16. January 2007Status Report On Compton Imaging Projects 1 Status Of Compton Imaging Projects Carried Out In The CIMA Collaboration HPD Brain PET Meeting Bari P. Weilhammer INFN Perugia/CERN

16. January 2007Status Report On Compton Imaging Projects 2 1. Photon Detection with Silicon Detectors (A few trivial notions) 2. Two Projects Involving the Compton Camera Concept Using Silicon Radiation Detectors OUTLINE of Presentation

16. January 2007Status Report On Compton Imaging Projects 3 Detection of Photons and Energetic Electrons in Semiconductor Detectors Medical imaging requires good ability of detection of photons, in reality detection of energetic electrons created inside the material ( an advantage!), over a wide range of energies. Energy Ranges: Single Photon Emission Tomography (SPECT): detect  -rays for a big variety of isotopes used in different tracer molecules, e.g. 99m Tc140 keV 111 In185 and 245 keV 31 I360 keV Positron Emission Tomography (PET): 511 keV  or ~50 keV to 350 keV Compton recoil electrons

16. January 2007Status Report On Compton Imaging Projects 4 Photon Interactions in Silicon Only two out of all photon interactions are important for medical imaging: In traditional imaging applications the “wanted” one: Photoelectric Absorption (total absorption of  or X-ray)  = 4√2  4    5  Th with the Thomson cross-section  Th. = 8  /3 r 2 0 = bars per electron.

16. January 2007Status Report On Compton Imaging Projects 5 The “unwanted” one: Compton scattering  The recoil electron ( from K-shell or L-shell or valence band) creates (eh) pairs in the semiconductor bulk through ionization Kinetic energy of recoil electron

16. January 2007Status Report On Compton Imaging Projects 6 Attenuation of incoming photons in material In 1mm thick silicon for 20 keV photons Photoelectric interaction: ~ 97% Compton interactions: ~3% Interactions/m for Si versus photon energy Interesting region for medical imaging

16. January 2007Status Report On Compton Imaging Projects 7 Range of Electrons in Materials The range of electrons in materials expressed as range * density is very similar for many different materials Typical Range of Compton recoils: 50 keV electron in silicon: ~20  m 200 keV : ~200  m 500 keV :~ 600  m For Compton interaction the medical imaging “point-like” domain is between 10 keV and 250 to 300 keV! Range*density [g/cm 2 ] 100 keV Si NaI

16. January 2007Status Report On Compton Imaging Projects 8 Some inherent physical limitations in different imaging modalities with scintillators are:  Spatial extension of the photon interaction in the detector material due to the nature of photon interactions (in most materials interaction cascades are frequent before final absorption). The typical extension of a photon interaction in many detector materials ( at 500 keV) can be considered to be confined in a sphere of ~1 cm in diameter.  Depth of Interaction  Parallax error  Finite path length of positrons and recoil electrons  Compton scattering in tissue.  In PET: Finite momentum of e + e - compound at the moment of decay  Acolinearity  Accidental coincidences.  ……… ~1cm through multiple interactions in scintillator Incoming 

16. January 2007Status Report On Compton Imaging Projects 9 An Observation: What about Silicon Radiation Sensors which are NOW part of Scanners or Cameras used in Hospitals Photo-diode arrays in present day X-Ray CT (mostly Hamamatsu ) Low Dose digital Mammography Scanner from SECTRA (single sided strip detectors and VLSI Front-end) See Autoradiography Camera from BIOMOLEX (double sided strips and VLSI Front-end) See And some more……… but it is a niche market except fror photo-diodes

16. January 2007Status Report On Compton Imaging Projects 10 R&D Projects using Silicon Detectors in Medical Imaging within the CIMA Collaboration Novel axial brain PET Scanner Compton Camera and Probes High resolution small animal PET scanner based on Compton interactions

16. January 2007Status Report On Compton Imaging Projects 11 Compton Imaging

16. January 2007Status Report On Compton Imaging Projects 12 The main features of Compton Imaging are: The Mechanical collimator in the Anger Camera is replaced by “Electronic Collimation”. This removes the coupling between sensitivity and spatial resolution. This is achieved by having two detectors in coincidence: In the first detector the gamma rays are scattered by Compton Scattering on electrons in the detector material In the second detector the scattered gamma ray is absorbed

16. January 2007Status Report On Compton Imaging Projects 13 The measured quantities in Compton imaging are: x, y, z-co-ordinates in the first detector x, y, z-co-ordinates in the second detector Energy of recoil electron in first detector Energy of scattered photon in second detector Not measurable with Compton Cameras for medical applications: Direction of recoil electron, which leads to the conical ambiguity. This leads to more complicated image reconstruction algorithms. Expected improvements over Anger Camera: Factor ~5 in spatial resolution for probes Factor 5 to 50 improvement in sensitivity

16. January 2007Status Report On Compton Imaging Projects 14 Results from a Demonstrator Test in 2005

16. January 2007Status Report On Compton Imaging Projects cm All possible solutions need to be cheap and standard technology readily available in Industry. Modifications to a technology need to be available in the standard industrial processes The Silicon Pad Sensors Schematic cross section of double metal pad sensor A processed wafer 1mm thick Details of routing technology on pads via double metal vias Routing lines end at external bond pad rows for connection to readout chip

16. January 2007Status Report On Compton Imaging Projects 16 Silicon detector and stack of 5 detectors

16. January 2007Status Report On Compton Imaging Projects 17 A Demonstrator set-up with stack of 5 Silicon pad sensor and 3 camera heads

16. January 2007Status Report On Compton Imaging Projects 18 Main Results Spatial resolution was measured for 4 energies; 57 Co (122 keV) and 133 Ba (272,302 and 356 keV). For the highest energy with a source-first detector distance of 11.3 cm: 5mm FWHM With a source Si distance of 3 cm this gives (simulation) 2 -3 mm FWHM

16. January 2007Status Report On Compton Imaging Projects 19 Status : Spatial resolution in Silicon Demonstrated Next Demonstrator test foreseen before end of 2006 with much improved camera head and improved silicon ( lower thresholds possible)

16. January 2007Status Report On Compton Imaging Projects 20 A High Resolution Small Animal PET Scanner based on Compton Scatter Events in Silicon Pad Detectors

Harris Kagan Imaging 2006, June 26-30, Stockholm Resolution Limitations for Conventional PET Scintillator ~ 1 cm Inter-Crystal Scattering Depth of Interaction Uncertainty Multiple Interactions Energy deposited over a volume ~ 1 cm mean path Penetration into crystals widens LOR Best Resolution ~ mm

16. January 2007Status Report On Compton Imaging Projects 22 BGO detector Si detector Si-Si BGO- BGO Si-BGO Si-Si : Very High Resolution Si-BGO : High Resolution BGO-BGO : Conventional PET Resolution Three Major Coincidence Events A Very High Resolution PET Scanner for small animals based on Compton Scattering events is proposed: The Concept

16. January 2007Status Report On Compton Imaging Projects 23 Detection Efficiency (%) Radial Posn. (mm) Single – SingleSingle – BGOBGO - BGO Calculated for point source in center plane. Only single scattering or absorption interactions in the silicon detector are included. Back scattered photons from BGO and events without full energy deposition are excluded. Simulation results with this configuration Efficiency for different event classes BGO ring

Harris Kagan Imaging 2006, June 26-30, Stockholm Compton PET Test Bench Silicon detector BGO detector 4.5 cm  2.2 cm and 1 mm thick 32  16 (512) pads, 1.4 mm  1.4 mm pixel size Energy Resolution 1.39 keV FWHM for Tc 99m 5.3 cm  5 cm and 3 cm thick 8  4 array, 12.5 mm  5.25 mm crystal size Energy Resolution 22% FWHM for Na-22 HAMAMATSU PMT R2497 VATAGP3

Harris Kagan Imaging 2006, June 26-30, Stockholm Prototype PET Instrument Single-slice instrument using silicon and BGO DisassembledAssembled Silicon detector

16. January 2007Status Report On Compton Imaging Projects 26

Harris Kagan Imaging 2006, June 26-30, Stockholm Resolution Uniformity cm Source pairs at 5, 10, 15, & 20mm off-axis Sinogram The sources in each pair are clearly separated at appropriate sinogram angles

Harris Kagan Imaging 2006, June 26-30, Stockholm Compton PET: Intrinsic Resolution Needle 25G (ID = mm, OD = 0.5mm, SS_steel wall = mm) cm mm mm Image Resolution = 700  m FWHM SS_steel wall F cm

16. January 2007Status Report On Compton Imaging Projects 29 Run PET in strong magnetic field (Raylman,Hammer,…) Positrons spiral transverse to magnetic B-Field vector Potentially useful for emitters with higher e + energy than 128 F like 124 I, 94m Tc,.. Measurement was done recently in a 9 Tesla magnet at OSU with one of the modules. The performance of the detector did not change. BUT: the bond wires need to be fixed with epoxy!

16. January 2007Status Report On Compton Imaging Projects 30 This work goes on For the PET project we got NIH funding and will go for FP7 European funduing For the ”PET in magnetic field” study a funding proposal has been submitted to NIH. For Compton PET project a funding request is prepared to develop a VATA”Compton” chip together with GM-I