Review Of CT Physics Flashcards
CT scanners need?
- Stable and fast detectors
- Exact source - detector geometrical relationship
- high resolution
- small focal spot
- mono-energetic beam
- large amount of computing power
- strong radiolucent beds
CT numbers show?
- attenuation coefficient of materials
- affected by KV
- Air has a CT number of -1000
- water has a CT umber of 0
CT numbers
CT numbers = [u(tissue) - u(water) / u(water] x 1000
- Measured in distribution of u
- u values are scaled to that of water to give CT numbers
- multiplayer of 1000 used to make whole numbers
- for a given tissue depends of the KV, scanner model, etc:
+ water = 0
+ Air = -100
+ bone = 1000
Everything else somewhere else in between
In filtered back projection?
A filter removes the streaks from the image
The more back projections the better the objects are represented in the image.
Image reconstruction
The more projections added - the better the representation
Simple back projection gives a streaky image
A filtered (image projection) is applied - FILTERED BACK-PROJECTION
Variety of filters available - smooth, fine
Iterative reconstruction (IR)
Uses acquired information for comparison
Can model physics
Very slow
“Plasticky look”
CTDI?
Is dependant on the irradiated slice width and dose profile
Dosimetry CTDI
CTDI = Computed tomography Dose index
Defined as CTDI = 1/NT integrated dose profile
NT = number of detector rows width of detector row
Dose profile - penumbra and scatter
Weighted CTDI
- doses greater towards the periphery
- measure in outer holes of phantom and add to central ones
CTDI w = 1/3 CTDI central/100 + 2/3 CTDI peripheral/100
- normally displayed on CT console
- indicates dose per rotations
DLP?
Is defined as the CTDI vol times the scan length
Can be converted to effective dose
DLP and CTDI
CTDI vol = CTDI w / pitch
DLP = dose length product
DLP = length of scan x CTDI vol
- commonly displayed
- indicates affect of settings on scan dose
- can be converted to effective dose if needed
Which factors affect dose and image quality?
- Slice thickness
- pitch
- KV/mAs
- contrast
- reconstruction filter
Slice thickness (Z-direction) ?
Thinner slices improve resolution in the Z-direction
Wider slices can increase partial volume effects
Slice thickness info - narrow slices
More noise
Better Z-axis resolution
Slice thickness info - wide slices
Less noise ( more x-ray photons contribute to image)
Worse Z-axis resolution
Partial volume effects? - depends on the size of the object being imaged
Faster coverage
Fewer slices
Helical Imaging?
Data is reconstructed using interpolation
Requires overscan so that there is sufficient data for thee beginning and finishing slices
Pitch = 1
Similar to axial scan at isocentre
No overlaps or gaps
Pitch < 1
Slices overlap
Higher dose
Better image quality
Slower scan
Pitch > 1
Gaps in between slices
Lower dose
More interpolation
Faster scan
Image reconstruction
- Helix scanning, produces a volume image
- Location of sliced reconstruction is arbitrary
- Data is collected from the projections
- Data is reconstructed into slices via interprolation
- Data from production angle at different table positions - Needs 180° of data (+fan) therefore needs to overscan to have this data for the beginning and finishing slice.
Kvp ?
Increasing KVP can reduce noise in the image
Can be altered automatically by the scanner
Tube potential (kVp)
~ non linear with radiation dose
~ radiation dose proportional to kvp^2
~ higher kvp means less noise in the image
~ however this may reduce low contrast resolution
~ can be manually fixed kvp or automatic selection
~ typically 120Kvp (between 80 - 140 kvp)
~ lower kvp for paeds and contrast protocols
MA ?
- Noise decrease as mA increases for a fixed rotation time
- dose increases linearly with mA
- is adjustable
MA is the
Tube current
Which of the following are true?
- Partial volume is where the values of a voxel are averaged
- Beam hardening causes areas deeper in the artefact to appear more attenuating
- Ring artefacts are caused by detector non-uniformities
- Photon starvation is caused by areas of high attenuation
- Metal can cause beam hardening, scanner, noise and photon starvation
- Ring artefacts are caused by jewellery
- Out of field artefacts are caused by anatomy outside the reconstructed FOV
- 5.7.
Beam hardening
Cupping - areas deeper within objects appear less attenuating
If medium is known correction can be applied
Photon starvation
- occurs in large patients or thick regions
- high attenuation gives parts of projections with low photon numbers
- noise is magnified during reconstruction, giving streak artefact.
Ring artefacts
Non-uniformities amongst detectors
Appears a a ring
Out of field artefacts
Cause of streaks and errors in reconstructed FOV due to presence of photon, interacting, medium outside FOV.
Material not accounted for in reconstruction
To mitigate this, try wide bore or arms up.
