C5&6: RT Considerations and Artifacts Flashcards
Each frame of US data is made up of what?
multiple scan lines
does a greater number of focal zones increase the time it takes for each frame to be produced
yes
what is the definition of frame rate
number of times the sweep of sound is produced by the transducer (or frames per second)
frame rates on US are approximately how many per second…. and how many scan lines
~30-60 fps
~120 scan lines
what are the 4 factors effecting frame rate
- depth
- sector angle
- # of lines
- # of focal zones
for every focus you add, how does it effect your frame rate
it will half it
how does the number of scan lines effect FR?
lower number will increase FR
how are frame rate and depth related
inversely
how are frame rate and sector width related
inversely (higher FR means narrow sector & vice versa))
how are frame rate and line density related
inversely
what is the range ambiguity formula
what should it be less than or equal to?
RA= Depth(cm) X LPF X # of foci X FR…
OR
FR= 77,000/s / LPF X depth X # of foci
77, 000 (1/2 of the speed of sound in soft tissue)
what is the time it takes to produce 1 frame
LPF X 13 micro seconds X depth (cm)
what is the reciprocal of frame rate
the time it takes to produce an image
what is the formula for frame rate
FR= 1/Tframe OR FR= 1/LPF X 13 microseconds X depth (cm) OR FR= c/2DN (D = depth, N = # of lines per frame)
what is scan line density
-the number of lines per degree or number of lines per centimetres
what is the importance of line density
the importance is the ~1 scan line for degree in needed for appropriate resolution
what is the consequence of a scan line density thats too low or too high
low: a greater need for interpolation
high: overwriting will occur (too many scan lines cross over one another)
are scan lines considered to be a part of spatial resolution
yes
does zooming increase the scan line density
yes
whats the advantage of Cine loop?
can improve the efficiency of seeing small structures missed during RT interrogations
how is a single frame/freeze frame displayed on the monitor and how is this display accomplished
- theres a continuous display of a single frame
- accomplished by the machine reading the same image over and over and continually writing it ijraster format over and over
at what rate does a CRT refresh a freeze frame image?
30 fps
what are side lobe and how do we compensate for them
- off axis beams that radiate away from the main beam… low energy
- occur in single disc mechanical probes due to radial mode vibration….
- compensate w/ insulator ring to suppress radial mode vibration
why are side lobes significant
they can place false reflectors/artifacts in the image as if they had come from the main beam because the machine assumes that sound travels in a straight line
what is responsible for grating lobes, how do we compensate for them
- width and length vibration which causes cross talk of the crystals
- compensate w/ apodization (fire weaker voltages at the periphery of the crystals) and sub dicing
what is subdicing and how is it accomplished
- another way to eliminate grating lobes
- accomplished by dividing each element into smaller pieces/sub elements that act as 1 crystal
to what dimension should we cut the sub-elements when sub dicing and why
- cut them to a dimension less than 1 wavelength
- this dimension will project the grating lobes at 90 degrees or greater from the main beam
what is a 3rd way we could reduce grading lobes (other than apodization and sub dicing)
scanning w/ harmonics helps reduce them
how do we accomplish mechanical isolation of the subdiced crystals (2 ways)
- space out crystals which somewhat allows for mechanical isolation…
- air is used as a spacing material which will minimize the penetrating of sound through to the adjacent crystal - Have the thickness of the crystal drastically different from the length and width dimensions so if there is any length and width vibrations they’ll ring at a different frequency from the central transducer frequency
what does the term channels refer to
the number of elements in the array and there respective delay mechanisms
how many channels do US systems usually have
48, 64 or 128
are more channels better? is yes, why
-more channels = more precise control over the beam characteristics
what is an artifact?
give some examples
anything that doesnt correctly display the structures or functions that are imaged… can refer to:
- echos that arent real (e.g. not representing actual interfaces)
- missing info
- improperly located structures
- improper brightness, shape, size
artifacts can be the result of, or related too, which 4 things
- Malfunctioning equipment
- improper operation of equipment
- defective recording device (film or VHS mostly)
- Acoustic properties of tissues and propagating of sound
what assumptions does the machine make about how sound propagates in tissue (4)
- all tissues have the same acoustic velocity
- sound travels in a straight line
- each strength indicates organ echogenicity only (example where this isn’t the case is acoustic enhancement)
- distance to each reflector equals the round tip time (13 microseconds/cm)
what are the 2 MAIN groups of artifacts
- propagation grp
2. attenuation grp
which aspect of axial resolution is considered to be an artifact
how do we correct for this artifact?
is it helpful or hurtful
- objects less than 1/2 SPL apart in the axis of the beam will be shown as a single echo.
- can increase the frequency to compensate, or reduce RD
- hurtful
which aspect of lateral resolution is considered to be an artifact.
what can accentuate this artifact?
how do we correct for this artifact
is it helpful or hurtful
- point spreading…. reflectors appear smeared across the screen in areas where a structure is smaller than the width of the beam
- increasing the gains or TGCs
- correct by lowering overall gains or TGC and scan in the near zone w/ properly placed focus
- hurtful
can point spreading happen at any point in the beam
yes…. occurs least at the focal point
whats another term for point spreading
image broadening
whats another term for slice thickness
volume averaging or partial volume
also section thickness
which aspect of slice thickness resolution is considered to be an artifact.
why does this occur?
how do we correct for this artifact
is it helpful or hurtful
- volume averaging… can account for filling in of anechoic structures w/ false debris
- occurs due to the assumption that all returning echos come back from the center of the beam, when there are echos that return from in front and behind the structure as well… these echos will be represented in the 2D image as explained
- correct for this by repositioning probe so that the narrowest part of the Z axis (and X/Y axis) is at the desired location
- hurtful
what is acoustic speckle
how do we correct for it
is it helpful or hurtful
- the result of constructive and destructive interference of off axis reflections of echo returning from parenchyma… this causes heterogenous brightness in the image
- correct for it w/ persistence (frame averaging) to help homogenize the tissue brightness
- hurtful
what is reverberation… where is it often seen
- the reflection of sound back and fourth b/w the transducer and a strong interface or b/w 2 strong interfaces
- often seen near the probe surface or in fluid filled structures
how does reverberation appear on the screen
how do we correct for it
- multiple equally spaces linear lines that get weaker w/ depth due to attenuation (looks like the rung of a ladder)
- correct by changing your window and using harmonics (for Main bang artifact ONLY)
where is the man bang artifact seen
-b/w the face of the probe and the skin surface
this is the first reverb artifact we talked about
is the reverberation artifact helpful or hurtful
-can be either, helpful to check for IUCD
why are the lines seen from the reverberation artifact equidistant from one another
-because the reflectors are at the same depth and the machine thinks that sound in traveling in a straight line from the probe to the structure and back….. so each time the line get deeper
do we often see reverb in vessels
yes, b/c the walls are strong reflectors
how do we accomplish electrical isolation of elements so that no cross talk occurs
-having separate electrical communication and grounding for each individual crystals
what thickness/length of thickness/width ratio will result in a significant reduction of cross talk
2:1
what type of artifact is a comet tail
is it helpful or hurtful
- a short range reverb artifact that occurs b/w the 2 closely spaced specular reflector (a metal object or calcium/chlesterol crystals)
- comes to a point
- metal object could be staples, metal pellets, biopsy needle, metal ICD
- can be helpful to diagnose
what type of artifact is ring down artifact
is it helpful or hurtful
- a reverb artifact that looks similar to comet tail but w/o the discrete echos… instead looks like a continuous stream of long echos (persistence throughout the image)
- resonance phenomena associated w/ gas bubbes where the gas vibrates and becomes a new source of sound
- can be helpful to diagnose because we can follow it to a source and see whats causing it.
why does mirror image artifact occur
can we correct for it?
- due to the sound hitting an odd angled strong reflector
- when the beam is reflected by a specular reflector towards an object and then returns to the probe along that same path it leads to a mirror image b/c the machine assumes the sound travelled in a straight line (angle of incidence = angle of divergence)
- not always
where is mirror image most commonly seen
-near the diaphragm and lungs
is mirror image helpful or hurtful
can be helpful to confirm no pleural effusion
why do multipath artifacts occur
how do we correct for it
- result of the assumption that sound travels in a straight line… since this isnt the case it can lead to the incorrect placement of echoes
- get more perpendicular to the structure so that sound comes straight back
why do refractive artifacts occur
-due to the redirection of the sound due to non perpendicular angle of incidence and different velocities of the two media
what are the 4 types of refractive artifacts
- edge shadowing
- refractive duplication
- refractive malposition
- refractive enhancement
what is edge shadowing
how do we correct for it
is it helpful or hurtful
- shadows seen on the edge of strong curves reflectors b/c the beam is changing direction w/ non perpendicular incidence and doest retune to the probe
- correct by changing windows to insinuate at a different angle
- helpful w/ isoechoic structures to identify borders
does calcium produce clean or dirty shadows
bowel?
- clean
- dirty
what is refractive duplication
why does it occur
- an artifact that creates the appearance of the lateral duplication of a structure on either side of its true location
- occurs b/c of refraction of the sound beam due to a strong interface and b/c the machine assumes the sound travels in a straight line
- hurtful
which structure often causes refractive duplication
how can we correct for it
the rectus abdominus muscles
-get perpendicular to the structure and change windows/angles
w/ refractive duplication, are either of the structure youre seeing real
no
what is refractive malposition
what causes it
how do we correct for it
Can we see this on US
- when the echos from a structures are false allocated lateral from there true position
- get perpendicular
- hurtful
No
how do side lobes appear in our images?
do grading lobes appear the same
how can we correct for it
-seen as linear echogenic bands or sheets in anechoic structures
+yes
- TGCs and gains
- hurtful
what is a propagation speed error
where does it commonly occur
how can we correct for it
- machine assumes the propagation speed is the same for all tissue which accounts for this artifact
- deep to fat
- cant correct it
- can be helpful
if the propagation speed of tissue is faster than 1540 m/s, where will it appear on the monitor
closer to the probe
if the propagation speed of tissue is slower than 1540 m/s, where will it appear on the monitor
further from the probe
why can a propagation speed error artifact sometimes be helpful?
when can it be hurtful
- it can indicate if a mass is likely fatty or if an organ may have fatty infiltration….
- for example, there can be a danger to over measure a mass (if fatty) because it will appear lower/longer than it actually is
whats another term for range ambiguity
high PRF mode
what does range ambiguity cause?
when is the artifact most commonly encountered?
when does RA occur
- can make echos appear closer to the probe than they actually are…. rare in 2D scanning but more commonly encountered in doppler
- when a second pulse is sent out before all the previous echos have come back (PRF is too hight and it exceeding its limit by the set depth)
how do you correct for range ambiguity
reduce FR or PRF
what can happen to our spectral doppler w/ RA
we can pick up waveforms from outside the vessel we’re sampling or you can pick up waveforms if you SV isnt even placed in a vessel
what are the 4 attenuation artifacts
- shadowing
- enhancement
- focal enhancement
- edge shadowing
in most cases, are we trying to enhance, or get rid of shadowing when scanning
-enhance to help w/ diagnosis (e.g. shadowing from a stone)….
but they can cover structures we’re trying to see so sometimes trying to get away from them (must use a different window)
how do we increase the shadowing from a strucutre
- use a higher frequency (including harmonics) b/c theres more absorption
- move focus to the source of the shadow
- reduce overall gains
- turn off SonoCT/compound imaging
whats another term for focal enhancement
focal banding
what is focal banding
how do we correct it
- a false brightness at the level of the focal zone… occurs b/c the intensity of the beam is greatest at this level
- TGCs
- hurtful
what are the 6 doppler artifacts we commonly see
- aliasing
- slice thickness
- reverb
- mirror image
- flash or clutter
- noise
why does aliasing occur
how can we correct for it
-we arent sampling fast enough
5 ways to correct:
- decrease frequency
- increase PRF (used when scanning)
- move baseline up (used when scanning)
- use CW
- increase angle of insonance
is aliasing helpful
yes, can determine where there are higher velocities or turbulent flow for diagnosis
what is doppler slice thickness artifact
how can we correct for it
- when multiple waveforms are seen w/in one another b/c of the beam width phenomenon (e.g. youre picking up flow from vessel infront or behind of where youre sampling)
- different window or change PW gains to reduce it
how does doppler reverb appear on US
-similar to 2D reverb and seen as a buzzing or equidistance colour lines
what is the twinkle artifact
why does it occur and how does it appear on US
- a type of doppler reverb that occurs ONLY w/ calcified stones…. helpful to diagnose stones
- reverb occurs b/w the front/anterior and back/posterior walls of the stone and will produce a mosaic of colour deep to the stone
does doppler image artifact effect both colour and spectral tracings
yes
how does spectral doppler mirroring appear and what causes it
-will have doppler flow on both sides of the baseline and is caused by over gaining w/ spectral gains or having an angle of incidence too close to 90
how does colour mirroring appear
-a false vessel will appear deep to the real one
what is doppler flash or clutter
how can you correct for it
- when low level echoes in the colour flow are seen in adjacent tissue due to tissue movement (e.g. heart, valve, wall motion)
- can correct by using a filter to eliminate it as the flow is usually slower in those areas… if filters are too high you risk eliminating real low flow info
why does colour doppler noise occur
why does spectral doppler noise occur
Colour
-when the doppler gains are set too high…
…..causes the colour to bloom outside of the vessel or appear in anechoic structures that have no flow
-can hide or create pathology
Spectral
-due to over gaining which can artificially increase peak velocity by creating a halo effect
what are the 4 reasons you may get artifacts related to the malfunctioning of equipment
- electronic interference (electronic interference by other equipment)
- faulty software
- faulty scan converted
- faulty equipment
what the term that refers to the spectral tracing generated by electronic interference
the 60 Hz bandwidth artifact
what type of faulty equipment can cause artifacts
- malfunctioning crystal elements
- cracked damping material
- defective recording device
- improper transducer shielding
where is acoustic speckle the most prominent and why
-the near field b/c this is where constructive and destructive interference is the strongest
in general terms, why do reverberation occur?
due to a difference in z values b/w two structures
whats another term for mirror image artifact
ghost artifact
is mirror image artifact a variant of reverberation?
yes
when does aliasing occur
when the doppler shift exceeds 1/2 the PRF
why does colour doppler mirror image occur
can occur when theres a strong specular reflector deep to the vessel being scanned…. w/ this, there will be a large impedance mismatch which causes the artifact
Which term describes the averaging of echo data due to an increased line density
Overwriting
At what rate will the average human detect flicker on a display
15-20 FPS
what type of artifact is edge shadowing
refractive artifact
and
attentuation artifact
What does overwriting do?
It averages echo data
In general terms, why does shadowing occur
occurs behind a highly attenuating structure due to absorption, reflection or scattering
If you want to double you frame rate, what can you change
1/2 the depth of interest Or 1/2 the number of scanlines Or Remove one focus
What’s responsible for pseudo-sludge in the GB?
Beam width artifact
