8/1/2013
2013 Annual Meeting of the American Association of Physicists in Medicine
Indianapolis, Indiana
Measurements and Indices in CT Dose
Disclosures Paid Consultant to:
Varian Imaging Systems
Alston and Bird LLC
CardioInsight
DXray
Royalty Income from:
Lippincott Williams and Wilkins
Samsung Corporation
John M. Boone
Professor and Vice Chair (Research) of Radiology
Professor of Biomedical Engineering
University of California Davis Medical Center
Sacramento, California
Research Funding from:
Stanford Research Institute
University of Pittsburgh
Siemens Medical Systems
Hologic Corporation
National Institutes of Health (NIBIB)
A brief history of the….
SB‐1237
TG‐233
TG‐220
2013
TG‐200
2010
2009 Epilation caused by CT
2007 Adaptive Dose Collimation
2006 Flying focal spot
2004 Dual Source CT
2000 64 slice CT
2000 8 to 40 slice CT
1997 Four Slice CT
1995 Mega HU x‐ray tubes
1994 mA Modulation
1992 Dual Slice CT (again)
Phantoms and radiation meters
1989 Spiral CT
Dose and CT scan length
1974 First 4rth Generation CDT
Dose dependency on patient size
1972 First Brain CT
Introduction & Historical CT Dose Metrics
1974
Measurements and Indices in CT Dose
First Whole Body CT
Evolution of CT Scanners and Dosimetry
ICRU extension to AAPM Report 111
Summary
CTDI
CTDIW
CTDIFDA
CTDIVOL
TG‐111
TG‐204
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CTDI ‐ based Dose Metrics
CTDI:
Computed Tomography Dose Index
32 cm diameter PMMA
100 mm pencil chamber
16 cm diameter PMMA
The Tools…….
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CTDI ‐ based Dose Metrics
CTDI100 peripheral
CTDI100 center
The Methods…….
Measuring CTDI100 in the real world
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CTDI ‐ based Dose Metrics
⅔ × CTDI100 peripheral
+ ⅓ × CTDI100 center
dose
weighted CTDI, CTDIw
dose = 1 / pitch
‐Z
Z=0
+Z
position
Volume CTDI, CTDIvol = CTDIw / pitch
The Mechanics…….
W Leitz, et al., CT dose assessment – a practical approach, Radiat Prot Dosim 1995; 57: 377
Which scan has more “dose”?
Dose Length Product (DLP):
L
L
…to first order, the dose is the same
DLP = CTDIvol × L (mGy • cm)
L
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DLP is related to the total energy deposited in the patient
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Effective Dose per DLP
(AAPM TG-96)
Region of Body
Head and neck
Head
Neck
Chest
Abdomen / pelvis
trunk
CTDI ‐ based Dose Metrics
k (mSv/[mGy‐cm])
0.0031
0.0021
0.0059
0.014
0.015
0.015
The Tools
The Methods
Effective Dose ≈ DLP × k
CTDI100 (center & peripheral)
CTDIw
The Metrics
CTDIvol
DLP
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CTDI is a good measure of CT dose to a large plastic phantom, but is not a stand‐alone metric for patient dose
Med Phys 2003
Measurements and Indices in CT Dose
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
etcetera….
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Dose Dependency on patient size
 = 1.19
32 cm
47”
waistline
120 cm
85% bigger by mass
 = 1.19
28 cm
≈
35”
waistline
88 cm
UC Davis Data
32 cm PMMA phantom
1138 cm2
 = 1.0
average patient
615 cm2
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Relative dose
CTDIvol
effective diameter
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TG‐204 Approach
Four Independent Research Groups Studied Size‐dependent CT Dose Keith Straus and Tom Toth
Mike McNitt‐Gray, et al.
Hong Zhou and John Boone
CTDIvol is indicated on most scanners…..
Conversion Factor
32 cm 120 kV
Cynthia McCollough et al.
CTDIvol
mGy
13.2
water‐equivalent diameter (cm)
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Size‐Specific Dose Estimate (SSDE)
SSDE conversion factor
CTDIvol (mGy) ×
air kerma
f
= SSDE (mGy) absorbed dose
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Measurements and Indices in CT Dose
Problems with CTDIvol
15 cm phantom
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
32 cm diameter PMMA phantom
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
10 cm scan length
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How long are the scatter tails in CT? 62%
10 cm
L
JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
How long are the scatter tails in CT? ~130 mm
10%
How long are the scatter tails in CT? 400 mm
80%
98%
10%
1%
JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
1%
JM Boone, Dose spread functions in computed tomography: A Monte Carlo study”, Med Phys 36, 4547-4554 (2009)
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Dose profiles as a function of Scan Length
Do(L)
Equilibrium Dose as a function of Scan Length
Deq
scan length
Integrating thimble chamber
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TG‐111 Method
TG‐111 Method
TG‐111 Method
TG‐111 Method
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Measurements and Indices in CT Dose
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
S
P

P
Summary
= scatter / primary
section A
section B
section C
ICRU / AAPM (TG‐200)
Dosimetry Phantom
Phantom is polyethylene 60 cm long by 30 cm in diameter
Each section is 20 cm long and weighs 13.7 kg (30 lbs) phantoms
compared to 32 cm diameter PMMA: 14.4 kg (5% lighter)
Total phantom 41.1 kg (90 lbs)
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The thimble chamber with signal
real time (>1000 Hz) readout rates
18 mm*
real time probes
time
*active length
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Measurements and Indices in CT Dose
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
ICRU Method
real time thimble chamber
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beam profile
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Measurements and Indices in CT Dose
ICRU method
Introduction & Historical CT Dose Metrics
Dose dependency on patient size
TG‐111 method
Dose and CT scan length
Phantoms and radiation meters
ICRU extension to AAPM Report 111
Summary
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Measurements and Indices in CT Dose
Measurements and Indices in CT Dose
CTDI‐based methods need to be updated
TG‐204/ICRU
SSDE is a method for adjusting for patient size
TG‐111/ICRU
Scan length dose dependencies h(L)
TG‐200/ICRU
Longer phantoms and faster radiation meters
TG‐220
Methods for automatic size detection
ICRU CT Report Available Q3 2013
John M. Boone
Professor and Vice Chairman of Radiology
Professor of Biomedical Engineering
University of California Medical Center
Sacramento, California
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