1. DME Bardo, MD1, KA Feinstein, MD2, D Pettersson, MD1, J Wiegert, PhD3, JH Yanof, PhD4
Philips Research3, Philips Healthcare4
Comparison of Patient-Specific &
Reference-Phantom Methods for
CT Dose Estimation in the
Pediatric Population
1 2

2. Purpose
 Compare the characteristics of
a) age-based reference phantoms used with Monte Carlo simulation to estimate organ doses with
b) patient-specific phantoms based on CT data sets.
 Compare the use of reference-phantom and patient-specific dose
distribution maps to estimate organ doses.
 Describe how differences in organ dose distribution affect the estimation
of effective dose.

3. Content Organization
Reference phantom &
Patient specific phantoms
Methods for estimating
organ dose
Effective dose
calculations
Standard
From
geometrically
defined organs
in stylized
phantoms
Patient
Specific
Using
segmentation
of organs in
patient-specific
dose maps
Standard
CTDI and
stylized
phantoms
Patient
Specific
Voxelized
models based
on patient data
sets
Standard
Regression
with
DLP & E
(k factors)
Patient
Specific
Weighted
sum of dose
map organ
doses

4. Morphology of
standard reference
phantoms used for CT
dose estimation
can differ greatly from
the anatomy of an
individual patient
A standard reference
(stylized, mathematical)
phantom (ORNL,
Cristy) is compared
with CT images of a 5-6
month old patient.
Striking anatomical
differences between
reference and the
patient can effect the
estimation of dose by
dosimetry simulation
(Monte Carlo).

5. Limitations of standard dose estimates
k factor – AAPM 95.6 Table 3
Volume CTDI & DLP
are based on PMMA
cylindrical phantoms
and are not intended to
be estimates of patient
dose. They do not
account for individual
patients body
habitus, attenuation
characteristics,
or specific
scanner
dosimetry.
PMMA CTDIvol phantoms
DLP conversion factors for
Effective dose (reviewed
later) – are based stylized
phantoms with fixed
geometry (modified ORNL
set for newborn, 1, 5, and 10
YO as shown). [1,2,3]

6. The x-ray beam has more attenuation as it traverses a larger patient (yellow to red to blue) in
comparison with a smaller patient (yellow to red). Thus, the average beam intensity and dose, as the
tube rotates, tends to increase with decreasing size for the same scan parameters
Dose
(mGy)
10
20
30
40
10 mGy
CtrLG
20 mGy,
PrphryLG
40 mGy
CtrSM
40 mGy
Prphry,SM
Background
Major factors that affect CT dose include size/diameter &
tissue/material
absorption

7. Background
Average patient dose tends to increasewith decreasingpatient
size/diameter
The Size Specific Dose estimates (SSDE) [6] also show that absorbed dose increases with decreasing size. A
regression model (above) relates dose to effective diameter. The model data was based on a range of dosimetry
methods (measured and simulated), four sets of phantoms, and four scanner vendors. All phantom sets included
pediatric sizes.
“Child”
Infant Child
Anthropomorphic CTDI
Voxelized (GSF) Cylinders
Adult

8. In contrast to a standard reference phantoms (left), a patient’s CT data set can be used to
create a patient-specific (virtual) phantom (middle). In addition to patient-specific
dosimetry, this enables and individualized dose maps (right).
Standard reference vs. Patient-specific
Phantoms and Dosimetry
Voxelized
Phantom
Patient
Data setStandard
reference phantom
Patient specific
dose map

9. Dose maps can also be displayed in units of CTDI normalized absorbed dose. In this case, dose map pixels are divided
by the simulated dose absorbed by the CTDI phantom. The basic trend of CTDI-normalized average dose increasing
with decreasing patient size (relative to the CTDI phantom) tends to agree with the SSDE correction factors.
Monte Carlo simulations with
patient-specific phantoms:
CTDI-Normalized Dose Maps
15 y/o
CTDIvol 32 = 6.3 mGy
Average
dose map value
Dose to 32 cm
CTDI phantom

10. A Monte Carlo tool is used to simulate the dose absorbed by the patient specific “virtual phantom” instead of the CTDI
phantom. This results in a patient specific dose map (in units of mGy). This example shows that the scan parameters
for a 13 day old resulted in less absorbed relative to a 15 year old.
Monte Carlo simulations with
patient-specific phantoms:
Individualized dose maps in mGy
13 day old
CTDIvol=1.5 mGy
15 year old
CTDIvol = 6.3 mGy

11. The CTDI of 2.5 mGy for a 13 day old and 6.3 mGy for a 15 year old would yield approximately the
same patient specific-dose map.
Monte Carlo simulations of
patient-specific phantoms
can be used to simulate new dose maps
13 day old
CTDIvol= 2.5 mGy
15 year old
CTDIvol = 6.3 mGy
Simulated
dose
maps

12. Morphological differences
between patient-specific & standard reference phantoms for
dosimetry simulation (Monte Carlo)
CT data sets are used to form patient specific virtual
phantom (previous slide). The size and shape of
organs and tissues can have wide variation.
Virtual phantoms do not extend
beyond reconstructed image
volume.
Organs are represented by patient generic,
fixed (stylized) geometric shapes.
Anatomy extends caudal-cranially,
enabling dose simulation and
scatter beyond scan range.
Oak Ridge National Lab Phantoms (Cristy)Virtual Patient (“Voxelized) Phantoms
newborn 1 YO 5 YO 10 YO 10 YO 5 YO 1 YO newborn

13. Acquisition of CT images
3
4
Monte Carlo simulations were
performed on voxelized image sets.
Dose Maps
displaying
CTDIvol
normalized
absorbed dose.
Organ doses can be segmented
from dose maps
CT image voxels were classified as 1 of 5
tissues based on attenuation
5
21
Background
Flowchartfor generation of dose maps: CT data acquisition,
creationof voxelized phantom,dose simulation
3

14. Dose Map:
Select
radiosensitive
organ or tissue
Average value of pixels
segmented in the organ
are used for organ
dose estimation
(in mGy)
Organ segmentation
Background
Patient-specificorgan doses can be estimatedwith dose maps
In standard reference phantoms such as those used for the DLP conversion factors, organs are defined
with fixed geometry using mathematical equations.

15. age 6 days 13 days 26 days 2 months
Lung dose 1.78 1.96 2.43 2.42
(mGy/mGy)
show patient-specific variability in organ doses that cannot
be shown in the Oak Ridge National Lab (ORNL)
references phantoms
Dose maps
Estimated lung dose (CTDI normalized) from patient specific dose maps varies from 1.78 to 2.43 mGy/mGy. CTDIvol
normalized organ doses simulated in the ORNL infant phantom (right) (new born) would not have any variability.

16. NRPB reference phantom (center [3]) extended the ORNL phantom concept (lower right) to include gender-based organs. The NRPB
phantoms were used by Jessen et al. (with IRCP 60 organ weighing factors) to determine widely used DLP conversion factors [5,11]
Standard Reference Phantoms
Can be modified to include additionalradiosensitiveorgans

17. Patient-specific voxelized phantoms
have featuresnot included in standard reference phantoms
Breast dose can be measured (green contours) in dose maps -- this tissue is represented in the NRCP phantoms (previous slide).
14 y/o female (left) and 15 y/o male (right) with approximately the same effective diameter (~27 mm). Bismuth shields (arrows) were
used in both exams (not represented in standard phantoms). Average lung dose is higher for the female patient due to relatively larger
lung parenchyma (lower beam attenuation).

18. Comparison of patient-specific &
reference phantom
methodsused for effectivedose calculation
Effective
Dose, Reference
Phantom
LEGEND:
CTDI = Computed Tomography Dose Index
DLP = Dose Length Product
ED = Effective Dose
ODi = Organ Dose
wi = tissue weighting factors, ICRP 60
compare
Effective Dose,
Patient-Specific
Phantom
ODi From Organ Segmentation
Dose Map
Monte Carlo
Simulation
(Scanner Specific)
Patient Data Set
CTDI, DLPscan
ED = k x DLPscan
Reference
Phantom
MonteCarlo
Simulation
ODi From Organ
Compartments
Dose
Map
CTDI, DLP
AAPM
Report
96.5
k

19. Background
Estimation of effectivedose for organ weighting factors
ED = ∑ wi x Odi
where
OD is the individual organ dose measured from non-patient specific
mathematical phantom or patient specific dose map
w is organ/tissue weighting factor (ICRP 60 or 103)
i = 13 (13 most radiosensitive organs)
The effective dose equivalent, therefore, represents a total body dose.

20. Relative Organ Dose Sensitivities, Wi
0
0.05
0.1
0.15
0.2
0.25
Relative Organ Sensitivities ICRP
(Used by both Pt. Specific and Std. Ref. Phantoms)
ICRP 60
ICRP 103
ICRP 60 and NRPB phantoms were used for DLP conversion coefficients (Jessen).
Patient-specific phantoms
Segmenting all the listed organs and
tissues for each individual voxelized
phantom can pose a challenge.
Standard reference phantoms
The NRPB phantoms do not include
all organs and tissues listed. And
they would need to be revised if the
ICRP adds new organs to the list.

21. Background
Determinationof DLPconversionfactors
Each body and age specific DLP conversion
factor (k factor in units of mSv mGy-1 cm-1)
was determined by dosimetry simulations
with varying DLP.
They are based on linear regression analysis
of body-region specific effective dose (from
the simulations, y-axis) and DLP (x-axis).
In this method, effective dose is assumed to be linearly proportional to DLP, i.e., E = DLP x k . DLP is linearly proportional to irradiated
scan length (includes helical over-ranging). For pediatrics, DLP is based on CTDI 16. Also, K-factors (i.e., DLP conversion) represent an
average over scanner types and are not gender specific. The organ dose weighting factors were described on the previous slide.
DLP input to Dose Simulation
Chest newborn
Slope = k, for each age
Chest 1 YO
Chest 5 YO
Chest 10 YO
EffectiveDose
asweightedsumoforgandoses

22. Effective dose using ICRP
weightingfactorscannot be patient-specific
• The organ sensitivity (weighting) factors are based on population data from survivors of
the atomic blast, where the sum of the weighting factors is one.
• A 0.12 value for lung tissue implies that the relative likelihood of developing lung cancer in
the population of blast survivors, in comparison with other listed organs, is 12%.
Therefore, any estimation of effective dose that uses these population based
weighting factors – patient-specific dose maps or DLP conversion k factors – cannot
be patient-specific.
Although effective dose is not patient specific, dose maps enable patient specific
organ dose estimates (next slide) and these increase the relative patient (as well
as scanner) specificity in comparison with DLP conversion factors.

23. Partial irradiation of an organ tends to decrease the organ
dose estimate. This is because organ dose is defined as the
average over the entire organ.
An advantage for the reference phantoms is that the caudal-
cranial range is not limited to a reconstructed scan volume as
with the voxelized phantom.
This can help estimate absorbed dose to partially irradiated
organs. Basing organ dose on only the fully irradiated voxels
will tend of overestimate the estimated organ dose.
Organ doses
partial irradiation, ICRPweighting factors
ICRP weighting factors are
based on full-body
irradiation. Tissues that
have wide distribution
throughout the body such as
red bone marrow are almost
certainly partially irradiated
in a CT examination.
partial irradiation of liver
scanlength

24. Summary comparison dose maps
and standard reference phantoms
Dose Map Method Standard Reference Phantom Method
Representation Voxelized Phantom Based on Data Set Four pre-defined geometric representation of organs
Morphology Patient-specific Not Patient Specific
Organs Organs must be segmented. Organs pre-defined mathematically
Caudal Cranial
End-effects
Not modeled (easily)
Extends beyond scan length to model partial organ
irradiation
Computation
Computed for each patient and each
examination
Can be pre-tabulated for set of examinations and stored
for future use
Material Models CT Numbers are mapped to ICRU 44 ICRP Publication 89
Effective Dose
Pt. specific organ dose can be used to
estimate eff. dose
Generic organ doses are used to determine DLP
conversion coefficients.

25. Estimation of CT dose is evolving …
10 cm CTDI phantom
Dose map sequence (z-axis) based on Monte
Carlo simulation with infant CT data set

26. Summary
 Patient specific voxelized phantoms can represent complex, patient specific
anatomy and materials that are not easily represented in standard reference
phantom.
 Organ doses estimated from patient specific dose maps ARE patient specific.
Patient-specific dose maps demonstrate the variability of organ doses and
highlight a key limitation of standard methods for estimating effective dose.
 Use of more patient-specific methods to estimate organ and effective doses
could lead to better metrics and reporting for CT dose management. Effective
doses estimated from ICRP wt. factors and NOT patient specific, but EDose Maps is
more patient- and scanner-specific than EDLP.

27. Clinical Relevance
 Patient-specific doses estimated by applying dosimetry
simulations to voxelized phantoms may have advantages when
patient morphology significantly differs from the reference
phantom.
 Quantitative evaluation of patient-specific dose maps are
underway. This will lead to a better understanding how more
accurate dose estimate methods will impact CT radiation dose
management.

28. References1. Cristy M . Mathematical phantoms representing children of various ages for use in estimates of internal dose. Report no. ORNL/ NUREG/TM-367. Oak Ridge,
Tenn: Oak Ridge National Laboratory, 1980 .
2. Cristy M , Eckerman KF . Specifi c absorbed fractions of energy at various ages from internal photon sources. I. Methods. Report no. ORNL/TM-8381/V1. Oak
Ridge, Tenn: Oak Ridge National Laboratory, 1987 .
3. A Khursheed, Phd, M C Hillier, P C Shrimpton, Phd And B F Wall, Bsc, Influence of patient age on normalized effective doses calculated for CT examinations
4. Maria Zankl , Handbook of Anatomical Models for Radiation Dosimetry Edited by Xie George Xu and Keith F Eckerman , 3] Taylor & Francis 2009
5. American Association of Physicists in Medicine. The measurement, reporting and management of radiation dose in CT. Report 96. AAPM Task Group 23 of the
Diagnostic Imaging Council CT Committee. College Park, MD. American Association of Physicists in Medicine, 2008.
6. American Association of Physicists in Medicine. Size-specific dose estimates (SSDE) in pediatric and adult body CT examinations. Report 204. AAPM Task
Group 204. College Park, MD. American Association of Physicists in Medicine, 2011.
7. McCollough CH, et al., CT Dose Index and Patient Dose: They Are Not the Same Thing. Radiology: Volume 259:(2) 311-316.
8. Morgan HT., Dose reduction for CT pediatric imaging, Pediatr Radiol. 2002 Oct;32(10):724-8; discussion 751-4. Epub 2002 Aug 29.,
9. Adam C. Turner1 and Michael McNitt-Gray, The feasibility of patient size-corrected, scanner-independent organ dose estimates for abdominal CT exams, Med
Phys. 2011 Feb;38(2):820-9.
10. Boone JM, Strauss KJ, Cody DD, McCollough CH, McNitt-Gray MF, Toth TL, Goske MJ, Frush DP. Size-specific dose estimates (SSDE) in pediatric and adult
body CT examination. Report No. 204. 2011
11. Cynthia H. McCollough et al. How Effective Is Effective Dose as a Predictor of Radiation Risk?, AJR:194, April 2010

29. air
adipose tissue
lung tissue
soft tissue
cortical bone
A patient-specific (tomographic) virtual phantom (i.e., model) is created by “voxelizing” and automatically segmenting patients CT
dataset. Each voxel is assigned one of five material types based on an a priori, global HU classification intervals (ICRU 44). These
material types are also assigned mass density to compute absorbed dose.
The resulting virtual patient phantoms are used for dose simulation (Monte Carlo) and the results are referred to as Dose
Maps (next slide).
CT image for 6 day old Virtual patient phantom
Appendix I
Creation of a Patient-specificVoxelized Models

30. Limitations of CTDIvol
The CTDIvol reports scanner output based on a standard, fixed-sized
phantom (32 cm for body), not patient-specific dose. Therefore, dose is
over- and underestimated for patients significantly larger or smaller
(respectively) than the phantom. (see AAPM report 201)
EstimatedCTDIvol[mGy](120kV)
24 32 50
15
10
5
0
Patient diameter [cm]
Actual dose
for 24 cm
diameter patient
Reported dose
Actual dose
for 50 cm
diameter patient

31. DLPand Effective Dose
Effective dose (ED) parameter shown here is also based on the plastic CTDI phantoms. It is a risk-related quantity used to indicate equivalent whole body
exposure that includes DLP as well as other factors such as the radiation sensitivities of the various organs in the body, age, and gender.
Notes: 1) Effective dose using DLP conversion coefficients are estimated with averages over gender and age and therefore do not estimate risk for an
individual patient. 2) Reference for ED are based on estimates for annual background radiation (3 mSv). 3) Another method to compute ED is based on the
summation of organ dose estimates.
CTDI (mGy)
Dose Length Product
(mGy *cm)
Effective Dose (mSv)
Equation
or dose
calculation
method
CTDIvol
CTDIvol is
presently
measured
with
16 and 32 cm
phantoms
DLP = Irradiated Scan Length x CTDIvol
Helical scan length: the reconstructed scan
length plus helical over-ranging
Axial scan length: the reconstructed scan
length for one “axial shot” * number of axial
“shots”. (The CTDIvol accounts for “over-
beaming”).
k = conversion
coefficient for the
DLP method of
estimation

32. Dose map reconstructions
Dose maps show representations of:
Absorbed dose map
[mGy]
Typical range: 0-20 mGy
Absorbed dose map/CTDIvol
[mGy / mGy]
Typical range: 0-2.5 mGy/mGy
Energy imparted map
[Joules]
Typical range: 10-5 J/pixel