Detector Development for Proton Computed Tomography (pct) Hartmut F.-W. Sadrozinski

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1 SCIPP Detector Development for Proton Computed Tomography (pct) Hartmut F.-W. Sadrozinski SCIPP, UC Santa Cruz, CA USA representing the pct Collaboration Update on my 2002 IEEE NSS-MIC talk: Toward Proton CT Hartmut F.-W. Sadrozinski: pct IEEE

2 Proton CT Basics Proton 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. Range Uncertainties (measured with PTR) > 5 mm > 10 mm > 15 mm RSP Schneider U. (1994), Proton radiography as a tool for quality control in proton therapy, Med Phys. 22, 353. Alderson Head Phantom H 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. Hartmut F.-W. Sadrozinski: pct IEEE

3 pct Challenge #1: Multiple Coulomb Scattering 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. From deflection and displacement, calculate the Most Likely Path MLP Beam test with sub-divided phantom: MLP can be predicted with sub-mm precision using tracking detectors with ~ 80μm resolution The most likely path of an energetic charged particle through a uniform medium Depth inside Absorber [cm] D C Williams Phys. Med. Biol. 49 (2004) Hartmut F.-W. Sadrozinski: pct IEEE M. Bruzzi et al IEEE Trans. Nucl. Sci.,54, 140 (2007) Displacement [cm] RM S = 490um M LP width = 380 um

4 pct Challenge #1a: Proton Data rate 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*10 8 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 Reconstruction To reconstruct images with > 10 7 voxels using ~10 9 protons is NOT trivial. Our reconstruction code is already running on GPU s in anticipation of the much higher data rates of the future. Hartmut F.-W. Sadrozinski: pct IEEE

5 Challenge #2 to pct: Range / Energy Straggling 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. Geant4 Study: Range straggling ~ 1% of range ~ 1mm for 100 MeV, ~ 3mm for 200 MeV Sigma (R) [g/cm 2 ] Range Straggling vs. Energy 2.0% 1.5% 1.0% 0.5% 0.0% Proton Energy [MeV] Sigma(R)/R Range counter always encounters the maximum range straggling: the error is independent of the WEPL of phantom (depends on proton energy) WEPL Resolution vs. WEPL for different Plate Thickness' (200 MeV Protons) WEPL RMS [mm] WEPL [mm] 1 mm 3 mm 4 mm 6 mm 5 mm proj. 6 mm proj. 8 mm proj. 10 mm proj WEPL = Water equivalent Path Length (of proton in phantom) Hartmut F.-W. Sadrozinski: pct IEEE

6 Instrument Solutions to the pct Challenge: Group Tracker Energy Detector Proton Energy [MeV] TERA / CERN + upgrade GEM Range (3mm) + WLSF + SiPM 100 under construction Firenze / LNS (V. Sipala et al., MIC15.S-305) SI SSD Fast crystal calorimeter + P.D. 68 LLU / UCSC / NIU Si SSD CsI + P.D NIU / FNAL SciFi + SiPM Range (3mm) + WLSF + SiPM under construction LLU /UCSC Si SSD Slim edges Range (>3mm) + direct SiPM or Polystyrene Calorimeter + PMT under construction Hartmut F.-W. Sadrozinski: pct IEEE

7 Proton Range Radiography PRR10 Prototype 10x10cm active area GEM detectors for tracking about 10kHz aquisition speed 30 3mm thick plastic scintillator for range 1x1mm SiPM 1mm WLS fiber Construction, test and operation of a proton range radiography system U. Amaldi et al., NIMA, 629 (2011) pp Hartmut F.-W. Sadrozinski: pct IEEE

8 Proton Range Radiography PSI beam test (June 2010) Images made with 100MeV protons and custom phantom. Smallest hole size is 1mm diameter. CNAO beam test (Aug 2011) Lung (.20) Trabecular bone (1.16) Imaging tissue equivalent cylinders. Breast 50/50 (.99) 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 Hartmut F.-W. Sadrozinski: pct IEEE

9 Fiber scanner, NIU - FNAL G. Blazey G. Coutrakon a, B. Erdelyi a, A. Dyshkant a, E. Johnson a N. Karonis a, Penfold d, V. Rykalin a, P. Rubinov e, G. Silberg,, V. Zutshi a, P. Wilson e a Northern Illinois University, d University of Wollongong, e FERMILAB Fiber tracker + Range detector FPGA based DAQ, 20 MHz acquisition rate, 100 MHz clock SiPM for both subsystem, 100 ns pulse separation (10 MHz seems possible), impressive vendor list Area 18 X 36 cm 2, no overlap, total number of channels: Tracker ~ 2160, Range Detector ~ 100 SFT TRACKER + SC RANGE DETECTOR 100 plates, 3 mm, Polystyrene Scintillator Hartmut F.-W. Sadrozinski: pct IEEE

10 Total number of channels ~ 120 One plate dimensions 27 X 36 X 0.3 cm 3 Digital or analog readout? SiPM readout through 1.2 mm WLS fiber SiPM is of 1.3 mm diam. ~ 22 PE P, 200 MeV 1 PE ~ 50 ADC counts Prototype, 18 X 36 cm 2 Sc plate with grooved WLS fiber Large signal dispersion: # of p.e.from width of curve: 13 Hartmut F.-W. Sadrozinski: pct IEEE

11 Position Resolution: ~ 4 6 x that of Si SSD Hartmut F.-W. Sadrozinski: pct IEEE

12 Scintillating Fiber ( SciFi( SciFi ) ) Tracker with SiPM Readout 11 PE ~ 21 ADC counts ~ 33 PE, p, 200 MeV SF, Green, Polystyrene, KURARAY, 1 mm, 2 clad., 3HF, Al spattering. ~ 33 PE, 36 cm Scint. Fiber, trigger fiber covers 5 cm from the far end. Large signal dispersion: # of p.e from width of curve: 9 Hartmut F.-W. Sadrozinski: pct IEEE

13 LLU-UCSC-NIU Collaboration SCIPP B. Colby, D. Fusi, R. Johnson, S. Kashiguine, F. Martinez-McKinney, J. Missaghian, H. F.-W. Sadrozinski, M. Scaringella SCIPP, UC Santa Cruz, CA USA V. Bashkirov, F. Hurley, S. Penfold, R. Schulte Loma Linda University Medical Center, CA USA G. Coutrakon, B. Erdelyi, V. Rykalin Northern Illinois University S. McAllister, K. Schubert CSU San Bernardino Hartmut F.-W. Sadrozinski: pct IEEE

14 The LLU-UCSC-NIU Prototype Scanner R. W. Schulte, et al.,, IEEE Trans. Nucl. Sci., 51,, pp 866, Optical Interface Photodiode Hartmut F.-W. Sadrozinski: pct IEEE

15 CT Image Reconstruction 1. WEPL calibration and cut 2. Correction for overlaps in Si tracker 3. Correction matrix with Calorimeter response 4. Angular and spatial binning 5. Filtered Back Projection and Iterative Algebraic reconstruction 6. MLP formalism for final reconstruction 2.5 mm slice 0.65 mm voxels air polyst. lucite bone 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 Bone Lucite Air Material Polystyrene Predicted RSP Hartmut F.-W. Sadrozinski: pct IEEE RSP reconstructed from Measurement

16 # per 10 AdC WEPL Calibration Response of CsI Calorimeter to different Degrader Depth CsI spectra for various degrader depth mm 52.9 mm mm mm mm mm mm ADC F. Hurley et al., subm. to MEDICAL PHYSICS Goal: Direct determination of WEPL from calorimeter response, without converting to MeV E(proton) = 200 MeV Polystyrene degraders Linearity of CsI calorimeter ADC [a.u.] CsI Calorimeter Response y = x Simulated Energy [MeV] Degrader WEPL vs. Calorimeter Response WEPL [mm] WEPL vs. CsI Calorimeter Response Calorimeter Response [a.u.] WEPL & Energy RMS vs. Degrader WEPL WEPL RMS [mm] Errors vs. WEPL WEPL Error Energy Error WEPL [mm] Hartmut F.-W. Sadrozinski: pct IEEE Energy RMS [MeV]

17 WEPL Resolution: CsI vs. Range Counter Geant4 Simulation for 200 MeV Protons, w/o detector threshold effects WEPL RMS [mm] WEPL Error vs. Degrader Thickness 4 mm Plate Resolution Range Counter with Straggling (4mm Polystyrene) WEPL RMS is constant as expected, range counter measures straggling in phantom/degrader + range counter! Plate thickness resolution < WEPL RMS Thicker plates viable for 200 MeV? WEPL [mm] CsI and Range Counter have similar relative WEPL resolution, at small WEPL (high proton energy) Range Counter superior at large WEPL (low proton energy) CsI superior, reaches 1%! Realtive WEPL RMS [%] WEPL Error vs. Degrader Thickness 100% CsI Calorimeter 4 mm Range Counter 10% 1% WEPL [mm] Hartmut F.-W. Sadrozinski: pct IEEE

18 LLU-UCSC-CSUSB Head Scanner SCIPP NIH Grant 1R01EB R. Johnson, H. F.-W. Sadrozinski, D. Steinberg, A. Zatserklanyi, V. Bashkirov, F. Hurley, S. Penfold, R. Schulte, S. McAllister, K. Schubert Increase Size 2x : 40 cm x 10 cm Improve data throughput 50x: 2MHz sustained proton rate with minimal pile-up Si sensors are intrinsically fast, built faster readout ASIC and distributed DAQ Data stream uses local FPGA for data collection, formatting and transmission Improve speed of energy detector: CsI calorimeter replaced with faster plastic scintillator Both range counter and calorimeter under test Polystyrene 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 Hartmut F.-W. Sadrozinski: pct IEEE

19 Range Counter with Direct SiPM Readout 4.2 mm Polystyrene Signal = 100 p.e.!! Very, very preliminary 1p.e. = 40 Width of signal distribution signal > 30 p.e. Hartmut F.-W. Sadrozinski: pct IEEE

20 Si Sensor Improvement: Slim Edges (with NRL) Si SSD with 900μm dead edge Cut within 50 μm Of Guard Ring Guard Ring Cut (!) Edge Treatment: Laser + XeF2 scribing Cleaving PECVD Passivation Slim edges: with guard ring reduce dead edge from 1mm to < 200 μm Excellent breakdown behavior Current at 150V: ~10 na/cm with guard ring ~100 na/cm without guard ring Charge collection unchanged See V. Fadeyev s talk N7-1 Hartmut F.-W. Sadrozinski: pct IEEE

21 Conclusions Proton 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 scanners Next (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 1R01EB Hartmut F.-W. Sadrozinski: pct IEEE