INITIAL STUDIES on PROTON COMPUTED TOMOGRAPHY USING SILICON STRIP DETECTORS

RESMDD'02 pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP

INITIAL STUDIES on PROTON COMPUTED TOMOGRAPHY USING

SILICON STRIP DETECTORSL. 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


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


SCIPPSCIPP Radiography: X-rays vs. Protons

10

100

1 10 100

Stopping Power for Protons

MuscleBone H2OFat

E [MeV]

0.01

0.1

1

10

100

1000

104

0.001 0.01 0.1 1

X-Ray Absorption Coefficient

Muscle

Bone

Water

Air

Energy [MeV]

[1/cm]

Attenuation of Photons, zN(x) = Noe- x

Energy Loss of Protons, dx

dx

dEE

NIST Data


SCIPPSCIPP Proton Radiography: Density Map

]}[:];/[:{ 2 cmlcmgxdldx

dEdx

dx

dEE

1

10

100

1000

1

10

100

1000

1 10 100 1000

Stopping Power : Density Effect

C

Si

Cu

C

Si

Cu

Energy [MeV]

rho = 9

rho = 2

NIST Data


SCIPPSCIPP 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

Protonbeam

Simodule 2

Simodule 1

Water phantom

Turntable

Scatteringfoil


SCIPPSCIPP 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


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


SCIPPSCIPP

TOT charge LET!

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]

Charge ~ Time-Over-Threshold (TOT):

Digitization of Position and Energy with large Dynamic Range

Pulse

Threshold

Time-over-Threshold TOT


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

TOT Spectra as for several proton energies

Mean TOT vs. Proton Energy

Good agreement between measurement and MC simulations


SCIPPSCIPPProton Energy Measurement with LET

0.01

0.1

1

10

10 100 1000 104

Resolution of TOT System

LETEnergy

0.01

0.1

1

10

Proton Energy [MeV]

TOT Saturation

0

20

40

60

80

100

0 50 100 150 200 250 300

TOT Measured

ToT measured

y = 964.83 * x^(-0.79073) R= 0.99983

E(MeV)

dTOT/dE = -965*0.79*E-1.79

=-763*E-1.79

TOTTOTE E

TOT

TOT

E 1

TOT Spectra vs. energy

TOT Resolution “~flat”.

Energy Measurement possible where slope dTOT/dE is large


SCIPPSCIPPProton Localisation: M.S. vs. Energy Resolution

0

0.1

0.2

0.3

0.4

0.5

0

0.01

0.02

0.03

0.04

0.05

0 100 200 300 400 500

Proton Energy Error and Mult. Scattering Angle L = 10cm H

2O

E error

M.S. Angle

E [MeV]

With 1000 EventsDetermine Energy Loss to % Level


SCIPPSCIPPExploratory Proton Radiography Set-up

Beam from Synchrotron

30 cm

1 2 - SSD modules

Object

Wax block

1 2 4 3 - SSD detector planes

y x x y

Air

27.3 cm

Air

250MeV 130MeV 60+130MeV

Use Loma Linda University Medical Ctr 250 MeV Proton BeamDegraded down to 130 MeV by Wax BlockObject is Aluminum pipe 5cm long, 3cm OD, 0.67cm ID Very large effects expected, but beam quite non-uniform


SCIPPSCIPPImage !

Subdivide SSD area into pixels1. Strip x strip 194um x 194um2. 4 x 4 strips (0.8mm x 0.8mm)Image given by average TOT or Energy in pixel


SCIPPSCIPPIssues

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


SCIPPSCIPPLoss of Resolution in Back: Data


SCIPPSCIPPMigration 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

Approx. Beam Profile

Excess Events

Energy lowered

Blurred Edges


SCIPPSCIPPMultiple Scattering: Emigration

Protons scatter OUT OF Target (not INTO). Those have

larger energy loss

larger angles

fill hole

dilute energy


SCIPPSCIPPEnergy Resolution = Position Resolution

Simulation reproduces spread of energy and loss of resolution

Data (LET converted to Energy) GEANT4 MC (LET in SSD)

RMS


SCIPPSCIPPEnergy Resolution

Data:LET converted to Energy

MC:LET in SSD plane

Object Background


SCIPPSCIPPMigration: MC

Dilution by events entering the Object but leaving it before the end


SCIPPSCIPPMC: Loss of Resolution in Back

First Plane, 2cm behind Object

Second Plane, 30cm behind Object


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 to be explored

Next steps: pCT

Documents

INITIAL STUDIES on PROTON COMPUTED TOMOGRAPHY USING SILICON STRIP DETECTORS