8/18/2011
Acknowledgements
Managing Pediatric CT Patient Doses
Marilyn Goske, MD
John Boone, PhD
Cynthia McCollough, PhD
Michael McNitt-Grey, PhD
Dianna Cody, PhD
Tom Toth, PhD
Mahesh Mahadevappa, PhD
G. Donald Frey, PhD
Keith J. Strauss, MSc, FAAPM, FACR
President
X-Ray Computations, Inc.
Boston, Massachusetts
MISSION STATEMENT
INTRODUCTION
A.
B.
C.
D.
E.
F.
G.
H.
Image Gently Campaign
Pediatric CT Public Health Concerns
Shortcomings of Dose Indices for CT
Affect of Patient Size on Dose Indices
Clinical Dilemma
Interim Solution: AAPM TG204
Applications of SSDE
Managing Pediatric CT Patient Doses
Alliance for Radiation Safety in Pediatric Imaging
is a coalition of health care organizations
dedicated to providing safe, high quality
pediatric imaging worldwide.
The primary objective is to
raise awareness in the imaging community of
the need to adjust radiation dose when
imaging children.
Adapted from Goske
1
8/18/2011
Does Practice Need to Change?
How much do we really understand?
Image Gently
A. The ultimate goal of the Alliance is
to accelerate the change of local
practice.
1.
2.
Problem?
Scientific observation to local
practice change ~ 17 years!1
1Greenberg
Under estimation by 75% of MDs!
SB. Trans Clin Climatol Assoc 119:2450261, 2008
Lee et al. Radiology. 2004; 231:393-398
Adapted from Goske
Does Practice Need to Change?
How much do we really understand?
Does Practice Need to Change?
How much do we really understand?
Which of the following provides a reasonable
estimate of a child‟s CT dose?
Which of the following provides a reasonable
estimate of a child‟s CT dose?
A.
B.
C.
D.
A.
B.
C.
D.
CTDIvol (mGy)
DLP (mGy-cm)
Effective Dose (mSv)
Values in DICOM Structured Report
E. None of the above!
CTDIvol (mGy)
DLP (mGy-cm)
Effective Dose (mSv)
Values in DICOM Structured Report
E. All of the above!
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8/18/2011
Critical Triad
Founding Organizations
The Society for Pediatric Radiology
To act together for radiation protection for children
American Society of Radiologic Technologists
American Association of Physicists in Medicine
American College of Radiology
Medical physicists
Adapted from Goske
Adapted from Goske
What is Image Gently
An education, awareness and advocacy campaign
To improve radiation protection for children
worldwide
Alliance for Radiation Safety in Pediatric Imaging
 58 health care organizations/agencies
800,000 radiologists,
radiology technologists,
medical physicists
worldwide
Adapted from Goske
Objectives
Decrease radiation dose in children
Positive message
..resulting dose to population will lead to
higher cancer rates, accounting for as many
as 2% of all cancers in the U.S.
Enroll key organizations
Increase awareness
educate
advocate
change practice
Adapted from Goske
3
8/18/2011
Use internal communication of
each member organization
Campaigns in
CT
CR / DR
Interventional Radiology
Flouroscopy
Nuclear medicine
Imagegently.org
Adapted from Goske
Adapted from Goske
PEDIATRIC CONSIDERATIONS
CLINICAL EDUCATIONAL MATERIALS
Abdomen
Baseline:
PA Thickness
(cm)
9
12
14
16
19
22
25
31
kVp
fill in
Approx
Age
newborn
1 yr
5 yr
10 yr
15 yr
small adult
med adult
large adult
mA
Time (sec)
fill in
fill in
Abdomen
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.43
#VALUE!
0.51
#VALUE!
0.59
#VALUE!
0.66
#VALUE!
0.76
#VALUE!
0.90
#VALUE!
1.0
fill in
1.27
#VALUE!
Abdomen
Baseline:
PA Thickness
(cm)
9
12
14
16
19
22
25
31
Pitch Abdomen
Pitch Thorax
fill in
fill in
Thorax
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.42
#VALUE!
0.49
#VALUE!
0.57
#VALUE!
0.64
#VALUE!
0.73
#VALUE!
0.82
#VALUE!
0.91
#VALUE!
1.16
#VALUE!
kVp
120
Approx
Age
newborn
1 yr
5 yr
10 yr
15 yr
small adult
med adult
large adult
IMAGE
GENTLY
BODY
mA
Time (sec)
400
1
Abdomen
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.43
172
0.51
204
0.59
236
0.66
264
0.76
304
0.90
360
1.0
400
1.27
508
Pitch Abdomen
Pitch Thorax
1.25
1.5
Thorax
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.42
202
0.49
235
0.57
274
0.64
307
0.73
350
0.82
394
0.91
437
1.16
557
PEDIATRIC CONSIDERATIONS
CLINICAL EDUCATIONAL MATERIALS
1. CTDIvol for Adults
a. < 25 mGy Body
b. < 75 mGy Head
2. Pediatric Patient Dose < Adult Dose
3. Conservatively high
4. Developed for adult department that
images children occasionally
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8/18/2011
Image Gently Vendor Summit
8 FREE online modules for CT technologists
August 20, 2008
CTDIvol is not an indication of patient dose
Pediatric Radiologists and Technologists
have been asking for a reasonable
estimate of CT radiation dose in
children for over 10 years.
You need to provide better training on
pediatric applications of your images!
Toshiba, GE Healthcare, Philips, Siemens
Adapted from Goske
TRAINING
Your Lender
Training is only as effective as the trainer‟s
understanding of their trainees
Each trainee may have a bit different
Your Buyer
Perspective!
Pediatric needs are
Different
Your Appraiser
YOUR HOUSE as
seen by...
Yourself
Your Tax Assessor
5
8/18/2011
Preliminary estimate of changes in
Medical radiation exposure to US population
US 1980*
Procedures vs Effective dose
contributions
Interventional
2%
US 2006
Other
0.06
CT
12%
Nuclear
Medicine
3%
Radiography &
Fluoroscopy*
19%
Interventional
12%
CT
46%
Interventional
0.4 mSv
Radiography
0.6 mSv
Medical
0.54
Nuclear Medicine
0.7 mSv
Natural
3.0 mSv
Natural
??
(3.0 mSv)
CT
1.5 mSv
Radiography &
Fluoroscopy*
83%
Nuclear
Medicine
23%
Percent Procedures
Medical 0.54 mSv per capita
Total 3.6 mSv per capita
Medical 3.2 mSv per capita
Total 6.2 mSv per capita
Effective Dose Contributions
17% of All Exams Deliver 81% of Total Effective dose
91% of Pediatric Dose in the ED comes from CT
* NCRP 93
Adapted from Mahesh
Adapted from Mahesh
GROWTH OF MEDICALRADIATION DOSE
C. General Radiography
1. 85% of dose
from exams
Pelvis/Hips
of Abdomen Upper GI
Pelvis/Hips, Spine
Abdomen
or Spine
Head Face
Chest
2. 280,000
IVP
person Sv
Barium enema
Extremities
3. ~ 0.62 mSv Mammography
Adapted from Mahesh
Number
millions
%
Collective
Person Sv
%
20
5
45,700
24.8
4
1
38,000
20.7
17
4
57,000
31.0
15
4
16,000
8.7
2.4
0.6
2,300
1.2
126
31
6,500
3.5
1.2
0.3
3,700
2.0
0.65
0.2
8,500
4.6
56
14
1,900
1.0
34
9
2,200
1.2
Dental
125
31
2,300
1.2
Total
~401
PEDIATRIC CONSIDERATIONS
A. Radiation Induced Cancer Lifetime Risk
From 1 Sv Dose
1. Average
a. 5% Males
b. 6% Females
2. First Decade
13 - 15%
3. Middle Age
2-3%
4. Children 3 – 5
times more sensitive
Adapted from Hall
~184,000
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PEDIATRIC CONSIDERATIONS
Radiation Risk based on Effective Dose?
A. Almen and S. Mattsson, J. Radiol. Prot. 16 (2), 81-89 (1996).
Somatic risk
%/Sv
Children
Aged 0-9
Hereditary risk
%/Sv
14.5
Total risk %/Sv
2.5
Real World ….
Radiation distribution crosses the imaged
volume
Peak dose
17
“Tails” of dose distribution
Children
Aged 10-19
8.5
2.5
11
Whole
population
5
1
6
- 5 mm
0
+ 5 mm
Adapted from Frey
CTDI = Integral under the
radiation dose profile along the z-axis
from a single axial scan of width nT.
radiation dose profile
nT
“nominal beam width”
=
“total nominal scan width”
Adapted from Frey
Z
CT SCANNER DOSE INDICES
B. Measurement of CT Radiation Dose
1. Plastic cylindrical phantoms: CTDI Phantoms
a. (PMMA)
b. 16 & 32 cm diameter
2. Pencil chamber moved into provided holes
to measure radiation dose
a. 1 cm under
peripheral
surface
PMMA
hole
b. Center of phantom
plug
center
c. Non measured
100 mm
hole
holes plugged
pencil
Adapted from TG204
chamber
7
8/18/2011
CTDIvol = [2 * surface dose / 3 + central dose / 3 ] / Pitch
CT SCANNER DOSE INDICES
C. Measure CTDIvol
1. Measure CTDIvol with identical scan
parameters
a.
b.
c.
d.
Measured
CTDIvol =
47
Measured
CTDIvol = 37
21.6 mGy
38 mGy
47 mGy
47 mGy
35 mGy
kVp
mA
Rotation time
Bow Tie Filter
2. Use phantom 10, 16, and 32 cm diameter
Measured
CTDIvol = 18
10.8 mGy
10 cm
Diameter
16 cm
Diameter
32 cm
Diameter
Measured CTDIvol increases 2.6 times as phantom size decreases!
8
8/18/2011
CT SCANNER DOSE INDICES
D. Displayed CTDIvol
1. Dose that represents distribution of dose
given to cross-sectional area of a slab of
the CTDI phantom (16 or 32 cm
diameter)
2. Reflects changes in:
a.
b.
c.
d.
e.
f.
High voltage to x-ray tube (kVp)
X-ray tube current (mA)
Rotation time (sec)
Bow tie filter shape, thickness, material
Pitch
Source to detector distance
CT SCANNER DOSE INDICES
D. Displayed CTDIvol
5. does not represent . . .
Patient dose!!
CT SCANNER DOSE INDICES
D. Displayed CTDIvol
3. Standardized method to estimate and
compare the radiation output of two
different CT scanners to same phantom.
4. Dose index of CT scanners if the fan beam
width in z direction of the patient is small
(< 1 cm)
a. AAPM TG111 addresses these
shortcomings
b. Beyond scope of this presentation.
CT SCANNER DOSE INDICES
E. Displayed Dose Length Product (DLP)
1. DLP (mGycm) = CTDIvol * Scan Length
a. Scan length is the length of phantom
irradiated.
b. „Represents‟ energy transferred.
2. DLP is not a patient dose index because
CTDIvol does not represent patient dose.
9
8/18/2011
Adapted from Frey
DLP = 200 mGy•cm
CTDIvol = 20 mGy
ten 1-cm slices
CT SCANNER DOSE INDICES
E. Displayed Dose Length Product (DLP)
DLP = 400 mGy•cm
CTDIvol STILL = 20 mGy
twenty 1-cm slices
So, DLP represents the greater biologic risk!
AFFECT of PATIENT SIZE on DISPLAYED
CTDIVOL & DLP
Large Adult
Adult
5 year
1. Previous example: radiation doses equal.
2. Second patient at greater risk because a
larger length of body was scanned.
3. While DLP is the dose to a standard
phantom, increases in DLP indicate
increases in risk to patient.
AFFECT of PATIENT SIZE on DISPLAYED
CTDIVOL & DLP
A. Patients of a given age vary
dramatically in size. Abdomens of the
largest 3 year olds are approximately
the same size as the
abdomens of the
smallest adults.
1 year
Neonate
4 cm
Size of patient, not
age should be used.
1 HVL @ 120 KVP ~ 70 keV
10
8/18/2011
AFFECT of PATIENT SIZE on DISPLAYED
CTDIVOL & DLP
Measured
CTDIvol =
47
Measured
CTDIvol = 37
Measured
CTDIvol = 18
21.6 mGy
38 mGy
47 mGy
35 mGy
47 mGy
10.8 mGy
AFFECT of PATIENT SIZE on DISPLAYED
CTDIVOL & DLP
CLINICAL DILEMMA
A. Displayed CTDIvol is independent of the
patient size; displayed CTDIvol assumes
either 16 or 32 cm CTDI phantom.
B. 16 cm CTDI phantom: adult dose over
while pediatric dose under estimated.
C. 32 cm CTDI phantom: adult and pediatric
dose under estimated ~ 2.5 times!
Displayed
CTDIvol16 = 37
Displayed
CTDIvol16 = 37
Displayed
CTDIvol16 = 37
Displayed
CTDIvol32 = 18
Displayed
CTDIvol32 = 18
Displayed
CTDIvol32 = 18
D. Propagated by DICOM Structured Reports
and CT scanner dose reports.
11
8/18/2011
TG 204
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E. Report does not:
!
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!
1. Address correction factors for heads
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2. Correct small (< 1%) doses from scanned
projection images
3. Correct for variation (~ 5%)
in attenuation of thorax
vs abdomen
4. Correct small variation in pre
and post contrast scans
TG 204
TG 204
E. Report does not address:
5. Changes in dose as a function of fan beam
a. 20 cm: relative dose ~ 2.7
b. 30 cm: relative dose ~ 3
c. Dose increases ~ 10%
d. Dose Profiles vs Fan Beam Width
F. Data from four independent investigators studying
patient size correction factors.
Adapted from TG 204
1. Physical
measurements
on phantoms
A. Anthropomorphic
Phantoms
(McCollough Laboratory “Mc”)
B. Cylindrical PMMA phantoms
(Toth / Strauss Collaboration “TS”)
2. Monte Carlo
computer
modeling
30 cm
20 cm
Adapted from J. Boone
C. Monte Carlo Voxelized
Phantoms
(McNitt-Gray Laboratory “MG”)
D. Monte Carlo
Mathematical Cylinders
(Boone Laboratory “Z-B”)
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TG 204
TG 204
age in years
0
1
5
1
0
1
5
32 cm 120 kVp
Adapted from TG 204
TG 204
1
5
1
0
1
5
Adapted from TG 204
TG 204
G. What about scans performed at 80, 100, or 140 kVp?
≥1
Year
16 cm 120 kVp
age in years
0
1. 5% difference
overall
2. 3% difference
between 1 yr
old (15 cm) &
adult (32 cm)
H. What about scans performed in the thorax?
1. Thorax data
from Huda et al.
2. 16% dif @ 12 cm
3. 7% dif @ 17 cm
4. < 3% dif > 17 cm
from 120 kVp
only
Combined TS / ZB: 80-140
kV
Adapted from TG 204
Adapted from Boone
13
8/18/2011
TG 204
TG 204
I. What is an effective diameter?
1. Circle with area of patient’s cross section
2. Effective diameter can be estimated if the
patient’s AP or lateral dimension is known.
J. Fitted equations calculate the effective diameter
from either AP or LAT dimension of patient based
on published data of actual patient sizes.
Adapted from TG 204
Adapted from TG 204
circle of
equal
area
ICRU 74
Strauss
Boone
Fit to All
ICRU 74
Strauss
Boone
Fit to All
AP
effective
diameter
lateral
204
K. What if I am TG
doing
retrospective dose
analysis and I only know age of patient?
1. Corrections based on patient size
are more accurate.
TG 204
AP or PA Projection Scan
Adapted from TG204
L. Determining patient size
1. Measure Lateral dimension
with mechanical calipers.
2. Measure Lateral or AP dimension
from AP or Lateral projection scan.
3. Measure AP or LAT dimension
from axial scan view.
4. In #2 & #3 patient must be
centered in gantry.
a. Magnification Error
Adapted from TG 204
AP or PA Projection
Scan
Adapted from TG204
Effective Diameter as a function of age per ICRU 74
14
8/18/2011
1. Failure to identify correct phantom, 16 or 32
cm leads to a systematic error of 100%.
2. No standard exists. Choice may depend on:
a.
b.
c.
d.
Selected protocol: adult or pediatric
Selected scan field of view
Year of manufacture
Software level
3. Make no assumptions: contact manufacturer
of your unit through its service organization.
TG 204
O.
Correction Factor based on 32 cm CTDI
Phantom
TG 204
M. Determining size of CTDI phantom your
CT scanner used to estimate CTDIvol
Adapted from TG 204
TG 204
TG 204
Q. SSDE Accuracy
1. 20%
Correction Factor based on 16 cm CTDI
Phantom
P.
2. Product is an estimate of patient dose
3. Report dose estimates with proper
number of significant digits
a. SSDE > 5 mGy: integers only, e.g.
7 or 23 mGy
b. SSDE < 5 mGy: one decimal point,
e.g. 2.7 or 4.5 mGy
Adapted from TG 204
15
8/18/2011
TG 204
R. Dose Reporting by Radiologists
The CTDIvol value reported on the scanner for
the [32 or 16] PMMA phantom was used with
correction factors obtained from AAPM Report
204. The correction factor for this patient was
based on the patient’s [AP, LAT, AP + LAT, or
effective dimension] This method is thought to
produce dose estimates with accuracy to within
20%. For this patient, the size corrected (SSDE)
estimate for this CT scan is _____ mGy.
SAMPLE CALCULATION: PRESCAN
A. Determine size of patient
1. AP Projection Scan: 16.8 cm
B. 16 cm CTDI phantom used by
scanner to calculate CTDIvol
C. Displayed CTDIvol = 9.29 mGy
D. 9.29 mGy x 1.08 = 10 mGy SSDE
Adapted from TG 204
Adapted from TG 204
SAMPLE CALCULATION: POST SCAN
SAMPLE CALCULATION: POST SCAN
A. Determine size of patient
1. AP = 9.9 cm; LAT = 12.3 cm
2. AP + LAT = 22.2 cm
A. Determine size of patient
1. AP = 9.9 cm; LAT = 12.3 cm
2. AP + LAT = 22.2 cm
B. 32 cm CTDI phantom assumed
B. 16 cm CTDI phantom assumed
C. Displayed CTDIvol = 5.4 mGy
C. Displayed CTDIvol = 10.8 mGy
D. 5.4 mGy x 2.5 = 13 mGy SSDE
D. 10.8 mGy x 1.24 = 13 mGy SSDE
Adapted from TG 204
Adapted from TG 204
16
8/18/2011
CLINICAL CHALLENGE
A. Child imaged on GE CT on Tuesday
followed by second examination on
Wednesday on Siemens CT in same
department.
1. GE: Displayed CTDIvol(16) = 10.8 mGy
2. Siemens: Displayed CTDIvol(32) = 5.4 mGy
CLINICAL CHALLENGE
D. Correction Factors for Heads
1. Will be published in the future
2. Correction factor not as large
32 cm CTDI
16 cm CTDI
B. Mom & Dad were not happy campers!
C. SSDE = 13 mGy for both studies!
Applications of SSDE
A. Understand difference between SSDE
and CTDIvol
1. SSDE may be useful as a first approximation of average organ dose to organs
completely contained in scan volume.
2. SSDE can NOT be substituted in place
of CTDIvol when using k-factors to
estimate Effective Doses from CT exam
Applications of SSDE
B. Start calculating SSDEs
1. More accurate estimates of patient dose
than CTDIvol
2. Recorded values of SSDE in patient
medical record have more applications
than CTDIvol.
3. DRLs could be based on SSDE values
4. Recorded SSDE values will lead to
department dose standards.
17
8/18/2011
Applications of SSDE
B. Start calculating SSDEs
5. SSDE values can be used as a tool to
manage patient doses from CT.
6. Work ongoing to get CT manufacturers
to calculate and display SSDE values.
Managing Pediatric CT Patient Doses
C. Establish Appropriate Pediatric Dose
Levels by reducing mAs appropriately
1. Reduce mAs until pediatric SSDE
< adult SSDE for department.
a. 10 cm AP
b. 1 / 2.5 = 40% original mAs
Managing Pediatric CT Patient Doses
A. CTDIvol Measurements
1. 32 cm adult body measurement
2. 16 cm adult head measurement
B. Compare/adjust wrt ACR Reference Values
Establish reasonable adult doses
Are they high, moderate, or low?
1.
< 25 mGy
2.
< 75 mGy
Managing Pediatric CT Patient Doses
C. Establish Appropriate Pediatric Dose
Levels by reducing mAs appropriately
1. Reduce mAs until pediatric SSDE
< adult SSDE for department.
2. IG website model of mAs reduction
a. 10 cm AP dimension
b. ~ 45% original mAs
18
8/18/2011
PEDIATRIC CONSIDERATIONS
CLINICAL EDUCATIONAL MATERIALS
Abdomen
Baseline:
PA Thickness
(cm)
9
12
14
16
19
22
25
31
kVp
fill in
Approx
Age
newborn
1 yr
5 yr
10 yr
15 yr
small adult
med adult
large adult
mA
fill in
Time (sec)
fill in
Abdomen
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.43
#VALUE!
0.51
#VALUE!
0.59
#VALUE!
0.66
#VALUE!
0.76
#VALUE!
0.90
#VALUE!
1.0
fill in
1.27
#VALUE!
Abdomen
Baseline:
PA Thickness
(cm)
9
12
14
16
19
22
25
31
Pitch Abdomen
Pitch Thorax
fill in
fill in
Thorax
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.42
#VALUE!
0.49
#VALUE!
0.57
#VALUE!
0.64
#VALUE!
0.73
#VALUE!
0.82
#VALUE!
0.91
#VALUE!
1.16
#VALUE!
kVp
120
Approx
Age
newborn
1 yr
5 yr
10 yr
15 yr
small adult
med adult
large adult
mA
400
Managing Pediatric CT Patient Doses
D. Dose reduction of IG website results in
conservatively high doses
IMAGE
GENTLY
BODY
Time (sec)
1
Abdomen
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.43
172
0.51
204
0.59
236
0.66
264
0.76
304
0.90
360
1.0
400
1.27
508
Pitch Abdomen
Pitch Thorax
1.25
1.5
Thorax
mAs Reduction Estimated mAs =
Factor (RF)
BL x RF
0.42
202
0.49
235
0.57
274
0.64
307
0.73
350
0.82
394
0.91
437
1.16
557
Managing Pediatric CT Patient Doses
F. Should voltages < 120 kVp be used for
Children?
1. Reduced high voltage at same dose
a. Dial up reduced mAs technique
b. Note displayed CTDIvol 120
c. Reduce kVp
d. mAs up until CTDIvol 80 = CTDIvol 120
e. Increased Contrast at same dose
E. Doses in a pediatric department might
be:
1.
2.
3.
4.
5.
10 cm (Newborn)
11 cm (1 yr old)
14 cm (5 yr old)
17 cm (15 yr old)
23 cm (Adult)
~ 0.6 x adult SSDE
~ 0.7 x adult SSDE
~ 0.8 x adult SSDE
~ 0.9 x adult SSDE
adult SSDE
Managing Pediatric CT Patient Doses
2. Reduced high voltage; reduced dose
a. Dial up reduced mAs technique
b. Note displayed CTDIvol 120
c. Measure increased contrast at 80
kVp compared to 120 kVp.
i. ACR accreditation phantom
ii. Clinical FoV / Bow tie Filter
19
8/18/2011
Managing Pediatric CT Patient Doses
2. Reduced high voltage; reduced dose
d. Estimate increase in noise by
comparing CTDIvol 120 & CTDIvol 80
e. Contrast Up 40% / Noise Up 60%
f. Increase mAs at 80 kVp until
CNR80 kVp = CNR 120 kVp
g. Same Image Quality; Reduced Dose
Conclusions
A. Adult hospitals performing 80% of all
pediatric patients should manage
pediatric radiation doses.
1. Use adult protocols and calculate
adult SSDE.
2. SSDE of pediatric patients prior to scan
< SSDE for adult patient.
3. Changing kVp in addition to mAs is more
involved, but allows both image quality
improvement and/or dose reduction.
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