Collot, J.

Jan, S.

Tournefier, E.

Hal - Université Grenoble Alpes

A liquid xenon PET camera for neuro-science

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A liquid xenon PET camera for neuro-scienceJ. Collot, S. Jan, E. TournefierTo cite this version:J. Collot, S. Jan, E. Tournefier. A liquid xenon PET camera for neuro-science. Aubert B.Colas J. Nedelec P. Poggioli L. International Conference on Calorimetry in Particle Physics9 CALOR2000, Oct 2000, Annecy, France. Infn Frascati, pp.305-313, 2000, Frascati PhysicsSeries. <in2p3-00010021>HAL Id: in2p3-00010021http://hal.in2p3.fr/in2p3-00010021Submitted on 6 Jul 2001HAL is a multi-disciplinary open accessarchive for the deposit and dissemination of sci-entific research documents, whether they are pub-lished or not. The documents may come fromteaching and research institutions in France orabroad, or from public or private research centers.L’archive ouverte pluridisciplinaire HAL, estdestine´e au de´poˆt et a` la diffusion de documentsscientifiques de niveau recherche, publie´s ou non,e´manant des e´tablissements d’enseignement et derecherche franc¸ais ou e´trangers, des laboratoirespublics ou prive´s.Frascati Physics Series Vol. nnn (2001), pp. 000-000IX Int. Conf. on Calorimetry in Part. Phys. - Annecy, Oct. 9-14, 2000A LIQUID XENON PET CAMERA FOR NEURO-SCIENCEJ. Collot , S. Jan , E. TourneerInstitut des Sciences NucleairesIN2P3/CNRS - Joseph Fourier University53 av. des Martyrs38026 Grenoble cedex , FranceABSTRACTA camera which makes use of liquid xenon in the scintillation mode is studiedfor Positron Emission Tomography of human brain.A simulation which takes into account the basic physical processes only , showsthat the intrinsic space resolution one can reach is rFWHMmin= 2:1 mm.Results on the performance of a Hamamatsu position sensitive PMT operatingin the UV range (180 nm) are presented.1 IntroductionThe aim of the project is to develop a Positron Emission Tomograph (PET)based on the use of liquid xenon (LXe) as an active medium. This PET will bededicated to human brain research, with a high spatial and timing resolution.Its application for an online (11C) PET camera in the context of12C hadron-therapy is also considered.This development is proposed by three laboratories of the Joseph Fourier Uni-versity of Grenoble ( ISN , IRM/CHU , Nuclear Medicine department/CHU)and one industrial partner, DTA/Air Liquide, for the cryogenic equipments.In this project, we only want to exploit the scintillation detection mode of liquidxenon. The scintillation decay time of LXe (3 ns) could result in a signicantprogress on the time resolution, the sensitivity and the selectivity of the de-tector. Beneting from this, we believe a high image resolution (' 3 mm onimage) and an increase by a factor 5 of the counting rate capability comparedto the present tomographs could be obtained.The execution of this project is organized along two phases : rst, the de-velopment of a full PET simulation (GEANT 4 - ROOT - IDL) and a R&Dinvestigation which includes the construction of a small prototype to conrmthe project feasibility, followed by the development and the construction of afull device which could be a micro-PET camera.We will present the operation principles of this device, its preliminary simulatedperformance and the rst results obtained during the ongoing R&D phase.2 Liquid xenon as compared to crystalsLiquid xenon is a known gamma detection medium which features gammainteraction properties comparable to NaI. However, its scintillation eÆciency istwice as high as that of NaI , which is the most eÆcient inorganic crystal, andits scintillation decay time is more than ten times shorter than the best valueof all crystals considered in PET development ( 40 ns for LSO ).An important aspect of this project is the fact that we only want to use theLXe scintillation properties2)and not its charge collection mode3), becausethe scintillation yield when compared to the charge collection eÆciency of LXeis much less sensitive to a pollution of the liquid : it may tolerate up to a fewppm of impurities ( O2...) versus a few ppb in charge mode. On top of this,the drift velocity of free electrons is much too slow.By comparison to crystal-based cameras, the use of a liquid active mediummay enable us to design novel detector geometries which could result in asizeable amelioration of the camera performance.The performance improvements one hopes to reach in our project, withLXe LSO (fast) (ns) 3 (98%) 40 (slow) (ns) 25 (2%)Photons/MeV 7.8 1043.2 104Wave length (nm) 178 420Table 1: Comparison between LSO and liquid xenon as an active medium.respect to the tomographs available today4), are: a factor 1.5 for the axialand transaxial space resolutions ( ' 3 mm on reconstructed images with18F) , a factor 5 for the counting rate , a time coincidence window  5 ns (reduction of random coincidences ). A good energy resolution to discriminatethe scattered photons and to lter the "Compton noise" could also be obtainedif one preserves a high collection eÆciency of the light emitted.3 Monte-Carlo simulationOur simulation program is based on the GEANT4 toolkit5). It takes care ofthe complete geometrical description of the apparatus , the generation and theinteraction of positons in a water standard phantom , their annihilation intophoton pairs and the interaction tracking of the subsequent 511 keV photons.ROOT is used to analyze the simulation results and construct sinograms6)which are then fed into IDL in order to build the images ( using the FilteredBack-projection method ).The liquid xenon is contained in a 5 cm thick ring of 30 cm of internalradius which covers 20 cm in the axial Field Of View (FOV). The total volumeof LXe is 20.5 l.A standard phantom for simulating the conditions prevailing during abrain study is a hollow cylinder, 20 cm in length and diameter, made of a thinPlexiglas vessel lled with water. The phantom is placed at the center of theFOV in the scanner. The18F +energy spectrum is sampled with the classicalVon Neumann algorithm (Fig. 1 ).One important physical eect which limits the PET spatial resolution isthe +range before it annihilates, and more precisely , the +distance of ightin the human tissue ( which is mainly composed of water ). On gure 2 canMeV0 0.1 0.2 0.3 0.4 0.5 0.6Counts020406080100120140160180200Mean  = 0.274Positron emission energiefor F18 radionuclide in waterFigure 1: Simulated kinetic energy distribution of the +from the18F spectrumsee that the mean distance of ight , in the case of18F, before annihilation is0.5 mm ( Fig. 2). The second physics process which has an important impacton the spatial resolution, is the acolinearity of the two annihilation 's. This isessentially due to the orbital motion of the atomic electrons which participateto the positon annihilations7). The acolinearity angle can be modeled bya Gaussian distribution with a 0.25Æstandard deviation. The contribution ofthis eect , at FWHM, is 1.6 mm for our geometry.3.1 First resultsThe energy deposited by the two  in the liquid xenon was simulated. 200000+events were generated in the water phantom. The deposited energy spec-trum for real ( photo-electric + Compton interactions ) coincidences with athreshold at 300 keV for each  is shown in gure 3. The fraction of selectedevents ( entering in this gure ) gives the upper sensitivity (Smax) limit of thetomograph : Smax= 1:8%Only the contribution of the physical eects to the spatial resolution ofthe detector was simulated. Figure 4 shows the space resolution obtained (for 100000 events simulated ) : 2.1 mm at FWHM, which is what we expectfrom the quadratic convolution of the positon distance of ight and the photonacolinearity.mm0 0.5 1 1.5 2 2.5Counts0500100015002000250030003500Mean  =  0.546Distance of flightfor F18 radionuclide in waterFigure 2: Simulated distribution of the +distance of ight in water from the18F spectrumMeV0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1 1.050200400600800100012001400Nent = 3678   Figure 3: Simulated deposited energy spectrum for real coincidences with athreshold of 300 keV per mm-10 -8 -6 -4 -2 0 2 4 6 8 10024681012141618Chi2 / ndf = 3.049 / 7Constant = 17.05 +- 2.409 Mean     = 0.07941 +- 0.1001 Sigma    = 0.8865 +- 0.07811 = 0. 7941 +- 0.1001 = 0.8865 +- 0. 7811 Figure 4: Space resolution for18F events simulated with a water phantom4 R&D phase4.1 Test of a Hamamatsu PMTA position sensitive photo-multiplier tube, equipped with a quartz window anda RbCs photo-cathode , (HAMAMATSU R5900-00-C12) was tested with theset-up shown in gure 5. Its anode is composed of two planes of crossed plateswhich enable us to detect the x and y barycenters of the light pulses with avery good resolution. The primary aim of these tests is to measure the intrinsicspatial resolution of this PMT at  = 180 nm.The output signals from the crossed-plate anodes are amplied and un-dergo Analog-to-Digital Conversion (Fig. 5). Then these signals are read outby a computer for digital processing to locate the center of gravity.The results presented in this paper were obtained at room temperature.The derived space resolution ( Fig. 6 ) at FWHM was 0.25 mm.ADCCOMPUTERPOSITION SENSITIVE PMTX-Y STAGEFILTER AT 180 nmUVLAMPX1X2Y1HTPREAMPY2QUARTZ OPTICAL FIBERFigure 5: Schematic block diagram for position detection set-up.div.1000 1020 1040 1060 1080 1100 1120Counts01020304050Chi2 / ndf = 26.56 / 32Constant = 42.77 +- 2.23 Mean   =  1038 +- 0.320 Sigma  = 5.901 +- 0.208 X bary.sigma = 5.9 div.0.018 mm/div.Figure 6: Spatial resolution at  = 180 nm.4.2 Cryogenic systemA liquid xenon cryogenic system is being built by the DTA branch of AirLiquide. It will be ready in a few weeks. We expect the rst results with thissystem before the end of the year. It will allow us to liquefy and monitor intemperature up to 5 l of LXe. This system can be used for prototyping as wellas for the construction of a small device ( micro-PET camera ).5 Conclusions and perspectivesThe preliminary results of a full simulation allowed us to determine the intrinsicperformance of this camera . For a point-like source located at the center ofthe tomograph, we have shown that the intrinsic achievable space resolution is2:1 mm.The next step in the simulation will be to study and optimize the instrumentalresponse of the camera so as to limit the degradation of its intrinsic spaceresolution as much as possible. It includes the simulation of the light collectionin optical guides (Al tubes, quartz tubes...).As a rst result of our R&D activities, we may conclude that the positionsensitive PMT Hamamatsu R5900 is a good candidate for the detection oflight at  = 180 nm. This has to be conrmed at the temperature of liquidxenon (165 K). In parallel, we also envisage to test Si photodiodes equippedwith quartz windows.As the test cryostat and the liquid xenon station will be shortly operational atthe laboratory, we foresee to build and test a small prototype cell (2 x 2 x 3cm3) at the beginning of 2001, so as to conrm the instrumental performanceobtained by simulation.6 AcknowledgementsThe authors would like to express their gratitude to their medical colleagues,J.F. Le Bas and D. Fagret , who by their solid support have contributed tomake this project start. We address special thanks to P. de Saintignon whocontributed to the design of the cryogenic equipments. We also wish to thankthe technical sta of ISN and in particular : J. Ballon, J. Berger, C. Barnoux,Y. Carcagno, P. Cavalli, A. Garrigue, G. Mondin, P. Petit, P. Stassi, F. Vezzu.This project is funded by the Rho^ne-Alpes regional council , IN2P3 andthe Joseph Fourier University of Grenoble.References1. S. Nagai, M. Watanabe, H Shimoi, H. Liu and Y. Yoshizawa, "A new com-pact Position-Sensitive PMT for scintillation Detectors, IEEE Transactionson nuclear Science, Vol.46, NO. 3, JUNE 19992. F. Favata, A. Smith, M. Davdaz, T.Z. Kowalski, Nucl. Instr. and Meth.A297 (1990) 595-6013. V.Yu. Chepel,M.I. Lopes, A. Kuchenkov, R. Ferreira Marques, A.J.P.L.Policarpo, "Performance study of liquid xenon detector for PET", Nucl.Instr. and Meth. A392 (1997) 427-4324. B. Bendriem, "les detecteurs de positons : camera PET - Camera PET-SPECT", revue de l'ACOMEN, 1998, vol.4, nÆ1, p. 345. GEANT-detector description and simulation tool, Copyright CERN,Geneve (1995)6. D.W. Townsend, M. Defrise, "Image Reconstruction Methods in PositronTomography", CERN 93-027. K.Iwata, R.G. Greaves, C.M. Surko, "-ray spectra from positron annihi-lation on atoms and molecules" Phys. Rev. A55 (1997) 3586

A liquid xenon PET camera for neuro-science

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