Physics Flashcards
Anechoic
-without internal echoes
Echogenic
- a region in an ultrasound image which has echoes
- synonymous terms: reflective, echo producing, echoic
Heterogenous
-variable levels of echogenicity
Homogeneous
-uniform echo texture
Hyperechoic
-displayed echoes that are relatively brighter than the surrounding tissue
Hypoechoic
-displayed echoes that are relatively darker than the surrounding tissue
Isoechoic
-having the same echogenicity (brightness) as the surrounding tissue
mega (M)
10^6
kilo (k)
10^3
deci (d)
10^-1
centi (c)
10^-2
milli (m)
10^-3
micro (u)
10^-6
What is sound?
- a travelling variation in acoustic variables
- a longitudinal, compressional wave
What are acoustic variables?
What are some examples of acoustic variables?
- quantities that vary in a sound wave
- examples: pressure, density, and particle motion
What is compression?
- when molecules are pushed together, it produces a region of increased density; creating a zone of high pressure
- compression describes the formation of a high pressure region
What is rarefaction?
- when molecules release (or bounce back) there is a zone of decreased density
- the rarefaction describes the creation of this low pressure
What is a longitudinal wave?
-a mechanical compressional wave in which back and forth particle motion is parallel to the direction of wave travel
What is frequency?
-a count of how many complete variations in pressure (cycles) go through in one second
What is the frequency range for audible sound?
20Hz - 20 000Hz
What is the frequency range for ultrasound?
> 20 000Hz
What is the period?
- a function time
- the time it takes for a sound wave to complete 1 cycle
As frequency increases, what happens to the period and why?
- T decreases
- more cycles per second will equal shorter cycles
- they are inversely proportional
What is wavelength?
-the length of space over which one cycle occurs
What is acoustic velocity (c)?
-the speed of a wave movement through a medium
If frequency increases, what will happen to wavelength and why?
-if frequency increases, wavelength decreases, because they are inversely related
What determines acoustic velocity?
-a medium’s stiffness
What is stiffness?
-a medium’s resistance to compression
Which materials have the highest acoustic velocity? (solids, liquids, gases)
solids > liquids > gases
What is the average acoustic velocity in soft tissue?
1.54 mm/us OR 1540 m/s
What materials are at the higher and lower ends of the scale for acoustic velocity?
- bone is higher than soft tissue
- lung is very low
- soft tissue is in the middle and are all within a tight range
Describe pulse echo technique in the use of ultrasound.
The transducer sends out a pulse which reflects off a boundary and returns to the transducer. The echo(es) is/are then represented as a dot on the screen. It’s brightness depends on the strength of the returning echo.
Describe harmonics production.
The more dense/high pressure areas of the sound beam move faster than the less dense areas. This changes the waveform and produces multiples of the original fundamental frequency.
What are the first 3 odd harmonic frequencies of a sound wave with fundamental frequency=5MHz?
15MHz, 25mHz, 35MHz
What is CW?
-continuous wave transmission continuously emits a constant frequency with constant peak pressure amplitude sound waves from the force
What is pulsed ultrasound?
- not a continuous wave
- a few cycles of ultrasound, separated by a gap of no sound
What is PRF and what is it’s unit?
- the number of pulses occurring in a second
- kHz
What is PRP and what is it’s unit?
- the time from the beginning of one pulse to the beginning of the next
- ms
If PRF increases, what happens to PRP, and why?
- PRP will decrease
- they are inversely proportional
What is PD?
- the time for one pulse to occur
- from the beginning to the end of one pulse
What is the typical number of cycles in an ultrasound pulse?
2 or 3
What is the typical number of cycles in a doppler pulse?
5 to 30
If frequency increases, what will happen to PD and why?
-PD will decrease with an increase in frequency, because a higher frequency will generate a shorter period, which will decrease the PD
What is the advantage of a shorter PD?
-shorter PD = better image (resolution)
What is DF?
-the fraction of time that a pulsed ultrasound is on
If PD increases, what happens to DF and why?
-increasing PD will increase DF, because they are directly proportional
What is SPL, and what is it’s significance?
- the length of space over which a pulse occurs
- unit is mm
- it is significant because it improves image resolution
- shorter pulse lengths improve resolution
If frequency increases, what happens to SPL?
- SPL will decrease with an increase in frequency
- SPL is directly related to wavelength and wavelength is inversely related to frequency, therefore any change in frequency will affect SPL indirectly
What affects bandwidth?
SPL
Which is better: a larger/smaller bandwidth?
-larger is better
Which is better: a QF of 3 or 0.4?
0.4
What is intensity?
-the rate at which energy passes through a unit area
If power increased, what would happen to intensity?
-intensity would increase proportionately with power
If the area increased, what would happen to intensity?
-intensity would decrease proportionately with the area
How do you change the area of an ultrasound beam?
-focus the beam
What is the amplitude of a wave?
- the maximum variation that occurs in an acoustic variable
- measured from the baseline to the peak
What is power?
-the rate at which work is performed or energy is transmitted
Attenuation
-weakening of sound
weaker signal =
weaker echoes
TGC
- Time Gain Compensation
- provides amplification of specific field echoes
What is attenuation is soft tissue?
0.5dB/cm MHz
Half Value Layer
- thickness of material that will half the original intensity
- distance it takes to drop 3dB
What is the opposite of reflection?
transmission
IRC + ITC =
1 or 100%
What is impedance directly proportional to?
density and propagation speed
What are specular reflectors?
- diaphragm
- fascia
- bone
- sound bounces back and produces a strong echo
When does scattering occur?
-when the boundary is not smooth
Scattering
-redirection of sound in many directions
What does scattering depend on?
-operational frequency and scatter size
Which is more likely to scatter? (a Lg or Sm wavelength)
-a large wavelength is more likely to scatter
Backscatter
-echo info that comes back to transducer
What does a large reflection mean?
-large difference between impedances (z)
What happens when there is a small difference between impedances (z)?
-most will transmit
What do contrast agents produce?
-harmonic frequencies
What is round trip travel time in soft tissue?
13us/cm
Is sound a mechanical wave?
Yes.
Is sound ionizing radiation?
No.
Is sound electromagnetic?
No. Electromagnetic waves do not require a medium.
Can sound converge/diverge?
Yes.
What are other words for a transducer?
- probe
- scan head
- transducer assembly
Transduce
- convert one form of energy into another
- voltage to sound
- sound to voltage
Piezoelectricity
piezo (press) + electron (amber)
Piezoelectric Effect
-sound to voltage
Converse/Reverse Piezoelectric Effect
-voltage to sound
Piezoelectric Element (aka crystal)
- adding a voltage causes thickness of the element to change
- creates a mechanical wave
- returning echo affects the element (creates a voltage)
1 cycle of alternating voltage =
2/3 cycles of pulse
longer AV =
longer pulse (doppler)
Quartz
-one of the first natural piezoelectric crystals
What piezoelectric elements do we use now?
-synthetic crystals (often ceramics)
1 Substance:
- lead
- zirconate
- titanate
- called PZT
-barium
What are piezoelectric elements usually made of?
- PZT
- aka lead zirconate titanate
How do we realign molecular dipoles?
-place a strong magnetic field at a high temp. (350 degrees)
Polarization
-aligned dipoles
What do piezoelectric elements do while in magnetic field?
-cooled
Curie Point
-the temp. in which the magnetic properties of a sound can be changed
What is the curie point go PZT?
350 degrees
What would happen if thou brought the crystal back to the Curie Point, but without the magnetic field?
- dipoles will scatter
- not piezoelectric anymore
What frequency does piezoelectric element (aka crystal) have?
- natural vibrational frequency
- fundamental frequency
- operational frequency
- resonance frequency
*they are all the same, just different names
What does operating frequency depend on?
-the element
Wavelength of US =
2x thickness of material (2 x th)
Multi Hertz Operation
-2 or 3 frequencies on the same element
How can operation frequency be adjusted?
-change voltage
Does PRF of voltage affect PRF of pulse?
Yes.
PRF volt =
PRF pulse
If you go further away, what do you have to decrease?
PRF
If you don’t decrease PRF when you go further, what happens?
- range ambiguity (artifact)
- echo misplacement (artifact)
-happens from sending pulses too fast (before the other now is back)
pen
- penetration
- aka depth
Damping Material
- aka backing material
- metal powder (tungsten) and plastic/epoxy resin
- on the back of the element
What is the damping materials impedance similar to?
crystal
What does a damping material reduce?
- ringing (ex. bell wrapped in rubber)
- cycles per pulse (n)
- amplitude (this is bad, weaker echo)
- sensitivity (this is bad, ability to detect weaker echoes)
What do a lower PD and a lower SPL result in?
better resolution
Does CW have damping material?
What does this result in?
No.
- better sensitivity
- worse penetration
House Unit/Case
- usually same material as damping
- prevents moisture
- protects internal structures
- absorbs energy from sides of crystal
What is the purpose of matching layer?
-to reduce the difference in z
Footprint
-width of probe head
Do higher frequency transducers have smaller or larger footprints?
-smaller
What does a smaller beam width mean?
- stronger intensity (picks up more signals)
- better resolution
What does focusing do?
-improves resolution (only in the NZL)
Where is beam width decreased?
-focal region
Focal Length
-distance from transducer to the focal region
Natural Focus
-beam will naturally come to focus
What are other names for the near zone?
- fresnel zone
- near field
NZL
-region from transducer to min beam width
Beam Convergence
-beam width decreases with increasing distance from the transducer
What are other names for the far zone?
- fraunhofer zone
- far field
Far Zone
-region beyond the NZL
Beam Divergence
-beam width increases with increasing distance from transducer
Aperture
- opening
- width of element(s)
Beam Profile
-width (wb)
What does beam width change with?
What does it affect?
- depth
- affects resolution and intensity
Is intensity uniform within a beam?
No
Is power uniform within a beam?
Yes
What is the width at the focus?
0.5 aperture size
What is NZL determined by?
- size of element (aperture)
- operating frequency
Focal Length
- aka NZL
- distance to focus
Why is transducer care important?
- prevention of nosocomial infections
- there is no infectious disease screening before exams
Critical
- device enters otherwise sterile tissue
- ex. intraoperative applications
Level of Disinfection: sterilization
Semi-Critical
- device contacts mucous membranes
- ex. endocavity applications
Level of Disinfection: high
Non-Critical
-device contacts intact skin
Level of Disinfection: intermediate or low
Transducer Care
- store in clean holders/racks
- keep cord off floor
- wipe probe and cord between patients
- use carrying case for transport
Low Level Disinfection
- use of sterilization wipes/solutions
- no bleach, ammonia or alcohol bases
- no sprays
- 5% hydrogen peroxide
- cavi wipes
- preempt (wipes)
High Level Disinfection
- invasive transducers are soaked
- cidex
- recert
Invasive Transducers
- transvaginal
- transrectal
- transesophageal
- catheter mounted
What are the benefits of invasive transducers?
- get us much closer to the tissue (higher resolution)
- can use high frequency without worrying about attenuation
What is a drawback for invasive transducers?
-high risk of infection without proper sterile techniques
What does resolution allow for?
- the ability to image fine detail
- ‘better’ picture
- being able to separate distinct echoes
Why is a smaller resolution better?
-smaller details can be discerned
What happens if 2 reflectors are not separated sufficiently?
-they produce overlapping echoes
Axial Resolution
-minimum reflector separation along scan line to produce separate echoes
What are other names for axial resolution?
- longitudinal
- radial
- depth
- range
If the axial resolution is 2mm, structures 2mm apart will be seen as ___ structure(s).
2
If the axial resolution is 2mm, structures 1mm apart will be seen as ___ structure(s).
1
Do we want a smaller or larger AR?
-smaller is better
AR
Axial Resolution
How do we improve AR?
-reduce SPL
How do we reduce SPL?
- reduce ‘n’ by reducing damping layer
- reduce wavelength by increasing operating f (will affect penetration)
What are some other names for lateral resolution?
- angular
- transverse
- azimuthal
Lateral Resolution
-minimum reflector separation perpendicular to scan LINE to produce separate echoes
LR
-lateral resolution
Do we want a sm or lg LR?
-smaller is better
How is LR improved?
-reducing beam width (by focusing)
Elevational Resolution
- minimum reflector separation perpendicular to scan PLANE to produce separate echoes
- 3rd dimension
A beam has slice thickness, which can also be known as __________ or __________.
- section thickness
- elevational plane
What can elevational resolution produce?
-section thickness artifact (aka partial volume artifact)
What does elevational thickness do?
-filling in of cysts or other anechoic structures (ex. GB, vessels, urinary bladder)
What will a poor evolutional resolution show?
-echoes from outside the intended scan plane (especially with anechoic structures)
How do we fix elevational resolution artifact?
THI
- narrower and thinner beam
- less likely to pick up echoes from other plane
Spatial Compounding
Temporal Resolution
-being able to separate echoes in real time
What does poor temporal resolution visualize as?
-lag
Contrast Resolution
-being able to separate 2 different shades of grey
Useful Frequency Range
2 to 10 MHz
What do higher frequencies increase?
-resolution
What do higher frequencies decrease?
-max imaging depth
When may a frequency up to 50MHz be used?
- opthalmologic imaging
- dermatologic imaging
- intravascular imaging
What does electronic focus eliminate the need for?
- a lens
- curved elements
How can focus be achieved in the third dimension (perpendicular scanning plane)?
- a lens
- curved element
How can phasing be applied to focus in the third dimension electronically?
-with at least 3 rows of elements
What do 2D rays have the ability to do?
-steer and focus in 2 dimensions
1D array is…
-2D imaging
2D array is…
-3D imaging
4D imaging =
3D imaging + time
What is 3D imaging mostly used for?
- obstetrics
- breast
What is 3D imaging also known as?
- volume imaging
- volumetric scanning
Reception Steering
-listening from a particular direction
Dynamic Focusing
-continual change of the focus with increasing depth
Dynamic Aperture
-as the focus continues to change during echo reception, the aperture will increase to maintain a constant focal width
Annular Arrays
- concentric rings
- piezoelectric material carved out in rings
Why aren’t annular arrays used anymore?
-we can do the same thing with phasing and electronic phasing
Hemodynamics
-study of blood flow
What is the motion of heart and blood flow detected with?
- doppler effect
- detects, quantify’s and evaluate’s blood flow
Circulatory System
- heart
- arteries
- arterioles
- capillaries
- venules
- veins
How many L of blood do humans have?
5L
What does blood consist of?
- plasma
- RBC
- WBC
- platelets
Blood Functions
- heat regulation
- oxygen
- carbon dioxide
- nutrients to cells
- removal of waste from cells
Heart
- 4 chambers: 2 atria, 2 ventricles
- cardiac muscles
- valves
Which arteries are oxygen poor?
-pulmonary arteries
Which veins are oxygen rich?
-pulmonary veins
Valves
- one way
- prevent back flow of blood
- in heart and veins
Stenotic Valves
-don’t open enough (pathology)
Insufficiency/Regurgitation (valves)
-don’t close enough (pathology)
Hydrostatic Pressure
- equivalent to the weight of a column of blood
- increases with distance below the heart
What is the hydrostatic pressure in a supine patient?
0 mmHg
What is the hydrostatic pressure in a standing patient?
100 mmHg
-pressure in veins is much higher
Respiration is also known as…
-phasicity
Inspiration
- diaphragm moves down
- increase in thorax volume, decrease in thoracic pressure
- allows air into lungs
- decrease in abdominal volume, increase in abdominal pressure
- stops venous return from the legs
Valsalva
- patient can be asked to hold their breath and ‘bear down’
- increase in abdominal pressure
- stops venous return from legs
Expiration
- diaphragm moves up
- decrease in thoracic volume, increase in thoracic pressure
- increase in abdominal volume, decease in abdominal pressure
- venous blood returns from legs
Pressure
-driving force behind fluid flow
What is required for fluid flow?
-pressure difference/gradient
Which way does fluid flow?
-area of high pressure to area of low pressure
Volumetric Flow Rate
- volume of blood passing a point per unit of time
- LONG STRAIGHT TUBE (we assume vessels in the body are long straight tubes)
R
resistance
Viscocity
-resistance to flow offered by fluid
What has the strangest effect on resistance?
-radius or diameter
What does resistance depend on?
-radius to the 4th power
If the radius doubles, what happens to R?
-decreases 16x
Vasoconstriction
- smaller blood vessels
- restricts blood flow
Vasodilation
- larger blood vessels
- allows more blood flow (when needed)
Types of Flow
- laminar
- turbulent (non laminar)
Types of Laminar Flow
- plug flow
- parabolic
- disturbed flow
Plug Flow
- at the entrance to tubes
- blood moves as a unit
- same speed across the vessel
Parabolic/Laminar
- after entering the straight tube
- fastest speeds in the centre of tube
Where do we assess in doppler?
-centre of the vessel (where it is fastest)
Where is the slowest speed?
- tube wall
- at 0
Average flow speed =
1/2 of fastest speed
Disturbed Flow
- at stenosis
- at bifurcation
- still laminar, but streamlines are not straight
- non parabolic
Turbulent Flow
- usually after a significant stenosis
- chaotic, multidirectional, multispeed flow
- non laminar
- eddies
- overal forward flow
What time of flow may physicians heart bruit with?
-turbulent flow
What does turbulent flow depend on?
-Reynolds #
What is critical Reynolds number?
2000 for blood
-flow must surpass a critical Reynolds # to cause turbulent flow
Stenosis
-partial blockage
Occlusion
-complete blockage
As heart beats…
-pressure and speed go up and down
what do we observe in compliant vessels?
1) added forward flow
2) reversal of flow
Compliance
-expansion and contraction of non ridging vessels during systole and diastole
Windkessel Effect
-aka added forward flow
Systole
-vessel expands
Diastole
- vessel contracts
- results in extended flow w/o driving pressure from heart
Flow Reversal
During Diastole
- in AO, blood doesn’t flow back because the aortic valve closes
- sometimes in distal circulation, when pressure decreases and vessel contracts, there will be reversal of flow (no valves to prevent back flow)
Continuity Rule
-speed goes up at a stenosis to keep volumetric flow rate (Q) constant at all 3 regions (before, at and after stenosis)
What happens to speed and pressure at a stenosis?
- speed goes up
- pressure goes down
What does the continuity rule apply to?
- volumetric flow rate for a short segment (constant)
- usually taking about a stenosis
Q = V x A
What happens when A decreases?
-V increases to keep the same amount of blood going through (continuity rule)
What is the point of the continuity rule?
- if we can figure out how fast the blood is going at a stenosis, we can figure out how much it is stenosed
- V will determine the severity
Bruit
-sound produced by a stenosis
Bernoulli Effect
-decrease in pressure in regions of high flow speed (at stenosis)
Before Stenosis:
Bernoulli Effect
-pressure goes up to push blood through stenosis
At Stenosis:
Bernoulli Effect
- pressure goes down to maintain energy
- pressure energy to flow energy
After Stenosis:
Bernoulli Effect
-flow energy to pressure energy
Ultrasound Basics
- sound wave is sent out at a particular frequency
- bounces off stationary structure
- returns at the same frequency that it was sent out
Doppler Effect
-change in frequency caused by the motion of a source, reflector or receiver
How do we know if the frequency will come back higher or lower?
-if the object is moving closer or further away
Doppler Shift
-difference between the sent a returning frequencies of a sound wave
What happens to the frequency if we are getting closer together?
-increases (positive shift)
What happens to the frequency if we are getting further apart?
-decreases (negative shift)
What shift does blood coming towards the transducer have?
-positive
what shift does blood moving away from the transducer have?
-negative
In Doppler…
- we fire a particular frequency
- measure the frequency that returns
- calculate how fast the object is moving
How do we affect the doppler shift?
-changing frequency
What can doppler shift calculate?
-velocity
Are we directly affecting the velocity by changing the frequency?
No.
Doppler effect Applications
- police speed detectors
- weather forecasting
- door openers
- burglar alarms
When does lesser doppler shift occur?
-if the angle of interrogation is non zero
What do we incorporate to compensate for lesser shift?
cos
Doppler Angle
-angle between the sound beam and the vessel
What does doppler shift depend on?
-cosine of doppler angle
cos 0 =
1
-hitting vessel straight on
cos 30 =
0.87
cos 60 =
0.5
cos 90 =
0
-hitting vessel perpendicular
What affects out calculated doppler shift?
-doppler angle
When is doppler shift less?
-when incorporating non zero angles
What does a decease in doppler shift measurement mean?
-decrease in velocity measurement
What happens if we don’t incorporate cos?
-velocity measurements are off
What degree do we want to hit vessels at?
0
-nearly impossible
What are some techniques to get close to a 0 doppler angle?
1) heel toe
2) phase (steer)
3) angle correct
Doppler Angle Correct
- we can tell the machine that we are off and it compensates
- increased our velocity back to where it should be
When will there be an increased error in speed calculation?
-with a higher angle
What angle do we work under?
60
Doppler Blood Flow
- presence (yes/no)
- speed (slow/fast)
- character (laminar/turbulent)
- direction
Doppler Displays
- colour
- spectral
- audible
Colour Doppler
- aka colour flow imaging
- presence, speed, character and direction of blood flow
- assess a very large area at a time (vs. spectral doppler)
- colour superimposed on grey scale image
How do we steer colour doppler?
- phasing
- to get closer to 0 degrees
- to avoid if vessels are parallel to surface
How do we know if doppler shift has occurred?
-if the retiring echoes have a different frequency from the emitted
How do we know if flow is moving toward or away from transducer?
- depending on the sign of the doppler shift
- colour is assigned to pixels
What is the difference in scan lines from colour and B mode?
Colour- multiple pulses are sent per scan line
B Mode- 1 pulse per scan line
what is doppler shift calculated within?
-signal processor
What does the signal processor detect?
Signal Processor Parameters:
- direction
- mean
- variance
- power
What is direction based on? (colour doppler)
+/- doppler shift
Mean/Average Velocity (colour doppler)
-average velocity of blood in an area is calculated and displayed
Variance (colour doppler)
- the variety within the blood flow
- soft tissue deviation
Power/Strength of Echo (colour doppler)
- related to amplitude
- depends on reflectors, impedance and concentration of RBC’s
Reflectors
-specular reflection vs scattering
Impedance
higher impedance mismatch = more reflection
Concentration of RBC’s
higher concentration = more reflection
Math Technique for Colour Doppler (autocorrelation)
- done with signal processor
- determines mean and variance
How many pulses per scan line are sent out for colour doppler?
- 3 to 32 per scan line
- usually 10 to 20
Ensemble Length
-# of pulses/scan line
More pulses for…
- accuracy of speed calculation
- sensitivity (picking out weaker echoes/shifts)
Where does the data go once the instrument process it?
-to the display
What does the display use?
- hue
- saturation
- luminance
Hue
-what colour is shown
Saturation
-amount of colour
Luminance
- brightness
- echo intensity/power
- similar to 2D echo intensity
- stronger echoes will be brighter
What can a monographer do to affect the effectiveness of the colour doppler examination?
Change the size of the colour box/window.
- changes coverage
- affects frame rate/temporal resolution
What does colour map help to determine?
-mean velocities
-direction of flow
variance (change in colour from LT to RT)
What colour spectrum is most commonly used?
-red/blue
What shift does red usually indicate?
-positive
What shift does blue usually indicate?
-negative
Where is the + and - on the colour map?
+ on top
- on bottom
Invert
- slips the colour map
- does NOT change direction of flow
Baseline
- zero point (no doppler shift)
- can move the baseline up or down based on what you want to fit on your map
Can you control PRF in doppler?
Yes.
Can you control PRF in 2D?
No.
What does changing the PRF in doppler do?
- changed the scale (cm/s)
- increases or decreases speeds
decrease in PRF is a ______ in scale.
-decrease
An increase in PRF is a ______ in scale.
-increase
What happens if the PRF scale is set too high?
-slower flows go undetected?
What happens if your PRF scale is too high?
-faster flows alias
Aliasing
-shows wrap around colour
What does aliasing depend on?
Nyquist Limit
Nyquist Limit =
1/2 PRF
Are higher or lower velocities more likely to alias?
higher
When will aliasing often show?
within a stenosis if PRF is set too low
When can aliasing be good?
- can adjust your PRF for the normal flow
- area of stenosis will show aliasing (highlights areas of highest velocity)
Colour Gain
- we can apply returning echo voltages (just like 2D)
- we want wall to wall filling
Priority (colour control)
-aka echo write priority
Grayscale vs Colour Threshold
- echoes below cut off amplitude will show as colour
- stronger echoes show as grey scale
Wall Filter
- filters out noise (non useful info)
- keeps the useful signal
- eliminates any movement in the tissue that will produce doppler shifts
- sets a cut off point
Movement in Tissues that Produces Doppler Shifts
- tissue vibration, vessel movement
- aka clutter
- have lower velocities
Are arterial or venous speeds higher?
-arterial
How can you fix aliasing?
-increase PRF (not too much)
What happens if you increase PRF too much?
-echo misplacement
What may a tortuous vessel look like in colour doppler?
- flow reversal
- aliasing
If you hit a vessel close o 90 degrees, what all it look like?
-no flow (occlusion)
What does an increase in doppler angle reduce?
- doppler shift
- corresponding flow
Power Doppler
- strength/intensity of echo
- related to amplitude
- determines by concentration of moving RBC’s
Can we display just the power?
Yes.
0shows only presence of flow
-no velocity, direction, flow character
Pro’s of Power Doppler
1) increased sensitivity
- slow flow
- small or deep vessels
2) angle dependant
3) no aliasing
Con’s of Power Doppler
- no speed/cannot quantify
- no direction
- no flow character to assess stenosis
Duplex Scanning
- doppler and gray scale
- we update between the 2 (transducer does 1 at a time)
- simultaneous (rapidly switches between the 2)
Spectral Doppler
-firing alone a single scan line
What is spectral doppler also called?
- pulsed doppler
- pulsed wave doppler
How many cycles/pulse does spectral doppler have?
5 to 30 cycles/pulse
Spectral Doppler Parameters
-presence, speed, character and direction of blood flow
Sample Volume
- specific area that is being assessed
- aka gate
Range Gating
-being able to select info from a specific depth
Effective Sample Length =
1/2 SPL + gate length
Sample Width =
beam width
Is sample volume (SV) user defined?
yes
Larger Gate Lengths
-looking for a vessel/signal
Smaller Gate Lengths
- more specific info
- less noise
- better quality of spectral waveform
With spectral doppler, samples are taken and then ______.
smoothes
Spectral Doppler
x axis =
y axis =
x axis = time
y axis = soppler shift/velocity
Quadrature Phase Detector
- allows detection of bidirectional doppler
- we can see +/- shifts
Spectral Analyzer
-determines each doppler shift frequency and it’s strength
Fast Fourier Transform
- occurs within the spectrum analyzer
- math technique to figure out the frequency spectrum
- results sent to be displayed on the spectral display
What does spectral display show?
-spectral waveforms
Each point of spectral display shows…
- direction (+/-)
- magnitude (how fast)
- amplitude (brightness)
What affects direction of spectral waveform?
- whether its coming towards or away from the transducer
- affected by direction of probe/steering
What affects magnitude (cm/s) of spectral display?
- speed of the blood flow
- doppler angle
Doppler Angle
- between the scan line and the blood flow
- can affect the calculated doppler velocity
larger doppler angle =
smaller doppler shift
slower velocity calculated (w/o angle correct)
What affects amplitude of spectral doppler?
- brightness/intensity of returning echoes
- concentration of RBC’s/reflectors/impedances
Spectral Doppler Controls
- gate size/sample volume
- gain
- spectral invert
- baseline
-very similar to colour doppler controls
Gate Size/Smaple Volume (spectral controls)
-affects rangle resolution
Range Resolution
- knowing exactly where the signal is coming from
- opposite of range ambiguity
Gain (spectral controls)
-amplification of incoming echo voltages
Invert (spectral controls)
- arteries are generally positive signals (above baseline)
- veins are generally negative signals (below baseline)
Baseline (special doppler)
- xero point
- ex. more + signals can move baseline down
Wall Filter (spectral controls)
- cuts off slower flows
- used to get rid of clutter (tissue motion, valves)
PRF (spectral controls)
-PRF affects scale (which velocities we can display)
What will happen to any doppler shift over Nyquist Limit?
-alias
What happens if we decrease the PRF too much?
-aliasing (wrap around)
What happens if we increase the PRF too much?
-poor signal (wasted space)
Angle Correct (spectral control)
- increased angle, decreased doppler shift
- makes it seem lower than it is
What happens when you correct the angle?
-brings the calculated speed back to it’s true value
Higher frequencies will have a ______ pitch.
higher
What else can spectral doppler be converted to?
-audible sound
Volume (spectral Controls)
- audible signal can be turned up or down
- we can listen for areas of high speeds (higher pitch)
Analyzing the Waveform
analyze spectral display with colour and grayscale info to assess for normal blood flow and pathology
Arterial Signals
- pulsatile
- higher velocity waveforms
Venous Signals
- phasic
- lower velocity waveforms
Analyzing the Resistance of Flow in a Waveform
-assess the systolic portion and the diastolic portion
High Resistance
- ECA, extremities
- quick upstroke
- low diastole
Low Resistance
- ICA/CCA, blood hungry organs (liver, renal, etc.)
- vasoldilation, slow upstroke
- higher diastole
Assessing Area of Stenosis
- remember continuity rule
- decrease in area, increase in velocity
- surpassing Reynolds # = turbulence
Spectral Broadening
wider range of spectra = more variety of shifts
- narrowing the window
- most obvious reason is stenosis
Spectral Limitations
- sonographers skill
- movement
- range gated to a specific depth
- aliasing
Fixing Aliasing
- adjust baseline
- lower operating f
- increase doppler angle (lowers all doppler shifts)
- increase PRF (increases Nyquist limit), allows for higher doppler shifts/velocities
Spectral Limitations
-body habits (depth) limits PRF
CW Doppler
- occasionally used
- 2 transducer elements (1 to send, 1 to receive)
- oscillator (CW beam former) produced a voltage
- sample volume = lg overlapping area
Can continuous wave doppler alias?
No.
-aliasing happens because the shift is more than Nyquist limit
Nyquist limit = 1/2 PRF
-no PRF in CW
-can pick up high maximum values without aliasing
CW Doppler Limitations
- large sample volume
- cant tell where its coming from
- poor range resolution
Does PW doppler have good range resolution?
excellent
