CT Flashcards
Is Compton scattering or the photoelectric effect responsible for more attenuation in CT?
Compton scattering
What term describes the average attenuation of a photon beam through an area?
linear attenuation
What symbol is used for linear attenuation in CT?
µ
What factors influence the linear attenuation through a medium?
physical density
atomic number
energy (of the photons)
What mathematical concept is utilised to calculate the incident energy of a photon beam that has travelled through something?
Beer’s law
What is Beer’s law
I = Io * e ^ (-µ t)
Transmiitted Intensity (I)
Incident intensity (Io)
path length (t)
How is Beer’s law utilised in CT?
Used to solve the linear attenuation where the intensity of the x-ray beam is known before and after patting through an object
What is type of data array is CT data initially stored in (before processing into an image)
a sinogram
How is data stored in a sinogram?
Each row represents all the x-rays that go through the patient from a certain angle (not for a certain point)
What methods can be used to process a sinogram into a CT image?
Back Projection
Filtered Back Projeciton
Fourier Transformation
Iterative
What characteristics are needed of CT x-ray generators (opposed to regular x-ray generators)?
high current fast rotations thin slices long scan times small focal spot size robust (up to 13Gs at times)
How are collimators used in CT?
used to define the slice that’s being CT’d, also used to restrict angle x-rays get through tot he detectors
How do CT detectors measure incident x-ray photons?
scintillator crystals convert into electricity
current read
What scintillator crystal is commonly used in CT
Na I
What is the name of the device that allows the CT detectors and x-ray generator to rotate around the patient?
slip ring
CTs that spin around the patient as they move through the machine?
helical CTs
What generation of CTs are helical CTs?
6th generation
What is the definition of pitch?
table travel per rotation/ normal slice width
What pitch level indicates not every point is scanned from every angle?
pitch >1
What are the advantages of pitch > 1 in CT scanning?
faster scan
lower dose
What are the advantages of pitch < 1 in CT scanning?
better quality (at cost of higher dose)
What are the advantages of helical CTs?
speed
potential dose reduction
reconstruction in any plane
What are 6th generation CT scanners
helical CT scanners
What are 7th generation CT scanners
multidetector CTs
What are the advantages of multi-detector CTs?
better resolution (thinner slices)
speed
less tube loading
