CT Principle 3

Question 1

CT based artifact types

Answer 1

1. patient
2. physics principles
3. hardware

Question 2

Examples of patient based artifacts

Answer 2

1. metallic implants
2. motion
3. MPR

Question 3

Examples of hardware based artifacts

Answer 3

1. ring artifact
2. cone beam artifact

Question 4

Examples of physic principle based artifacts

Answer 4

1. beam hardening
2. partial volume
3. photon starvation

Question 5

what artifacts come from beam hardening

Answer 5

streak & cupping artifacts

Question 6

beam hardening effect

Answer 6

when lower energy photons are absorbed faster than higher energy photons, resulting in a high mean energy and harder beam

Question 7

how does cupping artifact occur

Answer 7

beam hardens more than expected causing the attenuation profile to differ from ideal profile

Question 8

how to fix cupping artifact

Answer 8

1. higher kVp
2. reconstruction algorithm

Question 9

how does streaking artifacts occur

Answer 9

dark streaks appear between 1/2 very dense objects

Question 10

how to fix streaking artifact

Answer 10

1. metal artifact algorithm
2. higher kVp
3. remove artifact

Question 11

what causes metal streak artifacts

Answer 11

1. photon starvation
2. beam hardening
3. scatter
4. poison / white noise

Question 12

how do metal streak artifacts appear

Answer 12

starburst artifact where bright streaks originate from 1 point into nearby structures

Question 13

how does Poisson noise occur

Answer 13

statistical error of low photon count; random thin streaks along direction of greatest attenuation

Question 14

how to fix metal artifact from dense dental filling

Answer 14

metal algorithm reconstruction

Question 15

how do contrast density artifacts occur

Answer 15

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

Question 16

how to fix contrast density artifact

Answer 16

optimize contrast injection protocol for even distribution with saline flushes to clear contrast from vessels

Question 17

how do beam hardening artifacts occur

Answer 17

x-ray hardens from lower energy x-rays absorbed by dense tissues

Question 18

how to fix beam hardening artifact

Answer 18

1. advanced reconstruction algorithms
2. adjust energy settings
3. avoid high density objects in scanning area

Question 19

how do metal implant artifacts occur

Answer 19

metal implant/object absorbed x-rays leading to streaks and shades

Question 20

how to fix metal implant artifact

Answer 20

1. adjust scan parameters
2. metal artifact reduction algorithms
3. alternative modalities

Question 21

partial volume artifact AKA ___

Answer 21

streak-like artifacts

Question 22

where does partial volume effect occur most frequently at

Answer 22

bony structures at skull base & petrous bone region

Question 23

how does partial volume artifact occur

Answer 23

when very dense structures are partially included in the slice leading to high contrast errors

Question 24

how does photon starvation occur

Answer 24

more than expected x-ray beams are absorbed in 1 projection leading to less signal for detectors

Question 25

how do photon starvation artifacts appear

Answer 25

streak artifacts in widest part of object scanned

Question 26

what causes photon starvation artifacts

Answer 26

1. poor scan technique 2. reconstruction algorithm

Question 27

how to fix photon starvation artifacts

Answer 27

1. IR 2. patient positioning 3. hands up 4. increased dose 5. thicker slices

Question 28

how does patient motion artifact occur

Answer 28

voluntary / involuntary movement which disrupts spatial alignment of acquired data

Question 29

how to fix patient motion artifact

Answer 29

1. remain still 2. sedation / gating technique 3. faster scan technique

Question 30

what causes ring artifacts

Answer 30

1. rotate-rotate type 2. mis-calibrated DELs

Question 31

how to fix ring artifacts

Answer 31

recalibrate / replace DELs

Question 32

what causes cone beam artifact

Answer 32

1. larger cone angles 2. thinner slices 3. object seen at diff detectors at diff angles

Question 33

when are cone beam artifacts the most prominent

Answer 33

1. larger cone beam angle 2. thinner slices 3. 16+ slice scanner

Question 34

how to fix cone beam artifact

Answer 34

1. manufacturer reconstruction algorithm 2. position ROI in isocenter

Question 35

how are cone beam artifacts similar to partial volume effect

Answer 35

objects are off-center and more pronounced at image edges

Question 36

LCD

Answer 36

low contrast detectability

Question 37

what influences LCD performance

Answer 37

mAs, kVp, slice thickness, pitch, beam collimation, processing & visualization

Question 38

what does QC involve

Answer 38

monitoring, evaluation, maintenance

Question 39

what is HU

Answer 39

Hounsfield Units; standardized units to represent tissue radiodensity

Question 40

what is the HU value for air calibration

Answer 40

-1000HU

Question 41

what does weekly QC CT phantom testing involve

Answer 41

1. CT number accuracy 2. low contrast resolution 3. distance accuracy 4. high contrast resolution