COMPUTED TOMOGRAPHY EXAMINATION
QUESTIONS AND 100% VERIFIED ANSWERS
CT basics - • 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 - voxel thickness is equal to x-ray beam thickness
Slices - 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 - 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 - 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 - • CT creates a 2D map of linear attenuation coefficients ()
, • Then work out what is for each voxel
Reconstruction mechanisms - Back projection
Iterative methods
Analytic methods
Image display - • CT numbers
o Water is the reference CT #=0
o Bone: 1000
o Air: -1000
Windowing (CT image gray scale manipulation with CT #'s) - o Typically CT #'s will range
from -1000 to 1000
o Image is displayed in 256 shades of gray
o CT #'s are assigned gray scale values of 0-255 prior to display
o Assignment depends on window and width level
♣ Width: range of Ct #'s that are assigned a gray scale value of 0-255
♣ Level: Level of window centre (mid-range gray scale #)
o Narrow window: display ST within structures that contain varying tissues of similar density
image quality depends on (3) factors - ♣ spatial resolution: ability to detect the smallest
objects
• key parameter for describing scanner performance
contrast resolution: the ability to differentiate between two objects
artefacts: cause distortion and affect images
Factors that affect spatial resolution - o Focal spot size
o Detector size and spacing
o Magnification
o Pixel matrix
o Reconstruction algorithm
o Slice thickness
QUESTIONS AND 100% VERIFIED ANSWERS
CT basics - • 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 - voxel thickness is equal to x-ray beam thickness
Slices - 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 - 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 - 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 - • CT creates a 2D map of linear attenuation coefficients ()
, • Then work out what is for each voxel
Reconstruction mechanisms - Back projection
Iterative methods
Analytic methods
Image display - • CT numbers
o Water is the reference CT #=0
o Bone: 1000
o Air: -1000
Windowing (CT image gray scale manipulation with CT #'s) - o Typically CT #'s will range
from -1000 to 1000
o Image is displayed in 256 shades of gray
o CT #'s are assigned gray scale values of 0-255 prior to display
o Assignment depends on window and width level
♣ Width: range of Ct #'s that are assigned a gray scale value of 0-255
♣ Level: Level of window centre (mid-range gray scale #)
o Narrow window: display ST within structures that contain varying tissues of similar density
image quality depends on (3) factors - ♣ spatial resolution: ability to detect the smallest
objects
• key parameter for describing scanner performance
contrast resolution: the ability to differentiate between two objects
artefacts: cause distortion and affect images
Factors that affect spatial resolution - o Focal spot size
o Detector size and spacing
o Magnification
o Pixel matrix
o Reconstruction algorithm
o Slice thickness
