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John M. Boone, Ph.D., FAAPM, FSBI, FACR
Professor and Vice Chair (Research) of Radiology Professor of Biomedical Engineering
University of California Davis Medical Center Sacramento, California
Patient Dosimetry in CT: what to measure and estimate;
why and how?
2013 ICTP/IAEA Training Course on Radiation Protection of Patients Trieste
Disclosures Paid Consultant to:
Varian Imaging Systems
Alston and Bird LLC
CardioInsight
DXray
Royalty Income from:
Lippincott Williams and Wilkins
Samsung Corporation
Research Funding from:
Stanford Research Institute
University of Pittsburgh
Siemens Medical Systems
Hologic Corporation
National Institutes of Health (NIBIB)
Trieste 15 Sept 2013
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
5
80 x 80 matrix 8 grey scale (3 bit images)
EMI Mark 1: 1972
water bag for bolus 4 minute scan (120 sec/image) overnight reconstruction
Images courtesy ImPact Website (UK)
6
Modern helical multi-slice CT
7
8
axial coronal sagittal
high resolution three dimensional imaging
9
Allan M. Cormack
Godfrey N. Hounsfield
Nobel Prize for Medicine 1979
10
Helical CT
Multi-slice CT
technology advancements have led to greater CT use
531 mm
331 mm 200 mm
CT x-ray tube output
GE VCT 120 kV 32.3 mGy/100 mAs @ IC 600 mAs 1/6 rotation 432 mGy @ isocenter ISL = 2.57 1.1 Gy @ skin
~60°
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
Firs
t B
rain
CT
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ho
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T
Firs
t 4
rth
Gen
erat
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CD
T
Spir
al C
T
Du
al S
lice
CT
(aga
in)
mA
Mo
du
lati
on
Meg
a H
U x
-ray
tu
be
s
Fou
r Sl
ice
CT
8 t
o 4
0 s
lice
CT
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slic
e C
T
Du
al S
ou
rce
CT
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foca
l sp
ot
Ad
apti
ve D
ose
Co
llim
atio
n
Epila
tio
n c
ause
d b
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Evolution of CT Scanners and Dosimetry
CTDIFDA
CTDIW CTDIVOL
TG-204
TG-111 CTDI
TG-233
SB-1237
TG-200
TG-220
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72
19
74
19
74
19
89
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92
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97
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00
20
00
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07
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09
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10
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13
A brief history of the….
15
CTDI:
Computed Tomography Dose Index
16
The solid state CT detector
T
n=16
0.625 mm
nT = 16 × 0.625 mm = 10 mm
17
cone angle
fan angle
z-axis
Multiple Detector CT
Many detector arrays along z nT is the width of the x-ray beam at IC
nT
1( )CTDI D z dz
nT
do
se
position
Z=0 -Z +Z
1( )CTDI D z dz
nT
( )D z
CTDI
50
100
50
1( )
mm
mm
CTDI D z dznT
position
do
se
Z=0 -Z +Z
50
100
50
1( )
mm
mm
CTDI D z dznT
100 mm
position
do
se
Z=0 -Z +Z
50
100
50
1( )
mm
mm
CTDI D z dznT
100 mm
This equation requires table movement, either axial or helical CT scans which traverse 100 mm It is not valid for a stationary table scan such as with CT perfusion or CT fluoroscopy
50
100
50
1( )
mm
mm
CTDI D z dznT
100 mm
position
do
se
Z=0 -Z +Z
100 mm
CTDI - based Dose Metrics
32 cm diameter PMMA
16 cm diameter PMMA
100 mm pencil chamber
The Tools…….
Adult Body Phantom
Head Phantom
Peds Body Phantom
CTDI - based Dose Metrics
The Methods…….
CTDI100 peripheral
CTDI100 center
26 Measuring CTDI100 in the real world
The CTDI “Head” Phantom in position for CTDI100 measurement
The CTDI “Body” Phantom in position for CTDI100 measurement
CTDI - based Dose Metrics
The Mechanics…….
CTDI100 peripheral
CTDI100 center + ⅓ ×
⅔ ×
weighted CTDI, CTDIw
dose = 1 / pitch
Volume CTDI, CTDIvol = CTDIw / pitch
W Leitz, et al., CT dose assessment – a practical approach, Radiat Prot Dosim 1995; 57: 377
CTDI - based Dose Metrics
The CTDIw is the planar average dose to the central slice of the phantom in a series of slices spanning 100 mm
30
L
Which scan has more “dose”?
L
…to first order, the dose is the same
31
L
DLP = CTDIvol × L (mGy • cm)
Dose Length Product (DLP):
DLP is related to the total energy deposited in the patient
32
Effective Dose ≈ DLP × k
Effective Dose per DLP
(AAPM TG-96)
Region of Body
Head and neck
Head
Neck
Chest
Abdomen / pelvis
trunk
k (mSv/[mGy-cm])
0.0031
0.0021
0.0059
0.014
0.015
0.015
33
dose length product (mGy-cm)
The CT scanners now have a dose report that state the DLP. One can use these values with the conversion factors above to estimate the effective dose for the patient
Effective Dose per DLP
CTDI - based Dose Metrics Free-in-air measurement (no phantom)
CTDIfree-in-air or CTDIair
CTDI - based Dose Metrics
CTDI100 (center & peripheral)
The Tools
CTDIw
CTDIvol
DLP
The Metrics
The Methods
CTDIair
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
CTDI - based Dose Metrics
average peripheral measurement
effect of table attenuation
50
100
50
1( )
mm
mm
CTDI D z dznT
mGy mm
c mGy × 100 mm = 100 c mGy.mm
For a chamber reading out in air kerma
100cK L
CTDIn T
kerma reading chamber length
CTDI - based Dose Metrics
100 mm
1507.3 mR
corrections
1. exposure to kerma correction 1507.3 mR / (114.5 mR/mGy) = 13.16 mGy
2. chamber reading air kerma 3. correction for nT
13.16 mGy x 100 mm = 1316 mGy.mm
1316 mGy.mm / 20 = 65.8 mGy (CTDI100)
nT = 20 mm (e.g.)
data are approximated beam
detected e =
detected
beam
penumbra
detector arrays
CTDI - based Dose Metrics
Dependency on collimated beam width (nT)
CTDI - based Dose Metrics
Dependency on collimated beam width (nT) (but MDCT has reduced use of this)
thin collimation thick collimation
impactscan.org (UK)
Sue Edyvean
Why
What
How: conventional CT systems
How: cone beam CDT systems (stationary table) IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
Toshiba Aquillion 1: 320 x 0.5 mm = 160 mm
do
se
position
Z=0 -Z +Z
1( )CTDI D z dz
nT
50
100
50
1( )
mm
mm
CTDI D z dznT
100CTDI
CTDIe
CTDI - based Dose Metrics
100 mm
1507.3 mR
cone beam CT
nT > 100 mm
100CTDI
CTDIe
100CTDI
CTDIe
do
se
position
Z=0 -Z +Z
do
se
position
Z=0 -Z +Z
,( )
1
mc
free in air n T i
i
LCTDI D
n T
do
se
position
Z=0 -Z +Z
, ( 20 )c
free in air REF eg mm
K LCTDI
n T
50
,
100,
,50
1( )
( )
mm
free in air n T
REF
REF free in air REFmm
CTDICTDI D z dz
n T CTDI
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
do
se
position
Z=0 -Z +Z
100 mm ion chamber
18 mm thimble chamber
CT perfusion (stationary table)
do
se
position
Z=0 -Z +Z
CTDIw
peak dose
Stationary table perfusion CT (j rotations)
dose = peak dose × j rotations
position
do
se
Z=0 -Z +Z
100 mm
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
Measure versus Estimate
In general, it is best to say that doses are estimated, and not
calculated or measured. This conveys the proper notion that
CT dosimetry is an imprecise science, which it is.
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
62
CTDI100mm
center periphery
CTDIw = 1/3 CTDIcenter + 2/3 CTDIperiphery
CTDIvol = CTDIw / pitch
DLP = CTDIvol x length irradiated
dose length product (mGy-cm)
measure CTDI100 [center]
measure CTDI100 [periphery]
The phantom is either a 16 cm diameter acrylic (head) or a 32 cm diameter (body) phantom
63
CTDI is a good measure of CT dose to a large plastic
phantom, but is not a stand-alone metric for patient dose
etcetera….
Med Phys 2003
Why
What
How: conventional CT systems
How: cone beam CDT systems IEC methods
Other approaches
Measure versus Estimate
Summary
Patient Dosimetry in CT: what to measure and estimate; why and how?
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