CT Flashcards
role of filter
to account for the attenuation through the patients body
Removes low energy x-rays
Produces a more monochromatic beam
May be bowtie-shaped to even out attenuation once it passes through the body
in CT : where is a collimator placed
between filter and the patient.
Aim of collimator in CT
lower dose to patient
restrict scatter outside desired slice
what are the two types of detector in CT
SSD and ionisation chamber (no longer used)
Solid state detectors in CT operate how.
There is a solid scintillator layer that converts the x-rays into visible light photons.
The photodiode then converts the photon input into an electrical signal.
what type of CT scanner is most used?
3rd generation. wide row of detectors rotates with the xr beam.
takes 0.3s to image a slice as the whole beam is detected
advantages of spiral scanning in CT
no breathing artefact
better use of contrast
overlapping slices better reconstruction
high pitch can reduce scan rime / dose
what is Pitch
distance couch travels in a single rotation / width of slice
what is the difference betweena pitch of 0.5, 1 and 2
0.5 they are overlapping. travel 3cm / beam width 6cm
1 - equal
2 - bits are missed
Advantage of high pitch number
Lower radiation dose
quick scan
Disadvantage of low pitch number
more sparsely sampled
what do multislice scanners allow you to do?
Allows to choose the slice thickness (as well as the collimator doing this)
benefit of Multislice scanners?
Faster scanning (better dynamic imaging)
thinner slices
3D imaging
muliple slices all at the same time can be acquired.
What are the types of detector arrays?
Linear
Adaptive
Hybrid
Adapative array has what configuration of elements
The elements within the central detector rows are the thinnest and they get wider towards the outside.
What is the benefit of adaptive array
fewer detector rows so fewer septae dividing rows. This improves dose efficiency.
multislice pitch - two ways to measure it are what
Pitch d
Pitch x
pitch D =
couch travel per rotation / width of xr beam
doesnt’ represent the overlapping xr beam
pitch x =
couch travel per rotation / total width of simultaneously acquired slices.
How is slice thickness determined differently in Single slice CT and Multislice CT
Single slice CT = determined by collimation. Limited by detector row width.
Multisclice CT = determined by width of detector rows
HU are a representation of what
LAC
HU =
HU = 1000 x (μt – μw) / μw
uw is water.
What are the issues with back projection?
- Too few projections cause artefacts in the image as there are too few directions of summed LACs to accurately represent the image. Typically 2000 projections are used.
- averging of values means edges can be blurred.
How is the averaging of blurred lines in back projection correct?
Filtered backprojection
Iterative reconstruction
basically compares an initial image with what the detectors should have received for the image/patient. it continually updates the image.
iterative reconstruction benefit
reduces doses but is a lengthy calculation.
Mostly used
What is dual energy CT
Dual-energy CT (DECT) utilises the photoelectric effect to separate out different materials within the voxel based upon their different attenuations at different beam energies.
What are the different modalities for dual energy CT?
Dual source DECT
Singe source DECT
-Fast kVP switch. -
-Fast kVP switch. -
flips.
- detector based spectral CT.
- the detector has two layers than can detect two energies.
Singe source DECT
,
- two detectors. can be consecutive scans of different energies, sequential each rotation is repeated twice
Dual source DECT
- two tube, differnet voltages, 90 degress to each other. (increased doseand scatter)
