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)