1 | Page
Computed tomography Questions and
Correct Answers/ Latest Update /
Already Graded
CT basics
Ans: • Essentially an x-ray tube that rotates round the pt
• Table moves, feeding pt into the gantry
• Each large is data collected as a series of slices
o Each slice is divided into voxels (volume elements)
Multislice scanner
Ans: voxel thickness is equal to x-ray beam thickness
Slices
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Ans: o Thinner slice higher radiation dose better quality image
o Width and height depend on FOV
o Determined by the operator
o Thickness is determined by the image requirements
calculate linear attenuation coefficient for each voxel
Ans: o From this a T # is calculated this determines hoe the voxel appears
o Voxel appearance can be changed with windowing
♣ High CT numbers: brighter
♣ Low CT numbers: darker
• Linear attenuation coefficient is calculated in the reconstruction process
Linear attenuation coefficient calculation
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Ans: Xray tube passes around the pt, through the pt and detected when
they leave the pt
o By comparing their intensities to unattenuated rays, the net linear
attenuation is calculated for each ray
o This # is assigned to each voxel along the rays path
o By doing this from a number of different angles the true values for each
voxel is obtained
• Each voxel is assigned a CT # which depends on its linear attenuation
coefficient
o This is assigned a shade of gray for the final display
Reconstruction
Ans: • CT creates a 2D map of linear attenuation coefficients ()
• Then work out what is for each voxel
Reconstruction mechanisms
, 4 | Page
Ans: Back projection
Iterative methods
Analytic methods
Image display
Ans: • CT numbers
o Water is the reference CT #=0
o Bone: 1000
o Air: -1000
Windowing (CT image gray scale manipulation with CT #'s)
Computed tomography Questions and
Correct Answers/ Latest Update /
Already Graded
CT basics
Ans: • Essentially an x-ray tube that rotates round the pt
• Table moves, feeding pt into the gantry
• Each large is data collected as a series of slices
o Each slice is divided into voxels (volume elements)
Multislice scanner
Ans: voxel thickness is equal to x-ray beam thickness
Slices
,2 | Page
Ans: o Thinner slice higher radiation dose better quality image
o Width and height depend on FOV
o Determined by the operator
o Thickness is determined by the image requirements
calculate linear attenuation coefficient for each voxel
Ans: o From this a T # is calculated this determines hoe the voxel appears
o Voxel appearance can be changed with windowing
♣ High CT numbers: brighter
♣ Low CT numbers: darker
• Linear attenuation coefficient is calculated in the reconstruction process
Linear attenuation coefficient calculation
,3 | Page
Ans: Xray tube passes around the pt, through the pt and detected when
they leave the pt
o By comparing their intensities to unattenuated rays, the net linear
attenuation is calculated for each ray
o This # is assigned to each voxel along the rays path
o By doing this from a number of different angles the true values for each
voxel is obtained
• Each voxel is assigned a CT # which depends on its linear attenuation
coefficient
o This is assigned a shade of gray for the final display
Reconstruction
Ans: • CT creates a 2D map of linear attenuation coefficients ()
• Then work out what is for each voxel
Reconstruction mechanisms
, 4 | Page
Ans: Back projection
Iterative methods
Analytic methods
Image display
Ans: • CT numbers
o Water is the reference CT #=0
o Bone: 1000
o Air: -1000
Windowing (CT image gray scale manipulation with CT #'s)
