ct dose estimation
TRANSCRIPT
1
John M. Boone, Ph.D., FAAPM, FSBI, FACR
Professor and Vice Chair of Radiology
University of California Davis Medical Center
CT Dose Estimation
2
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
3
Introduction
4
Introduction
5
The radiation from CT has caught the eye of the public
6
California SB-1237, SB-58, AB-510
Dose reporting required as of July 1, 2012
Mad River Hospital
Cedars-Sinai
Alex Padilla
7
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
8
CTDI:
Computed Tomography Dose Index
where it all began….
Tom Shope
9
CTDIL =
-L/2
+L/2
1
nT ∫ D(z) dz
100 mm
CTDI100 =
-50mm
+50mm
1
nT ∫ D(z) dz
10
Phantoms
100 mm long ion
chamber
CTDI: Measurement tools
11
32 cm body PMMA phantom
16 cm head PMMA phantom
also, pediatric body phantom
100 mm pencil chamber
PMMA plug
peripheral hole
center hole
12 Measuring CTDI100 in the real world
13
32.1 mGy
14
100 mm pencil chamber
and thin CT x-ray beam
100 mm CT x-ray beam
and small radiation meter
100 mm
15
center CTDI100 periphery CTDI100
Weighted CTDI:
CTDIw = ⅓ × CTDI100 + ⅔ × CTDI100
center periphery
16
helical CT acquisition
low pitch high pitch
Dose 1
pitch
18
Weighted CTDI:
Volume CTDI:
CTDIvol =
CTDIw
pitch
CTDIw = ⅓ × CTDI100 + ⅔ × CTDI100
center periphery
19
CTDIvol is a useful parameter
because it is required to be
displayable on all modern CT
scanners due to IEC
regulations. It is also stored in
the header of the CT images.
Siemens
General Electric
20
CTDI is a good measure of CT x-ray tube radiation output,
but is not a stand-alone metric for patient dose
etcetera….
r = 1.19
47”
119 cm waistline
28 cm
34”
86 cm waistline
32 cm
typical patient
CTDI body phantom
CTDI is a good measure of dose to
a 32 cm diameter, 1.19 g/cm3 piece
of plastic. Most patients will be
smaller, and have higher doses.
Larger patients will have lower
doses (at the same techniques)
AAPM Report 204
5.40 mGy = CTDIvol
(32 cm phantom)
water equivalent
diameter
circle of equal area
(weighted)
24
Size-specific dose estimate (SSDE)
To perform patient size-specific
dose estimation, the conversion
factor above is multiplied by the
value of CTDIvol.
Dosepatient CTDIvol × f
SSDE
5.40 mGy = CTDIvol (32 cm phantom)
SSDE = CTDIvol × f
13.0 mGy = 5.40 × 2.4
26
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
27
L
Which scan has more “dose”?
L
…to first order, the dose is the same
D = De
Dm
28
L
DLP = CTDIvol × L (mGy • cm)
Dose Length Product (DLP):
DLP is related to the total energy deposited in the patient
a c
x-ray beam profile along z
f(z) DL(z)
L
b
-L/2 +L/2
DL(z) -L/2 +L/2
Dose profiles as a function of Scan Length
scan length DL(z
)
Equilibrium Dose as a function of Scan Length
Deq
D(L
)
TG-111 Method
TG-111 Method
TG-111 Method
L
AAP Task Group 200
& ICRU Phantom
39
ICRU Method: Real Time X-ray Meter
40 beam profile [ × RECT ]
41
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
42
L
DLP = CTDIvol × L (mGy • cm)
Dose Length Product (DLP):
DLP is related to the total energy deposited in the patient
43
Most modern dosimetry is Monte Carlo based
=
person mathematical model
of person
44
CT geometry is modeled and organ doses are estimated
45
Gonads
Bone marrow
Colon
Lung
Stomach
Bladder
Breast
Liver
Esophagus
Thyroid
Skin
Bone surface
Brain
Salivary Glands
remainder
0.08
0.12
0.12
0.12
0.12
0.04
0.12
0.04
0.04
0.04
0.01
0.01
0.01
0.01
0.12
Organ dose wt E
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
Y
YY mSv
absorbed
effective dose
organ doses computed
tissue weighting factors from ICRP 103
× =
46
other minor details
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
kerma
mGy
K
47
other minor details
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
units
quantities
kerma
mGy
K
48
other minor details
CTDIvol is the air kerma
measured inside the phantom
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
kerma
mGy
K
49
other minor details
Incident kerma leads to absorbed dose
estimate through the Monte Carlo process
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
kerma
mGy
K
50
other minor details
correction for different types of radiation
such as a, b, n: wr for x-rays is 1.0
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
kerma
mGy
K
51
other minor details
corrects for organ radiosensitivity using
tissue weighting factors, wt.
equivalent dose effective dose
mSv mSv
absorbed dose
mGy
D D × wr = H H × wt = E
kerma
mGy
K
abdomen
E = 0.015 × DLP
Effective dose (mSv) and dose length product (mGy.cm) for abdomen CT and the clinical
indication: rule out abscess.
The conversion factor is (1/59.1 =) 0.017 mSv / (mGy.cm)
Included, data from 46 scanners, 21 single slice, 25 multislice
Effective dose calculated with the ImPACT CT Dosimetry calculator
The short-cut calculation of E (abdomen)
53
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
DLP × k E
54
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
55
WORKFLOW in RADIOLOGY
All images are digital (on PACS)
Radiologists look at images on computers
As they look at the images, they dictate their
“findings” using voice recognition software
In California, CTDIvol & DLP need to be included
into the Radiologist’s Report
EXAM DATE [5/24/2012 12:35 pm]
INDICATION:
[]
DOSE:
[Dose information for this CT examination:
Series 1: (no contrast)
CTDIvol = 12.4 mGy
DLP = 496 mGy-cm
Diameter = 28.2 cm
SSDE = 16.1 mGy
Series 2: (with contrast)
CTDIvol = 13.2 mGy
DLP = 577 mGy-cm
Diameter = 28.2 cm
SSDE = 17.2
UC Davis Health System CT scanners ……..low dose CT practice.]
FINDINGS:
[]
IMPRESSION:
[]
Report: Gaga, Lady – MRN: 1234567
text and
format setup
by institution
The Structured Dose Report with Radimetrics Feed:
57
Institutional CT dose assessment
UC DOSE: Dose Monioring Software
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Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation
CTDIvol: (mGy)
Is a measurement of “dose” to a large plastic phantom, and for
normal size patients CTDIvol underestimates their dose. Should
not be considered as the “patient dose”
Uses two reference phantoms, 16 cm and 32 cm, depending on
scan protocol. The 16 cm reference phantom is used for all
heads and for some pediatric bodies (depends on scanner)
DLP: (mGy cm)
Includes CTDIvol and scan length
Scales linearly with effective dose
Summary
60
Size Specific Dose Estimate (SSDE)
Is an automatic way in which to correct for patient
body size, to produce a more accurate dose
estimate.
Effective Dose (E)
Is not a physical dose metric, but rather embodies
organ radio-sensitivities
Is controversial to use for specific patients
Summary
61
Introduction
The CTDI Family of Metrics
Dose Length Product
Effective Dose
Dose Reporting
Summary
CT Dose Estimation