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IEEE TRANSACTIONS ON NUCLEAR SCIENCE 1 Performance Study of a Wide-Area SiPM Array, ASICs Controlled A. J. González, S. Majewski, J. Barberá, P. Conde, C. Correcher, L. Hernández, C. Morera, L. F. Vidal, F. Sánchez, A. Stolin, and J. M. Benlloch

Abstract—In this paper, the capabilities of a wide-area gamma (PET) has boosted the research efforts of several groups to ray photosensor based on a SiPM array are investigated. For this demonstrate the capabilityofphotosensorstoworkinstrong purpose, we have mounted an array of 144 SiPMs with magnetic fields without impact on performance. individual active areaof mm and a pitch of mm, thus covering an active area of mm .Themeasurements There are mainly two types of photosensors capable of this were performed by coupling the SiPM array to LYSO crystal ar- compatibility, the so-called avalanche photodiodes (APDs) and rays of different pixel size ( mm , mm ,and silicon photomultipliers (SiPMs). Both are, in general, large ar- mm )and10–12mmthicknesses.The SiPM array was con- rays of micro-APDs, but the first type works in the avalanche trolled by means of three ASICs, and the SiPM signals were mul- regime, whereas the SiPMs operate in the Geiger mode, over the tiplexed in ordertodeterminethegammarayimpactpositionby means of implementing the Anger logic algorithm in the ASIC. The self-quenched breakdown voltage. Each micro-APD is referred optimum bias voltage and temperature dependence of the gamma to as a cell and produces a signal when it detects one photon. The ray sensor were determined. An energy resolution as good as 8%, SiPM output is provided as the sum of all output cells. SiPMs for individual crystal pixels, were reached at 5 V overvoltage. The have better timing response compared to APDs [1] as well as ASICs design allows one to “activate” different photosensor array high gain .However,bothdevicespresentthelimiting areas. Thisfeaturehasbeenusedtoevaluatethedetectorperfor- mance as a function of the crystal pixel size and the photosensor factor of dark noise. dark noise contribution. In this work we also show the system capa- Due to the incompatibility of PMTs to work in magnetic fields bility toprovidedepth-of-interaction(DOI)informationbymeans such as thosepresentedinMRIsystems,SiPMs(alsoAPDs) of implementing a two-layer staggered approach. We have found have been suggested to replace PMTs in the design of PET sys- that accurate DOI information is obtained when the ASICs en- tems [2]–[4]. There are several efforts focusing on the appli- abled an SiPM active area as high as mm ( SiPMs). cation ofindividualandsmallarraysofSiPMs( SiPMs Index Terms—Application-specificintegratedcircuits(ASICs), of mm area each) for this purpose with significant suc- Gamma-ray detectors, photodetectors, positron emission tomog- cess [5], [6]. A large-area, silicon-based detector of approxi- raphy (PET) instrumentation, scintillators. mately cm was recently presented [4]. These tests have demonstrated high spatial and energy performances but also I. INTRODUCTION temporal performance [7]. In particular, SiPMs are proposed for time-of-flight applications due to its fast response [8]. However, HOTOMULTIPLIER tube (PMT) technology has been there are not yet that many developments using large-area ar- P widely explored for a variety of applications, and nuclear rays of SiPMs achieving good performanc. In this paper, we will medicine has made use of them forabout50years.Recently,the show the good overall performance of an array of 144 SiPMs routine use of magnetic resonance imaging (MRI) in medical with final array dimensions of roughly cm . practice and especially when combined with other functional There are primarily two SiPM array readout approaches, imaging techniques such as positron emission tomography namely networks based on analogue devices [9], [10], but very recently application-specificintegratedcircuits(ASICs)have also appeared to be good candidates [11]–[14]. There are sev- Manuscript received February 21, 2014; revised July 10, 2014; accepted eral projects in which different resistor networks (also diodes) September 16, 2014. Project funded by the Spanish Ministry of Economy and have been studied in detail for both PMTs and SiPMs [9]–[16]. Competitiveness and co-funded with FEDER’s funds within the INNPACTO The most important advantages of using this approach are good 2011 program. This work was supported by the Spanish Plan Nacional de Investigación Científica, Desarrollo e Innovación Tecnológica (I+D+I) under linearity, high dynamic range, and also good timing resolution. Grant FIS2010-21216-CO2-01 and the Valencian Local Government under However, these methods are limited to provide information Grants PROMETEOII/2013/010 and ISIC 2011/013. further than planar impact position and time. Moreover, they A. J. González, P. Conde, L. Hernández, L. F. Vidal, F. Sánchez, and J. M. Benlloch are with Institute for Instrumentation in Molecular Imaging (I3M), may result in a high system cost when designing a large field CSIC -Universidad Politécnica de Valencia—CIEMAT, 46022 Valencia, Spain of view scanner. Depth-of-interaction (DOI) information is (e-mail: [email protected]). possible to be also determined using analogue readouts with S. Majewski and A. Stolin are with the Department of Radiology, West Vir- ginia University, Morgantown, WV 26506 USA. methods such as the phoswhich [17] or additional photosensors, J. Barberá, C. Correcher, and C. Morera are with Oncovision S. A., 46013 but also by methods in which the resistor network is upgraded Valencia, Spain. to provide information related to the DOI [18]. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. The alternative of using ASICs has recently showed good Digital Object Identifier 10.1109/TNS.2014.2359742 results for photosensor readout, especially SiPMs, due to their

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2 IEEE TRANSACTIONS ON NUCLEAR SCIENCE

entrance and exit faces were also polished, where the entrance additionally included an ESR reflector layer.

A. Application-SpecificIntegratedCircuit The SiPM array was controlled by means of three ASICs [12], as shown in Fig. 1, right. These ASICs are a CMOS integrated front-end architecture. The chip is PSPMT and SiPM compat- ible, presents low gain dispersion among inputs, low noise, and high speed response [13]. The underlying architecture calcu- lates the moments of the detected light distribution in an analog Fig. 1. (Left) Stack of three boards with the SiPM array in the front. (Right) mode. Due to the additive nature of the moment calculation, Photograph of the middle board showing the three ASICs. the operation can be carried out on a single device or split it into several devices, adding the partial results afterward. All the operations are carried out in current mode, and the weighting versatility [19]–[21]. They can be designed to provide high tem- operations are implemented using linear MOS current dividers. poral performance but also accurate determination of the photon Weighting coefficients are programmable via an I2C bus and impact. They have been used for both crystal arrays and mono- stored in 8-bit registers. Finally all the weighted currents are lithic scintillators. added together and introduced in an output current buffer [13]. In this work, we will describe a scalable ASIC that has been Selecting the proper set of weights allows one to estimate designed to control SiPMs [12]. We have put together arrays of many characteristic parameters ofthelightdistribution,e.g., 144 SiPMs that are ASICs controlled. In particular, three ASICs the centroids of the light distribution, their standard deviations, are required to control each 144-SiPM array. We will describe skewness, etc. The 144 ASIC inputs are replicated eight times. the design of the SiPM array and their potential use as a photode- All the input signals are added forming eight linear combina- tector for gamma rays. We will show the first results obtained tions of the 144 input signals. In these experiments, the ASIC with this assemble using crystal arrays. output 1 provided the data acquisition Ssystem (DAS) with the trigger signal. This output is obtained by loading identical co- II. MATERIALS efficients to all SiPMs. ASIC output channels from 2 to 5 were Our research team has accumulated experience in the de- programmed to deliver planar photon impact information. These sign and construction of dedicated PET systems, starting with signals were loaded with coefficient matrices representing the the small animal PET called Albira [22], and then the dedi- Anger logic (A, B, C, D), that is, diagonal gradient coefficients cated breast PET named MAMMI [23]. The photosensors used [26]. The sixth output provided DOI information, since it was in these systems were based on the well-established PSPMT calculated as the variance of the light distribution [27]. The re- technology. As with many other research groups, it was our re- maining two channels were not programmed and used in these cent goal to replace PMT sensors by SiPM technology in order experiments. to improve the photodetector performance. Arrays of SiPMs This particular ASIC allows one to connect SiPM devices are envisaged to deliver high intrinsic spatial resolutions and with a terminal capacitance below 40 pF. The new SensL SiPMs good timing response, but also immunity to magnetic fields. We provide, in addition to the standard output pF ,anad- carried out a preliminary work with arrays of 256 ditional output (named FAST) with a terminal capacitance of MPPCs of the type S10362-11-050 from Hamamatsu, with an only 30 pF. It has been shown the convenience of using this active area of mm and 50 mcellsize[11].Theywere output for high time resolution measurements. Unfortunately, it mounted on a cm PCB with a pitch of 3 mm. Due to the was not possible to directly use this signal with the ASIC, be- dead area and the scintillation light coupling to the photosensor cause it has a positive signal polarity, not matching the negative using special light guides [24], [25], we encountered limited de- input requirement of this particular ASIC. The solution we came tector performance. In order to improve operation, one alterna- with was to connect the FAST output to ground. In order to limit tive has been to use larger active-area SiPMs with a reduced gap the current flowing through the outputs in this new implemen- between them. In particular, we have designed and mounted an tation, an additional serial resistor was mounted with resistance array of 144 SiPMs from SensL. In this case we used values ranging from 0.1 to k ( k was the current se- SiPMs of the MicroFB-30035 SMT series that have an active lected value). In this way the final terminal capacitance of the area of mm with a total outside dimension of mm . anode-cathode signal is 30 pF, matching well with the ASIC re- These sensors are packaged in a mm clear molded reflow quirements. Fig. 2 depicts the circuitry schematic used for this lead frame. This package is soldered in standard reflow ovens. array and ASIC compatibility. The SiPMs were mounted with a pitch of 4.2 mm. The flatness of all 144 SiPMs was measured to be below m. The SiPM B. Data Acquisition System array active area covers mm .SeeFig.1. All output signals (trigger and impact positioning) are The measurements were performed with crystal arrays of dif- transferred to the DAS by means of coaxial cables with ferent pixel size ( mm , mm and mm )and impedance. The DAS used in the present experiments 10–12 mm thickness. In all cases, the individual lateral crystal was mainly composed of a trigger board and multichannel pixels were polished, and ESR (3M) reflectors were used. The analog-to-digital converters (ADCs) boards, both in CAMAC This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

GONZÁLEZ et al.:PERFORMANCESTUDYOFAWIDE-AREASIPMARRAY,ASICSCONTROLLED 3

Fig. 2. (Left) Circuit schematic with an output capacitance of pF in par- Fig. 3. (Left) Contour plot of mm crystal array pixels at 30 Vop, for allel with k and 700 pF. (Right) ASIC output trigger signals, 100 mV and an energy window of at the 511 keV photopeak. (Top right) Energy res- 50 ns plot divisions. olution of nine pixels as a function of the Vop. The red line is the first-order exponential decay fit to the data. (Bottom right) Individual pixels energy reso- lutions and average are plotted. format. The trigger signal (calculated from the sum of all SiPMs signals) uses a double leading-edge approach, and the main threshold was set to mV, suppressing most of the about 17 C). A gamma ray source was used based on a matrix electronic noise. The trigger signal threshold reduces low-en- of sources with a total activity of about Ci. ergy impacts that have been multiplexed along the SiPM array. The source array was located close to the entrance face of the As an energy window around the 511 keV peak is set when crystal array. working with PET applications, the effect of the threshold level As observed in Fig. 3 all crystal pixels could be distinguished. on the spatial resolution is low. The integration time for each The operational voltage (Vop) of the array was sequentially ADC channel was set to 192 ns. All measurements presented in varied from 25 V to 31 V and the data stored in list mode files. this report were performed in singles mode (no coincidence). Several regions of interest (ROI) around one crystal pixel and a Thus, all trigger signals above the selected threshold enabled group of nine pixels were analyzed in order to study the energy asynchronizedgatesignalthatstarted the digitization of the resolution. On the right side of Fig. 3 we have depicted the de- output ASIC channels on the ADC. These data were sent termined energy resolution (%) as a function of the Vop in volts through Ethernet to a personal computer workstation and stored for the group of pixels (top) and for individual pixels (bottom). in a list mode. Specially designed applications were used for The energy resolution for each histogram was determined by data presentation and analysis. fitting a Gaussian curve. The percentage is calculated as the ratio of full width at half maximum (FWHM) to the centroid value. In these figures, we observe an exponential decay of the III. MEASUREMENTS AND RESULTS energy resolution, improving with the increase of the Vop. The We have evaluated the performance of a wide-area SiPM ratio of the 1274/511 keV peaks in as a function of the array in terms of spatial resolution, energy resolution, and DOI Vop resulted on a standard deviation of only 2%, suggesting a capabilities, as a function of temperature, Vop, and crystal size. negligible nonlinearity effect of the detector assembly. Energy Since temporal resolution is also an important parameter for resolution values for individual pixels as good as 9% were PET applications, a deep analysis concerning timing perfor- measured, at Vop of 29–30 V. mance with these sensor arrays has been recently carried out Additionally to the energy resolution, the spatial resolution by the authors in a separate work [28]. was also evaluated as a function of the Vop. Here, profiles through one row of crystal pixels were analyzed. We observed A. SiPM Array Bias Adjustment an improvement of pixel identification for Vop higher than The used SiPMs have their breakdown voltage at about 28.5 V, with performance remaining almost constant up to 24.5 V. The whole SiPM arraywasbiasedtoacommon 30.5 V. Fig. 4 depicts profiles through one row of crystal voltage. By analyzing the energy and intrinsic detector spatial pixels as a function of different Vop. Based on these results, resolution, we determined the optimum operational overvoltage we selected an optimal Vop value of Vforthenext for these arrays. In these experiments, the SiPM arrays were experiments. coupled to a crystal array of elements with mm size each and through an acrylic spreader window of 1.7 mm B. Temperature Dependence thickness. Optical grease with an index of refraction of 1.46 SiPMs are known to be temperature sensitive, showing better (Visilox V-788) was used to couple all these elements. In the performance at lower temperatures. This effect is characteristic experiments carried out to define the best Vop and tempera- for solid-state-based detectors due to thermal charge generation. ture parameter, the ASICs were programmed to only return As for the tests on the optimum bias, we also analyzed here the signal information from one quadrant of the SiPMs array. This intrinsic detector spatial resolution and the energy resolution, configuration offers sufficient information about optimum bias but as a function of the detector block temperature. In this exper- and temperature without the need of programming the entire iment, the detector block temperature was controlled by means SiPM array. The detector block assembly was located inside of a closed box made out of Porexpan. This box was also cov- athermalcontrolledenvironmentatadetectortemperature of ered with a dark blanket. The temperature inside the box was This article has been accepted for inclusion in a future issue of this journal. 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4 IEEE TRANSACTIONS ON NUCLEAR SCIENCE

Fig. 4. Profiles through one row of crystal pixels as a function of Vop varying from about 25 V to 31 V, for a energy window around 511 keV. Fig. 6. Profiles through one row of crystal pixels as a function of detector tem- perature varying from 20 to 30 C. An energy window of around the 511 keV photopeak was selected. (Left) Twelve-pixel profile. (Right) Detail for three edge pixels.

temperatures below C(Fig.6).Asexpected,anoverall detector improvement is observed at low temperatures.

C. Spatial Resolution Dependence on SiPM Dark Noise In this section, we present a unique study in which we have investigated the intrinsic resolution of the detector block, as a function of the SiPM array size. The measurements were car- ried out at 30 Vop. The novelty of this work is the capability of varying the area of SiPMs contributing to the trigger signal and center-of-gravity (CoG) determination for the planar photon impact position (ASICs outputs second to fifth). This was pos- sible by programming the three ASICs with different trigger and CoG matrices according to the SiPM area under study. Areas of , , ,and SiPMs were selected. All Fig. 5. (Top) Energy resolution (FWHM in %) as a function of temperature for one (opened squares) and nine (full circles) crystal pixels, respectively. 144 SiPM signals are transferred to the ASICs without the capa- (Bottom) Photopeak centroid (ADC channel) as a function of the temperature. bility for individual amplitude thresholds. Individual thresholds for each SiPM prior to the ASIC [4], [10]or clustering SiPMs around the highest peak channels [29] to reduce SiPM noise regulated by means of a chiller that cyclically pumped glycolic contributions have been studied by other groups but not con- water to a cold plate located inside the box. The detectors were sidered in this ASIC development. After ASIC calculations, the positioned on top of such a cold plate. eight calculated signals are fed to the DAS, where only an am- The experiments were carried out with a crystal array com- plitude threshold can be used on the trigger signals. In order posed of pixels with mm transversal size. Two ROI to isolate selected SiPM array areas both the trigger and the around one and nine pixels were considered for all temperature four impact determination signals outside these areas were pro- measurements that ranged from 20 to 40 C. Fig. 5, bottom, grammed to zero, and the new matrices recalculated for the shows the 511 keV photopeak ADC channel as a function of planar impact characterization. the temperature. As expected, there is a gain increase of the The experiments were carried out for crystal arrays with pixel photon detection efficiency (PDE), proportional to the decrease sizes of , ,and mm ,respectively.Inorder of the system temperature. We have determined a gain increase to attain the best SiPM array response uniformity, spreader win- of C. In Fig. 5, top, we have plotted the energy dows of 1.7, 1.7, and 2.1 mm were used, respectively. Fig. 7 resolution for one and nine crystal pixels, as a function of the (left) shows, using contour plots, the results for crystal pixel detector temperature. An energy resolution improvement is ob- sizes of mm for different SiPM arrays active areas. The served at lower temperatures, reaching values of 7.5% for single data were taken in single mode, and the representations shown crystal pixels. in the following figures are depicted for an energy window of Aprofile of one row of crystal pixels provides a visual about 400–600 keV (at C). When SiPMs are en- demonstration of the detector intrinsic spatial resolution im- abled, some border effects and a poor resolution at the array provement with the temperature decrease. This is observed edges are observed. These effects are significantly reduced for as a more prominent signal-to-noise ratio (SNR) measure at an active area defined by SiPMs and almost completely This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

GONZÁLEZ et al.:PERFORMANCESTUDYOFAWIDE-AREASIPMARRAY,ASICSCONTROLLED 5

Fig. 7. Contour plots for crystal arrays of mm (four plots on the left) and of mm (four plots on the right) sizes, as a function of the SiPM active area. Fig. 9. SNR as a function of the number of SiPMs for the different crystal pixel sizes.

pixel cases are included in Fig. 9. The SNR values were calcu- lated as the peak-to-valley ratio of one profile. The error bars are the standard deviations of peaks and valleys counts, respec- tively. We observed an improvement of the signal when re- ducing the number of SiPMs. Another source of noise such as the natural radioactivity of the Lutetium in the scintillator is hardly contributing to the SNR data, since an energy window around the 511 keV photopeak was applied, in addi- tion to its low rate compared to the sources. These tests suggested that smaller crystal pixels of mm would hardly be resolved for the entire SiPM array. In order to show the system capability to resolve these small pixels, we directly analyzed the case for SiPM arrays, which is comparable with studies carried out by other groups. A Fig. 8. Profiles through one row of crystal pixels mm for different smaller area of SiPMs was also tried in order to observe if SiPM areas. The black dashed and red solid lines represent the average noise and signal values, respectively. an improvement could further be observed. However, as Fig. 9 depicts, there is no improvement for areas smaller than SiPMs. In Fig. 10 we have plotted the results for mm vanished for arrays smaller than SiPMs. Please note that pixels and two array areas. In the bottom, a profile of one row the area for SiPMs has dimension of roughly mm , of crystal pixels is plotted. and the one for still covers cm .Thelattersizeis The behavior of the SNR values for and SiPMs still large and comparable to sizes used in many studies carried in the case of a mm pixel area could be explained if we out elsewhere [29], [30], with active dimensions of mm consider that the relatively small amount of light (i.e., signal) and mm ,respectively. collected in that case cannot compensate for the noise reduction The experiments carried out with pixels of mm produced when the and SiPMs configurations are size were performed at a lower temperature C in order to used. In this sense, the mm pixel area configuration seems improve the system’s performance (Fig. 7, right). The results for to limit the SNR improvement when reducing the photosensor an area defined by SiPMs again showed lower spatial area. resolution compared to those obtained for smaller SiPM areas. D. Depth of Interaction With Crystal Arrays In both and mm pixel cases, when SiPMs and smaller areas were selected, the achieved spatial resolution In the search of replacing PSPMT by arrays of SiPMs, photon allowed one to resolve most of the pixels within the active field depth of interaction has to also be preferentially resolved. One of view. of the most popular approaches when using crystal arrays to re- In Fig. 8, the profiles through one row of crystal pixels for turn DOI information is the so-called phoswich concept [17]. the four measured cases when using mm pixel sizes Here, crystal arrays with different decay times are assembled to- are shown. These results confirm that the level of accumulated gether in a stack, one on top of the other. The impacted crystal noise is a constant background per unit of area that reduces the layer is determined by differentiating the distinct scintillation peak-to-valley determination when increasing the SiPM active decay time of that layer. When high intrinsic spatial resolutions area. are reached, an alternative to the phoswich is the staggered ap- The SNR data obtained through the profiles as those exempli- proach [31]. In this method, the pixels on one crystal layer are fied in Fig. 8, for the mm , mm ,and mm shifted in the X and Y directions, for example, by the size of half This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

6 IEEE TRANSACTIONS ON NUCLEAR SCIENCE

Fig. 11. Staggered approach reading SiPMs for the 511 keV and 1274 keV energy windows.

Fig. 10. (Top) Contour plots for crystal arrays of mm together with (bottom) the profiles through of one row. (Left) 6 6SiPMsarea.(Right)3 3 SiPMs. The measures were taken at an assembly temperature of 14 C. pixel. Therefore, the light of one top pixel is shared among four pixels on the bottom layer. The layer identification is typically done by CoG, although other methods like direct pixel identifi- cation could be enabled. There is also a possibility to combine the phoswich approach with the staggered approach to maxi- mize the number of distinguishable scintillation layers in the package. As a proof of concept, we mounted two blocks of mm Fig.12. (Top)Contourplotsforthestaggeredconfiguration at 511 keV window as a function of the number of activated SiPMs. (Bottom) The profiles for two LYSO pixel size following the staggered configuration. The consecutive rows of pixels. (Solid line) From top layer. (Dashed line) From block covered an area of approximately mm ,andthe bottom layer. entire assembly was kept at 13 C. The thicknesses of the two slabs were 10 mm each. We again run several tests varying the number of activated SiPMs. When the whole array was IV. DISCUSSION AND CONCLUSIONS enabled, it was hard to distinguish between the two crystal slabs. In this paper, we have shown the design of a gamma ray pho- Note that when using the described ASICs readout, even if only tosensor based on an array of SiPMs, ASICs controlled, in order aportionoftheSiPMarrayiscovered by crystals, all SiPM to replace PSPMTs in some dedicated PET systems. The de- signals (including noise) contribute to the CoG determination. tector module covers an active area of about cm ,well In Fig. 11 the results are shown for energy windows matching the area of H8500/H9500 PSPMT. A special circuit centered at 511 and at 1274 keV, respectively. Theresolution configuration of the SiPM array had to be implemented in order and pixel layer determination improves with the reduction of to make it compatible with our previously developed ASIC. This the SiPM array area, as expected.Whenasmalleractivearea new circuitry has not shown performance degradation at the rate of 8 8SiPMswasprogrammed,theyweredifferentiated. levels that these experiments were carried out at. The top of Fig. 12 shows the sequence of results for the stag- The bias voltage and temperature experiments showed the ex- gered approach with mm pixels, as a function of de- pected dependence of the SiPMs assembly with these parame- tection areas. In the bottom of Fig. 12, the profiles of two con- ters. Nevertheless, we found the present results to be of prac- secutive rows (i.e., crystal layers) for the SiPM case are tical importance, since there areonlyfewreportedstudieswith shown. Well-differentiated top and bottom scintillation layers SiPM arrays of similar dimensions cm studying these for this configuration are visible. The higher number of events effects [32]. An energy resolutionasgoodas8%,forindividual in the top curve correlates to the location of the gamma ray crystal pixels, was reached at 5 V overvoltage. The temperature source, being above this crystal slab, while the second layer was stabilization method that we have used has nicely worked in the shielded by the first array. laboratory environment, but other methods are being studied in This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

GONZÁLEZ et al.:PERFORMANCESTUDYOFAWIDE-AREASIPMARRAY,ASICSCONTROLLED 7

Summarizing, a detector block combining a SiPM array that is controlled with three ASICs, has been shown to return a good tradeoff performance. A unique ASIC versatility individually controlling the loaded matrix coefficients has allowed us to pro- vide information on the detector behavior as a function of the chosen SiPMs array area.

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Rodriguez-Al- performance in the transaxial field of view. varez, A. Ros, F. Sánchez, A. Soriano, L. F. Vidal, and J. M. Ben- In this paper, we have also shown the system’s capability to lloch, “Simulation study of resistor networks applied to an array of provide DOI information by means of implementing a staggered 256 SiPMs,” IEEE Trans. Nucl. Sci., vol. 60, no. 2, pp. 592–598, Apr., 2013. approach. We investigated this feature with crystal pixels of [16] S. Cherry, Y. Shao, S. B. Siegel, and R. W. Silverman, “High resolution mm .Here,weobservedthatthedetectorblockcould detector array for gamma-ray imaging,” US Patent 5 719 400, 1998. not provide accurate DOI information when the entire [17] C. M. Pepin, M. Bergeron, C. Thibaudeau, C. Bureau-Oxton, S. Shimizu, R. Fontaine, and R. Lecomte, “Digital identification of fast SiPM array was enabled, but it might be possible with an active scintillators in phoswich APD-based detectors,” IEEE Trans. Nucl. area of about mm ( SiPMs). Sci., vol. 57, no. 2, pp. 1435–1440, Apr. 2010. This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination.

8 IEEE TRANSACTIONS ON NUCLEAR SCIENCE

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