1	CT Scanner Workshop
2	CT Scanner Workshop Lastditch General Hospital has at last been given funding to replace its outdated single slice CT scanner, a Reliatom Basic. As they wish to undertake cardiac scanning, their business case for a 64 slice model has been approved. Of the four 64-slice models offered, they have shortlisted the following two: The Superwhiz TopSpin 64 The Ultra Volume Excel 64 Specifications of the two new scanners are given
3	CT Scanner Workshop Leaving aside financial issues, consider the pros and cons of the two scanners in terms of scanning capabilities (eg coverage - speed and length) image quality (resolution) patient dose. There is not necessarily any right answer, and you may think some information is irrelevant The aim is to stimulate discussion between delegates with different professional background and experience Same groups as for quiz
4	Disclaimer Whilst this is based on true life, any resemblances to current or redundant scanner models is purely co-incidental!
5	Academic Exercise In reality would not look at some of these specifications in so much detail Use it to back up performance data A suggestion to go through this is to 1. first find those aspects that you think are irrelevant 2. Since a number of the specifications are inter-related, identify those which relate to each of the topics (coverage, spatial resolution (scan plane and z), and dose). Then discuss. An alternative is to go through each specification one at a time.
6	1. Gantry Aperture UVE64 larger. Good for patient access for interventional work. Potential for RT treatment planning and bariatric patients. Would like also to know what largest field of view is, and image quality for largest field of view
7	2. Maximum Field of View UVE64 is larger. Good for RT planning and bariatric patients Might want to find out image quality of largest field of view. Can be reduced outside of main fov (Still suitable for identifying patient contour)
8	3. Rotation Times ST64 Shortest minimum scan time Less movement artefact, more contrast phases…. Short rotation time important in cardiac as determines temporal resolution A variety of times can be helpful for optimum cardiac scanning at all heart rates Make sure that the minimum scan time is for 360^o Don’t forget when operating at a fast scan time, tube will be operating at higher mA values (good heat capacity)
9	4. Minimum Slice Width ST 64, thinner slice ie better z-axis resolution Don’t just believe on face value if this is important to you. Look at measured data, and or rounding. Eg if image width is quoted to nearest 0.5 mm then both could be the same…..
10	5. Maximum number of simultaneous slices ‘image slices’ or ‘data slices’ ? ST may acquire 64 channels of data using the z-flying focal spot, earning it the right to be called a 64 (data) slice scanner Z-resolution potentially better – but not necessarily as interleaved sampling from complementary projections and low pitches can give a similar results (see item 13.)
11	6. Total length of detector array
12	7. Number of detectors per row Affects spatial resolution in scan plane
13	7. Number of detectors per row
14	Scan plane (x-y) Resolution Determined by Effective detector size at iso-centre, and this is dependent on No detector elements per row, geometry (FID, FDD) Focal spot size Number of samples or projections per rotation See also items 9 and 11 If assume same maximum field of view for full image quality (500). Then effective detector width for ST = 500/600, and for UVE = = 500/800. Therefore UVE potentially better spatial resolution.
15	Effective Detector Width Fundamental limitation of spatial resolution Depends on Focal spot size Scanner geometry (FID, FDD) Detector element size
16	7. Number of detectors per row
17	8. Detector Type Both solid state ceramic but nothing to say which is best afterglow ring artefacts detector efficiency
18	9. Focal spot size Affects resolution (x-y, and z) UVE64 smaller fs size. All things being equal Improved scan plane resolution Smaller penumbra - lower dose
19	10. Focus to Detector Distance UVE64 (110) vs ST64 (90) UVE64 Requires higher mAs for same noise (and dose) tube heat capacity and dissipation requirements important
20	10. Focus to Detector Distance UVE64 (110) vs ST64 (90) More room within gantry so collimators can be further away from focus-smaller penumbra ?Less cone bean artefact as beam in z-direction more parallel Potentially less scatter.
21	11. Sampling Rate Scan plane resolution depends on sampling interval Note ST64 is 2500 samples/sec At 0.3 sec, this is 750 samples/rotation At 0.4s, 1000samples/rotation. Neither as good as UVE
22	Sampling Rate Sampling rate and scan time determine number of projections per image Potential limit on spatial resolution
23	Scan plane (x-y) Resolution Spatial Resolution
24	Scan plane (x-y) Resolution
25	Scan plane (x-y) Resolution
26	12. Flying (Dynamic) Focal Spot (x-y) UVE 64 - samples at half detector width (in x-y). Potential for improved scan plane resolution,
27	13. Flying (Dynamic) Focal Spot (z) ST64 – samples at half detector width in z-axis. Potential for improved z-axis resolution
28	13. Flying (Dynamic) Focal Spot (z) ‘image slices’ or ‘data slices’ ? ST may acquire 64 channels of data using the z-flying focal spot, earning it the right to be called a 64 (data) slice scanner Z-resolution potentially better – but not necessarily as interleaved sampling from complementary projections and low pitch can give a similar effect.
29	Z-Axis flying focal spot vs interleaved samples Z-Axis Flying Focal Spot To improve spatial resolution in helical scanning
30	Z-Axis flying focal spot vs interleaved samples Z-axis interpolation To improve z-axis resolution in helical scanning
31	14. Helical Interpolation ST64. Potentially more exact, potentially takes longer. But don’t know which is best
32	15, 16, 17 X-ray tube 15. Anode Cooling UVE64 potentially much greater cooling efficiency as shown in next point 16. Anode heat capacity UVE64 Longer scan lengths possible Higher mAs possible Could lead to high doses if used carelessly 17. Anode cooling rate
33	18. Filtration UVE64 Harder beam more uniform dose distribution better dose:noise ratio ST Softer beam ? better low contrast differentiation ? higher surface dose
34	19. Maximum mA Good for bariatrics Good for short scan times
35	20. Maximum Continuous scan time ST better, but seems inconsistent with heat capacity. Question whether data is correct? Maybe limited because longer scan time would not be needed in practice?
36	21. CTDIvol Note mGy/100mAs ST64 has shorter fid, and lower filtration so is likely to operate at lower mA for a clinical scan (although it has a much higher max mA available)
37	21. CTDIvol
38	22. Dose modulation ST does not allow rotational modulation. Does not have benefit of streaking artefact reduction Does not make full use of possibility for dose reduction
39	CT Scanner Workshop