Slide 1
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The Physics and Technology of CT
Patient Dose Estimates From CT Examinations:
Estimating Organ Dose Using Monte CarloCarlo-Based
Methods
Michael F McNittMcNitt-Gray,
Gray, PhD, DABR
Professor
Department of Radiology
David Geffen School of Medicine at UCLA
Los Angeles, CA
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Slide 2
Background
Radiation Dose from CT
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Has always been an area of concern for Medical Physicists
Increased concern with MDCT
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Increased utilization
Pediatric CT
Cardiac CT
Screening applications.
How to accurately estimate radiation dose?
− Organ dose is a key building block
− How to estimate organ dose?
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Slide 3
Background
Monte Carlo methods
− Used in CT for some time
• NRPB report 250 (1990)
• GSF (Zankl)
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Slide 4
Background
These early reports used:
− Detailed Models of Single Detector, Axial Scanners
− Idealized (Nominal) slice collimation
− Standard Man Phantom
• MIRD V (geometric model)
• Eva, Adam
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Slide 5
Background
These form the basis for:
− CT Dose computer program
− ImPACT dose calculator
− k factor approach (Effective dose = k* DLP), which was derived
from NRPB simulated data
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Slide 6
Current Approaches
Model Scanner in detail
Model Patient
Simulate Scan
Tally Organ Dose
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Slide 7
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Modeling the CT scanner
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Photon Fluence Spectra
3.000E+11
Spectra
−Obviously a function of beam energy
Photon Fluence
2.500E+11
Geometry
−Focal spot to isocenter
−Fan angle
2.000E+11
80 kVp Spectra
1.500E+11
125 kVp Spectra
150 kVp
1.000E+11
5.000E+10
0
50
100
150
200
Ene rgy in keV
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128 mm in air at is o
Normalized Dose
1.100
1.250
1.000
0.900
1.000
Normalized Dose
0.800
r el ative dose
Beam Collimation
−Nominal or actual
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0.000E+00
0.700
0.600
0.500
Filtration
−Bowtie filter (typically propietary)
propietary)
−Other additional filtration (typically proprietary)
0.400
0.300
0.200
0.750
128 mm in air at iso
0.500
0.250
0.100
0.000
0
0.000
10
20
30
40
50
40
60
70
80
60
90
distance i n mm
Tube Current Modulation Scheme
− x-y only, zz-only, x-y-z, etc.
100
110
80
120
Distance (mm)
130
140
100 150
160
120
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Slide 8
Modeling the CT scanner
Source Path - dependent on scan parameters:
• Nominal collimation
• Pitch
• Start and Stop Locations (of the source)
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Slide 10
Modeling the Patient
Geometric
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e.g MIRD
Standard man
Often androgynous
Usually single size
Size and age variations have been created
− newborn, ages 1, 5, 10, and 15 years
− adult female, and adult male
− Including pregnant patient
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Slide 11
Modeling the Patient
All radiosensitive organs identified
− Location
− Size
− Composition and density
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Slide 12
Modeling the Patient
Voxelized Models
− Based on actual patient scans
− Identify radiosensitive organs – usually manually
− NonNon-geometric
Different age and gender
Different sizes
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Slide 13
Modeling the Patient
GSF models (PetoussiPetoussi-Henss N, Zankl M et al, 2002)
− Baby, Child, three adult females, two adult males, Visible Human
− All radiosensitive organs identified manually (ugh!)
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Slide 14
Modeling the Patient
Xu – pregnant patient
Bolch – UF Phantoms
Zubal – Adult male phantoms
Several others (see http://www.virtualphantoms.org
/)
http://www.virtualphantoms.org/)
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Slide 15
Modeling (Parts of) the Patient
Embryo/Fetus
Breast
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Slide 20
Simulating the Scan
Select Technical Parameters
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Type of scan (helical, axial)
Beam energy
Collimation
Pitch
Tube Current/rotation time (or tube current modulation)
Select Anatomic Region
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Head
Chest
Abdomen/Pelvis
Translate this to:
− Start and stop location
− Source Path
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Slide 23
Monte Carlo for CT Dose - Details
Monte Carlo Packages
− MCNP (Los Alamos)
− EGS
Model Transport of Photons from a modified (CT) source
Probabilistic interactions of photons with Tissues
− Photoelectric
− Compton Scatter
− Coherent Scatter
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Tissues need detailed descriptions
− Density
− Chemical composition (e.g. from NIST material web site)
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Slide 24
Dose Calculations
Score absorbed dose based upon collision kerma (Kc ).
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Calculated from MCNP tracktrack-length estimate of photon
energy fluence
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Multiply by the materialmaterial-specific and energyenergy-dependent
mass energyenergy-absorption coefficient.
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The mass energyenergy-absorption coefficients were taken
from the tables of Hubbel & Seltzer 1995.
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1 keV energy cutoff for photons
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Slide 25
Dose Calculations
MCNP reports relative dose
− In our case dose/photon from source
Have to convert that to absolute dose (mGy
).
(mGy).
Therefore, simulation results must be normalized to
absolute dose using measured and simulated air scans
( NF ) E,T =
( DAir,measured ) E,T
( DAir, simulated ) E ,T
To calculate Absolute Dose:
( Dabsolute) E ,T = ( NF) E,T * ( Dsimulated ) E ,T * (Total mAs)
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Slide 26
Dose Calculations
Dose results are reported on a “per 100 mAs”
mAs” basis.
We aim for an MCNP relative error < 1%
− typically requires between 106 and 108 source photons
− ~2 hours of computing time on a PC for small structures (<5mm)
− ≈ 15 minutes for large organ structures such as the lung or breast
breast
− This is also dependent upon the location of the tally structure
relative to the scan plane.
− (can take much longer for full resolution, small structures)
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Slide 27
Validating the CT Scanner Model
Benchmark MC Model against physical measurements
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− CTDI Phantoms
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Head and Body
Simulate a tally in a pencil chamber
Each kVp and beam collimation combination
Measured vs. Simulated
− Aim for < 5% difference between Simulated and Measured
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Slide 28
Monte Carlo Methods
Used to Estimate Organ Doses
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MDCT scanner details
Scan protocol details
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Different patient models
Extendable to new geometries/methods
Used to evaluate effectiveness of dose reduction methods on organ
organ
dose (and not just dose to phantoms)
Detailed organ dose approach will provide basis for determing
simpler relationships (curve fits, regression equations, etc.)
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