Computed Tomography II

Question 1

What is a pixel representing in a CT image?

Answer 1

a pixel in a CT image is a 2D representation of info obtained in a 3D voxel

Question 2

what is a CT number?

Answer 2

CT number represents the average linear attenuation of coefficient of tissues in voxel

Question 3

what is CT number measured in

Answer 3

Hounsfield units

Question 4

how many possible Hounsfield units are there?

Answer 4

there are 4000 possible Hounsfield units (-1000 - +3000)

Question 5

how many hounsfield units can the human eye see?

Answer 5

approximately 50

Question 6

CT number formula:

Answer 6

1000 x (linear attenuation of tissue - linear attenuation of water)
/
linear attenuation coefficient of water

Question 7

what is the linear attenuation of water?

Answer 7

0

Question 8

what colour does -1000Hu appear?

Answer 8

black

Question 9

what colour does 3000Hu appear?

Answer 9

white

Question 10

what is interpolation?

Answer 10

estimation of a value between two known values

Question 11

what is extrapolation?

Answer 11

estimation of a value beyond known values

Question 12

relevance of linear interpolation to CT?

Answer 12

we want to reconstruct the “missing” data between the data spirals

Question 13

two types of linear interpolation?

Answer 13

180 degree and 360 degree linear interpolation

Question 14

what data set is interpolated into planar images?

Answer 14

the helical data set

Question 15

in 360 degree linear interpolation, what data is weighted heavier?

Answer 15

the data closest to the z-axis has a heavier weighting

Question 16

in 360 interpolation, an axial slice is reconstructed ____ through the ____

Answer 16

in 360 interpolation, an axial slice is reconstructed midway through the spiral

Question 17

bigger the ____ the wider the SSP

Answer 17

The bigger the pitch the wider the SSP

Question 18

What data does the 180 degree linear interpolation use?

Answer 18

it uses data from 180 degrees of the tubes rotation

Question 19

what permits 180 degree linear interpolation?

Answer 19

this works because the beam absorption for the 0 degree projection is identical to the absorption for the 180 degree projection as the X-ray photons travel in opposite paths

Question 20

does 180 degree linear interpolation yield smaller or bigger effective slice width?

Answer 20

180 LI yields smaller effective slice widths

Question 21

advantages of helical CT

Answer 21

faster than conventional, sequential CT, able to reconstruct a volume data set, can reconstruct images in all planes, can reconstruct overlapping slices to improve 3D imaging and reduce partial volume artefact between axial slices

Question 22

disadvantages of helical CT

Answer 22

180 LI increases image noise slightly compared to sequential scanning, only half the data as half the photons used, slight increase in the SSP, can cause spiral artefact

Question 23

what is sequential scanning used for nowadays? and why?

Answer 23

brain CT, helical scanning can produce a spiral artefact