
the wall filter in Doppler ultrasound is used to reduce
“wall thump” … wall thump is a distracting thumping sound produced by large slow moving specular reflectors from vessel walls / heart muscles / heart valves
wall filter removes high intensity low velocity signals “clutter”
cross talk is a special form of…? appears as…
“mirror image” artifact, appearing as an identical Doppler spectrum both above and below baseline (from strong reflector)
the true flow pattern is unidirectional but it appears as bidirectional
mirroring and cross talk can result from two errors
doppler gain set too high
incident angle is near 90 degrees between the sound beam and the flow direction
This image is an obvious display of what artifact
Spectral mirroring. The flow below the baseline is purely artifactual. It is a mirror image artifact.
this type of artifact appears as diffuse echoes overlying signals of interest
clutter… occurs to motion of tissue, vessel, or heart wall motion. can also occur due to respiration… eliminated by increasing wall filter
