CONTENT ORGANIZATIONMorphology and tissues of a) reference phantoms used for organ doses and b) patients' voxelized data sets are compared. Review differences in body habitus (thin to obese), gender, and age.Methods for estimating dose to organs with a) pre-determined geometry in reference phantoms and b) segmentation of organs in patient-specific dose maps are compared. Review effects of organ size, shape and partial irradiation. Include comparisons with size-adjusted CTDI.Effective dose calculated by applying ICRP tissue weighing factors to a) reference-based organ doses and b) patient-specific organ doses are compared. Review implications for infant,1, 5, and 10 year-old DLP (“k”) conversion factors.SUMMARYEffective doses based on age-based reference phantoms are often used for relative dose comparisons. Use of more patient-specific methods to estimate organ and effective dose could lead to better metrics and reporting for CT dose management.
http://www.halls.md/chart/child-growth/pediatric.htmInfant 42 cm1 y/o 85 cm5 y/o 110 cm 10 y/o 140 cm
13 day old: 2.4 x 1.5 = 3.6 15 yr. old: 1.4 x 6.3 = 8.8
15 day old: 2.4 x 1.5 x X = 8.8 = 6.3 x 1.46.3/3.7 CTDI ratio = 1.7 = 2.4 / 1.4 CTDI Normalized Avg. Dose (un-weighted)
Rsna2012 pediatric dosemapvstdphantom
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
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.
Reference phantom &
Patient specific phantoms
Methods for estimating
of organs in
on patient data
DLP & E
sum of dose
phantoms used for CT
can differ greatly from
the anatomy of an
A standard reference
Cristy) is compared
with CT images of a 5-6
month old patient.
reference and the
patient can effect the
estimation of dose by
Limitations of standard dose estimates
k factor – AAPM 95.6 Table 3
Volume CTDI & DLP
are based on PMMA
and are not intended to
be estimates of patient
dose. They do not
account for individual
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]
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
Major factors that affect CT dose include size/diameter &
Average patient dose tends to increasewith decreasingpatient
The Size Specific Dose estimates (SSDE)  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
Voxelized (GSF) Cylinders
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
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
CTDI-Normalized Dose Maps
CTDIvol 32 = 6.3 mGy
dose map value
Dose to 32 cm
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
Individualized dose maps in mGy
13 day old
15 year old
CTDIvol = 6.3 mGy
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
can be used to simulate new dose maps
13 day old
CTDIvol= 2.5 mGy
15 year old
CTDIvol = 6.3 mGy
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
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
Acquisition of CT images
Monte Carlo simulations were
performed on voxelized image sets.
Organ doses can be segmented
from dose maps
CT image voxels were classified as 1 of 5
tissues based on attenuation
Flowchartfor generation of dose maps: CT data acquisition,
creationof voxelized phantom,dose simulation
organ or tissue
Average value of pixels
segmented in the organ
are used for organ
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.
age 6 days 13 days 26 days 2 months
Lung dose 1.78 1.96 2.43 2.42
show patient-specific variability in organ doses that cannot
be shown in the Oak Ridge National Lab (ORNL)
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.
NRPB reference phantom (center ) 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
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).
Comparison of patient-specific &
methodsused for effectivedose calculation
CTDI = Computed Tomography Dose Index
DLP = Dose Length Product
ED = Effective Dose
ODi = Organ Dose
wi = tissue weighting factors, ICRP 60
ODi From Organ Segmentation
Patient Data Set
ED = k x DLPscan
ODi From Organ
Estimation of effectivedose for organ weighting factors
ED = ∑ wi x Odi
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.
Relative Organ Dose Sensitivities, Wi
Relative Organ Sensitivities ICRP
(Used by both Pt. Specific and Std. Ref. Phantoms)
ICRP 60 and NRPB phantoms were used for DLP conversion coefficients (Jessen).
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.
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
Slope = k, for each age
Chest 1 YO
Chest 5 YO
Chest 10 YO
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
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.
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.
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
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
Not modeled (easily)
Extends beyond scan length to model partial organ
Computed for each patient and each
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
Pt. specific organ dose can be used to
estimate eff. dose
Generic organ doses are used to determine DLP
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
Patient specific voxelized phantoms can represent complex, patient specific
anatomy and materials that are not easily represented in standard reference
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.
Patient-specific doses estimated by applying dosimetry
simulations to voxelized phantoms may have advantages when
patient morphology significantly differs from the reference
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
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
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
Creation of a Patient-specificVoxelized Models
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)
24 32 50
Patient diameter [cm]
for 24 cm
for 50 cm
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.
Dose Length Product
Effective Dose (mSv)
16 and 32 cm
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-
k = conversion
coefficient for the
DLP method of