Artifacts Flashcards
What is an artifact?
Anything that does not correctly display the structures or functions that are imaged
What 4 things are artifacts related to? (With examples)
- Malfunctioning equipment
Ex. Broken crystal causing a shadow - Defective recording device
Ex. Discrepancy between the machine and PAX system - Improper operation
Ex. Bad techs - Acoustic properties of tissues and sound propagation
Ex. Many diff artifacts can occur
What 4 assumptions about the way sound propagates in tissue results in artifacts?
- All tissues have the same acoustic velocity of 1540 m/s
- The sound beam always travels in a straight line
- Echo strength only indicates the organs echogenicity
- The distance to the reflector equals the round trip time
What determines the velocity in tissue?
Compressibility and density
What are the 4 categories of artifacts?
- Not real
- Missing
- Improper location
- Improper brightness/size/shape
What two groups can artifacts be categorized into?
- Attenuation group
2. Propagation group
What is the axial resolution artifact?
Structures less than 1/2 the SPL apart in the axis of the beam are demonstrated as a single echo
What is the formula for SPL?
SPL = λ x RD
How can we correct the axial resolution artifact? (2)
SPL = λ x RD
- Manufacture the RD smaller in the first place
- Decrease the SPL by increasing the frequency which decreases the wavelength (λ)
What is the lateral resolution artifact?
A beam width artifact where reflectors appear smeared across the screen in areas where the beam is wide
What accentuates the lateral resolution artifact?
Increased gain or TGC
What is lateral resolution artifact also referred to as? (2)
- Point spreading
2. Image broadening
How do you correct for the lateral resolution artifact? (5)
- Lower gain
- Lower TGC
- Focus at correct level
- Increase frequency to extend the NZL
- Scan in the NZ (better beam width)
What is slice thickness artifact?
Cystic structures below the focus level can appear filled in due to echoes from in front and behind the structure (in the z axis) being spread where the beam is wide
What is the slice thickness artifact also referred to as?
The volume averaging artifact
What artifact is similar to the lateral resolution artifact?
The slice thickness artifact is also a beam width artifact but in the z-axis
Why does the slice thickness artifact occur?
Because of the assumption that all echoes return from the centre of the beam
What can fix the slice thickness artifact?
Repositioning the probe so that the narrowest portion of the z-axis is at the desired location
What is acoustic speckle?
The appearance of bright and dark dots (heterogenous brightness)
Why does acoustic speckle occur?
Due to the scatter reflection in the parenchyma due to constructive and deconstructive interference occurring from several off-axis reflections
What corrects acoustic speckle?
Persistence averages out the frames to help homogenize the tissue brightness
How does the size of the slice thickness effect the artifact fill in?
Greater slice thickness = greater fill in
Where is acoustic speckle predominant?
In the near field where constructive and deconstructive interference is the greatest
What is reverberation?
The reflection of sound bouncing back and forth between the transducer and a strong interface