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

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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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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)

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Relative ρe and RLSP