Towards Proton Computed Tomography - Welcome...

2002 IEEE NSS/MIC pCT: Hartmut F.-W. Sadrozinski , SCIPP

SCIPPSCIPP

Towards Proton Computed Tomography

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


SCIPPSCIPPComputed Tomography (CT)

X-ray tube

Detector array

CT:• 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 reconstruct z-

dependent features

Proton CT: • replaces X-ray absorption with

proton energy loss • reconstruct mass density (ρ)ρ)ρ)ρ)

distribution instead of electron distribution


SCIPPSCIPP

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]

Radiography: X-rays vs. Protons

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

Energy Loss of Protons, ρρρρ

NIST Data

dxdE

dldE ρ=

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

Measure statistical process of X-ray removal Measure energy loss on individual protons

∑∫ ∆≈=∆ ldxdEdx

dxdEE ρ

Bethe-Bloch


SCIPPSCIPPNegative Slope in the Bethe-Bloch Formula

•Relatively low entrance dose

(plateau)

• Maximum dose at depth

(Bragg peak)

• Rapid distal dose fall-off

• RBE close to unity0

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

Imaging Treatment


SCIPPSCIPPProtons vs. X-Rays in Therapy

Protons:

• Energy modulation

spreads the Bragg peak across the malignancy

X-rays:

• High entrance dose

• Reduced dose at depth

• Slow distal dose fall-off

leads to increased dose

in non-target tissue


SCIPPSCIPPMilestones of Proton Computed Tomography

• R. R. Wilson (1946)Points out the Bragg peak, defined range of protons

• A. M. Cormack (1963)Tomography

• M. Goitein (1972)2-D to 3-D, Simulations

• A. M. Cormack & Koehler (1976) Tomography, ∆ρ∆ρ∆ρ∆ρ ≈≈≈≈ 0.5 %

• K. M.Hanson et al. (1982)Human tissue, Dose advantage

• U. Schneider et al. (1996)Calibration of CT values, Stoichiometric method

• T. J. Satogata et al. (Poster M10-204) Reduced Dose of Proton CT compared to X-Ray CT


SCIPPSCIPP What is new in pCT ?

• Increased # of Facilities with gantries etc.See the following talk by Stephen G. Peggs)

• 2 Ph.D. Theses at PSI and Harvard Cyclotron(U. Schneider & P. Zygmanski)

• Existence of high bandwidth detector systems for protons– semiconductors – high rate data acquisition ( > MHz)– large-scale (6”wafers)– fine-grained (100’s um pitch)

• Concerted simulation effort – Exploitation of angular and energy correlations– Support of data analysis– Optimization of pCT set-up (detector, energy, ..)– Dose calculation


SCIPPSCIPP Potential of Proton 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

Proton CT can measure the density distribution needed for range calculation.

There is an expectation (hope?) that with pCT the required dose can be reduced.


SCIPPSCIPP Low Contrast in Proton CT

ρρρρ

132537.2158.92.0131737.7161.51.5

131138.1163.61.1

130938.2164.11.0

TOF[ps]

Range [cm]

Energy[MeV]

ρ∗ρ∗ρ∗ρ∗ l[g/cm2]

150

200

250

0 5 10 15 20 25

Proton Energy

Proton Energy[MeV]

Depth in H2O [cm]

0

0.2

0.4

0.6

0.8

1Energy Loss in Water

dE (250)dE (250)+1.1dE (250)+1.5dE (250)+2

Energy Loss

[MeV/mm]ρρρρ

1.01.01.01.01.11.11.11.11.51.51.51.52.02.02.02.0

Sensitivity Study:Inclusion of 1cm thickness and density ρρρρat midpoint of 20cm H2O


SCIPPSCIPPRequirements for pCT Measurements

Tracking of individual Protons requires Measurement of:

NE

Eσσ =><

%1≈Eσ

dxdE⋅=

AN D

• Proton angle to much better than a degreeMultiple Coulomb Scattering ΘΘΘΘMCS≈≈≈≈1o

• Proton location to few hundred um

• Average Proton Energy <E> to better than %

• In order to minimize the dose, the final system needs the best energy resolution! Energy straggling is 1- 2 %.

• Improve energy determination with statistics

• Issue: Dose D = Absorbed Energy / MassN/A = Fluence( for Voxel with diameter d = 1mm105 protons of 200 MeV = 7 [mGy])

0/6.13 Xlzp

MeVCMS ⋅

≈β

θ


SCIPPSCIPPDose vs. Voxel Size for pCT Measurements

10-6

0.0001

0.01

1

10-6

0.0001

0.01

1

0.01 0.1 1 10

pCT: Contrast - Voxel Size - Dose 200 MeV Protons, 3Stdv.

d=5mm, Sid=2mm, 2%d=5mm, 2%d=1cm, 2%

Dose [mGy]

Density Difference [g/cm 3]

Voxel Diameter

Define voxel of volume d3

Dose in voxel = DvTake n =20cm/d settings Total dose D = n* Dv

52

2

~ Dd

E

⋅∆ ρσ

Trade-off between Voxel size and Contrast (∆ρ∆ρ∆ρ∆ρ) to minimize the Dose

Require 3σσσσ Significance


SCIPPSCIPP Studies in Proton Computed Tomography

• Exploratory Study in Proton Transmission Radiography– Silicon detector telescope– Simple phantom in front– Understand influence of multiple scattering and energy resolution on image

• Theoretical Study (GEANT4 MC simulation)– Evaluation of MCS, range straggling, and need for angular measurements– Optimization of energy

Collaboration Loma Linda University Medical Center – UC Santa Cruz


SCIPPSCIPPExploratory Proton Radiography Set-up

Degraded down to 130 MeV by 10” wax block

27.3 cm30 cm

Beam from Synchrotron

Object

Wax block Air Air

250MeV 130MeV 50 + 130MeV

y x x ySi Modules

Proton Beam from Loma Linda University Medical Ctr @ 250 MeV

Object is aluminum annulus 5 cm long, 3 cm OD, 0.67 cm ID Very large effects expected, x = ρρρρ*l = 13.5 g/cm2

Traversing protons have 50 MeV, by-passing protons have 130 MeV

Silicon detector telescope with 2 x-y modules: measure energy and location of exiting protons


SCIPPSCIPPSilicon Detector Telescope

• GLAST Readout

1.3 µs shaping time

Binary readout

Time-over-Threshold TOT

Large dynamic range

6.4 cm

6.4

cm

5 x 64 channels

Simple 2D Silicon Strip Detector (SSD) Telescope of 2 x-y modules

built for Nanodosimetry

• 2 single-sided SSD / module

measure x-y coordinates

• GLAST Space Mission developed SSD

194 µm pitch, 400 µm thickness


SCIPPSCIPP

PulseThreshold

Time-over-Threshold TOT TOT ∝∝∝∝ charge ∝∝∝∝ LET

Time-Over-Threshold ~ Energy Transfer

Digitization of position (hit channel) and energy deposit (TOT)

0

20

40

60

80

100

120

0 50 100 150 200

TOT Measurement vs Charge

TOT [us]

Input Charge [fC]


SCIPPSCIPPCalibration of Proton Energy vs. TOT

Derive energy resolution from TOT vs. E plot

Good agreement between measurement and MC simulations

0

50

100

150

200

250

300

0 50 100 150

TOT Spectra for Low-energy Protons

250MeV40MeV24MeV17MeV

TOT [us]

10

100

10 100 1000 104

TOT vs Proton EnergyMeasurement and Expectation

ToT measuredTOT expected

TOT [us]

Proton Energy [MeV]

LLUMCSynchrotron P Beam

GLAST SLAC Test Beam

TOT Saturation

0

0.1

0.2

0.3

0.4

10 100 1000

Proton Energy Resolution

0

0.1

0.2

0.3

0.4

σσσσΕΕΕΕ/Ε/Ε/Ε/Ε

Proton Energy [MeV]


SCIPPSCIPPImage of Al Annulus

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

Image corresponds to average energy in pixel


SCIPPSCIPP Energy Resolution => Position Resolution

Slice of average pixel energyin 4x4 pixels(need to apply off-line calibration!)

Clear profile of pipe,but interfaces are blurred

Hole “filled in”

“Fuzzy” Edges


SCIPPSCIPP Multiple Scattering: Migration

Protons scatter OUT OF target (not INTO).

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

Image Features:

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

Energy diluted at interfaces

(Fuzzy edges, Large RMS, Hole filled in partially)

Migration of events

are explained by Multiple Coulomb Scattering MCS


SCIPPSCIPP GEANT4 MC: Use of Angular Information

Angular distributions well understood

Si Telescope allows reconstruction of beam divergence and scattering angles

Select 2 Areas in both MC and DataA = inside annulus : Wax + Al B = outside annulus : Wax only


SCIPPSCIPPGEANT4 MC: Migration

Protons entering the object in front facebut leaving it before the rear face

Beam profile in slice

Energy of protons

entering front face

Migration out of object


SCIPPSCIPP

Energy Profile before (after ) Angle Cut

Energy RMS before (after ) Angle Cut

GEANT4 MC: Use of Angular Information

Less Migration

Angular Cut at ΘΘΘΘMCS of the Wax

Sharp Edges (Energy Average)

Sharp Edges (Energy RMS)

Angular cut improves the contrast at the interfaces


SCIPPSCIPP ConclusionsPresent status of pCT:

• Long tradition, increased interest with many new proton accelerators

(see next talk by Stephen G. Peggs)

• pCT will be useful for treatment planning

(reconstruction of true density distribution)

• Potential dose advantage wrt X-rays

( see Poster M10-204 by Satogata et al. )

• Use of GEANT4 simulation program aids in planning of experiments

(correlation of energy and angle, “migration”)

(see Poster M6-2, L. R. Johnson et al.)

Our future plans:• Optimization of beam energy

• Investigation of optimal energy measurement method

• Dose – contrast - resolution relationship on realistic phantoms

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Towards Proton Computed Tomography - Welcome...

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