IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE

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1 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE Comparison of Fast Scintillators With TOF PET Potential Maurizio Conti, Member, IEEE, Lars Eriksson, Senior Member, IEEE, Harold Rothfuss, and Charles L. Melcher, Senior Member, IEEE Abstract The renewed interest in Time-Of-Flight (TOF) Positron Emission Tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. In this work we measure some intrinsic properties of these materials, such as decay time and light output, which have a direct effect on time resolution, the key performance parameter for a TOF-grade detector. In particular, we report on measurements on LSO:Ce, LuAG:Pr, LuYAP:Ce, LaBr 3 :Ce and LaCl 3 :Ce. The scintillators are characterized in terms of absolute light yield, decay time, energy resolution, emission and excitation spectra, and time resolution. A new figure of merit to compare scintillators based on their performance in a TOF PET scanner is introduced. This figure of merit, the TOF effective sensitivity, includes both interaction probability and timing characteristics. Index Terms [AUTHOR], please supply your own keywords or send a blank to keywords@ieee.org to receive a list of suggested keywords.. I. INTRODUCTION T HE renewed interest in Time-Of-Flight (TOF) Positron Emission Tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. A good candidate for TOF PET must offer high density and Z, fast rise and decay time, and high light output. Two classes of materials are particularly interesting for use in this application: lutetium based scintillators and lanthanum halides. The Lu-based materials have a high density, ranging from 6.7 to 8.3, which is associated with high detection efficiency for the 511 kev gamma rays used in PET. The La-based materials have a lower density, ranging from 3.8 to 5.1, but higher luminosity. All materials studied are Ce-doped (or Manuscript received June 27, 2008; revised September 09, Current version published June 10, M. Conti is with the Siemens Molecular Imaging, Knoxville, TN USA ( maurizioconti@siemens.com). L. Eriksson is with the Siemens Molecular Imaging, Knoxville, TN USA, with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA, and also with the Karolinska Institutet, Stockholm, Stockholm, Sweden, and the Institute of Physics, University of Stockholm, Stockhom, Sweden. H. Rothfuss is with the Siemens Molecular Imaging, Knoxville, TN USA, and also with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA. C. L. Melcher is with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TNS Pr-doped) with a fast primary decay time (20 40 ns), due to the 5d-4f transition of the (or ) ions. Lutetium oxyorthosilicate, (LSO), Ce-doped, is a dense scintillator (7.4 ) and is characterized by a decay time of between 30 and 50 ns, depending on calcium concentration as a co-dopant. It has a fairly high light output (up to photons/mev) [1] [3]. Lutetium aluminum garnet, (LuAG), Pr-doped, is characterized by more than one scintillation decay component: the fast decay time is around 20 ns. Its density is 6.7 and the total light output is below photons/mev [4] [7]. Lutetium aluminum yttrium perovskite, (LuYAP), Ce-doped, is also characterized by more than one component. The fast decay time is around 20 ns. It can reach very high density (up to 8.3 ), depending on the amount of the denser lutetium, but its light output is usually well below photons/mev [8] [11]. Lanthanum bromide,, Ce-doped, is a fast scintillator, with only one fast component and a decay time of less than 20 ns. It is also exceptionally bright (more than photons/mev). On the other hand, it is less dense than the lutetium compounds, 5.1 [12] [14]. Lanthanum chloride,, Ce-doped, has more than one scintillation decay component, the fastest decay time being around 20 ns. It has a very good total light output, more than photons/mev. Its density is 3.8 [15], [16]. Evaluating the possible performance of these scintillators in a TOF PET scanner is a complex process, since many variables are involved, and some of them are not intrinsic properties of the materials, such as cost or manufacturing reliability. The scope of this work is to measure the intrinsic properties of some materials, and propose a method to compare and rate the scintillators based on the effect of such intrinsic properties on the TOF PET scanner performance. II. MATERIALS AND METHODS This work presents measurements performed on a set of materials with TOF potential, namely LSO:Ce, LuYAP:Ce, LuAG:Pr, and. Information about the samples used is presented in Table I. LSO is activated. The crystals were grown at the University of Tennessee, in the Scintillation Materials Laboratory. In addition to standard LSO:Ce, samples with 0.1% and 0.3% Ca co-doping, which have shorter decay times, were used [3]. The samples used in the measurements were or mm, not polished. Web Version /$ IEEE

2 2 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 TABLE I COMPOSITION AND DESCRIPTION OF THE CRYSTAL SAMPLES STUDIED IN THIS WORK The LuAG crystals came from two sources: Furukawa Co., Ltd. and the Scintillation Materials Laboratory at the University of Tennessee. LuAG is activated. The samples used in the measurements were or mm. The samples from Furukawa Co. were polished, the samples from University of Tennessee were not The LuYAP is activated. The crystals were manufactured by SAES Srl. The samples had a low Lu content and were 5 5 5or mm, with polished surfaces. Lanthanum bromide and lanthanum chloride are activated. has 5% Ce doping. The crystals were manufactured by Saint-Gobain Crystals. The samples used in the measurements were cylindrical in shape, 13 mm in diameter and 13 mm in height, polished and factory canned. One base face and the side surface were wrapped in reflectant material, and the other face had a quartz window. The whole assembly was sealed (crystals are hygroscopic). A. Emission and Excitation Spectra A Hitachi fluorescence spectrophotometer, model F-4500 was used to acquire emission and excitation spectra. The scanning step was 0.6 nm. The emission spectrum was acquired using an excitation wavelength in the center of the excitation band. The excitation spectrum was acquired at a wavelength centered in the emission band. B. Scintillation Decay Time Scintillation decay time was performed on the largest samples available, in order to accumulate the most data. The experiment was performed in a light-tight box with a modified Thomas- Bollinger set-up [17]. The sample was placed between two Photonis XP2020Q photo-multiplier tubes (PMT), with high voltage applied. A source (662 kev) was placed on the crystal. One of the PMTs (PMT1), close to the crystal, was used as a trigger signal and the second PMT (PMT2), placed at about 10 cm from the crystal and partially covered with a shutter (in order to reduce the photon flux to a few percent), was used as a stop or measure signal. The anode signals from the PMTs were fed into a fast amplifier, Ortec 474, and then into a constant fraction discriminator (CFD), Ortec 935. The two discriminated signals were then used as a start and stop for a time-to-analog converter (TAC), Ortec 567, with a 2 end Fig. 1. Diagram of the experimental set-up for the scintillation decay time measurement. Fig. 2. Diagram of the experimental set-up for the absolute light output measurement. scale. The output voltage signal was fed into a multichannel analyzer Ortec Maestro, which produced the scintillation decay curves reported here. The experimental set-up is shown in Fig. 1. An exponential fit with one or more decay constants as free parameters was used to compute decay time values. The fraction of light emitted in each component was also a free parameter estimated by the fit. C. Absolute Light Output For the light output measurement, the samples were coupled to a H3177 PMT using optical grease. A Teflon reflective cap was placed over the sample crystal on the PMT photocathode, in order to reflect all the light coming out of the crystal back into the PMT. A source was placed above the Teflon cap. The assembly was inside a light-tight box. The anode signal was fed into a pre-amplifier Canberra 2005 and then into a spectroscopy amplifier Ortec 672 with the shaping time set to 3.A MCA Tukan-8 k on a USB port received the signal that was histogrammed on a personal computer. In Fig. 2, the experimental set-up is shown. For each energy spectrum, the energy resolution at 662 kev was obtained as the ratio between full-width-half-maximum (FWHM) and the centroid of the photopeak; a Gaussian fit to the photo peak was used. The position of the photoelectric peak for the 662 kev was proportional to the number of photoelectrons emitted by the photocathode. The peak position centroid was determined with a Gaussian fit, and it was divided by the Web Version

3 CONTI et al.: COMPARISON OF FAST SCINTILLATORS WITH TOF PET POTENTIAL 3 position of the single photoelectron peak position to determine the absolute number of emitted photoelectrons. The single photoelectron peak was measured acquiring data with no source. Knowing the incident energy and the quantum efficiency of the PMT at the emission wavelength, the absolute light output was obtained for each crystal in terms of photons per MeV. The quantum efficiency of the H3177 PMT was fairly uniform in the range of light emitted by all the materials used in this work, about 25%. This method for absolute light output measurement is fully described in reference [18]. D. Time Resolution Time resolution measurement was performed on the smallest available samples, in order to measure the best possible time resolution; however, additional measurements were also performed on larger samples to evaluate the effect of longer light paths. A source was placed between two crystal samples of the same size, each coupled to a Photonis XP2020Q photomultiplier. Each crystal was placed at the center of the PMT quartz window and was coupled together with optical grease. The crystal and the PMT window were covered with four layers of Teflon sheet as a reflectant, and the crystal-pmt assembly was carefully wrapped in black tape for optical isolation. The PMTs were supplied with. The anode outputs were used for timing, and the dynode outputs were used for energy window selection. The timing chain was comprised of a constant fraction discriminator Ortec 935 that received the anodic signal with the threshold set at the minimum possible, around 200 mv, which was between 5% and 10% of the anodic signal. The shaping delay was less than 1 ns. Two discriminated signals were then used as a start and stop for a time-to-analog converter (TAC) Ortec 567. The output voltage signal was fed into a multichannel analyzer Ortec Maestro, which produced a time histogram. Time resolution was obtained as the FWHM of a Gaussian fit to the time histogram. The time calibration of the MCA was 25.5 ps per channel. The TAC received a strobe signal from an energy channel, so that the system accepted only events in time coincidence and with energy within a narrow energy window around the 511 kev photo peak (the window width was about 15% 20% of the peak signal height). The energy channel was comprised of a pre-amplifier Ortec 113 (100 pf coupling capacitor), an amplifier Ortec 855 with a 1.5 shaping time, and a single channel analyzer Ortec 551, which selected the upper and lower limits of the accepted signal. Their digital signals were fed into a fast coincidence module Ortec 414A, which worked as a strobe to the TAC. The experimental set-up is shown in Fig. 3. III. RESULTS A. Emission and Excitation Spectra The emission and excitation spectra for the different materials are shown in Fig. 4. Emission spectra of the materials studied in this work range from 320 to 450 nm, all suitable for the XP2020Q and H3177 PMTs used. However, in general it can be observed that,, LuYAP:Ce and Fig. 3. Diagram of the experimental set-up for the time resolution measurement. particularly LuAG:Pr have a shorter wavelength emission spectrum, which requires special attention to the photocathode input window. LSO:Ce samples of different compositions produced similar spectra and only one is reported in Fig. 4. B. Scintillation Decay Time For each crystal, the decay time was obtained via an exponential fit to the data with one or more components. The experimental curves and the exponential fit for each material are shown in Fig. 5. LSO:Ce was fitted with only one exponential decay constant, and obtained 44 ns decay time for standard LSO:Ce, 37 ns decay time for 0.1% calcium co-doping, and 30 ns decay time for 0.3% calcium co-doping (Fig. 5(a)). Both samples (University of Tennessee and Furukawa Co.) of LuAG:Pr were described by a fast component with 22 ns decay time (60%) and a slow component with 419 ns (40%), as can be seen in Fig. 5(b). LuYAP:Ce was fitted with three components, and the results were a fast component with 16 ns decay time (57% of the light), and two slower components with 145 ns (22%) and 594 ns (21%), in Fig. 5(c). was fitted with only one decay time and obtained 17 ns (Fig. 5(d)). had three components: a fast 18 ns decay time (70%) and two slow components with 125 ns (21%) and 220 ns (9%) decay times (Fig. 5(e)). In Table II, a summary of all the components is presented. C. Absolute Light Output Different samples for each material were analyzed, acquiring an energy spectrum with a source. In Fig. 6, a typical energy spectrum is reported for LSO:Ce.In all spectra, the 662 kev peak is clearly resolved, and the energy resolution ranges from 4% to 9%, (FWHM). The energy resolution values for 662 kev are reported in Table III. Also in Table III, the average values of absolute light output are reported for each material. Since some LuYAP:Ce showed self-absorption, the average is computed, when possible, using measurements from the smaller 5 mm cubes. and Web Version

4 4 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 Fig. 4. Emission (solid line) and excitation (dotted line) spectra for (a) LSO:Ce, (b) LuAG:Pr, (c) LuYAP:Ce, (d) LaBr :Ce, and (e) LaCl :Ce. were available only in 13 mm cylinders.in Table IV, the absolute light output is presented for all the samples tested, Web Version Fig. 5. Scintillation decay spectra for (a) LSO:Ce, (b) LuAG:Pr, (c) LuYAP:Ce, (d) LaBr :Ce, and (e) LaCl :Ce. Experimental data (dots) and exponential fit (solid line) are shown. together with information about size and source of the crystal.

5 CONTI et al.: COMPARISON OF FAST SCINTILLATORS WITH TOF PET POTENTIAL 5 TABLE II SCINTILLATION DECAY TIME AND RELATIVE FRACTION OF THE TOTAL LIGHT OUTPUT FOR ALL COMPONENTS TABLE IV ABSOLUTE LIGHT OUTPUT FOR ALL MEASURED SAMPLES Fig. 6. Energy spectra of 662 kev photons from LSO:Ce. TABLE III ABSOLUTE LIGHT OUTPUT AND ENERGY RESOLUTION. Cs, measured with As explained in Section II, using the photoelectrons measured in the experiment, the energy of the incident photon and the quantum efficiency of the PMT, the absolute light output was computed and reported in Table IV. The absolute light output for each scintillator is the average of all measurements on 5 mm cubes (13 mm cylinders for and ). Energy resolution is measured with photons from. The absolute light output for LSO:Ce samples was measured to be generally between and photons/mev, regardless of the sample size, for 0.0%, 0.1%, and 0.3% calcium co-doping. These values are about 20% less than the best values ever reported in the literature for an LSO:Ce crystal [3]. Only one sample was found to have a lower light output of about photons/mev. The LuAG:Pr samples gave results that were independent of the crystal size, but were different between the two manufacturers. The crystals fabricated at University of Tennessee had a light output around photons/mev, the crystals from Furukawa had photons/mev. The 15% lower light output from the crystals produced at the University of Tennessee (UT) may be due to a difference in Pr concentration and a lack of polishing. Of all the material studied, LuYAP:Ce was the only one to exhibit a noticeable self-absorption: the 10 mm cubic crystals consistently showed lower light output (12000 photons/mev) than the 5 mm cubic crystals (16000 photons/mev). The two samples of yielded a light output of about photons/mev, only 10% less than the specifications on the Saint-Gobain Crystals data sheet of photons/mev. The two samples systematically produced lower light output than expected; photons/mev as compared to photons/mev reported in the manufacturer s data sheet. This could be partially due to the integration time being too short (3 ), but very likely occurred because the two samples came from a defective detector batch. D. Time Resolution In order to measure the best time resolution, the smaller 5 mm cubes were used for this measurement when possible. The two 13 mm cylinders were used for and. Time resolution (FWHM) for two crystals was measured. The single crystal time resolution (FWHM) was obtained assuming independent contributions of the two single detectors as. Time resolution and the emitted photoelectrons for each material are reported in Table V. Photoelectrons were estimated using the average fast light output for each crystal pair from data in Tables II and III, multiplied by the 511 kev energy and scaled by 25% quantum efficiency. In Fig. 7, the time resolution vs. the inverse of the square root of the number of photoelectrons was plotted. This is based on a model developed by Hyman and supported with experimental data by Moszynski [19], [20], and will be discussed in Section IV. Web Version

6 6 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 A. Conventional Detection Efficiency Fig. 7. Measured time resolution of a single crystal vs. 1= N ; N is the number of photoelectrons for 511 kev deposited energy: LSO:Ce (solid circle), LuAG:Pr (empty circle), LuYAP:Ce (solid square), LaBr :Ce (solid triangle), LaCl :Ce (empty triangle). TABLE V TIME RESOLUTION AND PHOTOELECTRON YIELD Time resolution improved as the number of photoelectrons increased, as expected, even though some samples appeared to deviate from the linear behavior. In particular, the and samples were larger than all the other samples, and the larger spread in light path may have had a negative effect on the time resolution. Time resolution of a single crystal and estimated number of photoelectrons for different materials and sample size. A source was used (511 kev). IV. DISCUSSION: A FIGURE OF MERIT TO COMPARE MATERIALS FOR TOF PET APPLICATIONS Time resolution is not the only parameter to be considered when comparing scintillating materials for TOF PET. The image quality of a PET image is directly related to the signal-to-noise ratio (SNR), which improves with the number of counts acquired. Therefore, the key to better image quality is the overall sensitivity of a PET scanner. From the point of view of the conventional detection efficiency, a good detector candidate for PET must have high density and high Z to increase the interaction probability of the 511 kev photons and the probability of photoelectric interaction (over Compton and coherent scattering). It is well known that TOF reconstruction in PET improves the SNR. TOF reconstruction is equivalent to a sensitivity amplifier characterized by a TOF gain, which is proportional to the inverse of the time resolution of the PET scanner [21] [24]. In this section we will introduce a figure of merit that combines the conventional efficiency and the TOF gain of different materials. A typical PET scanner architecture consists of rings of detectors made of scintillating crystals of radial thickness. The probability of an emitted photon to interact is proportional to the interaction efficiency, where is the material density and is the total attenuation coefficient for photoelectric absorption evaluated at 511 kev energy. In fact, only the fraction of events that deposit energy in a preset energy window centered on the photo peak is usually accepted. This fraction, called photo peak fraction (PPF), includes photons that undergo photoelectric interaction and part of the photons that undergo Compton interaction if the secondary photons are absorbed near the primary interaction point. Therefore, the interaction efficiency must be corrected by the photo peak fraction. The scanner sensitivity depends on the coincidence efficiency (the probability of having both coincidence photons detected), that is proportional to the square of the single count efficiency defined above, and can be expressed as: B. TOF Sensitivity Gain TOF reconstruction, which reduces the noise in an image, acts as a sensitivity gain factor that is known to be proportional to the inverse of the PET scanner system time resolution [21] [24]. The system time resolution depends on the intrinsic timing of the scintillating material. In Section III we reported the experimental values of time resolution for some scintillating materials. Such measurements depend on the experimental conditions, but the lower limit to the time resolution of a given material can be estimated using intrinsic material characteristics such as the decay time and the absolute light output (number of photons per MeV). Using the Hyman theory [19], [20], the time resolution can be modeled as where is the fraction of the signal used to trigger the discriminator; PMT represents the PMT gain, gain dispersion and timing characteristics; and represents the number of photoelectrons produced in the PMT. Since the Hyman function weakly decreases with increasing decay time with a power less than 1, one can assume that Web Version where. In fact, has been successfully fitted as proportional to the square root of [25]. Therefore, the system (1) (2) (3)

7 CONTI et al.: COMPARISON OF FAST SCINTILLATORS WITH TOF PET POTENTIAL 7 TABLE VI MAIN PROPERTIES OF SOME TOF-GRADE SCINTILLATORS AND TOF PET FIGURE OF MERIT. PERFORMANCE PARAMETERS FOR SOME TOF-GRADE SCINTILLATORS. DENSITY, TOTAL ABSORPTION COEFFICIENT AT 511 KEV (FROM TABLE IN [28]) AND PHOTO PEAK FRACTION (FROM MONTE CARLO SIMULATION) CONTRIBUTE TO THE CONVENTIONAL SENSITIVITY S, COMPUTED FOR A 2 CM THICK DETECTOR. THE FAST COMPONENT DECAY TIME AND THE LIGHT OUTPUT FOR THE FAST COMPONENT N CONTRIBUTE TO THE TOF SENSITIVITY GAIN. THE EFFECTIVE SENSITIVITY, OR FIGURE OF MERIT, IS THE PRODUCT OF CONVENTIONAL SENSITIVITY AND TOF SENSITIVITY GAIN time resolution of the PET scanner using such a material can be estimated as being proportional to: This has been demonstrated to be an accurate model for scintillators that have negligible rise time compared to decay time (for example LSO:Ce [25]), and less so for others [26], [27]. Also, the model was developed for materials with only one decay time component. Nevertheless, we can use this model to estimate the lower limit of the time resolution: in (4), we use the fast component decay time for, and the prompt fraction of the photoelectrons, associated with the fast part of the signal, for. This prompt fraction of the photoelectrons is proportional to, the fraction the absolute light output emitted in the fast component. Since the TOF sensitivity gain is inversely proportional to the system time resolution [21] [24], using (4) we can directly relate the TOF gain to the decay time and the light output with the following equation: where is now the fast component decay time. C. Figure of Merit for TOF PET Scintillators We propose to evaluate the potential use of a scintillating material in TOF PET using the TOF effective sensitivity as the key parameter, defined as the product of conventional coincidence efficiency (at 511 kev) times TOF sensitivity gain. Table VI summarizes all the main characteristics of the materials evaluated in this work: the density (from the material data sheet), the total absorption coefficient at 511 kev (data from [28]), the photo peak fraction (computed via a Geant4 Monte Carlo simulation), the overall coincidence detection efficiency computed for a typical 2 cm thick detector, the fast component decay time and the fast component light output measured (4) (5) (6) in this work, the resulting TOF sensitivity gain and the overall figure of merit effective sensitivity. In Table VI, we used the LSO:Ce with the highest light output (0.1% calcium co-doping) and the best results for LuAG:Pr obtained with the samples from Furukawa Co. LuYAP:Ce results are for the 52% lutetium and 48% yttrium material, but better interaction efficiency could be obtained with a higher percentage of lutetium. The Monte Carlo simulation of the photo peak fraction assumed a full collection of all the light emitted in a 2-cm thick scintillator ring. Even if not completely realistic, we believe that the simulation results can be used in our figure of merit that compares scintillators in the same scanner architecture. This figure of merit includes both interaction probability and timing characteristics, and allows the comparison of different scintillating materials as TOF PET detectors, assuming that all other parameters are kept constant (such as scanner geometry, electronics performance, quantum efficiency, timing of PMTs, etc.). This figure of merit does not include other parameters that can influence the choice of a scintillating material, such as: energy resolution, hygroscopicity, cost or reliability of the fabrication in terms of crystal uniformity, emission wavelength and resulting need for more expensive PMTs with quartz window, and others. In particular, energy resolution is very important in evaluating PET performance, since it has a key function in the rejection of radiation scattered in the patient: a complete evaluation of the design of a new PET scanner has to consider energy resolution performance. Also, in the design of a PET scanner, ad-hoc technological solutions could be used to overcome specific disadvantages associated with a scintillator. For example, if equipped with proper depth-of-interaction correction, a 3-cm thick detector is a viable alternative to a shorter detector made of a denser material: the figure of merit (3 cm) is 12.2, which well compare with a 2-cm thick LSO crystal (2 cm) of With all its limitations, we believe that the figure of merit, introduced in this work, includes the major components needed for the evaluation of a scintillator for TOF PET applications, and it is a good starting point for comparison. Web Version V. CONCLUSIONS While the halides, particularly, excelled in timing performance due to their high light output and fast decay time,

8 8 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 the lutetium compounds outperformed them in conventional PET efficiency due to their high density and high effective Z. LSO:Ce could be a good material for TOF PET. Calcium co-doping appears to be an interesting direction to follow in order to have faster LSO:Ce and good light output. LuAG:Pr would be a good candidate, if manufacturers could increase the amount of light in the fast component. The slow decay component has a double negative effect: it reduces the amount of light available for fast timing and makes problematic the use of LuAG:Pr for high count rate applications. The LuYAP:Ce available for this work had a low lutetium content and presented serious self-absorption. If self-absorption was overcome, LuYAP:Ce with a large ratio of Lu to Y could become very attractive. has optimal timing and brightness characteristics, but its low density can be overcome only by using thicker detectors, which implies higher costs and depth-of-interaction problems. has very low density and a large slow decay component, and appears to be the least suitable of the scintillating material analyzed. The TOF PET figure of merit introduced in Section IV shows that LSO:Ce and LuAG:Pr appear to be the best materials for this application, given the samples available for this study. ACKNOWLEDGMENT We thank Yasu Watanabe and Furukawa Co. Ltd for the loan of the LuAG:Pr crystals used in this work. REFERENCES [1] C. L. Melcher and J. S. Schweitzer, Cerium-doped lutetium orthosilicate: A fast, efficient new scintillator, IEEE Trans. Nucl. Sci., vol. 39, pp , [2] M. Kapusta, P. Szupryczynski, C. L. Melcher, M. Moszynski, M. Balcerzyk, A. A. Carey, W. Czarnacki, M. A. 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9 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE Comparison of Fast Scintillators With TOF PET Potential Maurizio Conti, Member, IEEE, Lars Eriksson, Senior Member, IEEE, Harold Rothfuss, and Charles L. Melcher, Senior Member, IEEE Abstract The renewed interest in Time-Of-Flight (TOF) Positron Emission Tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. In this work we measure some intrinsic properties of these materials, such as decay time and light output, which have a direct effect on time resolution, the key performance parameter for a TOF-grade detector. In particular, we report on measurements on LSO:Ce, LuAG:Pr, LuYAP:Ce, LaBr 3 :Ce and LaCl 3 :Ce. The scintillators are characterized in terms of absolute light yield, decay time, energy resolution, emission and excitation spectra, and time resolution. A new figure of merit to compare scintillators based on their performance in a TOF PET scanner is introduced. This figure of merit, the TOF effective sensitivity, includes both interaction probability and timing characteristics. Index Terms [AUTHOR], please supply your own keywords or send a blank to keywords@ieee.org to receive a list of suggested keywords.. I. INTRODUCTION T HE renewed interest in Time-Of-Flight (TOF) Positron Emission Tomography (PET) has been accompanied by new research in the development of fast scintillators, mainly halides and/or lutetium-based compounds doped with Ce or Pr. A good candidate for TOF PET must offer high density and Z, fast rise and decay time, and high light output. Two classes of materials are particularly interesting for use in this application: lutetium based scintillators and lanthanum halides. The Lu-based materials have a high density, ranging from 6.7 to 8.3, which is associated with high detection efficiency for the 511 kev gamma rays used in PET. The La-based materials have a lower density, ranging from 3.8 to 5.1, but higher luminosity. All materials studied are Ce-doped (or Manuscript received June 27, 2008; revised September 09, Current version published June 10, M. Conti is with the Siemens Molecular Imaging, Knoxville, TN USA ( maurizioconti@siemens.com). L. Eriksson is with the Siemens Molecular Imaging, Knoxville, TN USA, with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA, and also with the Karolinska Institutet, Stockholm, Stockholm, Sweden, and the Institute of Physics, University of Stockholm, Stockhom, Sweden. H. Rothfuss is with the Siemens Molecular Imaging, Knoxville, TN USA, and also with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA. C. L. Melcher is with the Scintillation Material Laboratory, University of Tennessee, Knoxville, TN USA. Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TNS Pr-doped) with a fast primary decay time (20 40 ns), due to the 5d-4f transition of the (or ) ions. Lutetium oxyorthosilicate, (LSO), Ce-doped, is a dense scintillator (7.4 ) and is characterized by a decay time of between 30 and 50 ns, depending on calcium concentration as a co-dopant. It has a fairly high light output (up to photons/mev) [1] [3]. Lutetium aluminum garnet, (LuAG), Pr-doped, is characterized by more than one scintillation decay component: the fast decay time is around 20 ns. Its density is 6.7 and the total light output is below photons/mev [4] [7]. Lutetium aluminum yttrium perovskite, (LuYAP), Ce-doped, is also characterized by more than one component. The fast decay time is around 20 ns. It can reach very high density (up to 8.3 ), depending on the amount of the denser lutetium, but its light output is usually well below photons/mev [8] [11]. Lanthanum bromide,, Ce-doped, is a fast scintillator, with only one fast component and a decay time of less than 20 ns. It is also exceptionally bright (more than photons/mev). On the other hand, it is less dense than the lutetium compounds, 5.1 [12] [14]. Lanthanum chloride,, Ce-doped, has more than one scintillation decay component, the fastest decay time being around 20 ns. It has a very good total light output, more than photons/mev. Its density is 3.8 [15], [16]. Evaluating the possible performance of these scintillators in a TOF PET scanner is a complex process, since many variables are involved, and some of them are not intrinsic properties of the materials, such as cost or manufacturing reliability. The scope of this work is to measure the intrinsic properties of some materials, and propose a method to compare and rate the scintillators based on the effect of such intrinsic properties on the TOF PET scanner performance. II. MATERIALS AND METHODS This work presents measurements performed on a set of materials with TOF potential, namely LSO:Ce, LuYAP:Ce, LuAG:Pr, and. Information about the samples used is presented in Table I. LSO is activated. The crystals were grown at the University of Tennessee, in the Scintillation Materials Laboratory. In addition to standard LSO:Ce, samples with 0.1% and 0.3% Ca co-doping, which have shorter decay times, were used [3]. The samples used in the measurements were or mm, not polished. Print Version /$ IEEE

10 2 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 TABLE I COMPOSITION AND DESCRIPTION OF THE CRYSTAL SAMPLES STUDIED IN THIS WORK The LuAG crystals came from two sources: Furukawa Co., Ltd. and the Scintillation Materials Laboratory at the University of Tennessee. LuAG is activated. The samples used in the measurements were or mm. The samples from Furukawa Co. were polished, the samples from University of Tennessee were not The LuYAP is activated. The crystals were manufactured by SAES Srl. The samples had a low Lu content and were 5 5 5or mm, with polished surfaces. Lanthanum bromide and lanthanum chloride are activated. has 5% Ce doping. The crystals were manufactured by Saint-Gobain Crystals. The samples used in the measurements were cylindrical in shape, 13 mm in diameter and 13 mm in height, polished and factory canned. One base face and the side surface were wrapped in reflectant material, and the other face had a quartz window. The whole assembly was sealed (crystals are hygroscopic). A. Emission and Excitation Spectra A Hitachi fluorescence spectrophotometer, model F-4500 was used to acquire emission and excitation spectra. The scanning step was 0.6 nm. The emission spectrum was acquired using an excitation wavelength in the center of the excitation band. The excitation spectrum was acquired at a wavelength centered in the emission band. B. Scintillation Decay Time Scintillation decay time was performed on the largest samples available, in order to accumulate the most data. The experiment was performed in a light-tight box with a modified Thomas- Bollinger set-up [17]. The sample was placed between two Photonis XP2020Q photo-multiplier tubes (PMT), with high voltage applied. A source (662 kev) was placed on the crystal. One of the PMTs (PMT1), close to the crystal, was used as a trigger signal and the second PMT (PMT2), placed at about 10 cm from the crystal and partially covered with a shutter (in order to reduce the photon flux to a few percent), was used as a stop or measure signal. The anode signals from the PMTs were fed into a fast amplifier, Ortec 474, and then into a constant fraction discriminator (CFD), Ortec 935. The two discriminated signals were then used as a start and stop for a time-to-analog converter (TAC), Ortec 567, with a 2 end Fig. 1. Diagram of the experimental set-up for the scintillation decay time measurement. Fig. 2. Diagram of the experimental set-up for the absolute light output measurement. scale. The output voltage signal was fed into a multichannel analyzer Ortec Maestro, which produced the scintillation decay curves reported here. The experimental set-up is shown in Fig. 1. An exponential fit with one or more decay constants as free parameters was used to compute decay time values. The fraction of light emitted in each component was also a free parameter estimated by the fit. C. Absolute Light Output For the light output measurement, the samples were coupled to a H3177 PMT using optical grease. A Teflon reflective cap was placed over the sample crystal on the PMT photocathode, in order to reflect all the light coming out of the crystal back into the PMT. A source was placed above the Teflon cap. The assembly was inside a light-tight box. The anode signal was fed into a pre-amplifier Canberra 2005 and then into a spectroscopy amplifier Ortec 672 with the shaping time set to 3.A MCA Tukan-8 k on a USB port received the signal that was histogrammed on a personal computer. In Fig. 2, the experimental set-up is shown. For each energy spectrum, the energy resolution at 662 kev was obtained as the ratio between full-width-half-maximum (FWHM) and the centroid of the photopeak; a Gaussian fit to the photo peak was used. The position of the photoelectric peak for the 662 kev was proportional to the number of photoelectrons emitted by the photocathode. The peak position centroid was determined with a Gaussian fit, and it was divided by the Print Version

11 CONTI et al.: COMPARISON OF FAST SCINTILLATORS WITH TOF PET POTENTIAL 3 position of the single photoelectron peak position to determine the absolute number of emitted photoelectrons. The single photoelectron peak was measured acquiring data with no source. Knowing the incident energy and the quantum efficiency of the PMT at the emission wavelength, the absolute light output was obtained for each crystal in terms of photons per MeV. The quantum efficiency of the H3177 PMT was fairly uniform in the range of light emitted by all the materials used in this work, about 25%. This method for absolute light output measurement is fully described in reference [18]. D. Time Resolution Time resolution measurement was performed on the smallest available samples, in order to measure the best possible time resolution; however, additional measurements were also performed on larger samples to evaluate the effect of longer light paths. A source was placed between two crystal samples of the same size, each coupled to a Photonis XP2020Q photomultiplier. Each crystal was placed at the center of the PMT quartz window and was coupled together with optical grease. The crystal and the PMT window were covered with four layers of Teflon sheet as a reflectant, and the crystal-pmt assembly was carefully wrapped in black tape for optical isolation. The PMTs were supplied with. The anode outputs were used for timing, and the dynode outputs were used for energy window selection. The timing chain was comprised of a constant fraction discriminator Ortec 935 that received the anodic signal with the threshold set at the minimum possible, around 200 mv, which was between 5% and 10% of the anodic signal. The shaping delay was less than 1 ns. Two discriminated signals were then used as a start and stop for a time-to-analog converter (TAC) Ortec 567. The output voltage signal was fed into a multichannel analyzer Ortec Maestro, which produced a time histogram. Time resolution was obtained as the FWHM of a Gaussian fit to the time histogram. The time calibration of the MCA was 25.5 ps per channel. The TAC received a strobe signal from an energy channel, so that the system accepted only events in time coincidence and with energy within a narrow energy window around the 511 kev photo peak (the window width was about 15% 20% of the peak signal height). The energy channel was comprised of a pre-amplifier Ortec 113 (100 pf coupling capacitor), an amplifier Ortec 855 with a 1.5 shaping time, and a single channel analyzer Ortec 551, which selected the upper and lower limits of the accepted signal. Their digital signals were fed into a fast coincidence module Ortec 414A, which worked as a strobe to the TAC. The experimental set-up is shown in Fig. 3. III. RESULTS A. Emission and Excitation Spectra The emission and excitation spectra for the different materials are shown in Fig. 4. Emission spectra of the materials studied in this work range from 320 to 450 nm, all suitable for the XP2020Q and H3177 PMTs used. However, in general it can be observed that,, LuYAP:Ce and Fig. 3. Diagram of the experimental set-up for the time resolution measurement. particularly LuAG:Pr have a shorter wavelength emission spectrum, which requires special attention to the photocathode input window. LSO:Ce samples of different compositions produced similar spectra and only one is reported in Fig. 4. B. Scintillation Decay Time For each crystal, the decay time was obtained via an exponential fit to the data with one or more components. The experimental curves and the exponential fit for each material are shown in Fig. 5. LSO:Ce was fitted with only one exponential decay constant, and obtained 44 ns decay time for standard LSO:Ce, 37 ns decay time for 0.1% calcium co-doping, and 30 ns decay time for 0.3% calcium co-doping (Fig. 5(a)). Both samples (University of Tennessee and Furukawa Co.) of LuAG:Pr were described by a fast component with 22 ns decay time (60%) and a slow component with 419 ns (40%), as can be seen in Fig. 5(b). LuYAP:Ce was fitted with three components, and the results were a fast component with 16 ns decay time (57% of the light), and two slower components with 145 ns (22%) and 594 ns (21%), in Fig. 5(c). was fitted with only one decay time and obtained 17 ns (Fig. 5(d)). had three components: a fast 18 ns decay time (70%) and two slow components with 125 ns (21%) and 220 ns (9%) decay times (Fig. 5(e)). In Table II, a summary of all the components is presented. C. Absolute Light Output Different samples for each material were analyzed, acquiring an energy spectrum with a source. In Fig. 6, a typical energy spectrum is reported for LSO:Ce.In all spectra, the 662 kev peak is clearly resolved, and the energy resolution ranges from 4% to 9%, (FWHM). The energy resolution values for 662 kev are reported in Table III. Also in Table III, the average values of absolute light output are reported for each material. Since some LuYAP:Ce showed self-absorption, the average is computed, when possible, using measurements from the smaller 5 mm cubes. and Print Version

12 4 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 56, NO. 3, JUNE 2009 Fig. 4. Emission (solid line) and excitation (dotted line) spectra for (a) LSO:Ce, (b) LuAG:Pr, (c) LuYAP:Ce, (d) LaBr :Ce, and (e) LaCl :Ce. were available only in 13 mm cylinders.in Table IV, the absolute light output is presented for all the samples tested, Print Version Fig. 5. Scintillation decay spectra for (a) LSO:Ce, (b) LuAG:Pr, (c) LuYAP:Ce, (d) LaBr :Ce, and (e) LaCl :Ce. Experimental data (dots) and exponential fit (solid line) are shown. together with information about size and source of the crystal.

13 CONTI et al.: COMPARISON OF FAST SCINTILLATORS WITH TOF PET POTENTIAL 5 TABLE II SCINTILLATION DECAY TIME AND RELATIVE FRACTION OF THE TOTAL LIGHT OUTPUT FOR ALL COMPONENTS TABLE IV ABSOLUTE LIGHT OUTPUT FOR ALL MEASURED SAMPLES Fig. 6. Energy spectra of 662 kev photons from LSO:Ce. TABLE III ABSOLUTE LIGHT OUTPUT AND ENERGY RESOLUTION. Cs, measured with As explained in Section II, using the photoelectrons measured in the experiment, the energy of the incident photon and the quantum efficiency of the PMT, the absolute light output was computed and reported in Table IV. The absolute light output for each scintillator is the average of all measurements on 5 mm cubes (13 mm cylinders for and ). Energy resolution is measured with photons from. The absolute light output for LSO:Ce samples was measured to be generally between and photons/mev, regardless of the sample size, for 0.0%, 0.1%, and 0.3% calcium co-doping. These values are about 20% less than the best values ever reported in the literature for an LSO:Ce crystal [3]. Only one sample was found to have a lower light output of about photons/mev. The LuAG:Pr samples gave results that were independent of the crystal size, but were different between the two manufacturers. The crystals fabricated at University of Tennessee had a light output around photons/mev, the crystals from Furukawa had photons/mev. The 15% lower light output from the crystals produced at the University of Tennessee (UT) may be due to a difference in Pr concentration and a lack of polishing. Of all the material studied, LuYAP:Ce was the only one to exhibit a noticeable self-absorption: the 10 mm cubic crystals consistently showed lower light output (12000 photons/mev) than the 5 mm cubic crystals (16000 photons/mev). The two samples of yielded a light output of about photons/mev, only 10% less than the specifications on the Saint-Gobain Crystals data sheet of photons/mev. The two samples systematically produced lower light output than expected; photons/mev as compared to photons/mev reported in the manufacturer s data sheet. This could be partially due to the integration time being too short (3 ), but very likely occurred because the two samples came from a defective detector batch. D. Time Resolution In order to measure the best time resolution, the smaller 5 mm cubes were used for this measurement when possible. The two 13 mm cylinders were used for and. Time resolution (FWHM) for two crystals was measured. The single crystal time resolution (FWHM) was obtained assuming independent contributions of the two single detectors as. Time resolution and the emitted photoelectrons for each material are reported in Table V. Photoelectrons were estimated using the average fast light output for each crystal pair from data in Tables II and III, multiplied by the 511 kev energy and scaled by 25% quantum efficiency. In Fig. 7, the time resolution vs. the inverse of the square root of the number of photoelectrons was plotted. This is based on a model developed by Hyman and supported with experimental data by Moszynski [19], [20], and will be discussed in Section IV. Print Version