Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP

Initial Studies in Proton ComputedTomography

L. R. 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 95064V. Bashkirov, R. W. M. Schulte, K. Shahnazi

Loma Linda University Medical Center, Loma Linda, CA 92354

• Proton Energy Loss in Matter

• Proton Tomography / Proton Transmission Radiography

• Proton Transmission Radiography Data

• Proton Transmission Radiography MC Study

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPComputed Tomography (CT)

X-ray tube

Detector array

• Based on X-ray absorption• Faithful reconstruction of

patient’s anatomy• Stacked 2D maps of linear

X-ray attenuation• Coupled linear equations• Invert Matrices and find

(hopefully) non-malignant structures

• Proton CT replaces X-ray absorption with proton energy loss

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Radiography: X-rays vs. Protons

Attenuation of Photons, ZN(x) = Noe- µ x

Energy Loss of Protons, ρ∑∫ ∆≈=∆ l

dxdEdx

dxdEE ρ

NIST Data

dxdE

dldE ρ=

0.01

1

100

104

1 10 100 1000

X-Ray Absorption Coefficient

BoneMuscleH2OFat

µ

X-Ray Energy [keV]

[1/cm]

1

10

100

10 100 1000

Stopping Power for Protons

Bone MuscleH2OFat

dE/dl[MeV/cm]

Proton Energy E [MeV]

Low Contrast: ∆ρ∆ρ∆ρ∆ρ = 0.1 for tissue, 0.5 for bone

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Advantages of Protons in Therapy

NIST Data

0

100

200

300

0

5

10

0 10 20 30 40 50

Proton Energy Loss in H2O

Proton Energy [MeV]

Energy Deposit in 1mm

[MeV/mm]

Water Depth [cm]

E = 130 MeV E = 250 MeV

• Relatively low entrance dose (plateau)

• Maximum dose at depth (Bragg peak)

• Rapid distal dose fall-off

• Energy modulation (Spread Bragg peak)

• RBE close to unity

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Use of Proton Beam CT: Treatment Planning

Alderson Head Phantom

Range Uncertainties(measured with PTR)

> 5 mm> 10 mm> 15 mm

Schneider U. & Pedroni E. (1995), “Proton radiography as a tool for quality control in proton therapy,” Med Phys. 22, 353.

X-ray CT use in Proton Cancer Therapy can lead to large Uncertainties in Range Determination

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP

0

0.2

0.4

0.6

0.8

1Energy Loss in Water

Energy Loss

[MeV/mm]

150

200

250

0 5 10 15 20 25

Proton Energy

Proton Energy[MeV]

Depth in H2O [cm]

Low Contrast in Proton CT

ρρρρ

ρρρρ

1.01.01.01.01.11.11.11.11.51.51.51.52.02.02.02.0

Inclusion of 1cm depth at midpoint of 20cm H2O

132537.2158.92.0131737.7161.51.5131138.1163.61.1130938.2164.11.0

TOF[ps]

Range [cm]

Energy[MeV]

ρ

[g/cm2]

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Proton CT Measurements

Requirements: • Proton location to few hundred um• Proton angle to a degree

• Average Proton Energy <E> to better than %

• Improve energy determination with statistics?

• Problem: Dose D = Absorbed Energy / MassVoxel with diameter d = 1mm 106 protons of 200 MeV = 7.2 [cGy]

• In order to minimize the dose, the final system needs to employ the best energy resolution!

NE

Eσσ =><

%1≈Eσ

52

2

~ D

AN D

d

dxdE

E

⋅∆

⋅=

ρσ

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Development of Proton Beam Computed Tomography

• Exploratory Study in Proton Radiography– two x-y detector modules– Crude phantom in front

• Theoretical Study– GEANT4 MC simulation– influence of MCS and range straggling– importance of angular measurements– Optimization of energy

• Experimental Study in pCT – Three or four x-y Si planes– water phantom on turntable

Collaboration Loma Linda University Medical Center – UC Santa Cruz

Protonbeam

Simodule 2

Simodule 1

Water phantom

Turntable

Scatteringfoil

Simodule 3

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP Proton Energy Measurement with LET in Si

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

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP GLAST Front-End Electronics ASIC

Binary Readout: •Low-power (~200uW/channel) •Peaking time ˜ 1.3 ms•Low noise (Noise occupancy <10-5)•Threshold set in every ASIC•Separate Masks for Trigger and Readout in every Channel•Self - Trigger = OR of one Si plane

(1536 channels)

Pulse Charge:Time – over-Threshold on the OR of every Si plane

Distinguish single tracks from two tracks in one strip

Electron Events

Photon Events

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP

TOT ∝∝∝∝ charge ∝∝∝∝ LET

Charge ~ Time-Over-Threshold (TOT):

Digitization of Position and Energy Deposit with large Dynamic Range

PulseThreshold

Time-over-Threshold TOT

0

20

40

60

80

100

120

0 50 100 150 200

TOT Measurement vs Charge in MIP'sEffect of Threshold and Voltage

TOT SLACTOT LLUMC

Input Charge [fC]

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPProton Energy Measurement with LET

10

100

1 10 100 1000 104

TOT vs. Proton EnergyMeasurement vs. Expectation

TOT & Resolution measuredTOT expected

Proton Energy [MeV]

LLUMCSynchrotron P Beam

GLAST SLAC Test Beam

TOT Saturation

0

0.1

0.2

0.3

0.4

10 100 10000

0.1

0.2

0.3

0.4

Proton Energy [MeV]

TOT Saturation

Derive Energy Resolution from TOT vs. E Plot

Good agreement between measurement and MC simulations

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPExploratory Proton Radiography Set-up

Use Loma Linda University Medical Ctr 250 MeV Proton BeamDegraded down to 130 MeV by 10” Wax BlockObject is Aluminum pipe 5cm long, 3cm OD, 0.67cm ID Very large effects expected, but beam quite non-uniformSilicon detector telescope with 2 x-y modules

Beam from Synchrotron

30 cm

Object

Wax block

y x x y

Air

27.3 cm

Air

250MeV 130MeV 60+130MeV

Si Modules

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPImage !

Subdivide SSD area into pixels1. Strip x strip 194um x 194um2. 4 x 4 strips (0.8mm x 0.8mm)

Image is average energy in pixel

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPEnergy Resolution = Position Resolution

Hole “filled in”

“Fuzzy” Edges

Slice of 4x4 pixels

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPGEANT4 MC: Energy Reconstruction

NIST Data

Energy Loss in Si Energy Reconstructed

from Energy Loss in Si

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPMC: Loss of Resolution in Back

First Plane, 2cm behind Object

Second Plane, 30cm behind Object: Fuzzy

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPMultiple Scattering: Migration

Protons scatter OUT OF Target (not INTO).

Those have larger energy loss, larger angles, fill hole, dilute energy

Features:Washed out image in 2nd plane (30cm downstream)

Energy diluted at edges and in hole (Fuzzy edges,Hole filled partially)

Migration of events

are all explained by Multiple Coulomb Scattering MCS

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPMigration: MC

Dilution: Protons entering the Object in Front Facebut leaving it before the Rear Face

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPMC: Use Angular Information

Effect of Angular Cut: Energy more uniform

Less Migration

Sharp edges (RMS and Energy)

Hit Profile after angle cut

Hit Profile before angle cut

ΘΘΘΘ

≈≈≈≈ MCS angle

Imaging with MCS Angle?

Vertex2002 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPPConclusions

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

Next steps: pCT