1	Image Quality and Dose Issues in MSCT
2	Image Quality and Dose Image quality Image noise Spatial resolution Contrast Artefacts Radiation Dose Organ dose Effective dose
3	Image Quality and Dose Image quality Image noise Spatial resolution Contrast Artefacts Radiation Dose Organ dose Effective dose
4	Image Quality and Dose Issues in MSCT Many issues are the same in ss and ms General comments Specific comments to msct tend to relate to z-axis features
5	Scanner parameters affecting IQ and Dose
6	Scanner parameters affecting IQ and Dose
7	Scanner parameters affecting IQ and Dose
8	Scanner parameters affecting IQ and Dose
9	IQ and Dose in MSCT Spatial resolution (z-axis) Pitch Dose issues Reconstruction algorithm What image quality do we want?
10	High contrast spatial resolution
11	Spatial Resolution – 3D Scan plane (limited by pixel size) Z-axis (image slice width)
12	Z-axis spatial resolution Imaged slice width Influences partial volume artefacts Affects contrast and noise In MSCT Flexibility of reconstructing different slice widths In helical generally (SS and MS) Optimised by reconstructing overlapping slices
13	Z-axis spatial resolution
14	Thinner slice minimises partial volume artefacts
15	Z-axis spatial resolution Image width affects contrast and noise of object Optimised slice width: imaged slice » object size
16	Thinner slice – improved contrast
17	Thinner slice - higher noise Object ~ 5 mm
18	Z-axis resolution in single-slice Image width depended on beam width And post patient collimation for thin slices
19	Z-axis resolution in multi-slice Image width depends on detector acquisition width eg 4 x 5mm, will not give a 2.5 mm slice! (Use 8 x 2.5) May be optimised in helical with closer z-axis sampling (eg z-sharp in Siemens, or certain overlapping pitches)
20	Z-axis resolution in multi-slice Image width depends on detector acquisition width eg 4 x 5mm, will not give a 2.5 mm slice! (Use 8 x 2.5) May be optimised in helical with closer z-axis sampling (eg z-sharp in Siemens, or certain overlapping pitches)
21	Optimising z-axis spatial resolution Visualisation optimised by overlapping reconstructions (viewed by cine or 3-D)
22	Optimising z-axis spatial resolution Overlapping reconstructions recommended for optimum contrast and z-axis resolution ½ to 2/3^rds overlap recommended
23	Effect of pitch SSCT vs MSCT Dose Noise Image slice thickness Artefacts
24	Pitch – dose Overlapping pitch – average dose increases Extended pitch – average dose lower
25	Pitch - single slice (increase pitch, mA const)
26	Pitch – multislice (inc. pitch, mA const.)
27	Pitch – multislice (inc. pitch, inc. mA)
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31	Dose issues in MSCT Beam width (overbeaming) Helical overscan (overranging)
32	Dose issues in MSCT - Beam width Penumbra typically 3 mm for all beam widths lower proportion of total dose with wider beam widths Wider is generally better
33	Dose issues in MSCT - Overranging
34	Effect of reconstruction filter Filter used in backprojection (convolution kernel) Smooth, standard, detail, bone AH30, AH40, AB50 FC41, FC43 etc, etc Used to optimise spatial resolution against noise
35	Effect of reconstruction filter higher spatial frequency Þ more noise eg Smooth ® Standard ® Sharp noise = ~ 7 HU ® 17 HU ® 70 HU
36	Tube current
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38	Compromise depending on requirements High spatial detail Low contrast resolution
39	Image noise What is an appropriate level of image noise ?
40	Image noise What is an appropriate level of image noise ? too low – high dose too high – no diagnosis / missed diagnosis How do we find the optimum level?
41	Systematic addition of image noise Systematic addition of noise to clinical images/raw data Simulate mA Studies for a variety of clinical conditions and scanners
42	Image quality required for diagnosis
43	Image quality required for diagnosis
44	"Frush et al ‘Computer..." Frush et al ‘Computer simulated radiation dose reduction for abdominal multidetector CT of Pediatric patients’ AJR:179, November 2002
45	What noise level is needed?
46	What noise level is needed?
47	What noise level is needed?
48	What noise level is needed?
49	What noise level is needed?
50	What noise level is needed?
51	What noise level is needed?
52	What noise level is needed?
53	What noise level is needed?
54	What noise level is needed?
55	What noise level is needed?
56	IQ and Dose in MSCT Spatial resolution (z-axis) Pitch MSCT dose issues Reconstruction algorithm What image quality do we want?
57	Image Quality and Dose Issues in MSCT