1
Q
What is this artifact? Why does it occur?
A
- Known as side lobe/side grating artifact (aka Clutter)
- Ultrasound waves outside of main beam may strike a strong reflector located outside of the main ultrasound beam and generate echoes that are detectable by the transducer
- Assumption that all signal is from main beam –> These echoes will be falsely displayed as having originated from within the main beam.
- This form of artifact is more likely to be recognized when the misplaced echoes overlap an expected anechoic structure
- Less prevalent with sector transducer
FIX: spatial compounding, THI
2
Q
What is this artifact? Why does it occur? How can it be minimized?
A
- Slice thickness artifact (aka volume averaging, beam width artifact)
- Ultrasound is bow-tie shape –> object located within the widened portion of beam may generate echoes, but the ultrasound display assumes that these echoes originated from within the narrow portion of the beam and displays them as such
- Artifact has curved surface and echoes may appear on nondependent surface
- How to minimize/differentiate:
- Image object of interest in the focal area
- Use positioning to differentiate from sludge
- Reduce overall gain
3
Q
What is the cause of reverberation, comet tail, and ring-down artifact?
A
- US assumes that an echo returns to the transducer after a single reflection and the depth of an object is related to the time for this round trip
- Two parallel highly reflective surfaces repeatedly reflect echoes back and forth before returning to the transducer for detection
- Echo returning to the transducer after a single reflection will be displayed in the proper location
- Sequential echoes will take longer to return to the transducer, and the ultrasound processor will erroneously place the delayed echoes at an increased distance from the transducer
4
Q
What is this artifact? What is responsible for its shape? When does it occur?
A
- Comet tail
- Form of reverberation
- Reflective echoes are closely space and individual signals are not perceivable
- Later echoes may have decreased amplitude secondary to attenuation -> result is a triangular, tapered shape
- Occurs with gas, metallic surgical clips, metallic pellets
5
Q
What is this artifact? When does it occur?
A
- Ring down artifact
- Form of reverberation
- Resonant vibrations within fluid trapped between a tetrahedron of air bubbles create a continuous sound wave
- Displayed as a line or series of parallel bands extending deep to a collection of gas
6
Q
What is this artifact?
A
- Reverberation artifact
- Produces a hyperechoic line at the tissue-gas interface with several equidistant hyperechoic lines
7
Q
What is this artifact? Why does it occur?
A
- Mirror image artifact
- Generated by the false assumption that an echo returns to the transducer after a single reflection. In this scenario, the primary beam encounters a highly reflective interface –> reflected echoes then encounter the “back side” of a structure –> reflected back toward the reflective interface –> reflected to the transducer
- Display shows a duplicated structure equidistant from but deep to the strongly reflective interface
- Commonly identified at the level of the diaphragm, with the pleural-air interface acting as the strong reflector
8
Q
What is this artifact? Why does it occur?
A
- Registration/propagation speed error
- When sound travels at a velocity significant slower than 1540 m/sec (e.g., fat), the returning echo will take longer to arrive at the transducer. The image processor assumes that the length of time for a single round trip of an echo is related only to the distance traveled by the echo. The echoes are thus displayed deeper on the image than they really are and may alter the shape of the object
9
Q
What is this artifact? Why does it occur? How can you minimize the effect?
A
- Refraction artifact
- Non-perpendicular incident ultrasound energy encounters an interface between two materials with different speeds of sound –> the incident ultrasound beam changes direction –> ultrasoud machine assumes beam travels in a straight line and thus misplaces the returning echoes to the side of their true location
- Can investigate from different angles to see if it persists
10
Q
What is this artifact? What causes it? How is it fixed?
A
- Edge shadowing
- Lower acoustic velocity through a fluid-filled structure with a curved surface, which refracts the ultrasonic beam at the fluid-tissue interface
- FIX: spatial compounding, change angle of insonation
11
Q
Name this artifact.
What is the cause of the artifact? How would you (try to) fix it?
A
Range ambiguity artifact: Occurs when the echo from a distant structure reaches the transducer after a second pulse has been emitted. Transducer thinks that this echo is associated with the second pulse and therefore in the near field instead of the far field.
FIX: reduce number of focal zone, especially when imaging fluid-filled structures
12
Q
Name this artifact.
What is the cause of the artifact? How would you (try to) fix it?
A
Aliasing artifact: occurs when the doppler shift frequency exceeds the Nyquist limit (1/2 PRF) –> information is mapped to the wrong side of baseline.
FIX: increase PRF, move baseline, decrease frequency, increase Doppler angle
13
Q
What is beam hardening artifact? What are two examples?
A
- Mean energy of the x-ray beam increases (“hardens”) as it passes through an object because lower energy photos are absorbed more rapidly than higher energy photons
- Examples:
- Cupping artifact
- Streaks and dark bands
14
Q
What is this artifact (left)? How is it corrected (right)?
A
- Cupping artifact
- Caused by differences in beam hardening between periphery and center
- Corrected by
- Filtration -
- Flat piece of metal can pre-harden the beam by filtering out lower energy photons
- Bowtie filter can harden the beam that passes through thinner areas of the patient
- Calibration correction (image on the right)
- Calibrate to phantom for expected beam-hardening effects in different regions of the body
- Filtration -
15
Q
What is this artifact (left)? How is it corrected (right)?
A
- Dark bands (aka Streaks)
- Occurs due to differences in beam-hardening at different tube positions
- i.e., in position 1 the beam may pass through one dense object (less hardening) whereas at position 2 it may pass through multiple dense objects (more hardening)
- Correction:
- Beam-hardening correction software
- Tilt gantry
16
Q
What is this artifact? Why does it occur? How is it fixed?
A
- Shading artifact
- Occurs when off-center, dense object protrudes partway into the beam and is only seen by the detectors in certain positions
- Avoid:
- Use thin slices for any part of body with rapidly changing anatomy in the z-axis
17
Q
What is this artifact? Why does it occur? How is it corrected?
A
- Photon starvation
- Occurs when photons hihgly attenuated by a dense region (pelvis, shoulder) and fewer reach the detector –> results in horizontal streaks
- Attenuation typically greatest when beam is traveling horizontally
- Minimize by:
- Automatic tube current modulation
- More photons pass through widest part of patient without unnecessary dose to narrower parts
- Adaptive filtration
- Corrects and smooths regions of high attenuation
- Automatic tube current modulation
18
Q
What is this artifact? Why does it occur? How is it fixed?
A
- View aliasing (type of undersampling)
- Insufficient # of views per beam rotation results in computer misregistration of info related to sharp edges and small objects
- Correct:
- Increase # of views by increasing tube rotation time (e.g., from 1s to 2s)
19
Q
What is this artifact? Why does it occur? How is it fixed?
A
- Ray aliasing
- Caused by undersampling
- Correction:
- High frequency objects should be avoided when possible
- Increase beam width
- Increase # of samples in a projection by:
- Quarter detector offset –> 2 samples per beam width
- Dynamic focal spot
- focal spot is relfected between 2 positions achieving 2 samples per beam width
20
Q
What causes metal artifact? How can you resolve it?
A
- Density of metal is beyond attenuation profile of the computer leading to an incomplete attenuation profile
- Correction:
- Avoid metallic structures by angling gantry
- Increase kV (variable in helpfulness)
- Manufacturer software
21
Q
How can you avoid motion artifact or minimize its appearance?
A
- Avoid
- Anesthesia/sedation
- Short scan time in regions prone to movement
- Breath hold for thorax
- Minimize
- Overscan and underscan modes
- Maximal motion occurs at beginning and end of 360 degree scan
- Software correction
- Cardiac gating
- Overscan and underscan modes
22
Q
What is this artifact? Why does it occur? How is it avoided?
A
- Incomplete projection
- Occurs when a portion of patient or a dense structure lies in the beam path, but outside FOV
- Avoided by positioning patient so no body part is outside scan FOV
23
Q
What is this artifact? Why does it occur? How is it fixed?
A
- Truncation artifact
- A line of abnormal signal intensity that occurs parallel to an interface between tissues of markedly different signal intensity (e.g., soft tissue-CSF)
- Cannot be eliminated bc due to Fourier transformation, but can minimized by:
- Increasing resolution (aka smaller pixel size)
- Apply pre-reconstruction filters
- Post-processing optimization
24
Q
- What is this artifact?
- Why does it occur?
- What direction does it occur in?
- How does it change with magnetic field strength?
A
- Chemical shift artifact
- Occurs only in the frequency-encoding (readout) direction
- Occurs due to differences between the chemical environments in water and fat –> fat and water precess at different frequencies –> esults in misregistration due to a chemical shift effect on the encoding of signal from the protons in fat and water
- Result is the appearance of a dark rim at one edge of an object and a bright rim at the opposite edge
- Worse at higher field strengths
25
What are three types of motion artifact in MRI?
1. Gross patient movement
2. Movement due to normal physiology (e.g., cardiac, respiratory, GI)
3. Microscopic motion (e.g., flow, diffusion)
26
* What is this artifact?
* Why does it occur? What phase direction does it occur in?
* How can it be minimized?
* Ghosting due to patient motion (gross motion or physiologic motion)
* Occurs in the phase encoding direction
* Minimization:
* Saturation band over vessels, heart, GI tract to null signal
* Use a faster acquisition time
* Use cardiac/respiratory gating or breath hold
* Average the signal
* Correction algorithms
* Can also swap PEG and FEG to move the ghost images away from the area of interest
27
What are 3 types of vascular flow artifacts?
1. Time of flight (TOF) effects
2. Entry slice phenomenon
3. Intravoxel dephasing effects
28
* What is this artifact?
* Why does it occur?
* How is it corrected?
* Vascular flow artifact
* Occurs due to time of flight (TOF) effects
* Some or all of the flow is replaced between RF pulses and the flow is through the plane of the image
* Spin echo sequences - tissue must receive the excitation (90) and refocusing (180) RF pulses to generate signal --\> in fast moving blood, variable parts of the blood experience both pulses --\> results in no or partial signal returned --\> signal loss
* Gradiet echo (GRE) --\> only one RF pulse, so never any flow-related signal loss
* Corrected by adding saturation bands in **slice direction**
29
* What is this artifact?
* Why does it occur?
* How is it corrected?
* Vascular flow artifact
* Occurs due to entry slice phenomenon
* Blood flowing perpendicular within a slice becomes progressively more saturated
* Blood within entry slice may show greater signal than blood further away from entry slice
* Degree of slice entry phenomenon depends on:
* TR
* Slice thickness
* Direction and velocity of flow
* Corrected by additing saturation bands in **slice direction**
30
What is this artifact? Why does it occur? How can it be reduced?
* Vascular flow artifact
* Due to intravoxel dephasing
* Nuclei of different velocities present in the same voxel --\> the nuclei will have traveled different distances to reach that voxel between the 90 degree RF pulse and signal readout --\> has variable amounts of phase --\> signal drop
* Reduced by gradient moment rephasing aka flow compensation
* Apply additional gradients to correct altered phases
31
What is this artifact (A) and how is it fixed (B, C)?
* Quantum mottle
* Underexposure results in grainy, noisy, mottled, and pixilated image
* Increase exposure (mAs)
32
What is this artifact?
* Uberschwinger or a rebound effect
* Radiolucent halo around metal or areas where there is a large density difference between adjacent objects
* Image processing error --\> induced by the unsharp masking that determines the degree of edge enhancement in the final image
* Improve by multifrequency processing
33
What is this artifact? Why does it occur?
* Saturation
* Exposure artifact - Occurs due to overexposure resulting in saturation of the detector
* Repeat radiograph at lower exposure
34
What is this artifact? Why does it occur? Does it occur in DR, CR, or both?
* Planking
* Due to over-exposure --\> sharply demarcated rectangles in varying shades of gray
* Occurs only in DR
* Repeat radiograph with decreased exposure
35
The following is an example of fading. Why does this occur? Does it occur in DR, CR, or both?
* Fading is a post-exposure artifact (Top image)
* Results when a CR cassette is left for a period of time before being read --\> gradual decrease in excitation of the molecules in the PSP layer --\> appears grainy due to loss of information
36
What is this artifact (left) and why does it occur?
* Grid cutoff
* Due to incorrect position of the grid relative to the x-ray source resulting in attenuation of the x-rays
37
What is this artifact? Does it occur in CR, DR, or both?
Light leak; only occurs in CR
38
What is this artifact? Why does it occur? How can you reduce it?
* Moire or courdory artifact
* Occurs when sampling frequency intersects grid lines
* More common with grids of low line density that are divergent from the sampling direction or plate reader scan lines
* Reduce by using an oscillating Potter-Bucky grid
39
What is this artifact? Why does it occur?
* Border detection error
* Incorrectly cropped image with border through ROI
* Occurs when:
* imaging plate and FOV are nonparallel by more than 3 degrees
* division of plate for multiple exposures
* off-centered object of interest
* highly attenuating linear objects such as bone or metallic implants
JS Boards
flashcards
Decks in class (9)
# Cards
