CT Flashcards
CT basic principle
images taken all around the patient
data reconstructed
tomogram: slice through patient
CT scanner
gantry:
tube
aperture
detectors
x-ray source
high voltages for high energies needed to scan full body
120-140kV
heavily filtered to get rid of low energies
use effective energy
compton scatter dominates attenuation
measures electron density
detectors
fast, no lag
high absorption efficiency
small and compact to fit in gantry
high stability to take more images with the same detector response
large dynamic range
xenon ionisation chamber
x rays ionise gas
electric field attracts ions
charge collected proportional to xray intensity
solving low quantum efficiency of gas
use high Z gas (xenon)
gas at high pressures
make detector long
solid state detectors
scintillator and photodiode
higher absorption efficiency
first generation CT
pencil beam
single detector
translate and rotate around object
configuration adv:
easy calibration
low cost
high scatter rejection due to beam collimation
parallel-beam reconstruction, geometry allows easier reconstruction
second generation CT
multiple detectors
fan beam - fewer rotations
translate and rotate
reduces acquisition time
third generation CT
more detectors
only rotate
fan beam
vulnerable to ring artefacts because the same physical detector channel measures all the rays that form a ring
fourth generation CT
detectors all around gantry
only source rotates
avoids ring artefacts
inflexible system, can’t change source-detector distance
fixed
helical/spiral scanning
continuously rotating xray tube
patient is moved through a rotating x-ray beam and detector set. From the perspective of the patient, the x-ray beam from the CT traces a helical path. The helical path results in a three-dimensional data set, which can then be reconstructed into sequential images for an image stack.
balance speed with loss of data
higher pitch (speed) lowers radiation dose but at expense of partial volume effect (loss of detail)
quick, done in one breath
less movement artefacts
slice interpolation
at every bed position, only 1 projection
360 degree linear interpolation
uses attenuation data from points 360 apart on the helix for interpolation
makes effective image width broader
180 degree linear interpolation
distance shorter
slice profile narrower (good thing)
but noisier
multislice CT
allow acquisition of multiple slices in a single rotation
cone beam artefacts
4-slice, 16-slice, 64-slice
faster
allows heart imaging
multislice CT slice thickness
thick slices have lower noise
thin slices reduce partial volume effects and allow off axis image creation with isotropic resolution
CT numbers
mu for each pixel converted to CT number
allows a greater range of gray levels to be displayed
in Hounsfield units
polychromatic beam
so scales values to reduce variation between difference scanners (different energies)
CT number for water is 0
lower CT number if less dense than water
why a high range of CT values for same material
consistency not the same
intra-tissue variation
inter-patient variation
windows and levels
window = width of CT numbers
level = centre of window
distributes grayscale across values
if CT number:
below window = black
above = white
within = gray shade
image quality
determined by
image contrast
spatial resolution: line pair test object, acrylic bars
noise: tube mA, scan time, slice thickness
artefacts
QA of CT systems
use phantoms to measure
noise
size dependence
spatial resolution
contrast scale
sensitivity
alignment
absorbed dose
linearity
slice thickness
artefacts
ring
motion
spectral
streak
partial volume
cone beam
noise
ring artefacts
3rd gen
non uniform detector response
concentric rings in image
removed by averaging or appropriate filters
identified and removed by software algorithms
spectral artefacts
beam not monochromatic
beam hardened across patient (the low-energy photons are absorbed more than the high-energy photons)
average energy higher in centre
CT numbers higher at edge
cupping effect
correct:
use bow tie filter
to modulate spectrum
software solutions
streaking
artefact
high density objects
cause streaks
eg bullet, surgical pin
partial volume effects
loss of apparent activity in small objects or regions because of the limited resolution of the imaging system
one 3D voxel occupied by more than one tissue type
CT number is intermediate
