1	Physics & Technology of Multi-slice CT James Weston ImPACT
2	Aims Some key factors about MSCT construction of scanners reconstruction techniques artefacts other factors Concepts and ideas keep it non-mathematical!
3	MSCT scanners 1991 Dual slice 1998 Four slice 2002 16 slice 2003 32 slice today 64 sub-mm slices 0.4 s rotation
4	Clinical scanners
5	The 3 Fs of CT Faster Further Finer
6	Isotropic imaging 2D pixel in a CT image represents a 3D voxel Resolution is ideal when equal in all 3 dimensions best results with slice thickness equal to (axial) pixel size routine 0.5 - 1 mm slice thickness achieves this goal
7	Scanner design
8	“Third generation” CT scanners Tube & detectors rotate around patient gathering x-ray projections Projection data used to form slice images filtered back projection
9	Helical CT Continuous gantry rotation + continuous table feed Scan data traces a helical path - or ‘spiral’ - around patient data used to form axial images
10	Multi-slice CT scanning Many features in common with single slice (SSCT) multiple parallel detector banks along z-axis enables a number of projections to be acquired simultaneously
11	MSCT scanning: in scale
12	Detector banks Array extends in 2 directions xy-plane arc to collect many samples for each projection z-axis along the patient length SSCT z-axis coverage: one element MSCT many z-axis elements
13	Slices & detectors Just 4 detectors reduces options for scanning Narrow coverage eg. 5 mm for d=1.25 mm
14	Slice width selection: 4 slice For more flexibility AND greater coverage need more detectors Can collect data from groupings of detectors individual detectors 4 x d pairs 4 x 2d triples 4 x 3d
15	Slice options: real example GE LightSpeed 4 slices 16 detectors in z-axis
16	Slice options: real example GE LightSpeed 4 slices 16 detectors Detector output combined to define data acquisition width Coverage up to 20 mm
17	Adaptive arrays Detector elements not all same size e.g. Toshiba Aquillion series
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19	64 slice scanners
20	64-Slice CT: double sampling z-flying focal spot 32 detectors -> 64 data channels
21	? CT Multi-slice CT MSCT Multi-detector CT MDCT Multi-channel CT MCCT Multi-row CT (MRCT less common as abbreviation) All effectively the same thing Note: care when using “SSCT” normally used for single slice can sometimes refer to single source check the context
22	Design considerations Scan gantry mechanical stresses data & power feed Tubes high currents narrow slices; fast rotations tube cooling generator response Detectors responsive efficient small Electronics / computers / reconstruction hardware
23	More challenges for MSCT Reconstruction Artefacts Dose efficiency Data management
24	Using helical data Single slice: interpolate using 2 nearest data points
25	Using helical data Single slice: interpolate using 2 nearest data points Up to 8 slice MSCT: use all data within a variable ‘filter width’ for interpolation
26	Flexibility of reconstruction ‘Overlapping’ reconstructions better z-axis resolution better 3D imaging
27	Artefacts All standard (SS) CT artefacts can still occur ring artefact beam hardening Specific issues for MSCT cone beam helical artefacts
28	Cone beam artefacts Seen as streaks in image as number of slices increases Due to large cone angles and narrow slices
29	"As number of slices increases" As number of slices increases, beam is more diverging, outer slices are distorted Negligible up to 8 slices, significant for 16 slice scanners
30	Cone beam artefact Beyond 8 slices, special reconstructions needed to avoid cone beam artefacts Range of techniques are used tilted (hyperplane, or non-orthogonal) 3D (Feldkamp / FDK) reconstructions
31	Tilted reconstruction ASSR techniques uses tilted reconstructions images back projected along optimal oblique planes reconstructed images then filtered to produce axial images
32	3D reconstruction Feldkamp based three dimensional reconstructions extension of back projection to third dimension requires more computing power
33	Effectiveness of cone beam algorithms
34	Helical artefacts Conical phantom single-slice helical Spherical air pocket 8 x 2.5 mm slice helical
35	Helical artefacts - clinically
36	Windmill artefact in consecutive slices Teflon rod at 60° to horizontal
37	Helical artefact Processing can compensate for helical scanning Reduces artefact
38	MSCT and dose CT is a high-dose exam more CT studies being undertaken even more exams with new MSCT apps Automatic exposure controls (AEC) Differences between single and multi-slice over-beaming over-ranging
39	Z-axis over-beaming Beams are wider than the nominal value due to finite size of focal spot Irradiated beam width ~ 3mm wider e.g. 4 x 2.5 mm slices, 12.5 mm beam Less significant as beam width increases wider collimations routinely used
40	Wider beams – lower dose Efficiency increases with collimation (beam width) More coverage means thin slices at lower dose
41	Overranging To image entire volume, data is needed at both ends of scan requires more rotations to acquire This is more significant for multi-slice, wider beams, and for short scan ranges
42	Data explosion! Scan data throughput from gantry to computer Single slice, 1 second rotation : ~ 2 megabytes per second 4 slice, 0.5 s rot : 16 MB/s 16 slice, 0.5 s rot : 64 MB/s 64 slice, 0.5 s rot : 256 MB/s Image production speed 2005: ~ 64 MB/s Data processing burden Network traffic … Archive issues… Images per exam Image viewing capacity?
43	Reporting & navigation tools
44	In summary Multislice CT scanning has progressed hugely since 1998 there are challenges that arise with MSCT – and have been met eg ConeBeam reconstructions 16 and 64 slice changes CT from slice to volume scanning image quality can now be routinely isotropic 3D data sets readily available data sets are there to be explored flexibly New applications still developing … and new scanners coming
45	Acknowledgements for scanner information & images GE Healthcare Philips Medical Siemens Toshiba University of Erlangen Matthew Benbow, RBCH Thanks also due to Sue, Maria and Margaret at ImPACT David Platten & Nick Keat
46	Physics & Technology of Multislice CT