SPI 3 Flashcards
Preprocessing
manipulating data before storage in scan converter
data is altered forever. cannot be reversed or undone.
examples of Preprocessing
TGC log compression write magnification fill-in interpolation persistence (frame averaging) spatial compounding
Log compression
related to the ability of humans to see the gray scale differences in anatomical structures. lowers the high level echoes and boosts the low level echoes. dynamic range reduced. controlled by sonographer
Postprocessing
manipulating data after it has been stored in the scan converter memory but prior to display. can be undone. preformed on frozen images.
examples of postprocessing
read magnification and 3-D rendering
Analog to digital conversion
electrical signals created by PZT are analog
but digital scan converter can only process computer info. So the analog signal must be converted or translated into digital form for input into the scan converter
example of an Analog to Digital scan converter
computer mouse (taking hand motions and converting them into computer skills)
Digital to Analog scan conversion
info in scan converter is digital however some displays are analog so image data must be reconverted to analog form prior to display
example of a Digital to Analog scan converter
iPod (taking computer files and converting them into sound waves in the form of music)
Read Magnification
Does not rescan only reads image in memory reads old data postprocessing same line density larger pixels spatial resolution not improved temporal resolution unchanged
Write Magnification
re-scans and acquires new data, discards old image data writes new data preprocessing increased line density more pixels improved spatial resolution temporal resolution can change
If image is shallower, write magnification can improve what?
temporal resolution
Fill-in interpolation
Pre-Processing
improves image detail (spatial resolution) by filling in missing data (less gaps)
images with low line density are most improved with this process.
see page (116)
Speckle
grainy/granular appearance
created by interference effects* of scattered sound, both constructive and destructive from many tiny tissue reflectors
(*) look for questions with these words
Spatial compounding
scan lines are steered by transducer in different directions or views, so structures are interrogated by multiple pulses from several different angles
what transducers are associated with spatial compounding?
phased array transducers only
Temporal Compounding/ Persistence (temporal averaging)
provides history of past frames that are overlaid or added on top of current frame overlaid.
What does persistence do?
displays smoother image and reduces noise and speckle
Improves dynamic range and contrast resolution useful for stationary or slow moving structures.
(pg 117)
Frequency compounding
Divides reflection into sub bands of smaller frequency ranges. Images are created from each of the sub bands
With frequency compounding what is the result?
Images averaged, improving signal-to-noise ratio
speckle and clutter artifacts are reduced
spatial resolution (detail) is improved (pg. 118)
Dynamic aperature
form of electronic receive focusing
varying number of elements used to receive reflected signal (changing size of listening hole, think about squinting improves image) (pg.118)
Edge enhancement
increases contrast at boundary to make images appear sharper
ideally suited to distinguish interfaces between structures with different gray scale characteristics (119)
Coded excitation
takes place in the pulser
improves signal-to-noise ratio
improves penetration, axial, spatial, and contrast resolution. (119) (-most important)
Elastography
based on deformation when force is applied to tissue
identifies tissue of different mechanical properties (not acoustic properties) (119)
Rendering
creates an element of realism to a 3-D 4-D image
constructs images with shadows, color, texture, and optical effects (120)
Dynamic Range
the ratio of the largest to the smallest signal strength that each component processes
indicates number of gray shades on an image
dB (120)
Narrow dynamic range
few choices
bistable (black and white)
high contrast
(120)
Wide dynamic range
many choices
gray scale
low contrast
(120)
Methods of recording and archiving
PACS-Picture archiving and communications system
DICOM-Digital imaging and communications in Medicine
PACS
digital lab stores distributes and displays data a computer network.
DICOM
provides standards and guidelines for imaging networks
Pulsatile flow
arterial
cardiac contraction
high rate
higher pressure (121)
Phasic flow
venous
respiration
low rate
lower pressure (121)
Flow
volume
how much?
volume/time
liters/min (121)
Velocity
speed
how fast?
distance/time
meters/sec (121)
Laminar flow
flow streamlines are layered, aligned and parallel
found in physiological states
plug or parabolic flow
(window clean) (122)
Parabolic flow
layers travel at individual speeds
speeds highest in center of lumen (122)
Turbulent flow
chaotic flow in many directions and speeds
flow varies from instant to instant and from location to location ex. stenosis (122) (window filled)
Turbulence may be identified as what?
spectral broadening (122)
Vortex
a swirling pattern of rotational flow (mini hurricane) (122)
eddys currents
turbulent flow (122)
Setting doppler gain too high can cause what?
spectral broadening (even with normal flow) (122)
Reynold’s number
unitless number indicating whether flow is laminar or turbulent
laminar- less than 1500
turbulent- over 2000
Energy gradient
blood flows when total fluid energy at one location differs from total fluid energy at another location (123)
Kinetic energy
motion energy (123)
Pressure energy
form of potential/stored energy that has the ability to preform work. (123)
Energy is imparted to blood by?
the contraction of the heart (left ventricle) called systole (123)
as blood flows through circulation what happens?
energy is dissipated (123)
Three forms of energy loss
Frictional
viscous
inertial
Frictional loss
Friction- the conversion of other forms of energy into heat.
Frictional losses occur when one object rubs against another (ex. blood sliding across vessel walls) (pg. 123)
Viscous loss
Viscosity- describes the thickness of fluid
Viscous loss results from fluid sticking to itself, internal friction. (p 123)
Inertial loss
Inertia- tendency of fluid to resist changes in velocity
Inertial loss is energy lost when velocity of fluid changes (p 123)
Inertial loss results from
pulsatile flow during both acceleration and deceleration and velocity changes at a stenosis. (p 123)
Stenosis
narrowing/ irregularity of lumen (p. 124)
What does a stenosis cause?
- change in flow direction
- increased velocity in stenosis, highest velocity at point of max narrowing
- turbulent flow at exit
- pressure gradient across stenosis
- loss of pulsatility in arterial flow (p. 124)
Factors that determine resistance
radius of lumen (most important)
length
viscosity of fluid
(p 124)
Arterioles are?
Resistance vessels in the circulation (p 124)
Bernoulli’s Principle
at the most narrowed location: velocity is highest kinetic energy is highest pressure energy is lowest Law of Conservation of Energy (p. 125)
Bernoulli’s equation
pressure gradient= 4(velocity)^2 (p 125)
Hydrostatic pressure equation
pressure measured= circulatory pressure+ hydrostatic pressure (p 125)
Hydrostatic pressure is
an error when measuring blood pressure at locations higher than or lower than heart level (p125)
Hydrostatic pressure supine patient
zero at all locations (p 125)
Hydrostatic pressure standing patient
-50 mmHg fingertip (hand above head)
-30 mmHg at head
0 mmHg at heart
75 mmHg at knee
100 mmHg at ankle
(p 126)
What is coaptation?
vessel collapse (p.126)
Venous flow- Inhale
diaphragm moves down
venous flow from legs decreases
venous return to heart increases
(p 127)
Venous flow- exhale
diaphragm moves up
venous flow from legs increases
venous return to heart decreases
(p. 128)
Doppler Shift/ Doppler frequency
change or difference in the frequency of sound as a result of motion between the sound source and receiver. (p. 129)
Positive doppler shift
when source and receiver are moving toward each other (p. 129)
Negative doppler shift
when source and receiver are moving away from each other (p. 129)
Doppler measures
frequency shift not amplitude (p 129)
velocity not speed* (p.130)
Doppler frequency units
Hertz (Hz)
Doppler shift equation
doppler shift= 2 x reflector speed x incident freq. x cos (angle) / prop. speed (p. 130)
demodulation
extracts the doppler frequency from the transducer frequency and is preformed by a demodulator.
Bidirectional Doppler is analyzed with?
phase quadrature processing
Speed measures
magnitude only (p. 130)
Velocity measures
magnitude and direction (p. 130) (what doppler measures)
Maximum doppler shift at what degree?
0 degrees (most accurate) (p. 130)
No doppler shift at what degree?
90 degrees (p.130)
Number and function of crystals in a Continuous Wave Doppler transducer?
2 crystals in the transducer
1 continuously transmits
1 continuously receives (p.131)
Most important advantage of Continuous Wave Doppler
High velocities are accurately measured (p.131)
Disadvantage of Continuous Wave Doppler
Range ambiguity ( cannot determine the depth) (p.131)
Other advantages of Continuous Wave Doppler
No damping Narrow bandwidth hi Q-factor higher sensitivity easier to detect small frequency shifts (p. 131)
Number and function of crystals in Pulsed Wave Doppler transducer?
One crystal
alternates between sending and receiving (p.131)
Most important advantage of Pulsed Wave Doppler
Echoes arise only from the area of interrogation. aka the sample volume or sample gate. we locate the gate (range resolution or range specificity) (p.131)
Disadvantage of Pulsed Wave Doppler
aliasing- errors in measuring high velocities (131-132)
Imaging and Doppler simultaneously is known as?
duplex imaging (p.131)
Other characteristics of Pulsed Wave Doppler
Uses damped, low Q, wide bandwidth transducer.
limit on max velocity (aliasing)
range resolution
min 1 crystal (p. 135)
What is aliasing?
High velocities appearing negative (PW Doppler) (p. 132)
What causes aliasing?
deeper sample volume and higher frequency (p.132)
Aliasing grows from where?
the top or the bottom of a spectrum, never the baseline. (p.132)
Nyquist limit (nyquist frequency)
the frequency at which aliasing occurs. (p.132)
Equation for Nyquist Limit
nyquist limit= PRF/2
p. 132
Aliasing appears when?
the doppler shift exceeds the nyquist limit
p.132
Aliasing can be eliminated when?
the doppler spectrum shrinks or when the nyquist limit is raised. (raise speed limit)
(p. 132)
5 Ways to eliminate Aliasing
select a transducer with lower frequency (reduces doppler shift and raises spectrum)
-
select a new view with a shallower sample volume (increases PRF and nyquist limit)
-
increase scale with same view (increases PRF and nyquist limit)
- baseline shift (moves line, for appearance only)
(p. 133-134)
What is wrap around?
When you cannot see top of waveform (not the same as aliasing) baseline shift does not correct this.
(p.133)
relationship between sample volume size and doppler spectrum?
-Smaller sample volumes (gates) create doppler spectra with cleaner spectral window
- Larger sample volumes create doppler spectra with filled-in spectral window ( spectral broadening)
(p. 135)
Doppler gain set too high creates the appearance of what?
spectral broadening.
p.135
Gray shades of a spectrum are related to what?
amplitude of a reflected signal
number of RBC’s creating a reflection
(p.135)
Color Flow Doppler does what?
Doppler shifts are superimposed onto a 2-D image
- black and white identifies anatomic structures
- color identifies blood flow velocities and function
p. 136
Color Flow Doppler is based on Pulse US and is subject to?
range resolution or specificity
aliasing
(p.136)
Color Flow Doppler provides info regarding?
direction of flow
and reports average velocities
(p.136)
Color Maps are?
a “dictionary” to convert measured velocities into colors
p.137
Velocity mode
colors present info on flow direction
p.137
Variance Mode
colors provide info on flow direction and presence or absence of turbulence (p.137-139)
Variance Mode colors
laminar flow-left sided colors
turbulent flow- right sided colors
(p. 137-139)
Doppler Packets
multiple US pulses are needed to accurately determine RBC velocities by Doppler. These multiple velocities are called a packet. (p.140)
Small Doppler Packet
- less accurate doppler
- less sensitive to low velocity flow
- higher frame rate, improved temporal res.
(p. 140)
Large Doppler Packet
- More accurate Doppler
- more sensitive to low velocity flow
- lower frame rate, reduced temporal res.
(p. 140)
More pulses in the packet has what two advantages?
- greater accuracy of the velocity measurement
- sensitivity to low flows is increased
(p. 140)
More pulses in the packet has what two disadvantages?
- more time is required to acquire information
- frame rate and temporal resolution are reduced
(p. 140)
Color Flow Doppler measures?
mean velocity
(PW and CW measures peak velocity)
(p.140)
Packet size must balance what?
between accurate velocity measurements and temporal resolution
(p.140)
Color Power Doppler is also known as?
energy mode or color angio
p.141
Advantages of Color Power Doppler
- Increased sensitivity to low flows (venous flow, flow in small vessels)
- Not affected by Doppler angles, unless angle=90 degrees
- No aliasing
(p. 141)
Limitations of Color Power Doppler
- No measurement of velocity or direction
- lower frame rates (reduced temp. res.) when compared to conventional color flow doppler
- Susceptible to motion of transducer, patient or soft tissues, this is called flash artifact.
(p. 141)
Roles of Continuous Wave Dopper
- identifies highest velocity jets everywhere along the length of the ultrasound beam. (jet-sniffer)
- Range ambiguity
- no aliasing
(p. 141)
Roles of Pulsed Wave Doppler
- accurately identifies location of flow (range resolution)
- has good temp. res.
- aliasing
(p. 141)
Roles for Color Flow Doppler
- Provides 2-D flow information directly on anatomic image.
- Size of color jet is most affected by color Doppler gain settings.
- Poorer temp. res. because of multiple sample volumes and packets.
- subject to range res. and aliasing
(p. 141)
Roles of Power Mode Doppler
- Allows the use of color with low velocities or small volumes of blood flow.
- Greatest sensitivity
(p. 141)
Spectral Analysis is?
- preformed to extract or identify the individual frequencies making up the complex signal.
(p. 142)
Spectral Analysis is used to?
interpret individual velocities in the signal.
p.142
Current methods of Spectral Analysis
- For Pulsed Wave and Continuous Wave: Fast Fourier Transform.
- For color flow Doppler: autocorrelation
(p. 142)
Fast Fourier Transform and Autocorrelation are?
digital techniques that are preformed by computers.
Autocorrelation is what?
used with color Doppler because of the enormous amount of Doppler information that requires processing. It is slightly less accurate but faster than FFT.
(p. 142-143)
What is the Doppler artifact called?
Ghosting
What is ghosting?
When color “bleeds” from a vessel into surrounding anatomy (color flash)
(p. 151)
Hyperechoic
portions of an image that are brighter than the surrounding tissues, or tissue that appears brighter than normal. (p. 153)
Hypoechoic
portions of an image that are not as bright as surrounding tissues, or tissues that appear less bright than normal.
(p.153)
Isoechoic
describes structures with equal echo brightness
p. 153
Homogeneous
portion of an image that has similar echo characteristics throughout. (p.153)
Heterogeneous
displaying a variety of different echo characteristics within the image. (p. 153)
Artifacts are?
Errors in imaging
Artifacts characteristics are?
- not real
- missing reflections
- improper brightness
- improper shape
- improper size
- improper position
(p. 153)
What causes Artifacts?
- Violation of assumptions
- Equipment malfunction or design
- Physics of US
- interpreter error
- operator error
(p. 153)
Six basic assumptions (artifacts)
- sound travels in a straight line
- sound travels directly to reflector and back
- sound travels exactly 1540 m/s
- Reflections arise from structures positioned along beam’s main axis
- intensity of reflections is related to scattering characteristics of tissue.
- imaging plane is extremely thin.
(p. 154)
Reverberations
multiple echoes appearing on the display as a result of US “ping-ponging” between two reflectors. (looks like a ladder or venetian blind)
(p. 154)
Characteristics of Reverberations
- multiple
- equally spaced
- parallel to the sound beam
- deeper and along a straight line
(p. 154)
Comet tail (Ring down) artifact characteristics
- single, solid hyperechoic line
- long echo
- parallel to sound beam
- reverberation with “space” squeezed out
(p. 155)
Shadowing artifact characteristics
- background color (too few reflections to scan)
- result of too much attenuation in structure above shadow.
- absence of true anatomy on scan in region of shadow
- parallel to sound beam
(p. 156)
Edge shadow artifact characteristics
Hypoechoic, background color (too many reflections on the scan)
due to refraction at edge of circular structure and beam spreading.
absence of true anatomy on scan in region of shadow.
(p. 157)
what can get rid of edge shadow artifact?
spatial compounding (p. 157)
Enhancement artifact characteristics
hyperechoic, foreground color (brighter echoes on scan)
results from too little attenuation in structure above artifact.
parallel to sound beam
beneath structure with reduced attenuation.
(p. 158)
Mirror image artifact characteristics
- Second copy of a reflector (extra reflections on scan)
- artifact (A) located deeper than the true reflector
- mirror lies on straight line between artifact and transducer.
- true reflector and artifact are equal distances from mirror.
(p. 159)
Propagation speed errors are?
When US doesn’t propagate a 1.54 km/s
assumed relationship between time and distance is invalid.
(p. 160)
Propagation speed errors result in?
- correct number of reflections on scan
- improper depth
(p. 160)
speed errors appear as?
step off, split or cut. (p.160)
reflector will be placed too shallow on display when?
speed is faster than soft tissue (p. 160)
reflector will be placed too deep on display when?
if speed is slower than soft tissue. (p. 160)
Refraction artifact sound changes direction striking a boundary how?
- obliquely
- when media have different prop. speeds
(p. 161)
Refraction artifact characteristics
- second copy of true reflector
- appears side by side with true anatomic structure
- degrades lateral resolution
(p. 161)
