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Scintillation Crystals for PET*

Charles L. Melcher

CT!Inc., Knoxville, Tennessee

crystal,a fractionof the energylocalizesonthe activator In PET, inorganic scintillator crystals are used to record -y-rays ions.Relaxationof the activatorionsresultsin the emission produced by the annihilation of positrons emitted by injected of scintillation photons, typically around 4 eV, correspond tracers. The ultimateperformanceof the camera is stronglytied ing to visible blue light. to boththe physicalandscintillationpropertiesofthe crystals.For In the early years of PET, detectors were made of single this reason, researchers have investigated virtually all known crystals ofthallium-doped sodium iodide (NaI[Tl]), individu scintillatorcrystalsfor possibleuse in PET.Despitethis massive researcheffort,only a few differentscintillatorshave beenfound ally coupledto photomultiplier tubes(PMTs).With thediscovery that have a suitable combinationof characteristics,and only 2 of bismuth germanate(Bi@Ge@O12orBGO), most detector (thallium-doped sodium iodide and bismuth germanate) have designers converted to this material because of its much greater found widespread use. A recently developed scintillatorcrystal, efficiency for detecting ‘y-rays.Ablockdetectorbecame the most cenum-doped lutetium oxyorthosilicate,appears to surpass all widely useddesign,in which a BGO block is segmentedinto as previously used materials in most respects and promisesto be many as 64 elements and coupled to 4 PMTs (1). Other the basisfor the next generationof PETcameras. scintillators have included barium fluoride (BaF2 [2]), yttrium KeyWords:PET;scintillationcrystals;lutetiumoxyorthosilicate aluminate (YA1O3[Ce]or YAP) (3), and cerium-dopedgadolin J NucIMed2000;41:1051—1055 mm oxyorthosilicate(Gd2SiO5[Ce]orGSO) (4). In recentyears, a promising new material, cerium-doped lutetium oxyorthosili cate (Lu25i05[Ce] or LSO) (5), has emerged and is likely to be used widely in future generation PET scanners. scintillator is a material with the ability to absorb ionizing radiation, such as x- or rny-rays,and to convert a THE SCINTiLLATION PROCESS fraction of the absorbed energy into visible or ultraviolet The process whereby a scintillator converts the energy photons. The conversion process typically takes place on a deposited in it by a ‘y-rayintoa pulse of visible (or ultraviolet time scale of nanoseconds to microseconds, thus producing [UV}) photons includes 3 main steps. According to Lem a brief pulse of photons corresponding to each rnl'-or x-ray picki et al. (6), the overall efficiency (‘ri)ofthe conversion that interacts with the scintillator material. The light pulse, process may be characterized as the product of 3 factors: the intensityof which is usually proportionalto the energy deposited in the scintillator, is sensedby a photodetector and ‘vi=I3SQ, converted into an electrical signal. where 1@isthe conversion efficiency of the ‘y-rayenergyto Scintillators may be liquid or solid, organic or inorganic, electron hole pairs, S is the transfer efficiency of the energy and crystalline or noncrystalline. Organic liquid and plastic held by the electron-hole pairs to the activator ions or other scintillatorsoften are usedfor detectionof @3particlesand luminescence centers, and Q is the quantum efficiency of the fast neutrons. For the detection of x- and -y-rays, such as the luminescence centers themselves. 511 keV -1'-raysusedin PET, inorganic single-crystal scintil Robbins (7) demonstrated that @3canbe calculated from lators are used, becauseof their generally higher density and thefundamentalphysicalpropertiesofthescintillatorcrys atomic number, which lead to better detection efficiency. tal, including the electronic band gap, the high-frequency A typical scintillator is a transparent single crystal in and static dielectric constants,and the optic longitudinal which valence and conduction bands are separated by a band photonenergy.For many scintillatormaterials,theseparam gap of 5 eV or more. In a perfect crystal, free of defects or eters are well known; consequently, the conversion effi impurities, there would be no electronic energy levels in this ciency can be calculated with reasonable confidence. For gap. However, most scintillators are doped with an activator somenewer scintillators,however, someof the parameters ion that provides energy levels in the otherwise forbidden are not known precisely, thus resulting in a significant band gap. After absorptionof ‘y-rayenergyby the bulk uncertainty in the calculated conversion efficiency. Despite the obvious importance of 5, no model exists to ReceivedJan. 7, 2000;revisionaccepted Feb. 9, 2000. Forcorrespondenceorreprintscontact:CharlesL Melcher,PhD, CTI, Inc., calculateit reliably.In fact, perhapstheprimary challengeof 810 InnovationDr.,Knoxville,TN37932. current scintillator research is to develop an accurate model *NOTE: FOR CE CREDIT, YOU CAN ACCESS THIS ARTICLE ON THE SNMWEBSITE(http://www.snm.org)UNTILDECEMBER2000. of this crucial process.

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QismeasuredbydirectlyexcitingthecenterswithUV TABLE 2 light, the energy of which matches the excitation energy of Physical Properties of Some Common Scintillator Crystals the center. In this way, the electron-hole creation step and the EffectiveCrystalDensity energy transfer step are bypassed, and the efficiency of the luminescence center itself can be observed directly. scopicRuggedCdWO47.9064NoNo(glcm3)atomicnumberHygro (cleaves CHARACTERISTiCS OF ThE IDEAL SCINTILLATOR easily)Lu2SiO5(Ce)(LSO)7.4065NoYesBi4Ge3O12(BGO)7.1375NoYesGd2SiO5(Ce)6.7159NoNo The ideal scintillator would have a combination of several physical and scintillation properties (Table 1). A high detection efficiency for the -y-rays of interest requires both high atomic number for a large photoelectron cross section (cleaves(GSO)easily)BaF24.8853NoYesCsF4.6453VeryNoCsl(Na)4.5154YesYesCsl(Tl)4.5154SlightlyYesNal(Tl)3.6751YesNoCaF2(Eu)3.1 and high density for a large Compton-scattering cross section. These are the 2 main interactions through which 511 keV rny-raysinteract with the scintillator crystals. For good coincidence timing and high count-rate capability, a short decay constant is required. In other words, the pulse of scintillation photons must be as brief as possible. A high 817NoNo light output allows a large number of crystal elements to be coupled to a single photodetector, and good energy resolu 5,000—10,000cc of scintillator crystals, the growth of large tion allows a clear identification of full energy events. The volumes of crystals at a reasonable cost must be feasible. transmission ofthe scintillation light pulses into the photode tector is best when the refractive index of the scintiulator PROPERTIES OF COMMERCIAL SCINTILLATORS material is similar to that of the entrance window and coupling material, usually near 1.5. In some materials, color Because the ideal scintillator does not actually exist, one centers may be easily produced by ionizing radiation, thus must look at the characteristics of materials that do exist and impeding the transmission of the scintillation light through choose the one best suited to the application. Table 2 shows the scintillator itself. Therefore, a resistance to this effect, the physical properties of some commonly available scintil known as radiation hardness, is desirable. Some scintillators lator materials, listed in order of decreasing density. Both are hygroscopic, i.e., they readily absorb water from the BGO and LSO have excellent physical properties. They atmosphere and therefore require special packaging to have high density and atomic number that result in efficient hermetically seal them. Nonhygroscopic materials have an detection of ‘y-raysandare also rugged and nonhygroscopic, advantage, in that simpler packaging may be used. Mechani which allows relatively simple detector fabrication. Cad cal ruggedness is desirable, because it makes fabrication of mium tungstate and GSO are also good candidates, except small crystals easier. Because a PET scanner may use that both cleave easily, which makes detector fabrication more difficult. TABLE 1 Table 3 shows the scintillation and optical properties of Properties of the Ideal Scintillation Crystal for PET some common scintillators, listed in order of increasing decay constant. BaF2has the shortest decay constant by far: Crystalproperty Purpose 0.8 ns. Unfortunately, the emission is weak and located in Highdensity High-y-raydetectionefficiency the far UV at 220 nm, which requires PMTs with more Highatomicnumber High -y-ray detection efficiency expensive quartz windows. It also has a long secondary Shortdecaytime Goodcoincidencetiming component of 600 ns. Cesium fluoride (CsF) hasa very short Highlightoutput Allowslargenumberofcrystal elementsperphotodetector decay constant of 4 ns, but its intensity is so weak that this Goodenergyresolution Clearidentificationoffull scintillator is seldom used. LSO has the best combination of energyevents a short decay constant, 40 ns, and high emission intensity. In Emissionwavelengthnear Goodmatchto photomultiplier addition, it has no secondary decay component. 400 nm tuberesponse Transparentatemission Allowslightto travelunim SCINTILLATORSUSEDIN PET wavelength pededto photomultiplier tube NaI(Tl) was discovered in 1948 by Hofstadter (8). It Indexof refractionnear1.5 Goodtransmissionoflight quickly became the scintillator of choice for radiation fromcrystalto photomulti detection because of its high light output, i.e., efficient pliertube Radiationhard Stablecrystalperformance conversion of deposited -y-ray energy to scintillation pho Nonhygroscopic Simplifiespackaging tons. The large light pulses are easily processed by conven Rugged Allowsfabricationofsmaller tional pulse-shaping electronics. The main disadvantage of crystalelements NaI(Tl) is its low detection efficiency for ‘y-raysabove200 Economicgrowthprocess Reasonablecost keV, as a result of low density and moderately low atomic

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TABLE 3 Scintillation and Optical Properties of Some Common Scintillator Crystals

decay decay emission of refractionBaF20.860012220and3101.49CsF453901.48Lu2SiO5(Ce)CrystalPnmary constant(ns)Secondaryconstant(ns)Relative intensityEmission wavelength(nm)Index

(LSO)40754201.82Gd2SiO5(Ce) (GSO)60600304301.85Nal(Tl)230—10,0001004101.85Bi4Ge3O12

(BGO)300154802.15Csl(Na)630754201.84CaF2(Eu)900404351.44Csl(Tl)1

000455651.80CdWO45000—20,000204802.20

number. At the energies typically used in SPECT (140 keV), crystalscleaveeasily.Thus, specialtechniquesareneededto the detection efficiency of NaI(Tl) is satisfactory, and it is avoid cracking the crystal elements during cutting. used almost exclusively in that application. However, for LSO offers the best combination of properties for PET of higher energy applications, such as PET (511 keY), NaI('fl) any scintillator known today (13). It has high density and hasbeenreplaced, for the most part, by materials with higher high atomic number for good -y-ray detection efficiency, a density and atomic number. An additional disadvantage of short decay constant for good coincidence timing, and high NaI(Tl) is that it is highly hygroscopic. As a result, a great light output that allows the use of many small elements per deal of effort has gone into the development of hermetic PMT. In addition, it is mechanically rugged and nonhygro packaging to protect the material from moisture in the scopic,thusallowing relatively simple fabricationof detec atmosphere. tors. LSO has a low level of natural radioactivity as a result BGO emerged in the early 1970s, with initial studies of the presence of ‘76Lu,butthe counting rate from this reported by Weber and Monchamp (9). Although the light isotope is a small fraction of the typical counting rates from output of BOO is only about 15% of that of NaI(Tl), its the injected tracers; thus, it is not a significant problem for dramatically higher detection efficiency, as a result of PET. LSO hasbeenusedin a high-resolutionbraintomo density almost twice that of Nal as well as a much higher graph (14), high-resolution animal tomographs (15), com atomic number, has made it a very popular choice for the binedPETIMRIdetectors(16),andcombinedPET/SPECT detection of radiation above a few hundred keV. PET is the cameras (17). Large-scale commercial production of LSO major ongoing application of BGO crystals today, despite has been realized during 1999 (18), and widespread use of the fact that their relatively long decay constant of 300 ns this scintillator is expected in the future. limits coincidence timing resolution. Scintillators with extremely short decay constants offer PROPERTIESOFSCINTILLATORSFORPET the possibility of time-of-flight PET, in which opposing Despite the investigation of virtually every known scintil detectors measure the difference in the arrival times of a pair lator for possible use in PET, only 2 have seen widespread of y rays. In this way, the location of the positron event can use so far, NaI(Tl) and BOO. A third, LSO(Ce), is expected be localized along the line connecting the 2 detectors. Two to see widespread use in the future. In this section, the possibilities that appear in Table 3 are CsF and BaF2. CsF properties of these 3 important scintillators are compared in has very low light output and is very hygroscopic and, more detail. consequently,hasseenlittle use despiteits shortdecay One of the most important properties of a scintillator for constant of 4 ns. BaF2has an even faster decay of less than 1 PET is y-raydetectionefficiency.Becauseofthedesireto ns, greater light output, and is nonhygroscopic. Therefore, in shorten scan times and maintain low tracer activity, the the early 1980s it was used in several PET scanners (10). crystals must detect as many of the -y-rays emitted as However, because of its relatively low density and atomic possible. This is the primary reason for the popularity of number, it eventually gave way to BGO. BGO. ‘y-rayswithenergy of 511 keV interact with solid One way to increase the spatial resolution of a tomograph matter primarily through 2 phenomena, the photoelectric istocouplemultiplescintillatorcrystalswithdifferentdecay effect and the Compton effect. In the photoelectric effect, the constants to a single photodetector. Pulse-shape discrimina -y-ray is absorbed by an atom that ejects an electron tion is used to identify the crystal element of interaction. (photoelectron) and also produces either characteristic GSO has been used in conjunction with BOO in this way for x-rays or Auger electrons. The end result is that the full a high-resolution tomograph (4,11). A tomographic design energy of the ‘y-rayisabsorbed. In Compton scattering, the usingGSOexclusivelyhasalsobeenreported(12).Fabrica -y-ray loses a fraction of its energy to an electron through tion of GSO detectors requires great care, because the scattering. The energy of the electron is likely to be absorbed

SCINTILLATION CRYSTALS FOR PET •Melcher 1053 Downloaded from jnm.snmjournals.org by on February 8, 2016. For personal use only. in the crystal, whereas the scattered -y-ray may or may not be absorbed. The distribution of energy between the electron and the ‘y-rayisdetermined by the scattering angle. The detection efficiency of a detector may be character ized by the fraction of incident -y-rays that are partially or fully absorbed by it. For a detector of thickness x, exposed to a mono-energetic beam of rny-rays,theinitial y-ray intensity, I@,is attenuated according to: 1(E) = I0(E) exp (—six), where I is the intensity of -y-rays passing through the detector without interacting at all, and pi is the linear attenuation coefficient. The y-rays that do interact in the detector by depositing either their full or partial energy are givenby: A= 1 —exp(—px). FIGURE2. ScintillationemissionspectraofNal(TI),BOO,and Thus, it is clear that the fraction of incident rny-raysthatare LSO(Ce),under-y-rayexcitation. partially or fully absorbed is determined by the linear attenuation coefficient (for an idealized geometry). Figure 1 near that wavelength, whereas BOO has a much weaker and compares the linear attenuation coefficients for Nal, BOO, longer wavelength emission (480 nm) (Fig. 2). The intensity and LSO. From these data, the advantagesof BOO and LSO of the scintillation emission strongly affects the number of over Nal are clear. At 511 keV, p = 0.96 cm ‘forBOO and crystal elementsthat can be coupled to a single PMT or, 1.1 = 0.87 for LSO, whereas for Nal, p is only 0.35 cm* stated another way, the ratio of scintillation elements to Consequently, to achieve similar efficiency, Na! detectors electronic channels. With BOO, block detectors today useup must be more than twice as thick compared with BOO and to 16 crystal elements per PMT, whereas LSO detectors use LSO detectors. up to 144 crystal elements per PMT. Thus, LSO makes In most PET scanners, the emission of the scintillation possible significant cost savings as a result of the reduced crystals is converted to electrical signals by PMTs. To number of photomultipliers. produce the largest signal, the scintillation emission should In PET, the decay constant of the scintillation emission is be as intense as possible, and the wavelength of the emission very important, because singles count rates are typically should match the wavelength of maximum photomultiplier very high, and coincidence resolving time should be as small sensitivity. Because bi-alkali photomultipliers with glass as possible to reject unwanted random events. A short entrance windows, the most commonly used type, have a scintillation decay constant is a benefit in both instances. maximum sensitivity near 400 nm, it is advantageous for the Figure 3 compares the scintillation decay of NaI(Tl), BOO, scintillator to have its emission maximum near this wave and LSO. BOO has the longest decay, 300 ns. The primary length. Both Nal and LSO have intense emissions that peak decay constant of NaI(Tl) is somewhat shorter, 230 ns, but

1000

10@ 100

I 10 p31 I .! 102

0.1 id 10 100 1000 0 @oo 2000 3000 Energy(k&/) Time(nil

FIGURE 1. Total linear attenuation coefficientsof Nal, BGO, FIGURE3. Decayofthe scintillationemissionofNaI(TI),BOO, and LSO. and LSO(Ce),after excitationby -y-rays.

1054 Ti@ [email protected]@i OF NUCLEARMEDICINE •Vol. 41 •No. 6 •June 2000 Downloaded from jnm.snmjournals.org by on February 8, 2016. For personal use only. an additional secondary decay of several microseconds is provides both PET and SPECT capability (18). The LSO also present. The decay constant of LSO is several times layer is used for PET imaging and the NaI(Tl) layer is used shorter, 40 ns, and no secondary component is present. for SPECT imaging. Similarly, researchershave constructed Because the quality of a PET image is strongly dependent prototype detectors of LSO and YSO for the same purpose on the coincidence resolving time of the detectors, a figure (22). of merit that has been used for PET scintillators is the ACKNOWLEDGMENTS number of photons emitted per nanosecond. In this way, both the overall intensity of the emission as well as the The author is grateful to Lars Eriksson and Mike Casey duration of the pulse are accounted for in 1 parameter. for several useful discussions. Figure 4 compares the photons emitted per nanosecond for REFERENCES Nal, BOO, and LSO. LSO has a large advantage over BOO 1. Casey ME, Nutt R. A multi-crystal two-dimensional BOO detector system for in this respect, becauseLSO's emission is both intense and positron emission tomography. IEEE TransNuclSci. 1985;NS33:460-463. fast, whereas BOO's emission is much weaker and slower. 2. Ter-PogossianMM,FickeDC,YamamotoM,HoodIT.SuperPETITI:Apositron LSO is also better than Nal in this respect, becausealthough emission tomograph utilizing photon time-of-ffight information. IEEE Trans Med Imaging. 1982;M1-1 :179—I87. Nal's emission is stronger than LSO, the decay constant is 3. Weber 5, Bauer A, Herzog H, et al. Recent results of the TierPET scanner. In: more than 5 times longer. Seibert JA, ed. 1999 IEEE Nuclear Science Symposium and Medical Imaging Conftrence Record. Piscataway, NJ: Institute of Electrical and Electronics Engineers, Inc.; 2000. FUTURE RESEARCH DIRECTIONS 4. Karp J, Adam L-E, Freifelder R, Muchilebner G, Liu F, Surti S. High resolution Researchers continue to actively investigate potential new GSO-based brain PET camera. In: Seibert JA, ed. 1999 IEEE Nuclear Science Symposium and Medicallmaging Conference Record. Piscataway, NJ: Institute of scintillator materials, because a tomograph's performance Electrical and Electronics Engineers, Inc.; 2000. depends so strongly on the characteristics of the detectors. 5. Melcher CL, SchweitzerJS.A promisingnew scintillator@cerium-dopedlutetium Most of the effort has focused on high-density and high oxyorthosilicate. NucI InsirMeth. l992;A3 14:212—214. 6. Lempicki A, Wojtowicz AJ, Berman E. Fundamentallimits of scintillator atomic-number materials as well as materials with poten performance. 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Scintillation Crystals for PET

Charles L. Melcher

J Nucl Med. 2000;41:1051-1055.

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