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Summary of the Compton-PET project

Summary of the Compton-PET project. V.Chepel. LIP-Coimbra and Department of Physics, University of Coimbra. V. Chepel. Jornadas do LIP, Lisboa, 22-24 April, 2012. Compton-PET stands for: “Feasibility study of using Compton scattering for medical imaging with positrons” (2008-2011).

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Summary of the Compton-PET project

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  1. Summary of the Compton-PET project V.Chepel LIP-Coimbra and Department of Physics, University of Coimbra V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  2. Compton-PET stands for:“Feasibility study of using Compton scattering for medical imaging with positrons” (2008-2011) V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  3. Motivation Scattering in the patient body: (~70 to 90%) Scattering in the detector: Probability of the first interaction to be a Compton: BGO – 53% LSO – 62% LXe – 79% LAr – 95% BGO – 78% LSO – 86% LXe – 95.6% LAr – 99.8% of coincidences involve scattered events Using these events for image reconstruction: improves statistics and/or allows reducing the dose administered to patient, but deteriorates resolution V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  4. (E2, q2) (E4) (E1, q1) (E2, q2) Possible solution: Compton tracing This is what we see (or not) in the detector: Reconstruction of the true sequence depends strongly on resolutions on E and q (i.e. position) p´g pg pg, p´g, pe are in the same plane pe Knowing electron track orientation can help V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  5. Objective: explore what can be done with the scattered events Compton tracing : • Compton electron track orientation: can we get some useful information on it ? • If one can measure polarization of the g-par – would it help ? • Do we really need to trace (i.e. reconstruct in detail) scattered g-rays ? Method: Monte Carlo and experimental • How Geant deals with the electron binding ? • (see talk by A.Mangiarotti on validation) V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  6. 1. Attempting to measure electron track orientation in LXe The idea: for 500 keV electrons 5.5 MeV a-s 511 keV g V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  7. 1. Attempting to measure electron track orientation in LXe 300 ns (expected ~25 ns) Neither field non-uniformity, nor diffusion in the gas explain such long rise time (unless diffusion in Xe vapour is much stronger than in non-saturated gas) LXe a-s, range 40 mm CHERNE2010, Coimbra, 7-9 June, 2010

  8. 2. Polarization of annihilation photons e+ + e-g + g What if we could measure polarization of each g ? Any help with Compton tracing ? Any information about scattering in the patient body ? V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  9. 2. Compton scattering cross section for polarized photons Eg = 511 keV Maximum anisotropy = 5.5 (for   82º) V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  10. 2. MC of g-pairs with GEANT4: Point source in vacuum 2 - 1 Point source in of water (20cm phantom) V. Chepel Jornadas do LIP, Lisboa, 22-24 April, 2012

  11. 3. Do we really need to do tracing ? Computation tool: Interactions (physics) Detector response Compton tracing and LOR definition Image reconstruction GEANT4 C++ Matlab CHERNE2010, Coimbra, 7-9 June, 2010

  12. 3. Choosing LOR: a “childish” approach CHERNE2010, Coimbra, 7-9 June, 2010

  13. 3. UTAH phantom simulated P r e l i m i n a r y Traditional PET: Single energy deposition in 350-650 keV Compton PET: Singles + doubles With total E= 350-650 keV Same contrast Statistics > x3 Detectors: LXe ring 80x5x5 cm3 Resolution 2 mm, dE/E=20% CHERNE2010, Coimbra, 7-9 June, 2010

  14. 3. Line source 4.5 cm off centre in 20 cm of water P r e l i m i n a r y Traditional PET: Scatter fraction  0.7 Compton PET: Scatter fraction  0.86 Detectors: LXe ring 80x5x5 cm3 Resolution 2 mm, dE/E=20% CHERNE2010, Coimbra, 7-9 June, 2010

  15. Conclusions • Compton electron tracking does not seem to be feasible for <=511 keV (at least with the readout we have tried). • The effect of gamma-ray polarization on the observable distributions is small. • Events involving Compton scattering in the detector can be recovered for the imaging with the advantage of higher image statistics and without jeopardizing the image quality. • Complex Compton tracing does not to be necessary. Simple methods can be used for that. CHERNE2010, Coimbra, 7-9 June, 2010

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