5/3/2011
Updating Image Quality
and Dose Metrics in CT
Updating Image Quality
and Dose Metrics in CT
John M. Boone, Ph.D., FAAPM, FSBI, FACR
Professor and Vice Chair of Radiology
Professor of Biomedical Engineering
University of California Davis Medical Center
John M. Boone, Ph.D., FAAPM, FSBI, FACR
Chair, AAPM Science Council
Member TG-111
Member TG-200
Co-chair TG-204
Chair, ICRU Committee on CT
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Corporate Disclosures (required by UC Davis):
Updating Image Quality
and Dose Metrics in CT
• Varian Imaging Systems, Consultant
• Artemis, Consultant
• Varian Imaging Systems, Research Funding
• Hologic Corporation, Research Funding
AAPM Report-96
ICRU & AAPM TG-200
ICRU & AAPM TG-111
ICRU & AAPM TG-204
ICRU
• Fuji Medical Systems, Research Funding
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
• Stanford Research Institute, Research Funding (R21 subcontract)
• Creativ Microtech, Research Funding (R21 subcontract)
Acknowledgements:
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California BCRP 7EB-0075
California BCRP 11I-0114
R21 CA89260
R01 EB002138- (BRP)
R01 CA089260R01 CA012955Susan G. Komen Foundation
University of Pittsburgh
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5/3/2011
Updating Image Quality
and Dose Metrics in CT
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
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Evolution of CT Scanners and Dosimetry
Use of CT: USA & UC Davis Trends
1989: Helical/Spiral CT
1972: First
clinical CT brain
scan
1998: Multi-Slice CT
1974: First wholebody
CT scanner
1981:
CTDI
1990: Helical CT
1974: 4th
1984:
generation
CT
CTDI
FDA
1970
Boone, J M et al J Am Col Radiology, 2008;5(2): 132–138
Brenner, D J et New Eng J Med, 2007;357: 2277-2284
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1980
2006: Dual
Source CT
1992: Dual Slice CT
1995:
CTDI100
1994: mA modulation
1997: 4 Slice CT
1995: CTDIw
2000: 8-40 Slice CT
1999:
CTDIvol
2000: 64 Slice CT
1990
2007: Adaptive
Dose Shield
2004: Flying
2010: Focal
TG111
Spot
2011: TG200
2011: TG204
2000
2010
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CTDI100 Dose Metrics and Its Derivatives
Updating Image Quality
and Dose Metrics in CT
CTDI100
(peripheral)
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
CTDI100 (center)
16 and 32 cm
diameter PMMA
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CTDIw =
1
3
CTDI100,center +
2
3
10
r = 1.19
CTDI100,periphery
47”
32 cm
waistline
119 cm
CTDIvol = CTDIw / pitch
Dose Length Product (DLP) = L × CTDIvol
Effective Dose ≈ DLP × k
scan location
k
Head
0.0021
Chest
0.014
Body
0.015
Abd-Pelvis
0.015
Pelvis
0.015
≈
11
34”
28 cm
waistline
86 cm
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5/3/2011
CTDI100 Dose Metrics and Its Derivatives
Updating Image Quality
and Dose Metrics in CT
15 cm
10 cm
CT dosimetry probe
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
40 cm
CT dosimetry phantom
CT scatter tails
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13.4 kg
29.5 lb
20 cm
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TG-200
30 cm
60 cm
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5/3/2011
section A
noise power spectrum (NPS)
section B
section C
modulation transfer function (MTF)
dosimetry
three sections of the TG-200/ICRU phantom
An Integrated CT Image Quality / Dosimetry Phantom
section A
section B
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section C
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section A
section B
section C
NPS Section
MTF insert
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Previous Era of CT
Modulation Transfer Function Assessment in CT
oversampled slit
LSF
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Modulation Transfer Function Assessment in CT
MTF
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Traditional method for noise / low contrast
detectability assessment in CT
effect of kernel
effect of slice thickness
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Noise Power Spectra Assessment of Noise in CT
step 1:
120 kV
400 mAs
Pitch ≈ 1.0
step 2:
Read out dose
adjust mAs
20 mGy
step 3:
step 4:
120 kV
xxx mAs
Pitch ≈ 1.0
Measure
NPS(f)
Noise Power Spectra Assessment of Noise in CT
2D NPS
3D NPS
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Noise Power Spectra Assessment of Noise in CT
region of interest
2D Fourier
DFT
1

N
Transform
N
N N P S 2 D (u , v ) 
i 1
2D
[D Ii ( x, y )  D Ii
2
2
x y
sagittal
Nx Ny
RAMP
apodizing
filter
CT image of phantom
2D Noise Power Spectrum
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f=fN
f=0
f=fN
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Noise Power Spectra Assessment of Noise in CT
Noise Power Spectra Assessment of Noise in CT
effect of technique
effect of kernel
effect of slice thickness
2D NPS
1D NPS
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Noise Power Spectra (3D)
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Noise Power Spectra (3D)
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CT image quality evaluation
Old Era
New Era
phantom
complicated
basic

M TF ( f )
analysis
results

L SF ( x )
e
 2  ifx
dx



L SF ( x )
dx

simple
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
perfunctory
more sophisticated
useful & quantitative
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TG-111
Updating Image Quality
and Dose Metrics in CT
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
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c
a
b
Axial CT Scanning
L
x-ray beam profile along z
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f(z)
-L/2
DL(z)
+L/238
Dose profiles as a function of Scan Length
L
Helical CT Scanning
scan length
f(z)
-L/2
D(z)
+L/2 39
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D(L)
Equilibrium Dose as a function of Scan Length
TG-111 Method
Deq
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TG-111 Method
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TG-111 Method
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TG-111 Method
Extensions to TG-111 Concepts
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dose spread functions
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weighted bi-exponential
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h
P
= scatter / primary
S
P
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Real Time X-ray Probe
Dose Probe Isotropy
x-ray detector
voltage
electronics
photodiode
time
isocenter
fiber optic bundle
probe rotation
dose probe
0.2 – 1.0 ms
Gd2O2S scintillator
x-ray tube
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•
•
•
•
Projection irradiation @ 120 kVp/7 mA
τ=5s
N=4
Average % error = 0.80%
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ICRU Method
Dose Probe Linearity
• Projection irradiation @ 80 kVp and 120 kVp
• Varied tube current at a constant tube potential
r2=0.999
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Correction Factor
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beam profile
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Updating Image Quality
and Dose Metrics in CT
CTDIvol
Relative dose
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
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Example Case of Size Specific Dose Estimation
effective diameter
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Example Case of Size Specific Dose Estimation
• Pediatric patient scanned initially with a Siemens
scanner in outpatient clinic
• CareDose 4D used
• CTDIvol = 4.78
• Post-surgery, patient scanned in-patient GE
scanner using “Auto mA”
• GE auto mA used
• CTDIvol = 17.7
20.5 cm
30.4 cm
32 cm PMMA Dose
Reference Phantom
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16 cm PMMA Dose
Reference Phantom
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TG-204 Size conversion factors for CTDIvol
Uncorrected data from scanners:
17.7 mGy-cm / 4.78 mGy-cm ≈ 3.7× difference in CTDIvol
20.5 cm
30.4 cm
TG-204 SSDE Corrections:
17.7 mGy-cm (16 cm PMMA reference) x 0.71 ≈ 12.5 mGy-cm
4.8 mGy-cm (32 cm PMMA reference) x 1.47 ≈ 7.1 mGy-cm
20.5 cm
12.5 / 7.1 ≈ 1.7× difference in CTDIvol
30.4 cm
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CT Scanner Output Measures along z-axis
Updating Image Quality
and Dose Metrics in CT
Influence of beam width / collimation / penumbra
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
F(q)
F(z)
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CT Scanner Output Measures along z-axis
CT Scanner Output Measures versus Fan Angle
Influence of beam width / collimation / penumbra
Influence of Bow Tie Filter
D(z)
F(q)
F(z)
z
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CT Scanner Output Measures versus Fan Angle
Influence of Bow Tie Filter
Influence of Bow Tie Filter
signal
CT Scanner Output Measures versus Fan Angle
signal
time
time
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CT Scanner Output Measure
Updating Image Quality
and Dose Metrics in CT
F(q)
F(z)
F(z)
ICRU CT REPORT CHAPTER 4
F(q)
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
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Monte Carlo modeling should be
f(z)
f(q)
the basis for patient CT dosimetry
Real time air kerma probe
Monte Carlo
useful beam characterization
CT scan & patient
parameters
data needed for MC simulation
organ doses
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CT beam profile
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practical methods to correct dosimetry estimates for CT scan length
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practical methods to correct dosimetry estimates for patient size
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Updating Image Quality
and Dose Metrics in CT
Introduction
CTDI100-based metrics
Image Quality and CT Dosimetry Phantom
CT Dose versus Scan Length
Correction for Patient Size
CT Scanner Output
Summary
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