loma linda university medical center dept. of radiation medicine
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Loma Linda University Medical CenterDept. of Radiation Medicine
and and Northern Illinois University
Dept. of Physics and Dept. of Computer SciencePresented by
George Coutrakon, PhDNIU Physics Dept.
Collaborators
� Northern Illinois University
Bela Erdelyi, Nick Karonis, Kirk Duffin (image reconstruction)
Victor Rykalin, Jerry Blazey, Vishnu Zutshi ( detector hardware)
� Univ. of California @ Santa Cruz, Physics Dept.
Hartmut Sadrozinski, Ford Hurley (1st prototype detector )Hartmut Sadrozinski, Ford Hurley (1 prototype detector )
� University of Wollongong, Radiological Physics.
Scott Penfold ( image reconstruction)
� Loma Linda University Medical Center, California
Reinhard Schulte, Vladimir Bashkirov, Ford Hurley
California State Univ. @ San Bernerdino
Keith Schubert ( Computer Science Dept.)
Uncertainty in RLSP from XCT data can exceed 5% (Moyers, Medical Dosimetry, Oct 2010)
Why does X-ray CT give ambiguities for relative stopping powers?
• Two materials with different composition, A and Z, can have the same X-ray absorption coefficient but different relative stopping powers which is required to calculate dose in the patient for proton therapy
• Reason: the X-ray mass absorption coefficient is linear with electron density, but also varies in a complicated with electron density, but also varies in a complicated way with A ( atomic weight) and Z ( atomic number)
• X-rays: µ=ρe[ f((A,Z,Eγ) + g(A,Z,Eγ)] for each voxel• Hounsfield Unit = 1000 (µ/µwater) for each voxel• Protons : dE/dx = ρe(Z/A)/β2 [ log(2me β
2/I(1- β2)) - β2 ]• Conventional solution: Use tissue substitutes to measureµ in X-ray CT scanner and then measure relative dE/dx (RSP) in proton beam, plot data points and then interpolate.
RSP � WEPL � Relative Dose
The importance of relative stopping power in accurate dose and range determination
• For heterogeneous materials, the water equivalent path length or WEPL for each proton is related to relative stopping power (RSP), relative to water
• For the jth track through the ith voxel• Knowing the entrance and exit proton energy, the
quantity WEPL can be determined. Then solve for RSP’s• RSP(l) is then used in Tx planning to look up the correct
dose in each voxel from the spread out Bragg curve in water.
CsI Calorimeter response vs. WEPL of tissue equivalent polystyrene blocks for E(in) =200 MeV,
E(out)>50 MeVData from LLUMC August 2010
y = -46.184x2 - 22.945x + 249.06250
300
y = -46.184x2 - 22.945x + 249.06
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Advantages of pCt over X-ray CT
• Decrease the range error from 3% to 1% for better electron density map for proton Tx Planning => better dose accuracy to target volume. Range error is caused by RSP error
• Reduce or eliminate CT artifacts due to metal/dental • Reduce or eliminate CT artifacts due to metal/dental implants with high Z materials
• Lower dose ( factor 5) to patient relative to X-ray CT• pCT imaging could replace Cone Beam CT for patient
alignment verification before Treatment
Matrix Equation for finding dE/dx in each voxel
WEPLi = Aij RSPj is equation for the ith trackAij is the the path length, dl, through the jth voxel ij
For the ith track
Reconstruction problem is solving for RSPj
How many tracks are needed to get 1% RSP resolution in each voxel?
Answer: 100 tracks per voxelHow many 1 mm3 voxels in a 23 cm head diameter with
20 cm length?Answer :10E7 voxels => 10E9 tracks
Density Resolution per voxel vs. dose
Data from Schulte et. al., Medical Physics, April 2005
Proton CT Detector Layout
Four tracker detectors are also shown but not labeled. The inner tracking detectors are 15 cm from the center of the phantom and the outer ones are 30
cm from the center of the phantom.
High Level pCT detector specs for a head scanner
Design for maximum head size 23 cm diameter by 20 cm long (sup/inf direction)Number of 1 mm cubic voxels = 10E7Number of protons/voxel =100 ( for 1% density res.)Total Number protons = 10E9Data rate capability = 2 MHz ( protons per second)Total scan time = 7.5 minutes Total number of gantry angles => ContinuousGantry speed = 1/10 RPM
Specs on current pCT Scanner
• Maximum data rate is 100 kHz � 2 hours for a small head ( 14 cm diam.) using LLUMC synchrotron; much larger time for 20 cm area � 2 scans for each adult size 20 cm area � 2 scans for each adult size head
• Imaging area 9 x 18 cm• Position resolution ; 0.24 mm Si Strip pitch• CsI Energy resolution ≈ 1% above 100
MeV
Silicon Strip detector (1 of 8) and 18 channel CsIcalorimeter
Track reconstruction and WEPL Calibration
y = -46.184x2 - 22.945x + 249.06
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Response
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CsI calorimeter resolution vs. proton energy
Readout Scheme for pCT
Trigger Scheme for Data Acquisition
Prototype pCT detector built byLoma Linda University Medical Center
Northern Illinois UniversityUniv. of California, Santa Cruz
Lucy phantom for 1st 3D image reconstruction using 200 MeV protons. Phantom is 14 cm
polystyrene sphere
1st 3D pCT image slices from prototype detector (87.5 million tracks)
Proton CT ( on left) X-ray CT ( on right)
Reconstructed Stopping powersdata from Penfold
RSP(measured)
RSP(true)
Polystyrene 1.065 1.035
Bone 1.68 1.70
Lucite 1.19 1.20
Air 0.05 0.004
Relative stopping power, electron density and water equivalent thickness (WET)
S(Imedium,β)/S(Iwater,β) vs. proton velocity in v/cwhere dE/dx=S/ρ
Relative Stopping Power for Bone (ylw) and Muscle (blue)
1.02
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Velocity
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Relative ρe and RLSP