Chapter 7 Doppler Effect
The higher the scan angle , the greater the?
% underestimation and the more the scanner has to compensate for the error
The contact angle
(0, 30, 45, 60, 90 degrees produce positive shifts) and 90 no shift (-30, -45, -60, -90 degrees produce negative shifts)
Percentage underestimation equation
(1- Cos value)*100
Memorize cosine values
0 & 180=1 30=.866, 150=30 degrees in opposite -.866 45=.707, 135=60 degrees in opposite -.707 60=.5, 120=60 degrees in opposite -.5 90=0
Flow towards transducer is assumed to be _______ when not specified.
0 degrees (cosine 1), positive shift
Doppler angle
0 degrees=1 (max positive shift) 90 degrees=0 180 degrees =-1 (max positive shift)
For Cardiac Doppler (echocardiography) the Doppler angle is
0 or 180 usually no angle corrections needed
Advantages of autocorrelation
100-400 Doppler samples per scan line 5-50 fps depends on depth and width However, doubling with will half FR A minimum of 3 pulses are required to average one Doppler estimate, the more pulses obtained the better accuracy of flow
Flow away from transducer is assumed ____ when not specified.
180 degrees (cosine -1), negative shift
Maximum negative Doppler shift =
180 degrees parallel facing opposite the direction of flow (I.e same effect as 0 producing maxim shifts only +/-
For vascular ideal imaging is between
30-60 degrees or zero whenever possible, this is most often used in Doppler evaluation
What equation is used when velocity is given in cm/sec and or frequency shift is given in KHz
77/fd(KHz)/fo(MHz)Cos0 From RBCs that reflect back to transducer causing a frequency shift
Doppler Effect
A change in frequency or wavelength of a reflected sound wave (echo) caused by a motion of a wave source, receiver or reflector. An apparent change in frequency as a result of a change in wavelength caused by motion of a wave source
wall filter
A control that varies the sensitivity of detection of the returing frequency signals. Area above and below baseline
Spectrum
A group of components of a wave separates and arranged in order of increasing wavelength or frequency
Shadowing
A lack of echoes behind strongly attenuating or reflecting structures If it shadows in grayscale it will shadow on colorflow
A color packet or ensemble length
A packet of lines transmitted so an average velocity can be determined at each depth along the line and displays a color. Packet sizes range from 4-12 minimum 3-4. Color boxes can range from 100-300 color display lines depending on width.
Sound Spectrum
A range of sound frequencies arranged in order from lowest to highest, or highest to lowest
Turbulent flow causes a wide range of velocities to occur in blood flow this causes
A wide range of velocities to occur in blood flow, and higher variance
Angle artifact
Aka directional ambiguity, 90 degree angle causes absence of flow to appear in an image
Color flow artifacts
Aliasing (most common) Color bleed Attenuation/shadowing Color noise/saturation Mirror image Angle artifacts
Sound is emitted in concentric circles from a central point in
All directions
region of interest (ROI)
Also known as the color box. Can steer the beam to the right or left by 30 degrees and select the vessel to be interrogated (prevents 90-degree angles). Angling Beam decreases Doppler angle to give a better Doppler shift. Manually done with curved or vector probe. Width of ROI must be narrow and little depth. Keep ROI in center of box. Framerate not affected by lengthening box
Disadvantages of Color flow
An increase in ensemble length/packet size decreases frame rate Lack of complete spectral information (no peak velocity or spectral broadening), angle dependent (can't use 90 degree angles unless using electronic steering) Can only show mean velocities
The control used for underestimation is the
Angle correction control, at angles over 60 degrees, the error is so great that even compensation cannot dependably correct the error, so avoid scanning at angles over 60
Autocorrelation unit
Attached to the outputs of the phase quadrature detector(PQD). Two channels off the PQD that feed the autocorrelator is the Direct Channel and Quadrature Channel
Typical Ranges
Average flow: up to 100cm/sec Doppler angle: 30 to 60 degrees, 0 when possible Operating frequency: 2-15 MHz Doppler shifts: 100Hz to 11Khz
Frequency shift depends on
Blood flow velocity (speed of RBCs) Angle between the waves propagation direction and the direction of motion (Cos 0) contact angle Transmitted frequency (from transducer) Propagatspeed of moving reflector
BART=
Blue Away, Red Toward Transducer
Luminescence
Brightness of hue and saturation, represents echo amplitude. Determined by concentration of RBCs. More RBCs= higher amplitude (stronger Rayleigh scattering). Weak echoes that don't meet threshold of wall filter are not assigned color and appear black
3 standard modalities
CW Doppler PW Doppler Color Doppler
Compensated velocity =
Calculated velocity + [(1-Cos) * 100] OR calculated velocity + % underestimation
What vessels can be at an 0 degree angle
Celiac and Portal vein
Variance maps
Change from side to side, have vertical and horizontal gradients. Can "tag" specific color to a turbulent flow (green)
Standard maps
Change shades from top to bottom, vertical gradient only
Color bar
Color bar shows flow directionally and average flow velocities (frequency shifts), color bar baseline represents no flow. "Aliasing" results in a wrapping of the color around the baseline. Highest detectable mean velocity towards transducer is red/yellow and highest detectable mean velocity away from the transducer is blue/aqua
Color flow vs spectral analysis
Color flow is less sensitive Color flow uses Autocorrelation
Twinkle artifact
Color reverberation seen extending off calcified structure
Color Noise
Color sensitivity is optimal when the color gain is set just below the color noise threshold (level just below which color speckle becomes evident). If too high color shows up in soft tissues
Doppler shifts are determined by where the __________ is placed?
Colorbox, shifts are then assigned to specific color hue, shades and brightness
The Doppler shift equation doesn't take refraction into consideration unless
Comparing 2 different tissues, where the Foppler beam moves through each one separately.
What equation is used if the Doppler beam is not zero degrees to the reflector (not parallel)
Complete Doppler equation
Lower Doppler shifts =
Darker shades and hues that are closer to the baseline
A moving object pushes sound cycles closer together in front of it (circles are closer together with less space between) which causes
Decreased wavelength, which equates to an increased pitch (I.e. as a blowing train whistle moves toward an observer, the whistle has a higher frequency and thus has a higher sounding pitch due to the compression of cycles in front of the train)
Hue (Color)
Depends on frequency (wavelength) of reflected light, and direction of flow. Frequency ranges from 400 to 800 million MHz.
Velocity has a _________ and _________ relationship with Doppler shift
Direct and linear. As velocity increases, the Doppler shift (frequency change) Increases
Power Doppler (Color power angio, color Doppler energy)
Displays amplitude of power shifts (not frequency) tells if a flow is present but does not give directional or velocity information. Good for low flow areas. Doesn't aliase because frequency shift is not used. Brighter=higher intensity Lighter=lower intensity based on concentration of RBCs
Steering control for spectral and color boxes steer
Doppler beam to right or left by 30 degrees making a 90 degree angle change to a 60 degree angle. This is necessary due to not being able to rock the flat transducer, since peripheral vessels are parallel to skin surface it creates a 90 degree angle which is removed by steering
Color Doppler vs B-scale
Doppler is color coded frequency shifts, gives average of velocities w/ colorflow B-scale shows actual blood flow
Types of color Doppler
Doppler shift :uses autocorrelation Time shift :uses cross correlation technique
If both operator frequency and reflector velocity double then
Doppler shift doubles
Color Flow Doppler
Doppler shift information in a two-dimensional presentation superimposed on a real-time gray scale anatomic cross-sectional image.
Doubling the cosine value=
Doubles the Doppler shift, as cos0 increases, Doppler shift increases. As Doppler angle increases, cos0 decreases, thus decreases Doppler shift
Triplex=
Duplex + color flow
Dopplers idea angle is
Either 0 or 180 degrees, unlike 2D ideal imaging of 90 degrees
velocity equation
Fd(MHz)c/2fo Cos0 Fd=Frequency change in MHz Fo=Original frequency in MHz Cos0=Doppler angle
Simplified doppler equation (cardiac imaging)
Fdop= 2FoV/c, used when flow is directly towards or away from transducers . Used when scanning at a zero degree angle (parallel to flow)
Color persistence
Frames averaged over time, improves SNR (increases sensitivity to weak Doppler shifts) used when signal is weak and it Color noise is present. Low persistence is required for cardiac imaging
Doppler angle is measured
From flow direction
Duplex =
Grayscale+ Spectral Analysis
Velocity of a moving object affects the Doppler frequency shift, faster moving objects will produce
Greater Doppler shifts. Doppler frequency is related to velocity
Gateing Technique
HPRF-up to 3 gates (one true and 2 ambiguous) CW-one large gate(overlaps echoes) PW- one small gate (unambiguous, range specific) Color flow-100 or more sample gates depending on width and depth of colorbox
Higher frequencies are scattered more strongly than lower frequencies and produce
Higher Doppler shifts
3 components of color
Hue (Color), Saturation (Shade) and Luminance (brightness)
As frequency increases, attenuation?
Increases, which causes attenuation prior to returning to transducer
Hues from lowest to highest
Infrared (too low to be seen by eyes) Red Orange Yellow Green Blue Violet Ultraviolet (too high to be seen by eyes)
Flow speed and angle DO NOT affect
Intensity of the wave
color priority
Is a threshold technique which allows the user to determine above what grayscale level 2-Ddata is presented and below which color data isn't presented. Appears as a green line high priority shows it higher up on bar which tells system which is more important Grayscale or color scale
Mean velocity
Is measured by by auto trace function. Required for some measurements. An average velocity of each Doppler shifted frequency range in the spectrum
Saturation
Is the shade of color, amount of hue present in a mix with white Depends on velocity of flow (dark=slower vs light=faster) White= a combination of all visible hues Black= absence of all visible hues Grayscale saturation =0 no color
Try not to scan less than 30 degrees unless it's 0 because
It may come too close to the critical angle and reduce the number of Doppler signals making it back to transducer
Doppler shifts occur in the
KHz range, typically 100Hz to 11,000 Hz. (11kHz)
Higher Doppler shifts =
Lighter shades and hues that are farther from the baseline
Color bleed
Looks like color extending beyond the vessel wall Due to overgain or low filter settings Improved by decreasing the color gain
Causes of Color Aliasing
Low color scales (low PRF) High velocity flow (in center of vessel or in stenotic segment) Regions of acceleration (bends, kinks, twists, narrowing) Angles which result in greater frequency shifts (close to 0 & 180) Higher frequency transducers (anything that increases Doppler shift fdop is proportional to fo)
Lower Doppler shifts produce
Lower (shorter) waveforms on the spectral graph velocity displayed depends on the Doppler shift that's detected
Approaching 90 degrees Doppler angles result in lower cosine value thus
Lower Doppler shifts
Autocorrelator gives the
Mean, Variance between samples, power and sign of the frequency shifts ( +\- shift above or below baseline). The two channels are required to separate frequency shifts into positive and negative components. The remaking parts determine power and sign of shifts
Resistive Index
Measures amount of resistance to blood flow RI=(PSV-EDV)/PSV PSV=Peak systolic velocity and EDV=End Diastolic Velocity
Pulsatality Index
Measures how pulsatile the flow is (how much it changes during the heart cycle) Requires auto trace function, since mean velocity is needed. PI=(PSV-EDV)/MV MV=mean average
Color Flash Artifact
Motion artifact when vessel walls move fast enough to produce Doppler shift I.e portal vein color flow in breathing patient use inheld respiration
Quadrature channel determines
Negative mean and variance
Cosines over 90 have
Negative values l, cosines range from-1-1
Colorflow & Diagnosing
Never use alone as it: Can appear normal in the presence of pathology Only an estimate of flow speed Uses an average of flow speed Lacks full spectrum of info
No motion=
No Doppler frequency shift because wavelength doesn't change
Slowest speed + highest speed=
Nyquist limit =PRF/2
Aliasing
Occurs when velocity flow exceeds the display scale (Nyquist limit) of the color flow. Occurs: when in center of vessel, tortuous vessel, when angle is closer to 0 or 180 degrees, when the scales are low, when the transmit frequency is high
Disadvantages of Power Doppler
Only shows presence of flow (no directional or velocity information) Motion artifacts (must be still)
These variables are included in the equation because they all effect the wavelength, and Doppler shift is determined by the wavelength
Original transmitted frequency Velocity of the reflector Propagation speed of tissue
Calculated (Uncompensated) Velocity =
Original velocity *COS value
Color Doppler controls
PRF (scale) sets Nyquist limit low scale increases sensitivity Baseline shift moves 0 baseline up & down ROI- aka colorbox where Samples are taken Color persistence-frame averaging for color flow increases SNR Color gain-increases brightness of color flowbh increasing amplitude of recvd echoes Post processing-choose diff color maps Priority determines which modality has priority to be displayed (color or gray) Packet size/ensemble length improves color flow by taking more samples slowing down FR Color invert
is measured by placing a caliper at the highest
Peak velocity
Spectral Analysis shows:
Peak velocity and mean velocity Amplitude of the Doppler shift (brightness) Character of flow (Amount of spectral broadening) Calculators: RI, PI, Acceleration
Higher frequencies produce greater frequency shifts at shallower depths but echoes from higher transmit frequencies
Penetrate less, so must use lower frequencies when scanning deeper than 3cm to get more echoes back
What color flow Doppler shows
Presence or absence of blood flow Direction (Red or Blue) Mean (average) velocity (by its hue and shade) Character (laminar or turbulent)
What does Doppler tell us
Presence or absence of flow Direction of flow (- or +) above baseline red or blue color hues Character of flow (turbulent-more than 1 direction or laminar-normal) amount of spectral broadening & resistance Velocity of flow (height of waveforms) Amplitude of Doppler signal (brightness of waveform and color flow) Amplitude (strength) of echoes determined by concentration of RBCs
Color Doppler
Provides flow information over a scanned region of the patient and estimates a mean velocity by analyzing Doppler shifts.
The reflectors in sonography are
RBC's which reflect as Rayleigh scatter which is very weak in amplitude but not prop. speed
Doppler Applications
Radar, Sonar, Cardiac Sonography and Vascular Sonography
The apparent change in frequency (pitch) of a wave is caused by a
Relative change ( motion) between the source and receiver
Color wheel same as color bar but stretched around into a circle. The highest positive and negative shifts touch onwhat side of the circle?
Right side, Aliasing colors are in the right
Color flow is both?
Scanned and non-scanned modalities. Non scanned because packet size causes beam to travel multiple times along scan line. Scanned because after it's packet pulsing if one scanned line, it moves laterally and repeats process
Advantages of Power Doppler
Shows renal blushing Not angle dependent (shows complete filling of vessel, no 90 degree artifact) No background noise Flow in tortuous vessels Better Edge definition
Critical angles can be reached at <30 degrees due to severe refraction which causes
Sound no longer emerges blood at all but is totally reflected at the vessel wall. 27 is the critical angle for vascular imaging
_____________ is more accurate than color flow in calculating velocity, because color only displays mean (average) velocity (by shades of a color), whereas spectral analysis gives both peak and mean (average) velocities.
Spectral Analysis
The ultrasound machine knows or assumes all values except
The Cosine of angle (Cos0) and velocity
The angular effect on the Doppler shift can be determined from the cosine function and depends on the angle formed between
The Doppler beam (incident ray) and the blood flow (medium)
Spectral Analysis
The breaking up of the components of a complex waveform or signal and spreading them out in order by frequency
The smallest angle =
The direction steered
What does spectral Doppler refer to
The fact that the Doppler frequency shifts are separated and displayed as a spectrum
Determining color flow direction
The field of view determines the direction of the pulses and thus the scanlines. We must use electronic steering to get away from 90 degree Doppler angle. Determined by looking at the smallest angle and direction of flow
If stationary the frequency emitted=
The frequency observed (heard) in all directions, so the same pitch would be heard no matter where they are standing in relation to the source
If the source of echoes (RBCs) are moving toward the receiver (transducer) then
The returning echoes will have a higher frequency shift than the sound waves that were transmitted (Positive Doppler shift )
Calculating percentage error
The scanner knows that there is a percentage of underestimation for all angles other than 0 or 180, so it adds the underestimates amount back to the calculated velocity that it gets from the Doppler equation
In cases of refraction
The tissue with the highest prop speed will have the highest Doppler shift. Refraction is related to speed and Doppler shift due to the way beam bends
In cases of no refraction
The tissue with the lowest prop speed produces the highest frequency shift per Doppler equation
The source and receiver in sonography is
The transducer
oblique incidence and different propagation speeds
The two things must be present for refraction to occur. Refraction causes transmission angle to increase, Doppler angle to decrease resulting in a higher Doppler shift
If Doppler shift=0
Then reflected frequency =original frequency (no shift)
What happens if sound only moves in one direction?
Then you would have to be directly in front of it to hear it
What happens to sound behind a moving object?
This causes increased wavelength (circles are farther apart with more space between them) , thus equates to a decreased or lower pitch (frequency).
How does the ultrasound machine calculate the cosine value
Through the sampling angle, so it can calculate the cosine value and plug it into the Doppler equation to calculate velocity
Other modalities include:
Tissue Doppler Power Doppler
Analysis
To take apart or break up into basic components
Relationship Summary
Transmitted frequency is directly and linearly related Wavelength is inversely and nonlinearly related Velocity is directly and linearly related C is inversely and nonlineraly
Color aliasing vs true flow reversal
True flow reversal shows a black boundary line separating the red and blue hues. Aliasing goes from red to blue without being separated by a black boundary
Best way to optimize color
Turn up color gain until you get color flow in tissues then click-by-click reduce until just above noise floor
To obtain the best Doppler shift
Use the highest possible frequency transducer Minimize Doppler angle as much as possible (steering the box, rocking the transducer, scan windows)
Tissue Doppler
Used in echocardiography permits an assessment of myocardial motion using Doppler ultrasound imaging with color coding Uses frequency shifts of ultrasound Uses filters to display low frequency shifts and blocks high frequency shifts
Autocorrelation
Uses math process that gives the Mean and Variance of the Doppler shifts for each pulse. Provides magnitude (power) by brightness and hue, sign (+|-) determines color, variance velocities (detect turbulent flow), mean velocities
color wall filter
Usually automatic in most systems set at about 10% of the velocity scale. Filters out velocity speeds less than 8cm/s. Filters too high can filter out diagnostic signals and filters too low increases noise on image
The color bars
Velocities are assigned colors above and below the scale. Shades change from top to bottom and side to side.
Mirror image
When a strong signal in one direction channel leaks into another, a mirror image on the opposite side of the baseline occurs. Causes a ghost image
Alianasing occurs
When velocities exceed scale causing a wraparound effect to other side of scale.. so increase scale to change velocity numbers
So pitch heard from a moving sound source depends on
Where the observer is standing in relation to the sources motion..for sonography it depends on direction that the blood moves in relation to transducer
Number of scanlines when averaging mean velocity is determined by ?
Width of the colorbox (gating), an increased packet size (more than 3) = better accuracy but slower Frame rate
Rouleau Formation
With low shear flow, blood cells form a "rouleau" formation (roll of coins), increasing the reflectivity. This results in the ability to visualize blood, but patients may develop DVT and susceptible to blood clots
If angle correction isn't used or when solving problem when angle is not given the machine assumes
Zero degrees
In pulsed wave the minimal detectable blood flow velocity is
about 6cm/sec, flows slower than this will not show up on color flow or spectral analysis (undetected)
Compensated velocity =
actual blood flow velocity (Compensated velocity is the corrected and actual velocity of blood flow
Where does Doppler shifts occur
at the vessel wall/blood boundary's
Doppler equation (used in vascular and non-cardiac modalities)
doppler shift = 2 x velocity of blood x transducer frequency x cos0 / propagation speed 2f0(MHz)V(m/s)Cos0/c(m/s)
Doppler shift equation
doppler shift = reflected frequency - transmitted frequency
Higher Doppler shifts produce
higher (taller) waveforms on the spectral graph. Velocity displayed depends on the Doppler shift that's detected
Greater flow speeds and smaller Doppler angles=
higher Doppler shifts, but not stronger echoes
As sound moves away from an observer the frequency becomes
lower, so pitch is lower due to the spreading apart (elongation) of the cycles behind the train
Direct channel determines
positive mean and variance
Maximum Doppler shift for PW=
the Nyquist limit (1/2 PRF)
Doppler shifts are dependent upon
the angle from which the observer is from the received echoe
If the source of echoes (RBCs) are moving away from the receiver (transducer) then
the frequency of returning echoes will have a lower frequency than the sound wave that was transmitted (Negative Doppler shift)
Doppler shifts are proportional to
the velocity of the blood flow (even extremely high blood flow rates are less than 1% of the prop speed of soft tissue) usually= .3% velocity =d/t
If soundwave reflects off a moving structure, the wavelength is changed,
thus a change in frequency Which results in a change in pitch with audible sound waves
Maximum Positive Doppler shift =
zero degrees directly parallel Facing the direction of flow
