Mersey CT Flashcards
3rd Gen Design
- XR Tube
- Bow tie filter - Copper or Tin to remove low energy breasking radiation XRs - to shape intensity ACROSS the patient,
- Collimation - to reduce width of beam in direction of patient
- Reconstruct with a FOV of 50cm - Gantry is normally 70cm
- Bank of XR detectors - in front of each is an ANTI-SCATTER COllimator
XR Tube in CT - MUCH HIGHER power
Focal spot of 0.6mm or 1.2mm
X-RAY Tube and Gantry - rotate synchornously
adult - 120kVp
paediatric - 80 kVp
MULTI-SLICE ACQUISITION
Multiple slices acquired in one rotation
Gadolinium oxysulfide scintillation crystal connected to a photodiode. sits on a bank on a board of detectors.
You must irradiate the detector back with a uniform beam of X rays.
You must exclude the penumbra from the from the beam. You overbeam so penumbra is well away from detector edges
Smaller beams have a larger penumbra in proportion
Slice thicknesses with a single slice CT scanner, the slice thickness was determined by the width of the collinator
o there’s lead collinators here, which restrict the width of the beam along the z axis of the patient. That the long axis of the patient
Different sized elements = provides different sized slices
AXIAL CT - STEP AND SHOOT
Best quality
BUT SLOWER
PITCH
Table feed per rotation DIVIDED BY
Width of collimated beam
SPIRAL or HELICAL CT
Thorax and abdomen - faster times
Areas of tissue not properly sampled - so INTERPOLATION from adjacent samples
PITCH Vs Patient Dose
Pitch <1 = Overlapping - good data sampling but more patient dose
Pitch = 1
Table movement SAME as beam width - complete xray sampling with no overlapping
Pitch >1 Under sampling but LOWEST patient dose
Spiral CT Overranging
Because we we don’t acquire full full acquisition when the gantry rotates, we’re not rotating with the bed in a single position.
The bed is moving during that rotation, and that means to reconstruct data in one slice, we need to add extra rotations at the end of the study BEYOND AREA of interest
CT Reconstruction
1. Initially using filtered back protection
Simple back projections - you get streaks
Filtered back projection to get rid of noise
Comparing the two
Iterative reconstruction techniques
Iterative reconstruction
Image in the right = Filtered back projection + statistically reconstructed image to reduce noise - allows lower doses
Fat is LESS attenuating that water
Narrower CT window = HIGHER CONTRAST but MORE NOISE as well
Display window settings
CT DOSE INDEX
CT DOSE INDEX PHANTOMS
Put an ionisation chamber in each of the positions and then acquire a single slice - MEASURE CTDI in 5 locations within each phantom
You then take a WEIGHTED average
CTDI Vol - takes into account gaps or overlaps
CTDI = CTDW / PITCH
Conversion factors to change DLP to EFFECTIVE DOSE
DOSE LENGTH PRODUCT
How different MACHINE PARAMETERS will affect patient dose
Dose is proportional to
- SQUARE of kVp
- 1:1 of ma
- Gantry rotation time
Angular Tube Modulation
Current adjustment on the fly IN PLANE depending on patient thickness
Z AXIS BEAM MODULATION
Do a SCOUT
- identify which areas of patient require additional radiation
E.g. Shoulder - more current needed than lungs
Beam Modulation reduces dose without compromising image quality
BOW TIE FILTER
More attentuation at the patient edges
BOW TIE FILTER
Assumes patient is in the centre - if you are offset
SPATIAL RESOLUTION IN CT
Well, again, it’s an x ray imaging modality, so the focal spot size that will affect geometric and sharpness.
Smaller detectors = higher sampling frequency of the transmitted X ray = better resolution.
Thinner slices = improve the resolution along the z axis of the patient, the long axis of the patient = reduction of partial volume effects
pixel size and the field of view, basically the
Smaller pixels = better resolution
Patient movement will reduce spatial resolution, cause artefacts
Filter that you use in your reconstruction method that has effects on spatial resolution, you can either enhance edges or you can blur them to reduce noise.
How different filters look
Noise in CT
Exposure factors -
Pixel Size
Slice thickness
Beam hardening Artifact
Reduction in CT number between 2 bony structures
Motion Artefact - directional shadowing or streaking artefact
Ring Artifact
One of the detectors performs poorly o rmiscalibrated
Photon Starvation
Excessive attenuation
Partial Volume Artifact
Large voxels - where CT number is to specific for one tissue as it becomes an average of multiple tissues within the voxel
Metal Artifact
Metal artifact streaking
Metal artifact software adjusts for position of metal
Cone Beam Artifact
Corrections for cone beam artefact
