Stylized Vs Tomographic Models: Experience At RPI

STYLIZED vs TOMOGRAPHIC:
AN EXPERIENCE AT RPI
X. George Xu
Nuclear Engineering and Engineering Physics Program,
Rensselaer Polytechnic Institute, Troy, New York, USA
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Introduction
Radiation protection dosimetry relies on fluence-todose-equivalent conversion coefficients that are
affected by three major factors:
1) human anatomy and tissue properties;
- stylized models that are based on surface equations
- tomographic models that are based on medical image
2) Monte Carlo code;
3) Algorithms to derive dose quantities.
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Stylized Models Developed for MIRD
Had Been the de facto Standard for 30 Years
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- Stomach - Small Intestine - Upper Large Intestine -Lower Large Intestine
Stylized Pregnant Woman Models
Existing models are crude, simplified shape, size
and locations of organs (Stabin’s 1995)
6-month model
3-month model
9-month model
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From Stylized To Tomographic
The past decade saw the emergence of a
new class of human models called
tomographic (or voxel) models
The Hypothesis was:
The greater realism in anatomical models would
improve the radiation dosimetry.
Questions remain today:
- Are dose estimates really improved?
- How much improvement?
- Is it worthy the effort?
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Objective of This
Presentation
To compare the effective doses from
the stylized and two tomographic
models for exposures to photons, as
well as selected electrons, neutrons
and protons
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The Visible Human Project®
- National Library of Medicine
Spitzer, V.M., and Whitlock, D.G. Atlas of the Visible Human Male. Jones and Bartlett Publishers, 1998.
Color
Photography
Computed
Tomography
(0.33mm x 0.33mm x1mm)
(1mm x 1mm x 1mm)
Magnetic
Resonance
Imaging
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(4mm x 4mm x 4mm)
Challenges With Tomographic Models
Segmentation is laborious
- At least months or years are needed
Computer Memory
- VIP-Man (0.33mm x 0.33mm x 1mm) is 3.7 GB data set
- The max. allowable RAM is < 2 GB
Monte Carlo Codes
- EGS4 and MCNP4b/MCNPX had to be enhanced for
handling such huge voxel data
Computer Timing
- The smaller the voxel size, the longer to run (~several hours)
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An adult male whole-body voxel model, called
Visible Photographic Man (VIP-Man), has been
constructed and adopted for Monte Carlo calculations
Xu, X. G. et al (2000). VIP-Man: An image-based whole-body adult male model constructed from color photographs of
the visible human project for multi-particle Monte Carlo calculations. Health Physics, 78(5):476-486.
EGS4
Photon / Electron
0.286 mm x 0.286
mm x 0.946 mm
(scaled voxel size)
MCNP
And
MCNPX
Photon/Electron/
Neutron/Protons
3.45 mm x 3.45 mm
x 3.785 mm
(scaled voxel size)
Images shown here are plotted from input data in EGS4 and MCNP/X, respectively.
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Stylized
vs.
Tomographic
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Method: External Parallel Beams
•
Particle types
– photons, electrons, neutrons and protons
•
Energy ranged from 20 keV to 10 GeV
•
Standard irradiation geometries (vacuum for electrons)
AP
PA
RLAT
LLAT
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ROT
ISO
Results: Effective Dose for Photon Beams
ED (Stylized)
ED (Scaled VIP-Man)
AP
PA
RLAT
LLAT
ROT
ISO
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
10
100
1000
10000
Photon Energy (keV)
• Data for stylized model were from ICRP 74/ICRU 57
• Above 1 MeV, EDs within 10%
• Between 0.1 –1 MeV, EDs differ within 20%
• Below 0.1 MeV, ED differ up to 80%
• In agreement with Jones (1995, 1997), Zankl et al. (2002) and Kramer et 12
al (2004)
Segmentation of Organs for the pregnant woman
model (Shi and Xu 2004)
34 organs and tissues have been segmented
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SAF RESULTS FOR PHOTON, SOURCE=UTERUS
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The Hypothesis:
The greater realism in anatomical models
would improve the radiation dosimetry.
Questions:
- Are dose estimates really improved?
- How much improvement?
- Is it worthy the effort?
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Problems with Voxels
• Many internal organs difficult to segment in CT
images
• Small structures (e.g., skin,eyes) difficult to model
• Geometric uncertainty and computational tradeoffs
inherent to voxel use remain
• Difficult to standardize
• Very difficult to deform
Moving organs
Different voxel sizes show variation in shape
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The Future ? Advanced Boundary Representation
Vol: 317.54 ml
Deformable Organ
Models Using NURBS
Segars et al (2001)
Vol: 211.72 ml
CAD Deformation of Lung
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Conclusions
• For external photon, the scaled VIP-Man model does
not have practical impact on the radiation protection
dosimetry
• Any single tomographic model is not representative
enough
• Tomographic models only provide info on anatomical
variations
• Future models will have advanced surface
representations
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Acknowledgements
Technical Supports
• National Library of Medicine on VHP Images
• Los Alamos National Laboratory on MC Codes
Grants
• NSF, CAREER/Biomedical Engineering (BES-9875532) 19992003
• EPRI, 2003-2005
• NIH, NLM/R03 (1 R03 LM007964-01) 2003-2005
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