Comparison of CT Scanner Image Noise,
Image Width, Dose and Spatial Resolution
Using Standard Test Methods
Sue Edyvean, Nicholas Keat
ImPACT (Imaging Performance Assessment of CT Scanners)
London
UK
www.impactscan.org
ImPACT
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AAPM 2004
ImPACT
• An evaluation group of the UK Department of Health
– MHRA (Medicines and Healthcare products Regulatory Agency)
• St George’s Hospital, London
• Perform independent evaluations of CT scanners
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AAPM 2004
Comparison of image quality and dose
•
•
•
•
Issues of comparison
Test methods
Scanning approach
Dealing with the data
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How can we compare image quality?
?
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AAPM 2004
Image quality comparison
• Defining image quality
– perception or numbers
• Image quality descriptors
– Image noise
– Spatial resolution (scan plane)
speckle
sharpness
– Image thickness (spatial resolution in z-axis)
– Contrast
– Artefacts
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Image quality and radiation dose
• Differences in image quality and dose
– Intrinsic differences in scanner design
– Due to scan protocol and scan parameters
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Intrinsic factors
• Scanner design
– Detectors
• material
• numbers, rows
• configuration
Whizzo CT
x-ray tube
filtration
FAD
– Data acquisition rates
– Software corrections
FDD
• e.g. beam hardening
– X-ray tube
• filtration
• focal spot
detectors
– Geometry
• focus-axis, focus-detector distances
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AAPM 2004
Scan protocols
• Clinical application
– X-ray filter, software corrections (eg beam hardening)
• Scan and reconstruction parameters
– X-ray tube
• current, voltage, focal spot
–
–
–
–
–
–
–
Scan time
Field of view
Reconstruction algorithm
Image slice thickness
Collimation width
No. slices (→ detector acquisition width)
Helical pitch and interpolation algorithm
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Comparison of image quality and dose
•
•
•
•
Issues of comparison
Test methods
Scanning approach
Dealing with the data
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Image quality descriptors
• Standard test methods
– Noise
– Resolution
– Image slice width
imaged slice
sharp
image
MTF (%)
100
smooth
image
50
fwhm
10
aluminium
ramps
spatial frequency (cm-1)
Std. Dev. σ
fwhm of image profile
Average MTF50 and MTF10
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AAPM 2004
Image quality – helical scanning
• Noise
– Same test
• Spatial resolution
– Same test
• Image width
tungsten or gold disc, 0.05 mm
– Use helical test tool → fwhm
CT Numbers
200
2.5mm
150
100
z-axis
fwhm
50
0
-10 -8
-6
-4
-2
0
2
4
6
8
10
Z-axis position of image (mm) 11
AAPM 2004
Radiation Dose (CTDIw, CTDIvol)
• CTDIw = dose averaged in scan plane
• CTDIvol = CTDIw / Pitch
= dose averaged in scan plane and z-axis
ie volume averaged CTDI
Pitch 2
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Comparison of image quality and dose
•
•
•
•
Issues of comparison
Test methods
Scanning approach
Dealing with the data
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Scanning approach
• Start with same clinical protocol on each scanner
(e.g. head, abdomen etc)
– minimise built in factors such as beam hardening
• Standardise scan parameters
– to minimise their effect on image quality and dose
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AAPM 2004
Dependence on scan settings
Image
width
Noise
Scan plane
resolution
Dose
mA
kV
Focal spot selection
Scan time
^
Nominal image width
Beam collimation
*
Detector group width
Convolution kernel
Pitch
^^
^^
^^
Interpolation algorithm
^ If scan time affects no. samples, ^^ In some circumstances , * In almost all cases15
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Standardise scan parameters
• Noise and dose dependent on kV
• Extent depends on filtration
Noise Versus kV
2.00
Hard
Beam
Scanner
A
to 120 kV
for each
scanner
Noise
normalised
Scanner
Soft
BeamB
1.50
1.00
0.50
80
100
120
140
kV
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AAPM 2004
Standardise scan parameters
• Dose dependent on collimation (beam width)
– Different doses (eg 4 mm, 16 mm)
– Same image width and noise (eg 1 x 4 mm, 4 x 4 mm)
• Scan with same collimation
Relative CTDI (4 and 16 slice)
2.4
same image width, same noise
relative CTDI
2.2
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0
5
10
15
20
collimation
25
30
3517
AAPM 2004
Comparison of image quality and dose
•
•
•
•
Issues of comparison
Test methods
Scanning approach
Dealing with the data
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AAPM 2004
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Comparison of image quality and dose
• Scanner A and Scanner B
– Head protocol
– Standardised scan parameters
• kV, collimation, nominal image width, focal spot, scan time
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AAPM 2004
Comparison of image quality and dose
• Image quality and dose parameters all different
Noise
Resolution^
A
7.6 HU
5.4 c/cm
4.5 mm
28 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
AAPM 2004
Image width
Dose
20
^resolution = average 50% and 10% MTF
Dealing with the data
• Correct for known parameter relationships
– ie noise and image width, noise and dose (mAs)
noise ∝
1
image width
noise ∝
1
mAs
• Obtain trend data for other relationships
– noise and resolution
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Correct for known parameter relationships
• Correct noise for same image width and dose
Noise
Resolution
Image width
A
7.6 HU
5.4 c/cm
4.5 mm
Dose
28 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
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AAPM 2004
^resolution average 50% and 10% MTF
Correct for known parameter relationships
• Correct noise for same image width and dose
Noise
Resolution
Image width
A
7.6 HU
5.4 c/cm
4.5 mm
Dose
28 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
A
6.0 HU
5.4 c/cm
5.0 mm
40 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
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^resolution average 50% and 10% MTF
Obtain trend data for other relationships
• Noise and resolution both dependent on recon. filter
• Scan noise and resolution test objects
• Reconstruct with the range of filters
–
–
–
–
GE LightSpeed: soft, standard, lung, detail, bone, edge
Siemens Sensation: AH/AB..10,20,30,40,50,60,70
Toshiba Aquilion: FC 20,21….,FC 10,11……..
Philips Mx8000/Brilliance: A,EB,EC,B,C,D
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Noise and resolution with reconstruction filter
noise ( %σ ) at 40 mGy
and 5 mm
image
% noise
for 40 width
mGy
10.0
noise and resolution
Siemens S4
images reconstructed
with different filters
1.0
0.1
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Average (MTF50, MTF10)
Average of MTF
50 and MTF10 (cycles/cm)
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AAPM 2004
Noise and resolution with reconstruction filter
noise ( %σ ) at 40 mGy
Noise at 40 mGy (% SD)
and 5 mm image width
10.0
noise and resolution
images reconstructed
with different filters
noise = resolution
1.0
Scanner A
Scanner B
0.1
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Average
MTF
and10MTF
/ cm)
MeanofMTF
, MTF
(cycles
per cm)
5050
10 (cycles
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AAPM 2004
Comparison of image quality and dose
• Standard resolution, image width and dose
Noise
Resolution
Image width
Dose
A
7.6 HU
5.4 c/cm
4.5 mm
28 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
A
7.8 HU
5.9 c/cm
5.0 mm
40 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
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AAPM 2004
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^resolution average 50% and 10% MTF
9
Comparison of image quality and dose
• Standard noise, resolution and image width
Noise
Resolution
Image width
A
7.6 HU
5.4 c/cm
4.5 mm
Dose
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
A
5.4 HU
5.9 c/cm
5.0 mm
83 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
28 mGy
28
8
^resolution average 50% and 10% MTF
AAPM 2004
Is it always so easy ?
10.0
A
B
noise ( %σ ) at 40 mGy
and
5 mm
Noise
at image
40 mGywidth
(% SD)
5
C
D
1.0
sometimes there is
no localised trend
0.25
0.1
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Mean MTF50, MTF10 (cycles per cm)
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AAPM 2004
Defining the trend
10.0
Siemens
power S4
=
2
2.9
R = 0.9942
2
R = 0.9755
2
R = 0.9411
2
R = 0.9972
Philips
Mx8000
power
= 2.6
powerAquilion
= 2.3
Toshiba
% noise for 40 mGy
noise ( %σ ) at 40 mGy
and 5 mm image width
5
noise ∝
power
= 2.7
GE
LightSpeed
Plus
1
dose.imagewidth
1.0
a
a
noise
noise α∝ resolution
resolution
dose × image width
where a is ~ 2.7 for this protocol
0.25
where a is a
~ 2.7 for this protocol
0.1
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
Average (MTF50, MTF10)
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Defining the trend
• Rodney Brookes and Giovanni di-Chiro (1976)
– Statistical limitations in x-ray reconstructive tomography
• Medical Physics Vol 3, No 4 July 1976
• Riederer S.J., Pelc N.J. and Chesler D.A. (1978)
– The Noise Power Spectrum in Computed Tomography
• Physics in Medicine and Biology 1978 23(3), 446-454
limiting resolution
σ ∝ f
1.5
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AAPM 2004
Can we use a single number ?
• Noise, image width, dose and resolution
σ∝
1
zD
σ ∝
f
a
σ∝f
zD
a
• Q factor
– Image quality relative to dose
Q=
f
σ
1.5
zD
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AAPM 2004
Comparison of image quality and dose
• Q value
Noise
Resolution
Image width
Dose
Q
A
7.6 HU
5.4 c/cm
4.5 mm
28 mGy
4.7
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
5.9
A
7.8 HU
5.9 c/cm
5.0 mm
40 mGy
B
5.4 HU
5.9 c/cm
5.0 mm
40 mGy
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AAPM 2004
^resolution average 50% and 10% MTF
11
Comparison of image quality and dose
• Start with typical clinical protocol
• Standardise scan parameters where possible
• kV, collimation, nominal image width, focal spot, scan time
• Dealing with the data
– Correct noise for known parameter relationships
• Noise and dose, noise and image width
– Obtain trend data for other relationships
• Noise and resolution
– Use graph as the comparison
– Correct values to give single value of noise
– Calculate a single quality performance parameter
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AAPM 2004
Limitations of approach
• Numerical, not perceived image quality
• Standard image quality parameters
– Single number does not represent complete IQ
– Full descriptors of image quality
– Noise power spectrum
– Complete MTF curve
– Complete z-sensitivity profile
• Helical pitch and reconstruction techniques
– Noise and z-sensitivity dependence on pitch not always
easy to define
– Shape of z-sensitivity profile affected
• Artefacts
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AAPM 2004
Comparison of CT Scanner Image Noise,
Image Width, Dose and Spatial Resolution
Using Standard Test Methods
Sue Edyvean, Nicholas Keat
ImPACT (Imaging Performance assessment of CT Scanners)
London
UK
www.impactscan.org
ImPACT
36
AAPM 2004
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