Development of Silicon Detector Instrumentation for Medical Physics

1. Development of Silicon Detector Instrumentation for Medical Physics A.B. Rosenfeld Centre for Medical Radiation Physics, University of Wollongong, Australia New Zealand-Australian Semiconductor Instrumentation Workshop. Wellington , 21-23 June, 2004

2. Overview • Positron Emission Tomography • Microdosimetry • Hadron Therapy • Space Dosimetry • Intensity Modulated Radiotherapy (IMRT) • Synchrotron Microbeam X-Ray Radiation Therapy (MRT) • High and Low Dose Rate Brachytherapy • Neutron Dosimetry • Radiation Damage Monitoring

3. Positron Emission Tomography Development of Silicon Detector Instrumentation for Medical Physics

4. Introduction Huntington’s disease studies http://neurosurgery.mgh.harvard.edu/pet-hp.htm

5. Introduction Advantages of PET • Measures level of radiotracer uptake • High sensitivity to small lesions Disadvantages of PET • Poorer spatial resolution (5-8 mm) than other anatomical imaging techniques such as MRI (1 mm) http://www.nucmed.buffalo.edu/

6. Background of the PET detector module http://imasun.lbl.gov/~budinger/medTechdocs/PET.html • The injected radiotracer decays via the emission of a positron • Scintillation detectors are used to detect the gamma rays • For best images we must know precisely the gamma interaction point (DOI) in the scintillator crystal. • Some DOI information in the plane of the PMT surface is obtained. • No DOI information in the radial direction (perpendicular to the PMT surface) is obtained which impacts on the final image resolution.

7. Introduction • Replace PMTs with silicon pixel photodetectors • Optically couple the 8x8 Si PD array on the side of the pixellated scintillator 8x8 array • Determine the position of gamma ray interaction (POI) in the x-y plane of the array. • Reduce the uncertainty of the POI in the z-direction • Spatial resolution defined by scintillator element dimensions (3x3x3 mm3) Reflective coating Buffer Amplifier TA64 Chip VA64 Chip

8. SPAD photodiode design • Optical photons from scintillators like LYSO are absorbed within 0.6mm of silicon so a customised ion implantation profile has been used to extend the electric field very close to the surface • Optical entrance window on the p-layer reduces the hole charge collection time • SPAD 1,SPAD2 and SPAD 3 are identical except that SPAD 2 has p+ strips embedded in the surface p- layer and SPAD3 incorporates an antireflective layer on the surface

9. Light Collection Simulations • Monte Carlo Simulations were performed by G. Takacs to: • determine the ideal crystal geometry so as to maximise the light output • optimise detector pixel size to improve the DOI measurements. Interaction point above the corner of four pixels

10. Photodetector Transmission

11. Charge collection analysis using IBIC • IBIC – Ion Beam Induced Current analysis • 1 mm diam. spot raster scanned across area of interest • Scale of image is ~1x1 mm2 • Image is for SPAD2 which is identical to SPAD 1 except that p+ strips are embedded in the uniform p-layer • shows excellent charge collection even at 20 V (full depletion is at 50 V) 0.4 MeV He+ 380 310 V = 20V

12. Gamma Ray Spectroscopy - LYSO • SPAD 3 @ 50 V • 3x3x3 mm3 LYSO • LYSO painted • 22Na - Green • 137Cs - Purple • Pulser (+SPAD) - Blue • T = 298 K • 0.5 ms shaping time • DE/E(511keV) = 13.1% • DE/E(662keV) = 11.2% • DE/E(pulser) = 8.3% 662 keV 511 keV Counts Pulser peak Channel number

13. Light Collection Efficiency (LCE) • SPAD 3 @50 V • 3x3x3 mm3 LYSO • LYSO painted • 22Na - Green • 125I (direct)- Purple • Pulser (+SPAD) - Blue • T = 298 K • 0.5 ms shaping time • 511 keV peak is at CH.341 • 22.0 keVpeak is at CH = 300 • 27.0 keVpeak is at CH = 373 • 511 keV (LYSO) is equiv. to 24.8 keV (Si) • ~6850 e-h pairs created by the optical photons detected • SLICE =6850/13,300 ~ 53% 27 keV 511 keV Counts 22 keV Pulser peak Channel number

14. Detector Module Status

15. Detector Module Status

16. Scanning Gantry • Linear step - 12.5 mm, Angular step – 0.05 degrees • Simultaneous PET/SPECT with CT

17. Microdosimeter applications in Hadron Therapy and Space Development of Silicon Detector Instrumentation for Medical Physics

18. Microdosimetry • Biological effect of weakly ionising radiation described by physical quantity average dose • Distribution of ionisation homogenous on microscopic scale • D = E / m • Biological effect of densely ionising radiation • Distribution of ionisation is inhomogeneous • Pattern of energy deposition on microscopic scale is important • Lineal energy event • y = e / < l > • Microdosimetry is an experimental technique to measure the distribution of y in a microscopic sensitive volume subject to densely ionising radiation

19. Introduction • Microdosimeter measures the energy deposition events in a small (cell-sized) volume due to radiation interactions. • Produces spectra of counts versus energy (E). • Divide energy by mean chord length (l) gives lineal energy spectra f(y) • Dose distribution d(y)  yf(y) where y=lineal energy=E/l • Motivation: • Radiobiological effectiveness depends upon LET or lineal energy • Distribution of dose (d(y)) with lineal energy gives dose equivalent (H) l=10um Protons Alpha, Heavy ions

20. AMPTEK A250 Pre-amplifier Detector Silicon Microdosimetry • Tissue Equivalent Proportional Counter (TEPC) • Low density, tissue equivalent, gas filled proportional counter • Simulates microscopic volume • TEPC Disadvantages • Measurement in gas phase • Poor spatial resolution • Silicon Microdosimetry • Solid state, microscopic, silicon sensitive volumes • High spatial resolution • Measure ionisation energy loss in depletion region of pn junctions

21. Sensitive Volume Characterization Using an Ion Microbeam 2 MeV Alpha Microbeam Optical Microscope Image High Energy Low Energy pn junction • Note: Both images for a 10x10x2um SOI Array

22. The ANSTO Heavy Ion Microprobe • Scanning coils • Quadrupole triplet • Sample holder

23. Applications • Cancer treatment modalities which utilise high LET radiation (proton, boron neutron capture, fast neutron, heavy ion therapies) • Characterise high LET radiation fields encountered in space (to fly on NASA research satellite)

24. Mapping the Sensitive Volume with IBIC • Need to study charge collection following ion strike • Investigate how charge collection varies with location of ion strike on the device • Investigate how charge collection varies with ion LET • Ditto for bias applied to microdosimeter • IBIC using 3 MeV H+, 9 MeV He2+, 25 MeV C4+

25. Results: 3 MeV H+ IBIC imaging 30 mm • Frequency distribution of energy events and image of median energy for applied bias of 0 V, 5 V, 10 V, 20 V • 0 V, 5 V, 10 V: Max charge collected for strikes at centre of device • Increase in charge collection with applied bias • 20 V: Max at corners of n+ region. High field effect.

26. Results: 9 MeV He2+ IBIC imaging 30 mm • Magnitude of energy events increases with increased LET of helium • Median energy images similar to proton IBIC including effect at 20V

27. Results: 25 MeV C4+ IBIC imaging 30 mm • Magnitude of energy event increases with increased LET of carbon • Increased sensitivity to device over-layer. Energy loss in contact and track evident (need to minimize over-layer in next generation) • High field effect more pronounced in frequency distribution and median energy image than for proton and helium IBIC

28. Silicon Microdosimetry Fast Neutron Therapy • Gershenson Radiation Oncology Center, Harper Hospital, Detroit • Microdosimetry spectrum measured at 2.5 cm depth in water phantom (Bradley, Rosenfeld) • Measured spectrum compared favourably with TEPC

29. Silicon Microdosimetry Fast Neutron Therapy

30. Northeastern Proton Therapy Center (NPTC)Harvard Medical School, Boston, USA

31. NPTC Proton Beam Dose Profile • Measurements using Markus parallel plate detector in water phantom • LET of protons varies from 0.45keV/um at entrance to 78keV/um • Bragg peak broadening filter used to spread dose at end of range

32. Microdosimetry of NPTC Facility The area under the curve between two values of y is proportional to the fractional contribution of that region to the total dose

33. MOSFET Dosimetry in Radiotherapy Development of Silicon Detector Instrumentation for Medical Physics

34. MOSFET Structure

35. Electron microscope image of MOSFET

36. On-line MOSFET Dosimetry System • Measurement over a wide dynamic range of accumulated dose and dose rate • Ability to operate in an on-line capacity • Excellent spatial resolution down to ~ 0.1 microns • Ability to work in pulsed mode with consecutive dosimetry after each pulse • Able to avoid temperature and fading error

37. MOSFET Response

38. Penumbra formation of the Siemens X-ray machine

39. Measurement of the profile of an IMRT radiation beam penumbra

40. Multileaf Collimator used in IMRT

41. Experimental set up to investigate interleaf leakage and stop gap leakage in the Varian MLC Stop-gap leakage Interleaf leakage

42. Measurement of the stopgap radiation leakage from an IMRT collimator

43. Synchrotron Microbeam X-Ray Radiation Therapy (MRT) Development of Silicon Detector Instrumentation for Medical Physics

44. MRT

45. Medical Beamline at the ESRF

46. Horizontal section CEREBELLUM 25 m-wide horizontal stripes, = paths of the microbeams Cells / nuclei in the paths were destroyed No tissue destruction present No signs of hemorrhage 210 m

47. Profile of Microbeam #1 FWHM~40 mm

48. Radiation microbeam profile using dual “edge-on” MOSFETs on the same chip FWHM ~ 25mm

49. Profile of Microbeam #1 – new MRT collimator FWHM~50 mm

50. Beam Number Theory GaFTM Film MOSFET System 25 44 63 70 65 100 1 110 Peak-to-valley ratio