Computed Tomography II Flashcards
What is a pixel representing in a CT image?
a pixel in a CT image is a 2D representation of info obtained in a 3D voxel
what is a CT number?
CT number represents the average linear attenuation of coefficient of tissues in voxel
what is CT number measured in
Hounsfield units
how many possible Hounsfield units are there?
there are 4000 possible Hounsfield units (-1000 - +3000)
how many hounsfield units can the human eye see?
approximately 50
CT number formula:
1000 x (linear attenuation of tissue - linear attenuation of water)
/
linear attenuation coefficient of water
what is the linear attenuation of water?
0
what colour does -1000Hu appear?
black
what colour does 3000Hu appear?
white
what is interpolation?
estimation of a value between two known values
what is extrapolation?
estimation of a value beyond known values
relevance of linear interpolation to CT?
we want to reconstruct the “missing” data between the data spirals
two types of linear interpolation?
180 degree and 360 degree linear interpolation
what data set is interpolated into planar images?
the helical data set
in 360 degree linear interpolation, what data is weighted heavier?
the data closest to the z-axis has a heavier weighting
in 360 interpolation, an axial slice is reconstructed ____ through the ____
in 360 interpolation, an axial slice is reconstructed midway through the spiral
bigger the ____ the wider the SSP
The bigger the pitch the wider the SSP
What data does the 180 degree linear interpolation use?
it uses data from 180 degrees of the tubes rotation
what permits 180 degree linear interpolation?
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
does 180 degree linear interpolation yield smaller or bigger effective slice width?
180 LI yields smaller effective slice widths
advantages of helical CT
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
disadvantages of helical CT
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
what is sequential scanning used for nowadays? and why?
brain CT, helical scanning can produce a spiral artefact
