CT 2

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Physics

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33 Terms

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Axial CT
Gantry stops + rotates to get data from single slice, X-rays switched off, pt moves to next slice, Rotates to acquire data from next slice
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Helical CT
AKA spiral/volume CT
Gantry rotating continuously releasing x-ray beams
table simultaneously moves
results in a continuous spiral scanning pattern
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MDCT
Multi-Detector CT
2/more rows of parallel detector arrays
Allows acquisition of multiple slices in a single rotation
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Advantages of MDCT
Advantages of MDCT
Faster scanning time due to wider total active detector width
Fewer motion artefacts
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Reduced patient risk
Ideal for trauma imaging – cover entire pt in 1 scan
Fast scan times minimise time on table for critically ill patients
Paediatric scanning can be done with less sedation
Less contrast required reduces risk of adverse reaction
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Thinner slices
Improved z-axis resolution
Isotropic imaging (equal voxel dimensions)
Improved multi-planar reformats (MPRs)
Improved 3D image rendering
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MDCT detector
MDCT detector
slice thickness is determined by collimation
Electronic detector selection (detector switching)
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configuration
3 types:
Uniform/linear
Non-uniform/adaptive
Hybrid/mixed
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Uniform
All rows have same size, width, thickness
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Non-uniform

Not all equal
Smaller in middle, larger outside
Improves dose efficiency
Less division/ dead space
Expensive
Flexible
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Hybrid

Set of narrow + set of non-uniform
Main type
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Pitch
= ratio of distance moves per rotation to total w/ beam width
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Higher pitch
less pt dose + quicker
Lower imaging quality because less images acquired
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lower pitch
more pt dose + quicker better imaging quality
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Beam pitch
able distance travelled in 1 360 by gantry rotation divided by total thickness of all simultaneous acquired slice.
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Pitch determination
Pitch determined by how quickly table moves ---> in MDCT, factor in total thickness because there are more than 1 detector
e.g. in multi: 7.5/4 * 2.5= 0.75 while in single 7.5/5.0=1.5
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cone beam acquisition
cone beam acquisition
Having more detectors and more slices means a having wider beam width
A cone beam is required to cover the whole detector width
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Cone Beam Reconstruction
With increased number of slices, the cone beam generates cone beam artefacts
As tube rotates, off-centre objects are visualised by different detector rows
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Cone Beam Interpolation (2)
Tilted Reconstruction- produces non-axial images which are then filtered to produce standard axial images

‘Feldkamp Algorithm’ - a 3D back projection (standard FBP is planar + therefore 2D)
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Tilted (Oblique) Reconstruction
Reconstruct using BP, at an angle to the axial plane
Overlap reconstructions and filter along the z-axis
Basis of GE and Siemens techniques
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Feldkamp Algorithm
Measurements are being taken from different angles for each patient ‘voxel’.
The section of pt being imaged is divided into 3D voxels rather than a 2D matrix of pixels as happens in back projection
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Image Quality in CT
image showing visibility of anatomical structures, various tissues, + signs of pathology
A measure of how suitable an image is for its intended diagnostic purpose
Suitability is determined if specific relevant criteria are met
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Desired attributes
Good image Q but low dose
Low noise
Fast scanning
Free of artefact
High spatial resolution
Less blurring
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Factors affecting CT
pt factor, reconstruction, scan parameters, viewing conditions, re solution
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Noise
Variation in CT no. which isn't related to true attenuation co-efficient
Amount of ‘mottle’ in image
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Noise occurs because...
Random variation in photons detection – stochastic noise
stat fluctuation in x-ray production /interaction/detection
Electronic noise=measuring system
Reconstruction noise
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disadvantage of noise
Lower noise= better LCD (low contrast detection)
Smooth image does not vary from the value
Noise= can mask detail
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Quantifying noise
Measure noise/deviation
Can be quantified: standard deviation in %
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Factors affecting noise
Scanner specifications and design
Scanning acquisition parameters
Reconstruction parameters
Patient factors
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Scanner specifications and design
Efficiency of detectors
X-ray beam filtration
Scanner geometry
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Scanning acquisition parameters
Tube voltage
Tube current
Scan time
Slice thickness
Pitch
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Reconstruction parameters
Back projection algorithms
Noise filters
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Patient factors
pt size