MPPC Arrays in PET Detectors With LSO and BGO Scintillators

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MPPC Arrays in PET Detectors With LSO and BGO Scintillators T. Szczęśniak, Member, IEEE, M. Kapusta, Member, IEEE, M. Moszyński, Fellow, IEEE, M. Grodzicka, Member, IEEE, M. Szawłowski, Member, IEEE, D. Wolski, J. Baszak, and N. Zhang, Member, IEEE

Abstract—Presently, a majority of studies concerning application of silicon photomultipliers (SiPMs) in positron emission tomography detectors are focused on scintillators containing lanthanum LaBr or lutetium (LSO, LYSO, or LFS). However, the modules with the well-known BGO in combination with SiPM light readout are also interesting, owing unique features of SiPMs. In this paper, the two types of detectors, based on BGO and LSO scintillators, are compared in terms of requirements for positron emission tomography scanners. The presented studies are performed with two Hamamatsu Multi Pixel Photon Counter (MPPC) arrays of 2 2 channels, with the total area of mm and micro-pixel size of 25 m (S10985-025C) and 50 m (S10985-050C). The measurements of a number of photoelectrons, energy resolution at 511 keV and Na time resolution are reported for various sizes of the mm pixel and single scintillators, including crystals covering the whole MPPC active area. The paper is more focused on optimization of the system with BGO as its performance in combination with SiPM light readout is less known. The aim of this work is to show advantages of a SiPM-based detector, especially in combination with BGO, in respect to the block detectors where classic photomultipliers are used. Index Terms—Energy resolution, number of photoelectrons, PET, silicon photomultipliers, time resolution.

I. INTRODUCTION

A

silicon photomultiplier (SiPM), trademarked by Hamamatsu as a Multi Pixel Photon Counter (MPPC) became the candidate for a classic photomultiplier (PMT) successor in many applications. Positron emission tomography (PET) is one of the areas where detector modules with SiPM light readout are intensively studied. Presently, a majority of research studies concerning new SiPM-based PET detectors focus on modules with LaBr or with scintillators containing lutetium (LSO, LYSO or LFS-3 [1]), which are currently the most commonly used in commercial PET systems. Although BGO is the second most popular PET scintillator, its performance with SiPM light readout is much less known.

Manuscript received February 07, 2012; revised December 17, 2012; accepted February 25, 2013. Date of publication April 05, 2013; date of current version June 12, 2013. This work was supported in part by the EU Structural Funds Project no POIG.01.01.02-14-012/08-00. T. Szczęśniak, M. Moszyński, M. Grodzicka, M. Szawłowski, and D. Wolski are with the National Centre for Nuclear Research (formerly the Soltan Institute for Nuclear Studies), PL 05-400 Otwock-Świerk, Poland (e-mail: [email protected]). M. Kapusta and N. Zhang are with the Siemens Medical Solutions, Rockford, TN 37853 USA. J. Baszak is with the Hamamatsu Photonics Deutschland GmbH, D-82211 Herrsching am Ammersee, Germany. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2013.2251002

TABLE I MAIN PROPERTIES OF MPPCS USED DURING THE STUDIES

Full body time-of-flight (TOF) PET could be built only by means of LSO detectors due to its high light output of 30 000 ph/MeV and fast decay time of 40 ns. However, in many PET studies where TOF information usefulness is limited by the size of the tested object or by the geometry of the system, like in PET mammography, PET prostate probes, brain studies in combination with MRI, or in small animal PET imaging, the BGO crystals read by SiPM array could be the perfect solution. The biggest advantage of this crystal is its 1.5 times higher, compared with LSO, detection efficiency of coincident 511 keV gamma rays. Moreover, BGO’s low-light output of 9000 ph/MeV and rather slow decay time of 300 ns became advantageous in combination with SiPM, extending its linear range. BGO also does not contain internal radioactivity, which is observed in the case of LSO due to naturally occurring (2.6%) Lu isotope. Finally, the price of BGO is considerably lower than LSO, which is especially important if the volume of scintillators is involved, as in the case of nuclear medicine devices. The aim of this work is to present and compare properties of detector modules based on BGO and LSO scintillators whose light is read by means of MPPC arrays. The studies are focused on measurements of photoelectrons’ number, energy resolution, and time resolution. The obtained results are benchmarked by PET requirements. II. EXPERIMENTAL DETAILS A. Photodetectors and Scintillators The studies were carried out with two Hamamatsu MPPC arrays of 2 2 active elements (channels) format. The total active mm , and the micro-pixel area of each device is equal to

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TABLE II MAIN PROPERTIES OF BGO AND LSO SCINTILLATORS

size is 25 m and 50 m for S10985-025C and S10985-050C devices, respectively. This type of MPPC array is made on one substrate with separate structures of elements. Such technology assures uniformity and minimizes the dead area of the MPPC array having an obvious advantage over an array built as a mosaic of individual SiPMs. The main properties of the tested devices, following the Hamamatsu data sheet [2], are presented in Table I. The breakdown voltages showed in Table I were evaluated on the basis of the single photoelectron peak positions measured at various bias voltages. In order to understand the performance limits of the MPPC based detectors, as well as parameters of the future PET block detector, LSO and BGO crystals of different dimensions have been used in the experiments, including long pixels and single crystals covering the whole MPPC active area. The following scintillator sizes were used: mm , mm , mm , mm , mm , and 2 2 matrix of mm . The scintillators were wrapped in Teflon tape and coupled to the MPPC by means of silicone grease. The 2 2 matrices were “homemade” block detectors consisting of four mm crystals, each wrapped with one layer of Teflon. These crystals were simply stacked together (without any light guide) and wrapped with few layers of Teflon. The main properties of the BGO and LSO scintillators are listed in Table II. B. Experimental Setup Measurements of the energy resolution were made in a standard energy spectroscopy setup using the Canberra 2005 chargesensitive preamplifier and Tennelec TC244 spectroscopy amplifier (NIM module). Energy spectra were recorded using a PC-based multichannel analyzer Tukan8k [4]. The timing resolution data were obtained in Na coincidence measurements with a fast reference detector comprising a truncated cone BaF crystal (20 and 25 mm in diameter and 15 mm high) coupled to an XP20Y0Q/DA PMT. Its time resolution for 511 keV was of 145 4 ps. The measurements were done using a slow–fast arrangement [3] with a constant fraction discriminator (CFD) Ortec 935 operating in a leading edge mode. In the fast section of an experimental setup, related to the CFD signals, the variations in time at which coincident events are sensed by the MPPC-based detector and reference detector were measured using a time-to-amplitude converter (TAC). In the slow part, the coincidence arranged between the PMT’s dynode pulse and the MPPC’s response generated the gate signal. This signal was used for gating the TAC output, enabling selection of the coincident events for the energy range of interest (511 keV in this case). In order to get MPPC signal in both fast and slow electronic chains, a Mini-Circuits ZFSC-2-4+ splitter module was used.

During the timing studies, two types of preamplifiers were used to form the MPPC signal sent to the discriminator input. Measurements with LSO required high bandwidth, hence Mini Circuits ERA-4SM+ preamplifier was applied. In the case of BGO the Analog Devices preamplifier configured for transimpedance mode of operation was used in order to assure high gain. Since the preamplifier bandwidth is not crucial for slow BGO scintillation pulse, the MPPC signal after the preamplifier was further amplified using a fast amplifier Caen N978. The summing of signals from all sections of MPPCs was made by soldering of output pins. Input impedance of the preamplifiers was equal to 50 . The rise time of the MPPC output pulse for an LSO scintillation signal and all channel readout was equal to about 20 ns. The time spectra were measured with an Ortec 566 TAC and recorded by a PC-based multichannel analyzer Tukan8k [4]. Time-scale calibrations of the TAC were done using a precise Time Calibrator Ortec 462, based on a quartz clock. III. RESULTS AND DISCUSSION A. Energy Resolution The energy resolution of scintillation detector improves with an increase of the photoelectrons number created in a readout detector. The number of generated photoelectrons (or fired pixels) in MPPC is proportional to a photon detection efficiency (PDE). PDE is specified as a product of MPPC quantum efficiency (QE), geometrical fill-factor (FF), and the combined probability of electrons and holes to initiate Geiger breakdown (GP): PDE

QE

FF

GP

(1)

FF and QE (for a typical scintillation light) do not depend on the device operating bias voltage (HV) and are constant for a given MPPC. An increase of the bias voltage leads to an increase of the GP [5] and then the photoelectrons number. Hence, the energy resolution of the MPPC-based scintillation detector should improve with bias voltage. However, the energy resolution of a scintillator coupled to the MPPC also depends on MPPC excess noise factor (ENF). This noise factor increases with the applied voltage [6], [7], resulting in degradation of the energy resolution. Due to the aforementioned relationships, the best energy resolution of the MPPC detector corresponds to the bias voltage that is a tradeoff between contributions of PDE and ENF. The dependence of the measured energy resolution of 511 keV gamma ray versus applied bias voltage for MPPC 025C (a device with micro-pixel size of 25 m) is presented in Fig. 1. The presented data were recorded with mm BGO and LSO crystals. The optimal bias voltage is well pronounced for BGO scintillator at HV values between 73.4 and 73.8 V. Raw data collected with LSO crystal does not show

SZCZĘŚNIAK et al.: MPPC ARRAYS IN PET DETECTORS WITH LSO AND BGO SCINTILLATORS

Fig. 1. The dependence of the energy resolution at 511 keV recorded with mm BGO and LSO crystals coupled to mm MPPC 025C. Open squares show LSO data corrected for the nonlinearity of the MPPC response to a high number of incident photons.

Fig. 2. The dependence of the energy resolution at 511 keV recorded with mm BGO crystal coupled to mm MPPC 050C.

minimum and are not deteriorated at higher bias voltages. Such behavior is an effect caused by the nonlinear response of the MPPC to the high number of photons emitted by the fast LSO crystal. The same LSO data corrected for the nonlinearity (open squares) show worsening of energy resolution for bias voltages above 73.8 V, similarly as in the case of BGO. Taking into the account the BGO data and nonlinearity of the LSO points, a bias voltage of 73.4 V was chosen as optimal value for further measurements with the both scintillators and MPPC 025C. A similar plot, but obtained for BGO coupled to the MPPC 050C device with four times larger micro-pixel size is presented in Fig. 2. In the case of larger micro-cells, the MPPC readout of LSO is highly nonlinear so measurements have no purpose. The optimal bias voltage was established from BGO data, and the value of 71 V was chosen. Because the larger micro-pixel size assures a better fill factor, and hence higher PDE, the MPPC 050C was chosen for BGO

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Fig. 3. Number of photoelectrons in 511 keV gamma peak generated in difmm MPPC 025C (2 2 channels). The data was ferent channels of mm LFS-3 crystal coupled collected with a Na gamma source and to only one channel of the tested MPPC.

readout in further energy resolution measurements. A reliable comparison of the energy resolution of two detectors must be made in their linear range of operation; therefore, the MPPC with 25- m micro-pixels was chosen as a photodetector in the case of the LSO scintillator. Only such configuration assures linear response at 511 keV and sufficient linear dynamic range when an optimal bias voltage (see Fig. 1) of 73.4 V is applied. One has to remember that the required linearity of the LSObased detector is achieved at the expense of its PDE. Due to different fill factors (see Table I), application of an MPPC with 25- m subpixel size considerably reduces PDE, compared with the MPPC 050C used for the BGO light readout. Correction of an LSO detector for nonlinear response of the MPPC is possible (see Fig. 1); however, the algorithm requires two gamma lines with known energies (e.g., 511 and 1275 keV), whereas PET systems are optimized for detection of only 511 keV annihilation quanta. Initial experiments made with MPPC 025C and mm LFS-3 scintillator placed on one channel of the MPPC showed significant difference in collected light between readout of only this channel and readout of all four channels. This effect is caused by a 0.45-mm-thick layer of protecting, optical resin covering the photosensitive area of the MPPC. The resin acts as a light diffuser enabling light collection in neighboring channels. Fig. 3 presents a number of photoelectrons in 511 keV gamma peak generated in each channel when the mm LFS-3 crystal was placed on one channel only. The data presented in Fig. 3 triggered the tests of the mm crystal, whose size is slightly smaller than an entire MPPC-photosensive area. Such configuration may improve the light collection efficiency from the scintillator edges compared with mm crystal. Examples of Na energy spectra obtained with the two tested MPPCs (050C and 025C) and different LSO and BGO scintillators are presented in Figs. 4–7.

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Fig. 4. Example of the energy spectra recorded using all channels readout and mm LSO placed on one channel of MPPC 025C.

Fig. 5. Example of the energy spectra recorded using one channel readout and mm LSO placed on one channel of MPPC 025C.

Fig. 6. Example of the energy spectra recorded using all channels readout and mm BGO placed on one channel of MPPC 050C.

The spectra obtained with mm LSO crystal, using readout signals from one and sum of four MPPC channels, are

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Fig. 7. Example of the energy spectra recorded using all channels readout and mm BGO placed on one channel of MPPC 050C.

presented in Figs. 4 and 5. They show the performance advantage due to an increase of collected light by summing the signals from all four channels. The difference in values of 511 keV peak energy resolution reflects the light distribution over the readout channels area (see Fig. 3). The spectra in Figs. 4 and 5 also show the fitting procedure used during the energy resolution studies. The analysis of Na energy spectra recorded with both BGO and LSO included fits of double Gaussian function with common FWHM and distance between peaks defined by the energy of K X-rays from bismuth (77 keV) or lutetium (54 keV). Similar examples of Na spectra but obtained with mm and mm BGO crystals are presented in Figs. 6 and 7. In both BGO plots, the escape peaks are very well pronounced, which confirms very good energy resolution of the tested detector even for long, fingerlike scintillator (see Fig. 7). The escape peaks are less visible in the case of LSO, because the energy of lutetium X-rays is of 23 keV lower as this of bismuth. All the results of the energy resolution at 511 keV as well as the number of photoelectrons for various sizes of BGO and LSO crystals are collected in Table III. The photoelectron numbers presented in the Table III were calculated comparing 511 keV peak position with the single photoelectron position (Bertolaccini et al. method [8]) but were not corrected for crosstalk and afterpulses contribution. The results of the measurements made with a classic photomultiplier XP20D0 are presented in Table IV. The measurements made with cubic scintillators of a mm and mm size allow estimation of performance limits for a given scintillation detector, because an influence of the light transport inside such small crystals is negligible, and results are practically not affected by the light transport inside the scintillator. The tested configurations of BGO coupled to MPPC 050C and LSO coupled to MPPC 025C are characterized by similar energy resolution. For both mm crystals the value slightly below 11% was obtained. The observed small difference between BGO and LSO is inconsistent with the PMT data presented in Table IV and may be caused by not perfectly

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TABLE III ENERGY RESOLUTION

AND NUMBER OF PHOTOELECTRONS RECORDED CRYSTALS COUPLED TO MPPC 050C AND 025C

WITH BGO

AND

LSO

a) Including crosstalks and afterpulses. b) Single Gaussian fit. TABLE IV ENERGY RESOLUTION AND NUMBER OF PHOTOELECTRONS RECORDED WITH BGO LSO CRYSTALS COUPLED TO XP20D0 PHOTOMULTIPLIER

accurate coupling of the LSO crystal to the one channel area of the 025C MPPC. In the case of mm scintillators, the difference between LSO and BGO is larger and similar for PMT and MPPC. The tests made with monolithic scintillators with the length of 20 mm, typical for PET block detectors, show only a slight deterioration of energy resolution due to light transport inside the crystal. The change up to 12.2% and 12.3% was observed for the fingerlike BGO and LSO crystals, respectively. The results in the case of the rest of long BGO scintillators follow the number of photoelectrons leading to values slightly below 12%. The energy resolution of the long LSO crystals remained at the level of 10%, recorded earlier for the mm sample, whereas the photoelectron numbers changed significantly. It suggest that the quality of the crystals as well as crystal wrapping and coupling to MPPC are the main sources of the observed consistency of energy resolution values between long and cubic LSO scintillators. Nevertheless, all the results obtained for 511 keV with LSO show that the value of about 10% is possible to get using MPPC readout with 25 m micro-pixel size. Energy resolution of 12.2% in the case of fingerlike BGO as well as 14% obtained with 2 2 matrix coupled directly to the MPPC are very promising regarding application of BGO in PET systems. These values are comparable to those obtained with LSO. However, one has to remember that the energy resolution of 12.3% reported for the LSO crystal was

AND

obtained with single Gaussian fit and, hence, is slightly overestimated. It is worth underlining the MPPC array readout configuration, using summing of the signals from the array separate channels, demands the perfect uniformity of MPPC characteristics over entire photosensitive area. This is extremely important condition to obtain good energy resolution. B. Time Resolution The time resolution measurements were started with the optimization of the experimental setup, in particular the discriminator settings. First, the constant fraction discriminator was tested with different settings of shaping delay; however, the shortest delay (CFD in a leading edge mode) led to the best timing resolution. Since the leading-edge mode of operation was chosen, the influence of the threshold settings on the discriminator performance was also tested. The results for both LSO and BGO crystals are presented in Fig. 8. The measurements show that the lowest applicable threshold in the leading-edge mode of operation leads to the best time resolution, which is consistent with the Hyman theory of timing [9] and our previous studies made with LSO [3]. In the case of LSO, the threshold level was limited by the amplitude of the MPPC output pulse and was set at the level of 2.5% of its value. In the case of BGO, whose signal is not affected by the fast amplifier bandwidth, the high

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Fig. 8. Time resolution dependence on the threshold set on a discriminator working in a leading edge regime.

Fig. 9. Examples of timing spectra recorded with MPPC 050C and two BGO mm and mm . crystals:

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Fig. 10. Examples of timing spectra recorded with MPPC 050C and two LSO mm and mm . crystals:

Examples of timing spectra obtained with mm and mm BGO and LSO crystals are presented in Figs. 9 and 10. As in the case of energy resolution measurements, use of the small cubic BGO and LSO crystals allowed determination of the detector limits regarding its time resolution. Time resolution recorded with mm scintillators is not affected by the light transport inside the crystal and additional time jitter due to different points of gamma-ray absorption. All the results of the time resolution measurements made with different sizes of BGO and LSO crystals coupled to MPPC 050C are collected in Tables V and VI, respectively. The measured values of coincident time resolution are gathered in the third column. Next, these values are corrected for the contribution of the reference detector and presented in the fourth column. In the fifth column, the number of excited or fired pixels is shown, calculated using the well-known formula [10], [11] and corrected for the contribution of a scintillator decay time and effective dead time of MPPC cells ( 50 ns in the case of 050C device) [12]: PDE

gain could be achieved, and a threshold level below 1% was possible to apply. Optimization of the time resolution with MPPC (and any other photodetector) requires a large number of photoelectrons and, hence, high PDE of a readout device. This can be obtained using devices with a high fill factor and operating at a rather large overvoltage. For these reasons, in timing measurements, the MPPC 050C array was chosen as a readout device for both BGO and LSO scintillators (in energy resolution studies the 025C device was used with LSO). Since the LSO detector with MPPC 050C readout has a very limited linear range, the level of overvoltage was determined on the basis of Na energy spectra recorded at various voltages. The voltage was increased up to the point at which 511 keV gamma peak starts to saturate the signal. A bias voltage of 72 V, just before the saturation, was chosen. At this HV, the 511 keV gamma peak, detected in LSO, was still possible to distinguish from higher energy gammas. This allowed the proper setting of the energy gate.

(2) is the total number of pixels in a given MPPC where device, is the number of incident photons, and PDE is the number of detected photons . In the last columns, the single detector time resolution (fourth column) is normalized to the number of fired pixels (fifth column). The value of the time resolution below 1 ns obtained for mm BGO crystal is very promising. Such a good result was possible due to a very high number of 4400 photoelectrons (fired pixels) registered in MPPC, much larger compared with classic photomultipliers (see Table IV). Moreover, even with 20-mm-long scintillators, similar to the one used in commercial PET scanners, the time resolution of the tested BGO detectors remains below 1.5 ns. A comparison of the data obtained with the monolithic crystals covering the whole MPPC area ( mm and

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TABLE V TIME RESOLUTION AND NUMBER OF PHOTOELECTRONS RECORDED WITH BGO CRYSTALS COUPLED TO MPPC 050C

a) Corrected for the contribution of the b) Including crosstalks and afterpulses.

reference detector of 145

4 ps.

TABLE VI TIME RESOLUTION AND NUMBER OF PHOTOELECTRONS RECORDED WITH LSO CRYSTALS COUPLED TO MPPC 050C

a) Corrected for the contribution of the BaF reference detector of 145 b) Including crosstalks and afterpulses.

mm ) with the single pixel mm or the pixel matrix emphasizes the superiority of pixelated scintillation block detector application over a continuous crystal. The time resolution obtained with small pixels is only slightly worse compared with the monolithic single crystals; however, pixelated detector configuration greatly simplifies the spatial reconstruction algorithms. In the case of BGO, additional experiments were done for a bias voltage of 71 V, optimal in respect to energy resolution. It is worth noting that the lowering of the bias voltage to the value optimized for energy resolution of BGO-based detector still assures a time resolution well below 2 ns. It suggest that the scintillation detector consisting of BGO coupled to MPPC can simultaneously provide good energy resolution as well as good time resolution. Moreover, the use of the MPPC build of 100 m subpixels would bring further improvement of the BGO-based detector due to a better fill factor, and hence higher PDE. Together with the improved performance, such a BGO device should still keep linear response for gamma radiation with energies up to hundreds keV. The time resolution values obtained with LSO are also very promising and even better than the results obtained with the fastest classic photomultipliers [13]. Again, the biggest advantage of an MPPC is its high PDE and, thus, very high number of photoelectrons. It is worth noting that, in conditions of our measurements, the BGO readout was operated in linear range, i.e., the number of fired pixels was close to the number

4 ps.

. In the case of fast and bright of detected photons LSO, is much smaller than . IV. CONCLUSION The reported results are one of the first showing good energy and timing performance achieved using MPPC arrays to read out a BGO scintillator. They prove a very high potential of such configurations of scintillation detector. Currently, LSO crystals are practically the only solution regarding TOF-PET; however, in PET applications with less stringent timing demands, a BGO scintillator is a good choice. Measurements of the energy resolution showed that BGO scintillator is better suited for MPPC readout than LSO. This is because of its long, 300 ns decay time constant and moderate light output which allow application of an MPPC with high fill factor (62%) built of 50 or even 100 m square micro-pixels. In consequence, their high PDE, and hence high number of photoelectrons, led to good results, e.g., at 511 keV, the energy resolution of 10.6% and 12.2% were measured for small cubic crystal and fingerlike pixel, respectively. Moreover, MPPC 050C (4 3600 pixels) is fully linear up to 1 MeV gamma energy detected in the BGO. An MPPC with a high number of micro-pixels increases the linear range of LSO readout response, however, at the price of reduced fill factor and, therefore, the PDE. Nevertheless, the LSO detector, despite application of MPPC 025C readout with a

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low fill factor of 31%, showed slightly better energy resolution than the BGO where MPPC 050C was used. The results of the time resolution obtained with BGO crystals coupled to the MPPC gave ground for expectation of their good performance in PET. The resolution of about 1.5 ns, measured for 20-mm-long crystals as used in PET block detectors, shows the possibility of their operation applying a narrow coincidence window in the PET system. A time resolution below 1 ns, obtained with a small cubic crystal, is one of the best reported for this scintillator. The BGO, regardless of scintillation light reading device, is not applicable in TOF PET systems. The time resolution obtained for the LSO with an MPPC light readout is very promising, since it is better than the best values reported with PMTs. The additional advantage of application of MPPC instead of PMTs in a TOF PET block detector is a fact that the time resolution of the MPPC-based detector is practically not affected by the position of the crystal on a photosensitive area. However, it requires an excellent spatial gain uniformity of the used MPPC. The measurements also showed how important in optimization of the MPPC is selection of the optimal bias voltage, which is crucial for the energy resolution performance of the detector. Experiments with LSO crystals proved that the MPPC with a fill factor above 60% (25 m micro-pixel size) is needed in order to obtain reliable energy resolution and good linearity of the MPPC response with fast and bright scintillators. On the other hand, a comparison of the results obtained with the BGO at 71 and 72 V showed that, for this type of a scintillator, the MPPC can be optimized in the way that assures good energy resolution and time resolution at the same settings. In the case of the LSO, the optimal time resolution is obtained at high overvoltage and very high nonlinearity. Taking into account the reported results, the BGO crystal with the MPPC readout meets the needs for PET detectors in systems where the TOF is not required. The use of 100 m MPPC should further improve BGO detector performance constituting highly efficient device, characterized by both good energy and time

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resolution. Moreover, such a BGO block detector can be much cheaper compared with LSO.

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