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CT & MRI > Week 2 > Flashcards

Week 2 Flashcards

1
Q

CT image artifacts

A

patterns / structures that appear on the image but not found on the original object

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2
Q

CT image artifacts appear due to

A

1) patient based
2) physics principles based
3) hardware based

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3
Q

importance of identifying CT image artifacts

A

1) obscures important anatomical details
2) alters CT hounsfield units & tissue density
3) misdiagnosis

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4
Q

patient based artifacts

A

metallic implants, motion, MPR artifacts

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5
Q

hardware based artifacts

A

ring artifact, cone beam artifact

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6
Q

principles of physics based artifacts

A

beam hardening, partial volume, photon starvation

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7
Q

what is beam hardening effect

A

low energy photons are filtered out but high energy photons pass through, mean energy increases

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8
Q

2 types of artifacts from beam hardening effect

A

streak & cupping

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9
Q

how to fix cupping artifact

A
  • higher kVp
  • reconstruction algorithm
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10
Q

how to fix streaking artifact

A
  • metal artifact algorithm
  • higher kVp
  • remove any artifacts
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11
Q

how do streaking artifacts appear

A

dark streaks between 1 or 2 dense object in an image (lots of lines converging)

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12
Q

how do cupping artifacts occur

A

x-ray hardens more than detector expects causing resultant attenuation profile to differ from ideal profile

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13
Q

how do metal streak artifacts occur

A
  • low mAs (photon starvation)
  • beam hardening
  • scatter
  • poisson noise
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14
Q

poisson noise AKA

A

white noise

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15
Q

Metal Streaking Artifacts: If scanned material includes features that are higher density than surrounding matrix, what occurs

A

‘starburst’ artifact formed in which bright streaks spread out from object for short distance into nearby material

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16
Q

why does Poisson Noise occur

A

statistical error of low photon count resulting in thin bright & dark streaks that occur along direction of greatest attenuation

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17
Q

why do contrast density artifacts occur

A

non-uniform distribution of contrast agents leading to streaks & variation in tissue enhancement

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18
Q

how to fix contrast density artifacts

A

optimize contrast injection protocols to ensure even distribution with saline flushes to clear contrast from vessels

19
Q

what causes beam hardening artifact

A

hardened x-ray beam due to preferential absorption of low energy x-rays by dense tissues

20
Q

how to fix beam hardening effect

A
  • adjust exposure factors
  • avoid high density objects in scan area
  • use reconstruction algorithms
21
Q

what causes metal implant artifact

A

metal implants absorb x-rays = causes streaks & shading

22
Q

how to fix metal implant effect

A
  • adjust scan parameters
  • metal artifact reduction algorithms
  • change imaging modality
23
Q

what causes partial volume artifact

A

very dense structures partially included in the slice causing high contrast errors (off-center)

24
Q

where do partial volume artifacts occur most frequently in

A

bony structures such as skull base & petrous bone region

25
Q

how do noise / photon starvation artifacts appear as

A

streak artifacts in the direction of the widest part of object scanned due to less signal reaching detectors

26
Q

what causes noise / photon starvation artifacts

A
  • poor scan techniques
  • reconstruction algorithm
27
Q

how to correct noise / photon starvation artifacts

A
  • iterative reconstruction
  • patient positioning (hands up)
  • increase dose
  • use thicker slices
28
Q

what causes patient motion artifacts

A

patient motion during scan disrupts spatial alignment of acquired data

29
Q

how to fix patient motion artifacts

A
  • ask patient to remain still
  • sedation / gating techniques
  • faster scan
30
Q

how to fix ring artifacts

A
  • recalibrate detector
  • replace detector module
31
Q

what causes ring artifacts

A
  • common in rotate-rotate type
  • mis-calibrated / defective DELs
32
Q

how do ring artifacts appear as

A

rings centered on center of rotation

33
Q

what causes cone beam artifacts

A

object seen by diff detectors at diff angles

34
Q

what are cone beam artifacts similar to

A

partial effect artifacts for off-centered objects but more pronounced at image edges

35
Q

what are cone beam artifacts prominent with

A
  • large cone angle
  • thinner slices
  • 16+ slice scanner
36
Q

how do cone beam artifacts appear as

A

streaks & altered density in peripheral part of image

37
Q

how to fix cone beam artifacts

A
  • manufacturer reconstruction algorithms (cone beam reconstruction algorithms)
  • reposition ROI in isocenter
38
Q

Low Contrast Detectability (LCD) performance of CT
imaging systems is influenced by CT system specification

A
  • mAs
  • kVp
  • slice thickness
  • pitch
  • beam collimation
  • processing & visualization
39
Q

what is CT QC

A

series of distinct technical procedures that identifies defects or imperfections in a product that can be corrected to eliminate these defects

40
Q

why is CT QC necessary

A

max image quality, low dose

41
Q

what does CT QC include

A

monitoring, evaluation, maintenance

42
Q

what is used to ensure accurate measurement of HU for diff tissues & materials

A

air calibration

43
Q

what is HU value for air

A

-1000 HU

44
Q
A

CT & MRI (12 decks)

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