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Hartmut F.-W. Sadrozinski: pCT IEEE 2011 1

Detector Development for Proton Computed Tomography (pCT)

Hartmut F.-W. Sadrozinski

SCIPP, UC Santa Cruz, CA 95064 USArepresenting the pCT Collaboration

Update on my 2002 IEEE NSS-MIC talk: “Toward Proton CT”

SCIPPSCIPP


Alderson Head Phantom

Range Uncertainties(measured with PTR)

> 5 mm> 10 mm> 15 mm

Schneider U. (1994), “Proton radiography as a tool for quality control in proton therapy,” Med Phys. 22, 353.

RSP

H

Proton CT BasicsProton therapy and treatment planning requires the knowledge of the stopping power in the patient, so that the Bragg peak can be located within the tumor.

X-ray CT has been shown to give insufficiently accurate stopping power (S.P.) maps in complicated phantoms or from uncertainty in converting Hounsfield values to S.P.

The goal of Proton CT is to reconstruct a 3D map of the stopping power within the patient with as fine a voxel size as practical at a minimum dose, using protons (instead of x-rays) in transmission.

In a rotational scan the integrated stopping power is determined for every view by a measurement of the energy loss.


pCT Challenge #1: Multiple Coulomb Scattering

“The most likely path of an energetic charged particle through a uniform medium”D C Williams Phys. Med. Biol. 49 (2004) 2899–2911

The proton path inside the patient/phantom is not straight the path of every proton before and after the phantom

has to be measured and its path inside the patient reconstructed.

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14 16 18 20Depth inside Absorber [cm]

Disp

lacem

ent [

cm]

R M S = 4 9 0 u m

M L P w id th = 3 8 0 u m

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.40.5

0 2 4 6 8 10 12 14 16 18 20Depth inside Absorber [cm]

Disp

lacem

ent [

cm]

R M S = 4 9 0 u m

M L P w id th = 3 8 0 u m

M. Bruzzi et al IEEE Trans. Nucl. Sci.,54, 140 (2007)

From deflection and displacement, calculate the “Most Likely Path MLP”

Beam test with sub-divided phantom:MLP can be predicted with sub-mm precisionusing tracking detectors with ~ 80μm resolution


Tracking and measuring the residual energy of every proton requires fast sensors and fast data acquisition (DAQ).

Data Flow math:Assuming 100 protons / 1mm voxel and 180 views requires ~ 7*108 protons.With 10 kHz data rate, one pCT scan will take 20 hrs (requiring a very patient patient!).A scan with a proton rate of 2 MHz takes 6 min. N.B. such a scan will deliver a dose of 1.5 mGy.

Image ReconstructionTo reconstruct images with > 107 voxels using ~109 protons is NOT trivial. Our reconstruction code is already running on GPU’s in anticipation of the much higher data rates of the future.

pCT Challenge #1a: Proton Data rate


The proton energy loss is not fixed, but is a stochastic process. The straggling error is a function of depth, irreducible when energy is not measured.

the straggling within the phantom limits the precision of the energy loss measurement.

Challenge #2 to pCT: Range / Energy Straggling

WEPL Resolution vs. WEPL for different Plate Thickness' (200 MeV Protons)

3

3.2

3.4

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3.8

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0 50 100 150 200 250

WEPL [mm]

WEP

L R

MS

[mm

]

1 mm 3 mm 4 mm 6 mm5 mm proj.6 mm proj.8 mm proj.10 mm proj

Range straggling ~ 1% of range~ 1mm for 100 MeV, ~ 3mm for 200 MeV

Range counter always encounters the maximum range straggling: the error is independent of the WEPL of phantom (depends on proton energy)

WEPL = Water equivalent Path Length (of proton in phantom)

Geant4 Study:

Range Straggling vs. Energy

0.0

0.1

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0 50 100 150 200 250 300Proton Energy [MeV]

Sigm

a (R

) [g/

cm2 ]

0.0%

0.5%

1.0%

1.5%

2.0%

Sigm

a(R

)/R


Instrument Solutions to the pCT Challenge:

under constructionRange (>3mm) + direct SiPM orPolystyrene Calorimeter + PMT

Si SSD“Slim edges”

LLU /UCSC

under constructionRange (3mm) + WLSF + SiPM

SciFi + SiPMNIU / FNAL

100 - 200CsI + P.D.Si SSDLLU / UCSC / NIU

68Fast crystal calorimeter + P.D.

SI SSDFirenze / LNS(V. Sipala et al., MIC15.S-305)

100under construction

Range (3mm) + WLSF + SiPM

GEMTERA / CERN+ upgrade

Proton Energy [MeV]

Energy DetectorTrackerGroup


Proton Range Radiography

PRR10 Prototype

• 10x10cm active area• GEM detectors for tracking• about 10kHz aquisition speed• 30 3mm thick plastic scintillator for range

“Construction, test and operation of a proton range radiography system”U. Amaldi et al., NIMA, 629 (2011) pp 337‐344

1x1mm SiPM

1mm WLS fiber


Proton Range Radiography

New 30x30cm active area prototype is currently being developed. Along with being large enough to perform a full head‐sized proton radiography, an entirely new readout front‐end electronics is being developed which can achieve rates of 1MHz / channel. Expected completion, early 2012.

Images made with 100MeV protons and custom phantom. Smallest hole size is 1mm diameter.

PSI beam test (June 2010)

CNAO beam test (Aug 2011)

Imaging tissue equivalent cylinders.

Lung (.20)

Trabecular bone (1.16)

Breast 50/50 (.99)


Fiber tracker + Range detectorFiber tracker + Range detectorFPGA based DAQ, 20 MHz acquisition rate, 100 MHz clockSiPM for both subsystem, 100 ns pulse separation (10 MHz seems possible), impressive vendor listArea 18 X 36 cm2, no overlap, total number of channels: Tracker ~ 2160, Range Detector ~ 100

SFT TRACKER + SC RANGE DETECTOR

100 plates, 3 mm, Polystyrene Scintillator

““FiberFiber”” scanner, NIU scanner, NIU -- FNALFNALG. Blazey G. G. CoutrakonCoutrakonaa, B. , B. ErdelyiErdelyiaa, A. , A. DyshkantDyshkantaa, E. , E. JohnsonJohnsonaa N. N. KaronisKaronisaa, , PenfoldPenfolddd, , VV. . RykalinRykalinaa, ,

P. P. RubinovRubinovee , G. , G. SilbergSilberg, V. , V. ZutshiZutshiaa , P. , P. WilsonWilsonee

a a Northern Illinois University, Northern Illinois University, dd University of Wollongong, University of Wollongong, e e FERMILABFERMILAB


Total number of channels ~ 120One plate dimensions 27 X 36 X 0.3 cm3 Digital or analog readout ?SiPM readout through 1.2 mm WLS fiberSiPM is of 1.3 mm diam.

~ 22 PEP, 200 MeV

1 PE ~ 50 ADC counts

Prototype, 18 X 36 cmPrototype, 18 X 36 cm22 Sc plate with Sc plate with grooved WLS fiber grooved WLS fiber

Large signal dispersion: # of p.e.from width of curve: 13


Position Resolution:Position Resolution:~ 4 ~ 4 –– 6 x that of 6 x that of SiSi SSDSSD


~ 33 PE,p, 200 MeV

11 PE ~ 21 ADC counts

~ 33 PE, 36 cm Scint. Fiber , trigger fiber covers 5 cm from the far end. SF, Green, Polystyrene,

KURARAY, 1 mm, 2 clad., 3HF, Al spattering .

Scintillating Fiber (Scintillating Fiber (““SciFiSciFi””) Tracker ) Tracker with with SiPMSiPM ReadoutReadout

Large signal dispersion:# of p.e from width of curve: 9


LLU-UCSC-NIU CollaborationB. Colby, D. Fusi, R. Johnson, S. Kashiguine, F. Martinez-McKinney, J. Missaghian,

H. F.-W. Sadrozinski, M. ScaringellaSCIPP, UC Santa Cruz, CA 95064 USA

V. Bashkirov, F. Hurley, S. Penfold, R. Schulte Loma Linda University Medical Center, CA 92354 USA

G. Coutrakon, B. Erdelyi, V. RykalinNorthern Illinois University

S. McAllister, K. SchubertCSU San Bernardino

SCIPPSCIPP

2003 2010


The LLU-UCSC-NIU Prototype Scanner

Optical Interface Photodiode

R. W. Schulte, et al.,,

IEEE Trans. Nucl. Sci., 51,, pp 866, 2004.


CT Image Reconstruction1.1. WEPL calibration and cutWEPL calibration and cut2.2. Correction for overlaps in Correction for overlaps in SiSi

trackertracker3.3. Correction matrix with Correction matrix with

Calorimeter responseCalorimeter response4.4. Angular and Angular and spatial spatial binningbinning5.5. Filtered Back Projection and Filtered Back Projection and

Iterative Algebraic Iterative Algebraic reconstructionreconstruction6.6. MLP formalism for final MLP formalism for final

reconstructionreconstruction2.5 mm slice0.65 mm voxels

Reality Check:We accumulated data for this reconstructed image during 4 hours at 20 kHz trigger rate. This is not acceptable for clinical applications ! Next development step: 50x faster pCT scanner

15

0.050.004Air

1.191.20Lucite

1.681.70Bone

1.0351.037Polystyrene

RSP reconstructed from Measurement

Predicted RSPMaterial

air

bone

lucite

polyst.


WEPL Calibration Response of CsI Calorimeter to different Degrader Depth

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ADC

# pe

r 10

AdC

0 mm 52.9 mm132.3 mm 185.2 mm211.59 mm 224.9 mm243.16 mm

CsI Calorimeter Response

y = 10.574x

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Simulated Energy [MeV]

ADC

[a.u

.]

Errors vs. WEPL

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WEPL [mm]

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WEPL vs. CsI Calorimeter Response

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0 500 1000 1500 2000 2500Calorimeter Response [a.u.]

WEP

L [m

m]

F. Hurley et al., subm. to MEDICAL PHYSICS

CsI spectra for various degrader depth Linearity of CsI calorimeter

E(proton) = 200 MeVPolystyrene degraders

Degrader WEPL vs. Calorimeter Response WEPL & Energy RMS vs. Degrader WEPL

Goal:Direct determination of WEPL

from calorimeter response, without converting to MeV


WEPL Resolution: CsI vs. Range CounterGeant4 Simulation for 200 MeV Protons, w/o detector threshold effects

Plate thickness resolution < WEPL RMSThicker plates viable for 200 MeV?

WEPL RMS is constant as expected, range counter measures straggling in phantom/degrader + range counter!

CsI and Range Counter have similar relative WEPL resolution,at small WEPL (high proton energy)

Range Counter superiorat large WEPL (low proton energy)

CsI superior, reaches 1%!

(4mm Polystyrene)WEPL Error vs. Degrader Thickness

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WEPL [mm]

WEP

L R

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[mm

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4 mm Plate Resolution

Range Counter with Straggling

WEPL Error vs. Degrader Thickness

1%

10%

100%

0 50 100 150 200 250WEPL [mm]

Rea

ltive

WEP

L R

MS

[%] CsI Calorimeter

4 mm Range Counter


Increase Size 2x : 40 cm x 10 cmImprove data throughput 50x:

2MHz sustained proton rate with minimal pile-upSi sensors are intrinsically fast, built faster readout ASIC and distributed DAQData stream uses local FPGA for data collection, formatting and transmission

Improve speed of energy detector:CsI calorimeter replaced with faster plastic scintillatorBoth range counter and calorimeter under testPolystyrene Range Counter with direct SiPM readout looks very promising

(~3x p.e. wrt to WLSF readout?)Geant4 results on Range Counter with thicker tiles is intriguing

Improve tiling of Si sensors:Si SSD are attractive since they have low noise at good efficiency,

an important factor in a sparse system (no redundant space points)“slim edges” allow tiling without overlap

LLU-UCSC-CSUSB Head Scanner

R. Johnson, H. F.-W. Sadrozinski, D. Steinberg, A. Zatserklanyi,

V. Bashkirov, F. Hurley, S. Penfold, R. Schulte, S. McAllister, K. Schubert

NIH Grant 1R01EB013118-01SCIPPSCIPP


Range Counter with Direct SiPM Readout4.2 mm Polystyrene

1p.e. = 40

Signal = 100 p.e. !!

Width of signal distribution signal > 30 p.e.

Very,

very

prelim

inary


Si Sensor Improvement: Slim Edges (with NRL)Si SSD with 900μm dead edge

Slim edges:reduce dead edge from 1mm to < 200 μmExcellent breakdown behaviorCurrent at 150V:

~10 nA/cm with guard ring~100 nA/cm without guard ring

Charge collection unchanged

See V. Fadeyev’s talk N7-1

Edge Treatment:

Laser + XeF2 scribing

Cleaving

PECVD Passivation

with guard ring

Cut within 50 μm Of Guard Ring Guard Ring Cut (!)


ConclusionsProton CT has come a long way since my talk at the 2002 IEEE NSS-MIC Symposium in Norfolk, VA.We see very different approaches on instruments, motivated in part by a technology transfer from HEP. This has come with severe limitations (proton rate!).We are starting to reconstruct very clear and sophisticated radiographs AND CT images, and are actively improving reconstruction algorithms.We are now arriving at a new phase in pCT: we have dedicated detector development, with focus on speeding up the data taking to be useful in clinical applications.End-to-end simulation of the instrument has been essential for our understanding of the requirements and proper choice of the technical solution, yet many lessons were learned during operation of the scannersNext (big) step: clinical application.

Ongoing and unwavering support by Prof. James M. Slater (LLUMC) made this project possible.

We acknowledge support from the US National Institute of Health under the grant NIH Grant 1R01EB013118-01

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