Detector technology. Aim of this talk. Principle of a radiation detector. Interactions of gamma photons (gas) Gas-filled detectors: examples

Transcription

1 Aim of this tal Detector technology WMIC Educational Program Nuclear Imaging World Molecular Imaging Congress, Dublin, Ireland, Sep 5-8, 202 You can now the name of a bird in all the languages of the world, but when you're finished, you'll now absolutely nothing whatever about the bird... So let's loo at the bird and see what it's doing - that's what counts. I learned very early the difference between nowing the name of something and nowing something. Richard Feynman, Nobel Prize Physicist, Principle of a radiation detector A radiation detector converts the energy of ionizing particles into charge pulses Interactions of gamma photons (gas) ion Αlpha-particle Proton Electron Interaction => Ionization trac Free charge carriers gas-filled detectors semi-conductors Conversion to luminescence Scintillator + photosensor X-ray Gamma-ray => electric signal electron 3 4 Ionization chamber: gas charge transport Gas-filled detectors: examples signal formation Geiger-Muller detector (saturation detection) particle e ion q dv dq = qdv/v i = dq/dt gas Example: free-air dose meter -V 0 5 6

2 Interaction of gamma s (semiconductor) hole Semiconductor detector n + contact charge transport signal formation particle e h q e dv dq = (q e dv +q h dv 2 )/V i = dq/dt q h dv 2 Examples: silicon diode electron p + n junction -V 0 germanium detector reverse bias, fully depleted 8 Semiconductor detectors e.g. in digital radiography Principle of a scintillation detector X-Ray Αlpha-particle Proton Electron Scintillation photons electric signal Direct Conversion X-ray Gamma-ray scintillator photosensor 9 0 Components of a scintillation detector Inorganic scintillation crystals Scintillators Light sensor inorganic crystals organic plastics glass liquid gas human eye photomultiplier tubes photodiodes avalance photodiodes silicon photomultipliers CCDs gas-filled detectors etc CsI(Tl) NaI(Tl) BGO CdWO 4 Various, under UV excitation 2 2

3 The scintillation process Scintillation pulse shape (simple case) Three phases:. The interaction phase + thermalization phase (ps) 2. The charge carrier and energy migration phase (ns-ms) 3. The luminescence phase (ns-μs) I I 0 I 0 /e 0.3 I 0 I(t) I 0 exp (-t /) E g =3-0 ev 2 3 Conduction band Luminescence center (LC) photon VIS nm ev UV nm ev Valence band VUV < 80 nm > 6.9 ev Intensity It ( ) Iexp( t/ ) 0 0 with I given by I( t) dt E Y 0 where E is the absorbed gamma energy (in MeV) and Y the scintillator light yield (in photons/mev) t 3 4 Important scintillator parameters Properties of some inorganic scintillators high light output Y (photons/mev) fast scintillation speed (ns) good energy resolution R FWHM (%) high density for γ detection (g/cm 3 ) large size of crystal cm 3 low cost per cm 3 low afterglow (low phosphorescence) low bacground count rate (low intrinsic activity) absence of radioactive isotopes Relative importance depends on application scintillator NaI (at 80 K) NaI(Tl) CsI(Tl) BaF (valence e to core) Bi 4Ge 3O 2 (BGO) PbWO 4 Lu 2SiO 5:Ce (LSO) YAlO 3:Ce (YAP) LaCl 3 :Ce LaBr 3 :Ce mass density index of refraction decay constant emission (nm) ph/mev Properties of some inorganic scintillators Photomultiplier tubes The main PMT elements S.R. Cherry, J.A. Sorenson, M.A. Phelps, Physics of Nuclear Medicine, 3 rd ed., 2003 photocathode photon photoelectron quantum efficiency (e.g. ~25%) electron-optics focusing of photoelectron on the first dynode preservation of time information multiplication stage 6 to 2 dynodes charge pulse at anode output 8 3

4 Conversion and multiplication Scintillation detectors in nuclear medicine Gamma camera (planar scintigraphy) SPECT scanner (single photon emission computed tomography) PET scanner (positron emission tomography) 2 3 Quantum efficiency of photocathode photoelectrons/photon Overall electron gain is sensitive to applied voltage (typically V 2.5 V) Secondary emission factor of dynodes δ typically 4-8 Typical gain = (number of dynodes N = 8-2) Time-of-Flight PET (TOF-PET) 9 20 Scintillation detectors in nuclear medicine Simplest case: planar scintigraphy 2D position sensitive detector: gamma camera Single Photon Emission CT (SPECT) rotation Collimator Photomultiplier tubes Light guide Scintillator plate Radiopharmaceutical 2 22 Gamma camera Position Estimation (Anger logic) NaI:Tl crystal of gamma camera to 6 photomultipliers a 0.8, a a 0.8, a 3 4 a 0.3, a a 0. Energy: E a y a a2 a3 a4 a5 a6 a x NaI:Tl + crystal cm diameter, 2-25 mm thic (often 9.5 mm) Position: X E a x Y E a y

5 Positron emission tomography (PET) PET detectors: classic bloc detector Neutron-deficient radionuclide n n p np p p n p n p n + e + + e positron range e + e - ~80 o Detector 5 ev annihilation photon Detector 5 ev annihilation photon Several bloc detectors are assembled into a ring A scanner may consist of several detector rings PET detectors: classic bloc detector PET detectors: Anger logic Saw cuts direct light toward PMTs. Depth of cut determines light spread at PMTs. Crystal of interaction found with Anger logic (i.e. PMT light ratio). 4 PMTs (25 mm square) 50 mm 50 mm 30 mm Scintillator Bloc Identify crystal of interaction using looup table Position given by crystal ID A B C D Energy: E = A + B + C + D Position: Y = (A + B) / E X = (B + D) / E Y X Courtesy of Bill Moses, LBNL 2 Courtesy of Bill Moses, LBNL 28 Energy: pulse height spectrum Example: NaI:Tl pulse height spectrum The height (amplitude) of the charge pulses produced by a scintillation detector are proportional to the number of scintillation photons detected and, thus, to the energy of the energy deposited by the gamma photon NaI:Tl + Photopea Counts Compton edge Detector surrounding Energy (ev) Scint

6 Scattering in patient Energy discrimination energy window line of response (LOR) incorrect LOR Compton scattering 3 32 PET: coincidence detection Random coincidences detector d Coincidence window Tube or Line Of Response (LOR) d d2 t t e - e + t -t 2 <t? detector d2 Coincidences t 2 yes coincidence t < 0 ns time e.g. scattering out of system Random coincidences: R ~ 2 S S 2 where 2 is the width of the coincidence time window and S and S 2 the singles rates of two opposing detectors detector timing resolution must be high time resolution (~ns) needed! incorrect LOR From: R. Boellaard, VUmc PET Centre Time-of-flight PET Time-of-flight PET: concept of CRT LOR t 2 5 ev The accuracy of source position localization along line of response depends on the coincidence resolving time (CRT) x = uncertainty in position along LOR = c. CRT/2, where c is the speed of light. Annihilation x The TOF benefit is proportional to x/d, where D is the effective patient diameter. t 2 -t t 5 ev => The smaller the CRT, the better. D State-of-the-art: CRT 500 ps x.5 cm

7 Silicon Photomultiplier (SiPM) SiPM-array based PET detectors mm - 3 mm 20 m 00 m Array of many single-photon avalanche diodes (microcells) connected in parallel Increasingly interesting as replacement for PMTs: high gain (~0 6 ) high PDE compact and rugged transparent to γ-photons fast response (ns) insensitive to magnetic fields For example: crystal matrix composed of e.g. 4 mm x 4 mm x 20 mm crystals each crystal coupled -to- to an individual SiPM => high spatial resolution => high energy resolution => excellent timing 3 Image courtesy of Philips ps barrier broen using SiPMs Multimodality: PET + MRI Made possible by the combination of: Small LaBr 3 :Ce(5%) crystals (3 mm x 3 mm x 5 mm) Silicon Photomultipliers (Hamamatsu MPPC-S C) Digital Signal Processing (DSP) Now: avalanche photodiodes (APDs) Next generation systems: SiPMs!!! 00 ps FWHM => 5 mm FWHM D.R. Schaart et al, Phys Med Biol 55, N9-N89, Images: Siemens 40