3D position readout from thick scintillators

AbstractAnoveltechniquehasbeendevelopedandtestedforthethree-dimensionalmeasurementofpositioninSPECT-PETdetectors.Resultsarepresentedfor2and20mmthickNaI(Tl)planarcrystals.Inaplaneofcrystal,acoordinateresolutionofabout1mm(rmserror)isdemonstrated.Thedepthofinteraction(DOI)ismeasuredwithanrmserrorofabout2mmusinglightconeparameters.r 2002ElsevierScienceB.V.Allrightsreserved. PACS: 29.40.Mc;29.40.GxKeywords: Scintillationdetectors;Depthofinteraction;Positionresolution 1. IntroductionThree-dimensionalmeasurementsofinteractionpositions of g-particles or X-rays is a verychallenging task. In high resolution positronemissiontomography(PET),itisveryimportanttoknowthedepthofinteraction(DOI),toavoidthe so-called parallax errors which degrade theimageresolutionfromthecentertotheouteredge.Severalmethodshavebeenproposedtosolvetheproblem. These include the utilization of atemperaturegradientalongthecrystal[1];theuseofaphoswichdesign[2]andtheconnectionoftwoendsofthe crystals toseparatephotosensors[3].Othermethodsincludetheuseofalightabsorbingmaterial around each crystal [4] or drug a lightabsorbing medium between the detector [5].However,allthemethodsrequireeitherextensiveworkwithdetectorcrystalsoradditionalelectro-nics, and the DOI resolution is still not satisfac-tory.Weemployyetanotherapproachtosolvetheproblem.This work is a continuation of the researchdescribed in Ref. [6]. In a search for an optimaldesign of a multi-modality scintillating detector,we have studied several geometries of positionsensitivedevices.Amongcurrentlyavailablecrys-tals, NaI(Tl) crystal is a good candidate for thecombined PET-SPECT modality. It has a highdensity and a good light yield with an emissionwavelengthspectrumwellmatchedtothephoto-cathode of a photomultiplier tube. To avoidhydrophilicdeteriorationofNaI,ourtestcrystalswere packed in metal–glass or glass–glass envel-opes.LimitationsinPETimagingduetoslowlightemission of NaI(Tl) crystals could be overcomeusingasectioneddesignwithtile-typescintillators.