factors affecting CT image quality

Question 1

factors affecting CT image quality

Answer 1

• spatial resolution
• contrast resolution
• radiation dose
• noise
• image artifacts

Question 2

how to increase spatial resolution in CT

Answer 2

• reduce sampling aperture
• increase sampling freq
• decrease slice thickness
• increase matrix size
• reduce RFOV

Question 3

how to increase contrast resolution in CT

Answer 3

• increase photon flux
• increase slice thickness/ z-axis collimation
• reduce patient size
• increase sensitivity of detector
• reduce matrix size

Question 4

noise explained in CT

Answer 4

• uniform phantom of water = uniform HU value
• if this does not occur = fluctuation = noise

Question 5

what is noise level expressed as

Answer 5

%

Question 6

how to reduce noise levels in CT

Answer 6

• increase mA
• increase slice thickness
• reduce helical pitch
• reduce object size
• increase pixel size
• increase number of photons reaching detectors

Question 7

effect of low mAs

Answer 7

grainy

Question 8

increase in kV

Answer 8

intensity of beam increases and beams ability to penetrate thick anatomical parts
- increases photon flux and could reduce noise

Question 9

what is pixel size determined by

Answer 9

RFOV and matrix size

Question 10

the effect of reconstructed algorithm

Answer 10

• image quality - sharpness
• noise reduction
• artifact reduction
• contrast enhancement
• spatial resolution
• artificial smoothing
• processing time

Question 11

CT artefacts

Answer 11

• motion
• out of field view
• beam hardening
• metallic foreign bodies
• partial volume
• photon starvation
• ring
• multislice

Question 12

what kind of energy photons are absorbed with beam hardening

Answer 12

low energy photons
they have a lower penetrating power than high-energy photons making them more likely to be absorbed by body tissues

Question 13

why is beam hardening undesirable

Answer 13

• CT can distort images and hinder accurate diagnosis
• can cause shadowy artefacts

Question 14

ways to solve beam hardening

Answer 14

• use appropriate energy filters that allow you to selectively remove low-energy filters
• calibration helps correct variations in x-ray beam + detector response
• preprocessing to reduce streak artifacts and post processing
• iterative reconstruction

Question 15

what is volume averaging

Answer 15

where multiple structures with varying densities within a single voxel are averaged together leading to a loss of detail and contrast in CT image

Question 16

how can volume averaging be resolved

Answer 16

• using thinner CT slices during acquisition process - represent smaller volumes + containing mix of different tissues in single voxel reduced
• metal decomposition - reduce effects of volume averaging by providing more accurate information about composition of tissues within each voxel
• iterative reconstruction
• post processing

Question 17

what is photon starvation in CT

Answer 17

where there are not enough photons reaching CT detector resultign in poor image quality
- thick/ dense patient anatomy
- low x ray exposure

Question 18

how can photon starvation be resolved

Answer 18

• have a CT system that varies the mA according to patient attenuation
• patient size adapted mA
• Z-axis mA modulation - some areas along patients length receive lower mA (low attenuation) than others
• angular modulation

Question 19

what are ring artefacts

Answer 19

caused by defective detector and happens mostly in 3rd gen scanners
- unbalanced isocenter

Question 20

how to resolve ring artefacts

Answer 20

• regular air calibration
• engineer

Question 21

what is multi slice cone beam artefact

Answer 21

cone-beam ct systems have multi-slice arrays and several detector rows/ columns to capture multiple slices simultaneously
- artifacts arise through misalignment in detector components

Question 22

how are multi-slice cone beam artefacts resolved

Answer 22

manufacturers have developed special cone beam reconstruction software algorithms to reduce this effect