Artifacts Flashcards
What is a reverberation artifact caused by?
Occur when high intensity returning sound beam hits transducer and it sent back into the body another (or multiple times).
Will usually result from a large acoustic impedance mismatch (gas, metal, bone)
Transducer will received second/third trip and assume that the tissue was deeper because it took longer to return - and places equal distances, twice as deep.
What is the difference between ring-down and comet-tail artifact?
Comet tail - high acoustic impendance (metal/soft tissue) which results in reverberation. In addition, have reverberation from withint he object - causing a solid, echogenic streak behind the object.
Ring-down - result from continuous sound waves returning from a bugle shaped fluid trapped between 2 layers of gas bubbles - GI tract
What is a reverberation artifact? Give example.
High intensity returning sound beam hits transducer and is reflected and travels back to surface of high intensity surface. Multiple orund trips occur, and transducer interprets each trip as taking 2x, 3x, 4x as long and has equally spaced lines.
Example: Gas, metal/soft tissue
What is acoustic shadowing?
Reduction in amplitude of reflected sound, due to strongly reflecting or attenuating structure - metal/gas
What is edge shadowing/acoustic reflection
Give example:
Shadowing in which shadow is created by edge of a highly reflective surface.
Part of the sound beam is refracted, and the othe rpart is reflected.
Bladder/gallbladder/kidney
What is snell’s law - what artifact does it pertain to?
Refraction artifact - can be associated with edge shadowing as well.
Change in sound direction at a boundary due to tissues having different speed of sound.
What is snell’s law equation?
Transmitted angle / Incident angle = C (second tissue) / C (first tissue)
When does snell’s law occur? when can it not occur?
Can occur: oblique angles
Cannot occur: When sound is perpendicular to the object
Using snells law…
When will the refraction angle be smaller than the incident angle?
Speed of sound through the second tissue is slower than the first tissue.
Beam will be directed ventrally.
Using Snells law
When will refraction angle be larger than the incident angle?
Large refraction: speed of sound is faster through the second tissue.
Beam will be directed more laterally.
What is a side lobe artifact?
Give example?
The secondary, off-center ‘weaker’ beams interact with a highly reflective surface (such as colon).
Sound returns to transducer quickly and is propagated in direction of the primary beam
Example: colon and bladder.
What is a slice thickness artifact?
Partial volume averaging - 3d image into 2d image.
When beam includes a cystic and a solid structure - scan consists of echoes from both.
When is a slice thickness artifact going to occur?
Use gallbladder and liver as example.
When imaging outside of the focal zone (diverging) .
Using focal zone - will only evaluate the gallbladder. If after the focal zone - will include echoes from the liver.
What is the mirror image artifact?
Use liver/gallbladder as example.
Sound bounces off of highly reflective surface, such as the diaphragm. Rather than going straight to transducer, may reflect off of gallbladder and then return.
Transducer assumes everything travels in straight line and interprets gallbladder (off-axis) as being on the main axis, but just taking longer to return.
What is range ambiguity artifact?
When using higher PRF - the PRP decreases. Deep sound waves from first PRF return after the next PRF has already occurred.
Transducer interprets these sound waves from the first section (which are very deep) as being very shallow.
What is propagation speed error?
Give examples of faster and slower tissues
US assumes that everything is 1540m/s.
If >1540m/s - the transducer will place the structure too shallow.
If
Why does the magnetic suspcetibility artifact occur?
Occurs due to metallic objects, or due to tissue interfaces (bone-tissue, tissue-air) because of differences in magnetic susceptilibty
Which sequences are magnetic susceptibility artifacts seen most frequently, and why?
Gradient sequences (GRE- T2*) - lack the re-focusing pulse that cancels out any inhomogeneities in the magnetic field
T2 spin echoes will have more of this artifact than T1 because of longer phase encoding gradient time.
How do you reduce the magnetic susceptibility artifact?
Using a spin-echo sequence, decrease voxel size (increases spatial resolution, decreases SNR), increasing the receiver band width, changing phase encoding direction
Which MRI artifacts occur in phase encoding direction? Frequency encoding direction?
Phase encoding direction: Gibbs truncation, flow artifacts (ghost/CSF flow)
Frequency: chemical shift, zipper
Describe ghosting/ flow artifact in MRI.
Give an example.
Pulsation that causes structures to be in different places during the phase encoding process. This will result in vessels being represented in different regions/numbers.
Distance from originating structure, number of replicas and brightness depends on magnitude and frequency of pulsations.
Examples: cardiovascular pulsations (internal carotid over brain)
What is the CSF signal void sign in MRI, give example
In turbulen regions - CSF in region will be excited by 90 RF pulse, but will not be present for re-focusing 180 RF pulse. No signal from either CSF leaving this region, or the CSF coming into this region.
example: High/turbulent CSF flow in COMS, hydrocephalic patients.
What is a chemical shift artifact - give an example.
Fat and water have different Larmor frequencies. The difference between these frequencies is linearly related to MRI strength. Because of this, will result in mis-registration of fat at a fat-soft tissue interface in the frequency encoding direction. (will have separation between fluid [csf] and fat [epidural] on MR)
Example: Signal void surrounding CSF
Which MR artifacts are in the phase encoding direction?
Gibbs truncation Flow artifacts (ghost/CSF flow)
Which MR artifacts are in the frequency encoding direction?
Zipper
Chemical shift
What is the volume averaging artifact on MR
Signal intensity of voxel is average of all different signal intensities from different tissues within the slice thickness. Intensity mapped into voxel is a misrepresentation of different structures (bone and air together)
What is aliasing in MRI?
How can it be prevented?
Portion of anatomy is outside selected image field of view. Anatomy will appear wrapped around to opposite side of image as a mirror image – can be superimposed on area of interest.
How to remove it: Include in FOV, or pre-sat
What is the Gibbs truncation artifact, give an example.
# of phase encoding steps is decreased in relation to frequency encoding steps to save time. With excessive reduction – may be mis-mapping of image in phase direction. Other article: errors in Fourier transformation in structures with abrupt transitions (high to low signal – spinal cord and CSF)
Example: T2 weighted images of spinal cord - linear hyperintensity superimposed in central region.
When does the zipper artifact occur? Which direction does it generally occur in?
Defects in the RF shielding with leakage of electromagnetic energy into the magnet room.
Occurs in frequency encoding direction
What is cross-talk in MR. Give an example.
Artifact occurs when multiple stacks of images and fields of view intersect – results in loss of signal
Example: when cutting through lumbosacral spine on transverse sequences, and want to set up two ‘blocks’ – and portions of the cuts intersect
What is the magic angle artifact? Which sequences is it most common, and how can these be prevented?
Affects structures of low signal intensity that are oriented at 55 degree angle of main magnetic field.
True signal is misrepresented as hyperintense on short TE sequences (T1) .
Avoided by including long TE sequences (T2)
What is the definition of quantum mottle?
Radiographic (quantum) mottle: [AKA Noise] radiographic recording of the statistical fluctuation in a beam of photons absorbed by the intensifying screen
What is the quantum mottle artifact?
Low signal to noise ratio (low proton # reaching the screen - underexposure)
Image will be grainy
What is the saturation artifact?
As we increase technique - pixels will become darker. At some point, the pixels will be saturated and cant get any darker
Pixels will reach their maximum value and structures (especially thin structures) are no longer visualized, even with re-windowing
AKA clipping?
What is planking artifact?
When a radiograph is over exposed and the amplifiers are visualized (as rectangular squares)
What is a fading artifact?
Seen in CR cassettes
If a cassette is not exposed somewhat immediately - the excited electrons in the scintillator - holding the latent image will fade (be lighter and less quality)
What is a light leak artifact in CR
Light erases the film. (causes de-excitation of the electrons)
When film is accidentally exposed to light - lightening of the film due to the electrons being released.
**Different then screen film - where optical density will be increased
What is a dirty light guide artifact?
Light guide blocks the light emitted from the film from reaching the photomultiplier tube.
Usually runs in direction that the film is being read in.
What is a faulty transfer artifact?
Multiple pixels, lines are missing or are replaced by noise. Usually due to bad memory, digitazation errors, or communication
What is misplacement artifact?
When whole sections of a radiographic projection are out of order
Check cables, power fluctuations
What are the 3 modes of border detection?
Automatic - image borders may be incorrectly chosen within the FOV (choosing spine as the edge of the film)
Semi-automatic - central regions of the film are included
Fixed - applied when post-exposure processing is not necessary.
What are dead pixels, what does the remaining detectors do to compensate?
Dead pixels are not able to capture electrons or produce light appropriately
The surrounding pixels will blur over this pixel
What is another term for the Moire artifact?
Corduroy artifact
What is the Moire artifact?
Essentially - visualization of the grid lines
Regions of linear/wavy lines of high attenuation
Prevent by using an oscillating Bucky, use correct orientation of the grid, use high grid ratios
What are other names for the Uberschwinger artifact?
Rebound or halo artifact
What is the Uberwschwinger artifact caused by?
When is it most seen?
Unsharp masking technique are utilized to accentuate sharp margins (usually subtracting low LUT from high LUT)
Dark borders will surround highly intense objects such as metallic implants
What is the density threshold artifact?
Image with a highly attenuating structure (metal) will automatically make machine select for a long scale of contrast.
This will result in the rest of the items on the film (organs/fat) to be on the low end of the histogram - and difficult to evaluate due to lack of latitude.
What is ghosting artifact?
Occurs because of afterglow from CR screens
Immediately after a film is exposed/read and reloaded with fresh film - the photodiodes may retain some of the charge and cause a light exposure of the film.
On the next image performed, a lightly appearing image of the previous film is on the new film.
What is the difference between table top contaminants and collimator contaminants?
Collimator - will have penumbra
table top - no penumbra, sharp/distinct artifacts.
How do you correct a border detection artifact?
properly centering the image, exclude highly attenuating linear objects (bone/metallic implants)
