INITIAL STUDIES on PROTON COMPUTED TOMOGRAPHY USING SILICON STRIP DETECTORS

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• INITIAL STUDIES on PROTON COMPUTED TOMOGRAPHY
  USING SILICON STRIP DETECTORS
• L. Johnson, B. Keeney, G. Ross, H. F.-W.
  Sadrozinski, A. Seiden,
• D.C. Williams, L. Zhang
• Santa Cruz Institute for Particle Physics, UC
  Santa Cruz, CA 95064
• V. Bashkirov, R. W. M. Schulte, K. Shahnazi
• Loma Linda University Medical Center, Loma Linda,
  CA 92354

• Proton Tomography / Proton Transmission
  Radiography
• Proton Transmission Radiography Data
• Proton Transmission Radiography MC Study

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Computed Tomography (CT)

• Based on X-ray absorption
• Faithful reconstruction of patients anatomy
• Stacked 2D maps of linear X-ray attenuation
• Coupled linear equations
• Invert Matrices and find (hopefully)
  non-malignant structures

X-ray tube
Detector array
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Radiography X-rays vs. Protons
Energy Loss of Protons, r
Attenuation of Photons, z N(x) Noe- m x
NIST Data
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Proton Radiography Density Map
NIST Data
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Development of Proton Beam Computed Tomography

• Exploratory Study in Proton Radiography
• two detector planes
• Crude phantom in front
• Experimental Study
• two detector planes
• water phantom on turntable
• Theoretical Study
• GEANT4 MC simulation
• influence of MCS and range straggling
• importance of angular measurements
• Optimization of energy

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Proton Energy Measurement with LET
Simple 2D Silicon Strip Detector Telescope
built for Nanodosimetry (based on GLAST
Design) 2 single-sided SSD 194um Pitch 400um
thick 1.3us shaping time Binary
readout Time-over-Threshold TOT Large dynamic
range Measure particle energy via LET
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GLAST Front-End Electronics ASIC

• Binary Readout
• Low-power (200uW/channel)
• Peaking time 1.3 ms
• Low noise (Noise occupancy lt10-5)
• Threshold set in every ASIC
• Separate Masks for Trigger and Readout in every
  Channel
• Self - Trigger OR of one Si plane (1536
  channels)

Electron Events
Pulse Charge Time over-Threshold on the OR of
every Si plane Distinguish single tracks
from two tracks in one strip
Photon Events
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Charge Time-Over-Threshold (TOT)
Digitization of Position and Energy with large
Dynamic Range
Time-over-Threshold TOT
Pulse
Threshold
TOT ? charge ? LET!
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Proton Energy Measurement with LET
TOT Spectra as for several proton energies
Mean TOT vs. Proton Energy Good agreement between
measurement and MC simulations
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Proton Energy Measurement with LET
TOT Resolution flat. Energy Measurement
possible where slope dTOT/dE is large
TOT Spectra vs. energy
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Proton Localisation M.S. vs. Energy Resolution
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Exploratory Proton Radiography Set-up
Use Loma Linda University Medical Ctr 250 MeV
Proton Beam Degraded down to 130 MeV by Wax
Block Object is Aluminum pipe 5cm long, 3cm OD,
0.67cm ID Very large effects expected, but beam
quite non-uniform
 
 
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Image !

• Subdivide SSD area into pixels
• Strip x strip 194um x 194um
• 4 x 4 strips (0.8mm x 0.8mm)
• Image given by average
• TOT or Energy in pixel

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Issues
Features Washed out image in 2nd plane Fuzzy
edges Hole filled partially Energy diluted at
edges and in hole Migration of events All
explained by Multiple Coulomb Scattering
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Loss of Resolution in Back Data
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Migration and Energy Dilution in Slice
Data shows increased frequency of hits in
boundary of the pipe the hole and the outside
perimeter. These events are associated with a
dilution of the energy profile
Hit frequency vs. Location
Mean Energy vs. Location
Energy lowered
Excess Events
Blurred Edges
Approx. Beam Profile
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Multiple Scattering Emigration
Protons scatter OUT OF Target (not INTO). Those
have larger energy loss larger angles fill
hole dilute energy
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Energy Resolution Position Resolution
Data (LET converted to Energy) GEANT4 MC (LET
in SSD)
RMS
Simulation reproduces spread of energy and loss
of resolution
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Energy Resolution
Object Background
Data LET converted to Energy
MC LET in SSD plane
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Migration MC
Dilution by events entering the Object but
leaving it before the end
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MC Loss of Resolution in Back
First Plane, 2cm behind Object
Second Plane, 30cm behind Object
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Conclusions

• Imaging with protons is working!
• GEANT4 program describes the data well
• (energy and position resolution, migration)
• Issues
• Energy needs Optimization depending on Target
• Improve resolution with cut on exit angle?
• Investigate independent Energy measurement
• Dose Contrast - Resolution Relationship to be
  explored
• Next steps pCT