megavoltage computed tomography at site 300 · 2 llnl-pres-751684 x-ray computed tomography at site...
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LLNL-PRES-751684
This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344. Lawrence Livermore National Security, LLC
Megaelectron Volt Computed Tomography at Site 300
Alex Dooraghi, Jerel Smith, Joe Tringe and Bill Brown
March 2, 2018
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X-ray computed tomography at Site 300
• At Site 300 we use Megaelectron Volt (MeV) CT to diagnose operation of conventional weapons
• X-ray computed tomography (CT) is a nondestructive imaging modality that requires x-ray projections at various orientations to reconstruct an object
• MeV x-ray CT systems are used to image radiographically dense objects
• The Site 300 system uses a dual capability (6 MeV or 9 MeV) x-ray source with a digital detector panel
Varex Linatron M9
Varex XRD 1621 detector panel
• Concern: As x-ray energy increases, (1) image contrast decreases and (2) safety considerations increase due to expanded radiation dose fields
• Our interest is examining Monte Carlo methods, specifically MCNP, to characterize image contrast and radiation dose field with the objective of maximizing image quality and minimizing radiation field
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Three concerns motivate our investigation
6 MeV
1 MeV
0.121 MeV
Dzierma et al (2014)
Concern 1: Contrast is reduced with higher energy x-ray systems
Without Scatter
With Scatter
Concern 2: Contrast is reduced by object scatter, which becomes more significant with increasing energy
Concern 3: High radiation doses
• LD50 is the amount of a material, given all at once, which causes the death of 50% (one half) of a population
• The LD50 for x-ray radiation is 5 Gy
• The x-ray system at Site 300 can produce 30 Gy per minute 1 meter from target. Dose is reduced by:
DistanceShielding
Time
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Goal: Simulate MeV CT system design to maximize the image quality and minimize radiation dose field
MCNP6 is a general-purpose Monte Carlo code used to simulate neutron, photon, electron, or coupled neutron/photon/electron transport
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MCNP has a built-in framework for Monte Carlo radiographic simulation to accelerate convergence
Framework allows for scatter evaluation
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Simulation Performed
1. How well does the MCNP simulation match the measurements?
2. How much of an improvement in image quality does MCNP predict from scatter reduction?
3. How well can we simulate the radiation field at site 300?
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Simulation Performed
1. How well does the MCNP simulation match the measurements?
2. How much of an improvement in image quality does MCNP predict from scatter reduction?
3. How well can we simulate the radiation field at site 300?
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Measurement data was taken using a 6 MeV x-ray spectrum
Object:Cylindrical munition component principally composed of HMX, tungsten and aluminum encased in carbon fiber
X-Ray Spectrum:6 MeV spectrum filtered with 3.175 mm of tantalum
VarexLinatron M9
Mid-StreamCollimator
VarexXRD 1621 Panel Object
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1. Measured images (Im) were processed as follow:
𝐼𝑚 = −𝑙𝑛(𝐼 − 𝐼𝑑)
(𝐼𝑜−𝐼𝑑)= න𝜇 𝑑𝐿
where I : Image with x-rays on and object in field of viewIo : Image with x-rays on but no object in the field of viewId : Image with x-rays offµ : Linear attenuation coefficient (mm-1), which is a function of
atomic number, density and x-ray energyL : Material thickness (mm)
2. Similarly, simulated images (Is) were process as:
𝐼𝑠 = −𝑙𝑛𝐼
𝐼𝑜= න𝜇 𝑑𝐿
Measured and simulated radiographs were processed to compare attenuation
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Simulation that includes an energy dependent detector response and scatter best approximates measurement
SimulationMeasurement
Percent deviation from measurement (%)
Detector Response (DR) Primary Primary and Scatter
Energy Independent (EI) 27 20
Energy Dependent (ED) 5 0.6
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Simulated reconstruction deviates by < 1 %
Measured
Simulated
µ(m
m-1
)µ
(mm
-1)
Simulation assumes energy dependent detector response and includes scatter
Explosive
Carbon Fiber
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Simulation Performed
1. How well does the MCNP simulation match the measurements?
2. How much of an improvement in image quality does MCNP predict from scatter reduction?
3. How well can we simulate the radiation field at site 300?
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At Site 300 we acquired CT data of a conventional explosive warhead using a 9 MeV spectrum
Detector
Warhead
Linear Accelerator
Mid Stream Collimator
Longitudinal Cross Section
TransverseCross Section Rendering of
radial plate
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Focus of analysis was on radial plate
Explosive 2(Boosters)
Explosive 3(Main Charge)
Explosive 1(Detonation
Chain)
Aluminum
Measured CT slice through radial plate
µ(m
m-1
)
Polycarbonate
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Simulation 1 considers primary and scatter
Simulation 1(Primary and Scatter)
MeasurementProvided models:1. Energy spectrum2. Detector response
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Simulation 2 considers only primary
Simulation 2 (Primary only)
Simulation 1(Primary and Scatter)
Measurement Scatter reduces linear attenuation values (µ) for all materials near and within the radial plate.
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Slit collimation was simulated to evaluate effect on contrast
5 cm of W
2 mm
Vertical
Collimation
Detector
Source
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Slit aperture effectively reduces scatter
Simulation 2 (Primary only)
Simulation 1(Primary and Scatter)
Measurement
Simulation 3 (Primary and Scatter, Slit)
Profile of simulation 3 (primary and Scatter, Slit) overlaps simulation 2 (primary only)
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Slit collimator can recover reduced contrastdue to object scatter
𝐶𝑜𝑛𝑡𝑟𝑎𝑠𝑡 =𝑅𝑂𝐼2 − 𝑅𝑂𝐼1𝑅𝑂𝐼2 + 𝑅𝑂𝐼1
Contrast Results: Polycarbonate to Explosive 1
Polycarbonate to Explosive 2
Polycarbonate to Explosive 3
PE Simulation 1 (P & S) 0.0691 0.1313 0.1079
PE Simulation 2 (P) 0.0889 0.1691 0.1254
Percent Change (%) 29 29 16
PE Simulation 3 (P & S, slit) 0.0878 0.1677 0.1246
Percent Change (%) 29 29 15
Note:ROIi: region of interest in material i
Simulation
Explosive 2(Boosters)
Explosive 3 (Main Charge)Explosive 1
(Detonation Chain)
Polycarbonate
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Summary
• Site 300 uses MeV CT to diagnose operation of non-nuclear weapon components
• MeV CT enables interrogation of items that may be too radiographically dense for keV systems
• Three concerns when using MeV CT:1. contrast is reduced with higher energy x-ray systems2. contrast is reduced by scattered radiation which becomes more significant
with higher energy x-rays3. high radiation fields
• Goal of this work is to simulate MeV CT system to optimize design that maximizes image quality and minimizes radiation dose field
• Currently, we are using the MCNP Monte Carlo radiation transport code to evaluate:1. radiographic framework to assess detector response and scatter2. benefits of scatter reduction techniques 3. radiation field at Site 300
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Acknowledgements
NDE Radiographer TeamJohn Rodriguez (Senior Supervisor)Kenn Morales Luke Schrimsher
Health Physicists Chad HopponenEric Galicia
FeedbackHarry MartzAditya MohanOwen MaysRay Obenauf Rosa MoralesIan LadnerLogan BekkerJoshua Deotte
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