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
New cards
2
Helical CT
AKA spiral/volume CT Gantry rotating continuously releasing x-ray beams table simultaneously moves results in a continuous spiral scanning pattern
New cards
3
MDCT
Multi-Detector CT 2/more rows of parallel detector arrays Allows acquisition of multiple slices in a single rotation
New cards
4
Advantages of MDCT
Faster scanning time due to wider total active detector width Fewer motion artefacts
New cards
5
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
Not all equal Smaller in middle, larger outside Improves dose efficiency Less division/ dead space Expensive Flexible
New cards
11
Hybrid
Set of narrow + set of non-uniform Main type
New cards
12
Pitch
= ratio of distance moves per rotation to total w/ beam width
New cards
13
Higher pitch
less pt dose + quicker Lower imaging quality because less images acquired
New cards
14
lower pitch
more pt dose + quicker better imaging quality
New cards
15
Beam pitch
able distance travelled in 1 360 by gantry rotation divided by total thickness of all simultaneous acquired slice.
New cards
16
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
New cards
17
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
New cards
18
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
New cards
19
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)
New cards
20
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
New cards
21
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
New cards
22
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
New cards
23
Desired attributes
Good image Q but low dose Low noise Fast scanning Free of artefact High spatial resolution Less blurring
New cards
24
Factors affecting CT
pt factor, reconstruction, scan parameters, viewing conditions, re solution
New cards
25
Noise
Variation in CT no. which isn't related to true attenuation co-efficient Amount of ‘mottle’ in image
New cards
26
Noise occurs because...
Random variation in photons detection – stochastic noise stat fluctuation in x-ray production /interaction/detection Electronic noise=measuring system Reconstruction noise
New cards
27
disadvantage of noise
Lower noise= better LCD (low contrast detection) Smooth image does not vary from the value Noise= can mask detail
New cards
28
Quantifying noise
Measure noise/deviation Can be quantified: standard deviation in %
Studied by 23 people
