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what artifacts can be fixed by harmonics?
slice thickness
side lobes
grating lobes
reverberation
what artifacts can be fixed by spatial compounding?
speckle
edge shadowing
enhancement
dirty shadowing
what artifacts can be fixed by adjusting TGC’s?
focal banding
enhancement
what artifacts can be fixed by using 1 focal zone?
range ambiguity
what artifacts can be fixed by adjusting the power input or gain?
range ambiguity
enhancement
what artifacts can be fixed by sliding the transducer laterally?
refraction
what artifacts can be fixed by persistence?
speckle
what artifacts can be fixed by changing your approach?
mirror image
what artifacts can be fixed by changing probes?
side lobes
how can propagation speed error be reduced?
Trick question
CANNOT BE ELIMINATED
what is a slice thickness/partial volume artifact?
displays as echoes within an anechoic structure (fills lumen)
3rd dimension
beam width perpendicular to the scan plan is greater than 2 adjacent reflectors
what is the speckle artifact?
granular appearance of images that results from interference of echoes from the distribution of scatterers
constructive and destructive interference
diagnostic
what is the reverberation artifact?
equally spaced reflections of diminishing amplitude with increasing imaging depth
2 or more strong reflectors cause multiple reflections
ex: comet tail and twinkle (color)
what is ring down artifact?
type of reverberation
appears as multiple parallel lines or a solid band behind a reflector
caused by vibrations of air bubbles or a strong reflector (stone)
what is the mirror image artifact?
vertical duplication of a structure on the opposite side of a strong reflector
form of reverberation
common around the pleura and diaphragm
also occurs in doppler
what is the refraction artifact?
change of direction of the sound beam from one medium to the next (diff prop speeds)
displays structures laterally from their correct location
ex: duplication
what is the multipath artifact?
displays a structure deeper than it should
beam bounces of multiple structures so it takes longer to return
oblique incidence causes the beam to reflect at an angle
beam later reflects off another structure
what is the focal banding artifact?
region of increased brightness at the focal zone
caused by increased intensity of the beam (I = P/A)
what are grating lobe artifacts?
additional weaker beams outside the main beam path
from a multielement array transducer
strong reflector duplicates laterally creating an arc appearance
what is a side lobe artifact?
beams in different directions than the beam
from a single element transducer
strong reflector duplicates laterally creating an arc appearance
what is the propagation speed error artifact?
occurs when speed of sound in ST is faster or slower than the assumed 1540 m/s
slower speeds place echo deeper (takes longer to return)
faster speeds place echoes closer (return sooner)
can also change shape of the reflector (oval instead of round)
what is the range ambiguity artifact?
all echoes are not received before the next pulse is emitted
places structures much closer to the surface than they should be
PRF is too high
also occurs with doppler
what is the shadowing artifact?
weakening of echoes distal to:
strongly attenuating structure
strongly reflecting structure
from the edges of a refracting
ex: bone, metal
what is the edge shadowing artifact?
caused by refraction along the edge of a curved structure
decreases intensity of sound posterior to the curved edge
what is the enhancement artifact?
strengthening of echoes distal to a weakly attenuating structure
increased brightness behind a weakly attenuating structure
ex: cyst
what is a damaged elements artifact?
crystal/transducer element is damaged
displays on the screen as vertical anechoic line
NOT reverberation
no fixing it, get a new probe after testing it
overall gain
determines amount of amplification of echoes
ratio of output to input (dB)
3 dB → x2
10 dB → x10
time gain compensation
amplifies selectivity based on arrival time (depth)
compensates for the effect of attenuation on an image
allows for display of echoes from similar reflectors at different depths in a similar way
creates uniform brightness
power
amount of energy transmitted by an ultrasound machine over time
controls the strength of the sound waves which can be changed by the sonographer
controlled by voltage amplitude which is controlled by the pulser
depth
distance between the transducer and the structures being imaged
controls how far into the body the waves can penetrate
inversely related to PRF and f
pulser automatically adjusts PRF
zoom
write: increased # of pixels or scan lines (pre-processing
read: # of pixels or scan lines is the same as the original picture but can look like less (post-processing)
harmonics
allows for better penetration
transmitted at lower f and received at higher f
image quality is improved in 3 ways:
LR improvement (narrower beams)
grating lobes are eliminated
superficial reverberation reduced or eliminated
dynamic range
ratio of largest to smallest amplitude or power a system can handle
dB
can change to control the # of shades of gray displayed
inversely relate to compression
edge enhancement
sharpens boundres to make them more detectable and measurements more precise
persistence
reduces noise/speckle and smooths the image by frame averaging
multiple frames at the same angle are averaged together creating one image
uses weighted average
higher levels are appropriate for slow moving structures (needs minimal movement)
frequency compounding
2 or more frames at different f are averaged together
processed separately but at the same time and then recombined
echo frequency spectrum is divided into f bands by filters
commonly used to reduce noise and improve CR
extended field of view
trapezoid (adds wings to the sides of the image)
panoramic imaging
spatial compounding
averages frames that view anatomy from different angles through phasing and beam steering
anatomy is scanned more than once
improves image quality
improves presentation of specular (smooth) reflections
increases likelihood of 90 degree incidence
coded excitation
uses a series of pulses and gaps to generate a scan line
improves penetration
better AR, CR, and S-to-N ratio
results in a more sensitive receiving system of weak echoes
image memory
image frames are stored in the memory
divides the image into pixels (more pixels = finer spatial detail)
holding and displaying 1 frame out of a sequence is called a freeze
storing the last several frames acquired before freezing is called a cine loop
image display
brightness is proportional to echo strength
presenting in A, B, and M mode
flat panel display and cathode ray tube
frame: each individual image, 2 fields (odd and even), 525 horizontal lines
frame rate and refresh rate
what functions does coded excitation accomplish?
multiple foci
separation of harmonic echo bandwidth from transmitted pulse BW
increased penetration
reducing of speckle with improved contrast resolution
gray scale imaging of blood flow
what is A-mode?
amplitude mode
x-axis: depth
y-axis: amplitude
shown as sharp spikes on a graph
used in ophthalmology
what is B-mode?
brightness mode
AKA gray scale
2D or 3D
x-axis: side to side OR sup to inf aspect of the body
y-axis: penetration depth
what is M-mode?
motion mode
combines A-mode and B-mode to show motion
can also be considered B-mode
x-axis: time
y-axis: depth
What does the thickness of the element effect?
effects the resonant frequency
thin = higher f to visualize small structures
thick = lower f to penetrate deeper
What assumptions does the machine make?
sound travels in a straight line
echoes originate only from objects located on the beam axis
the amplitude of the returning echoes is directly related to the reflecting scattering properties of distant objects
the distance to reflecting or scattering objects is proportional to the round trip travel time
What determines the acoustic power?
the amplitude of the wave or voltage applied to the transducer
higher voltage produces a higher amplitude wave so greater power
I = A²
panoramic imaging
extends the image beyond the normal limits of the field of view of the transducer
new echoes are added to the image in the direction in which the scan plane is moving
how does spatial compounding improve image quality?
reduces speckle and clutter artifacts
smooths imaging surfaces
visualizes structures behind a highly attenuating structure
how are harmonics generated?
generated within the tissue during non linear wave propagation
due to pressure changes
what negatively affects frame rate & TR?
line density
lines per frame
sector width
increased depth
multiple foci
what positively affects frame rate?
PRF
increased temporal resolution
What does changing to a higher frequency transducer result in?
decreased depth (image superficial structures)
better image resolution
can see smaller details
shorter wavelength
sounds waves are more easily absorbed by the tissue obstructing penetration
How does a sonographer compensate when changing to a higher frequency?
reduce signal amplitude to get rid of excess noise
modify the gain settings
high pass filter to reduce amplitude
what does depth control?
controls how deep into the tissue you can visualize on the screen
controls upper limit of f (but lowers resolution)
optimize power and TR
pulser automatically adjusts PRF
inversely related to PRF
principle 1
one to one correspondence
physical beam forming is directly coupled with displayed scan lines
pulse echo scan line
principle 2
virtual beam forming does not rely on one to one relationship
uses weakly focused or unfocused transmit beams and computed reception “beams”
images are in focus throughout, improved quality
what is operating principle 1 system composed of?
beam former
signal processor
image processor
display
what is the beam former?
where the action originates
generates voltages that drive the transducer
determines PRF, coding, f, and I
scanning, focusing, and apodizing the transmitted beam
amplifying the returning echo voltages
compensating for attenuation
digitizing the echo voltage stream
directing, focusing, and apodizing the reception beam
what does the beam former consist of?
pulser
pulse delays
transmit/receive switch (T/R switch)
amplifiers
analog to digital converters (ADCs)
echo delays
summer (sum)
what is the pulser?
generates the voltages that drive the transducer
f determines PRF and PRP
voltage amplitude controls transducer:
amplitude
intensity
power
directly related
how does the pulser avoid echo misplacement?
all echoes from one pulse must be received before the next pulse is emitted
depth and PRF are inversely related
so, the pulse automatically adjusts the PRF based on imaging depth
pulser and pulse delays functions
sequencing
phase delays
variations in pulse amplitudes
the pulser and pulse delays allow electronic control of:
beam scanning
steering
transmission focusing
aperture
apodization
what is apodization?
sending different voltage amplitudes to minimize grating lobes
what is a channel?
an independent signal path consisting of a transducer element, delay, and possible other electronic components (transmit and receive)
an increase allows more precise control of beam characteristics
modern systems typically consist of 64, 128, or 192
what is the T/R switch?
during transmission, this opens the path from the pulser to the transducer elements
during reception, this opens the path from the elements to the reception amplifiers
protects the sensitive input components of the amplifiers from the large driving voltages from the pulser
what are amplifiers in the beam fomer?
increase voltage amplitudes
beam former has one amplifier for each channel
basically turns up the gain/volume
what are the digitizers?
analog to digital converters convert the analog voltages representing echoes to numbers
these numbers are sent for digital signal processing and storage
what is the echo delay?
reception dynamic focus
steering
what is the summer?
signals are added together to produce a scan line
reception apodization
dynamic aperture
what is the signal processor?
receives digital signals from the beam former
functions:
filtering
detection (demodulation)
compression
how does filtering work in the signal processor?
bandpass filter
passes a range of frequencies (BW)
rejects frequencies above and below
reduces noise
eliminates the fundamental frequency in harmonic imaging
how does detection work in the signal processor?
conversion of echo voltages from radio f form to video form
AKA demodulation
keeps amplitude of echoes
not operator controlled
includes:
rectification: inversion of negative values into positive (all above the baseline)
smoothing: allows for one mean signal to be recorded
how does compression work in the signal processor?
reduces the dynamic range to a usable range
operator control
small (narrow) DR = fewer shades of gray, black to white high contrast
high (broad) DR = more shades of gray, good CR
what is the image processor?
converts scan line data to images
processes the images before storing them in the image memory
processes images as they come out of memory
converts images from digital format to analog
sends images to the display
what does an “image” go through after the signal processor and before the display?
preprocessor
scan converter
image memory
postprocessor
digital to analog converter
preprocessing
image processing done before storing in memory
pixel interpolation (fills in missing pixels and assigns brightness based on average of adjacent pixels)
persistence
volume imaging
4D
edge enhancement
write zoom
panoramic imaging
spatial compounding
volume imaging (3D)
acquiring several 2 dimensional scans for 3D volume of information in the image memory
4D imaging
allows for real time scanning in 3D
3D in the 4th dimension of time
what is the scan converter?
converts scan line data into image format data (matrix)
properly locates each series of echoes corresponding to each location in depth along the scan line
what is post processing?
performed after the echoes are stored in memory
assignment of display brightness to numbers from memory
operator controlled
post processing functions
gray scale maps
B-color
3D presentation
read zoom
measurement calipers
what is the digital to analog converter?
converts the digital data received from the image memory to analog voltages (amplitude)
determines the brightness of the echoes on the display
what is the refresh rate?
number of times images are retrieved from the memory and presented on display per second
what is frame rate?
number of images entered into memory per second
directly affects temporal resolution
flicker free must be > 20 frames/sec
typical TV = 30 frames/sec
flat panel display
back lighted liquid crystal display (LCD)
found in the moniter
contrast resolution
ability of a gray scale display to distinguish between echoes of slightly different intensities
depends on the number of bits per pixel in the image memory
improves with an increase in the number of bits per pixel
temporal resolution
ability of a display to distinguish closely spaced events in time and to present rapidly moving structures correctly
dependent on frame rate and PRF
temporal resolution equations
what is elastography?
imaging means through palpation
presents qualitative tissue stiffness information
“stress” and “strain”
what is cardiac strain imaging?
presents information regarding contraction and relaxation strain and strain rate info for the myocardium of the heart
what is fusion imaging?
a combined presentation of a sonographic anatomic image with another imaging form
CT or MRI
what can frequency compounding be adjusted to emphasize?
penetration
resolution
tissue texture
