- Email: [email protected]
A phoswich detector design for improved spatial sampling in PET
Nuclear Inst. and Methods in Physics Research, A 882 (2018) 124–128
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A phoswich detector design for improved spatial sampling in PET Jonathan D. Thiessen a,b,c , Merry A. Koschan d , Charles L. Melcher d , Fang Meng d , Graham Schellenberg e , Andrew L. Goertzen c,e, * a b c d e
Imaging Program, Lawson Health Research Institute, London, Ontario, Canada Department of Medical Biophysics, University of Western Ontario, London, Ontario, Canada Department of Radiology, University of Manitoba, Winnipeg, Manitoba, Canada Scintillation Materials Research Center, University of Tennessee, Knoxville, TN, USA Department of Physics & Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada
a r t i c l e
i n f o
Keywords: Positron emission tomography Phoswich detector LSO
a b s t r a c t Block detector designs, utilizing a pixelated scintillator array coupled to a photosensor array in a light-sharing design, are commonly used for positron emission tomography (PET) imaging applications. In practice, the spatial sampling of these designs is limited by the crystal pitch, which must be large enough for individual crystals to be resolved in the detector flood image. Replacing the conventional 2D scintillator array with an array of phoswich elements, each consisting of an optically coupled side-by-side scintillator pair, may improve spatial sampling in one direction of the array without requiring resolving smaller crystal elements. To test the feasibility of this design, a 4 × 4 phoswich array was constructed, with each phoswich element consisting of two optically coupled, 3.17 × 1.58 × 10 mm3 LSO crystals co-doped with cerium and calcium. The amount of calcium doping was varied to create a ‘fast’ LSO crystal with decay time of 32.9 ns and a ‘slow’ LSO crystal with decay time of 41.2 ns. Using a Hamamatsu R8900U-00-C12 position-sensitive photomultiplier tube (PS-PMT) and a CAEN V1720 250 MS/s waveform digitizer, we were able to show effective discrimination of the fast and slow LSO crystals in the phoswich array. Although a side-by-side phoswich array is feasible, reflections at the crystal boundary due to a mismatch between the refractive index of the optical adhesive (𝑛 = 1.5) and LSO (𝑛 = 1.82) caused it to behave optically as an 8 × 4 array rather than a 4 × 4 array. Direct coupling of each phoswich element to individual photodetector elements may be necessary with the current phoswich array design. Alternatively, in order to implement this phoswich design with a conventional light sharing PET block detector, a high refractive index optical adhesive is necessary to closely match the refractive index of LSO. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Phoswich detectors, combining two scintillator materials with different light output properties, have been used successfully in both commercial and research PET systems due to their ability to use scintillator decay time differences to provide an additional binary degree of spatial sampling. In most approaches, the phoswich design is used to provide depth-of-interaction (DOI) information. Successful examples of this design include: (i) the Siemens/CTI High Resolution Research Tomograph (HRRT) [1,2], a dedicated human brain PET system that used a dual layer design with either an LSO/LSO or LSO/GSO phoswich; (ii) the General Electric eXplore VISTA, a small animal PET that used
a dual-layer LYSO/GSO phoswich [3] to avoid severe degradation of spatial resolution due to DOI effects caused by the small 118 mm ring diameter; and (iii) the Raytest ClearPET [4,5], a small animal PET that used a dual-layer LYSO/LuYAP phoswich, where LuYAP has two decay modes (𝑡decay ∼ 20∕250 ns) and LYSO has only one (𝑡decay ∼ 40 ns). An alternative phoswich detector design places the scintillator pairs in a side-by-side configuration rather than a multi-layer configuration. Perhaps the best known application of this detector design in PET is in the LabPET systems from the Université de Sherbrooke group. Their detector consists of a single channel avalanche photodiode (APD) coupled to a LYSO/LGSO phoswich [6]. Two crystals are placed side by side with the APD coupled to the end, effectively reducing the number
* Correspondence to: Department of Radiology, University of Manitoba, 702 John Buhler Research Centre-715 McDermot Ave. Winnipeg, Manitoba, Canada R3E 3P4.
E-mail address: [email protected] (A.L. Goertzen). Abbreviations: PET, positron emission tomography; PS-PMT, position-sensitive photomultiplier tube; DOI, depth-of-interaction; APD, avalanche photodiode; DLO, dual-layer offset; ROI, region of interest https://doi.org/10.1016/j.nima.2017.11.036 Received 24 February 2017; Received in revised form 21 September 2017; Accepted 11 November 2017 Available online 16 November 2017 0168-9002/© 2017 Elsevier B.V. All rights reserved.
J.D. Thiessen et al.
Nuclear Inst. and Methods in Physics Research, A 882 (2018) 124–128
Fig. 1. (A) Schematic diagram of the phoswich scintillator array. A phoswich crystal pair (left) is bonded with an optically transparent adhesive (dashed lines) and assembled into an array (right) with reflective material between phoswich pairs (solid lines), in this diagram creating a 4 × 4 array. (B) Photograph of a 4 × 4 array of this design in which each element is a pair of LSO:Ce,Ca crystals with different Ca concentrations. Each phoswich element is 3.17 ×3.17× 10 mm3 .
this technique, a time-to-amplitude converter was triggered by the rising edge of the scintillation light pulse measured by the ‘‘start PMT’’, and then the arrival times of single photons, correlated with each start signal, at the ‘‘stop PMT’’ were histogrammed to map the scintillation time profile. A least-squares fit of the data to an exponential function was used to calculate the decay time constants. The decay times of the fast and slow scintillators were measured as 32.9 and 41.2 ns, respectively. The crystals were processed by cutting into elements of size 3.17 × 1.58 × 10 mm3 and mechanically polished on all sides. Crystals were assembled into phoswich pairs by bonding the fast/slow crystals using Dymax OP-20 UV curing glue (index of refraction 1.50), creating scintillator elements of size 3.17 × 3.17 × 10 mm3 . A 4 × 4 element array, shown in Fig. 1B, was assembled with the reflector between crystals being a laminate of 11 μm aluminum sandwiched by 3M Enhanced Specular Reflector (ESR). All reflectors and crystals were bonded with Dymax OP-20 UV curing glue. The top surface of the array was covered with bonded ESR. The array processing and assembly was done by Agile Technologies Inc. of Knoxville, TN. The scintillator array was coupled using optical grease to a Hamamatsu R8900U-100-C12 position-sensitive photomultiplier tube (PSPMT) (Hamamatsu Photonics K.K., Hamamatsu City, Japan) with outputs multiplexed to 4 channels using a Siemens Inveon preamplifier board [11]. A 10 μCi 137 Cs sealed source was placed at a suitable distance from the center of the array. A bias voltage of 800 V was provided by a CAEN programmable power supply. The four channels from the PS-PMT and the sum signal were outputted by a custom-made amplifier breakout board (Niagara Engineering, Winnipeg, Canada). The 4 analog output signals from the amplifier (𝑋+ , 𝑋− , 𝑌+ , 𝑌− ) were digitized using a CAEN V1720 waveform digitizer with a 12-bit, 250 MSPS sampling frequency yielding a 4 ns sample time (CAEN, Viareggio, Italy). Data were collected on a PC connected to the CAEN digitizer with a fiber optic link via a PCIe card and consisted of 4 × 106 waveforms. Waveform data were processed using MATLAB R2015a (The Mathworks, Natick, MA). Each waveform is represented by 64 samples with a 4 ns sample time (256 ns long) and a 12-bit scale representing the voltage of the 4 channels from the PS-PMT (Fig. 2A). A baseline signal was calculated as the average of the first 10 samples and subtracted from the signal in each channel. The sum of the signals was calculated (Fig. 2B) and the start of each rise time defined as the point where the summed signal was greater than 10 (ADC units). The expected number of photoelectrons between the time 0 and t is proportional to the integral of the signal as a function of time, 𝑆(𝑡). A bi-exponential timing model was used to solve for both the rise time and decay time in the signal [12]: [ ] 𝑡 𝜏 −𝑡∕ 𝜏1 𝜏2 𝜏 + 𝜏2 −𝑡∕𝜏 𝑒 2 + 1 𝑒 𝜏1 +𝜏2 𝑓 (𝑡) = 𝑆 (𝑡) 𝑑𝑡 = 𝑅 1 − 1 (1) ∫0 𝜏2 𝜏2
of electronics channels in the system by a factor of 2:1. The higher detection efficiency of LYSO compared to LGSO [7] is partially compensated for by having longer LGSO than LYSO crystals, however this complicates reconstruction algorithms that model the system physical response. In recent years there have been significant advances in using codoping approaches to modulate the decay time of LSO scintillator crystals. The best known of these approaches is to use LSO co-doped with cerium (Ce) and calcium (Ca) (LSO:Ce,Ca) with Ca concentrations ranging from 0 to 4% [8], allowing approximately 10 ns variation in the scintillator decay times. In the same paper, the measured light output was reported as 30,900 photons/MeV for 0% Ca concentration and 34,800 photons/MeV for 0.4% Ca concentration [8]. The key benefit of these co-doping approaches is that the attenuation and light output properties of the two materials are similar, thus avoiding the complications of accounting for materials with vastly different properties. We have previously proposed and studied using simulations a phoswich detector design that consists of a 2D array of phoswich crystal pairs [9] as shown in Fig. 1. This detector design resembles conventional 2D scintillator arrays used in PET detector designs, however each crystal element is actually a phoswich pair bonded together with an optically transparent glue. In this design the phoswich discrimination would provide a two-fold improvement in spatial sampling in one dimension of the array without the need to resolve smaller crystals, thus allowing use in conventional block detector designs. In this earlier work, we proposed building dual-layer offset (DLO) detector arrays in which the top and bottom layers could have their phoswich pairs rotated at 90◦ to each other, allowing for improved sampling in both the axial and transverse directions. The simulation studies demonstrated a clear improvement in the ability to resolve small objects using this detector design. In this present work, we present results from a single layer prototype of this phoswich array design. 2. Materials and methods LSO scintillator crystal materials co-doped with cerium (Ce) and calcium (Ca) (LSO:Ce,Ca) were obtained from the Scintillation Materials Research Center at the University of Tennessee in Knoxville [8]. Ca co-doped LSO scintillator crystals were grown in iridium crucibles that were inductively heated to ∼2070 ◦ C in a Cyberstar Oxypuller Czochralski growth station with growth atmosphere of flowing nitrogen to which was added a small fraction of a percent of oxygen. The raw materials used were stoichiometric mixtures of Lu2 O3 , SiO2 , CeO2 , and CaO oxide powders of at least 99.99% purity. In both crystals the cerium concentration in the initial melt was 0.1 atomic percent with respect to lutetium. The fast LSO was co-doped with 0.4 atomic percent Ca while the slow LSO was co-doped with 0.05 atomic percent Ca. The dopant concentrations in the finished crystals will differ from that of the melt due to segregation at the solid–liquid interface during growth. The scintillation time profiles of bulk pieces of LSO:Ce and LSO:Ce,Ca were measured with the time-correlated single photon counting technique that was originally developed by Bollinger and Thomas [10]. In
where 𝜏 1 is the rise time constant, 𝜏 2 is the decay time constant, 𝑅 = 𝜏2
2 𝑎 𝜏 +𝜏 is the total photoelectron yield, and a is a scaling constant. Eq. (1) 1 2 was fit to the integral of each waveform signal from 0 to 160 ns after the initial rise using a non-linear and bounded minimization search. See
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Fig. 2. (A) Example of waveforms captured from a single scintillation event from all 4 channels of the PS-PMT and (B) sum of the 4 channels. (C) The integral of the summed signal starting at the leading edge of the waveform (circles) was fit to a bi-exponential timing model (solid line) in order to determine a scaling factor (𝑅), rise time (𝜏 1 ) and decay time (𝜏 2 ) for the waveform. The decay time was subsequently used to determine the origin of the event, from either fast or slow decaying crystals in each phoswich element. Signal, S(t) has ADC units with a 12-bit scale representing the voltage of the 4 channels from the PS-PMT.
3. Results
Fig. 2C for an example of the integral of the summed signal and fit to this model. The position of each signal was calculated using the maximum value found in each of the 4 waveforms representing the 𝑋+ , 𝑋− , 𝑌+ , 𝑌− outputs on the PS-PMT. The 𝑋 and 𝑌 coordinates were calculated as:
The bi-exponential timing model provided a robust fit to the integral of the signal from each scintillation event (Fig. 2), resulting in equal rise times and scaling constants for both LSO crystals in the phoswich element (Table 1). Similarly, the calculated mean decay times had low variability for both the fast LSO (35.3 ± 0.4 ns) and the slow LSO (41.2 ± 0.7 ns). Decay times showed good agreement with the 32.9 and 41.2 ns values measured at the University of Tennessee. The mean decay time threshold was (37.4 ± 0.5) ns with a range of 36.5–38.3 ns across all of the phoswich pairs. There was ∼15% misidentification of events based on timing discrimination, better than phoswich results reported by Shimizu et al. [13], who reported a 22.4% error for a LGSO/LGSO single crystal phoswich with 5.9 ns 𝛥tdecay . Assuming the second peak of the double Gaussian distribution equals 661.7 keV (gamma ray energy from decay of 137 Cs), the mean energy threshold for fast LSO was 𝐸fast = (500±28) keV with a range of 456–543 keV and for slow LSO was 𝐸slow = (550±14) keV with a range of 515–567 keV. Rejecting events below the energy threshold removed scattering artifacts from the flood histograms and resulted in a cleaner separation of the two scintillators in the phoswich. The energy resolution for the fast and slow LSO was 13.4 ± 0.5% and 14.9 ± 1.0%, respectively. Each phoswich element appeared as two distinct ‘blobs’ in the detector flood images, suggesting that there is significant total internal reflection at the crystal/crystal boundary caused by the lower index of refraction glue that was used to bond the crystal pairs together. Application of both decay time and energy thresholds created reasonably clean flood histograms for both the fast and slow LSO in the phoswich elements (Fig. 3).
𝑋 = (𝑋+ − 𝑋− )∕(𝑋+ + 𝑋− ) 𝑌 = (𝑌+ − 𝑌− )∕(𝑌+ + 𝑌− )
(2)
and binned into a 100 × 100 pixel flood histogram (Fig. 3A), creating a 4 × 4 array of phoswich elements with two distinct blobs per element. A square region of interest (ROI) was defined around each pair of phoswich blobs and the indices of all the contributing signals in the ROI were determined. A histogram of all of the decay times in each pair of phoswich blobs was fit to a double Gaussian distribution. The threshold value, 𝜏2thresh , was determined as the crossing point of the two Gaussian curves, with all decay times shorter than the threshold classified as ‘‘fast’’ and all decay times longer classified as ‘‘slow’’. An example of the decay time histogram with double Gaussian fit for one phoswich element is shown in Fig. 3B. An energy histogram was calculated using the maximum values of the summed signals for each scintillation event in individual fast and slow LSO crystals. A double Gaussian fit was used to identify the 661.7 keV gamma rays photopeak region and the lower energy Compton scatter region. An energy threshold was automatically defined based on the crossing point of the two Gaussian peaks for both the fast (𝐸fast ) and slow (𝐸slow ) LSO (example energy histograms and double Gaussian fit can be seen in Fig. 3C). Only signals with energy greater than 𝐸slow and 𝐸fast were used in the slow and fast decaying flood histograms, respectively. Energy thresholds applied to individual crystals account for small differences in light output from different concentrations of Ca. 126
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Fig. 3. (A) Flood histogram of array acquired on Hamamatsu R8900U-100-C12 PS-PMT with no timing discrimination applied (left panel). Flood histograms were subsequently generated based on events identified as being in the ‘fast’ LSO crystal (𝜏 2 ≤ 𝜏2thresh ) (middle panel) or in the ‘slow’ LSO crystal (𝜏 2 >𝜏2thresh ) (right panel) with events below energy thresholds, 𝐸fast and 𝐸slow , also removed. (B) Example of decay time histogram from a single phoswich pair with decay time threshold, 𝜏2thresh , defined as the crossing point of a double Gaussian fit. (C) Example of energy histograms from the same phoswich pair with energy thresholds, 𝐸fast and 𝐸slow , defined as the crossing point of a double Gaussian fit. Table 1 Phoswich timing parameters determined using a bi-exponential timing model (mean ± standard deviation across 16 phoswich elements). Fast LSO 𝜏2 ≤ 𝜏2thresh and 𝐸 > 𝐸fast
Slow LSO 𝜏2 > 𝜏2thresh and 𝐸 > 𝐸slow
𝑅
𝜏1 (ns)
𝜏2 (ns)
𝑅
𝜏1 (ns)
𝜏2 (ns)
4.043 ± 0.003
4.1 ± 0.2
35.3 ± 0.5
4.093 ± 0.005
4.1 ± 0.1
41.2 ± 0.7
4. Discussion and conclusions
resolution in a PET system [8]. These earlier simulations assumed that light was perfectly shared between the two crystals in the phoswich element. Based on our experimental results, this does not appear to be the case, ruling out the use of this phoswich design for improved spatial sampling in DLO configurations. In the LabPET family of small animal PET systems, it is assumed that light is shared between the two optically coupled crystals [6]. The LabPET benefits from a pseudo one-to-one coupling of crystals to the photodetector elements, an approach that may be necessary for our phoswich array, as opposed to the light sharing approach presented here. Given that optical adhesive tends to have an index of refraction equal to 1.5 and LSO has an index of refraction closer to 1.82 [15], it is not surprising that there is refraction and light tunneling at the interface between the two crystals. In order to facilitate light sharing between a reduced number of photodetector elements for either a single-layer or DLO array, a high refractive index optical adhesive is necessary to closely match the refractive index of LSO.
A 4 × 4 array of phoswich elements was constructed and tested using a Hamamatsu PS-PMT and CAEN waveform digitizer with 4 ns sampling period. We were able to achieve excellent separation of the two LSO crystals with an average decay time difference of 5.9 ns. In the future, since full waveform capture is not practical in a complete PET system, a real-time approach could be adapted from [14], calculating partial and full integrals to determine a crystal identification ratio using an FPGA. Although we successfully separated fast and slow LSO in each phoswich element, the light spread patterns from each half of the phoswich pairs were also clearly distinguished in the full flood histogram. This suggests that, although a side by side phoswich is feasible, reflections at the phoswich pair boundary cause it to behave optically as an 8 × 4 array rather than a 4 × 4 array. For the crystal array tested in this work, the crystal elements were large compared with the spatial resolving capability of the PS-PMT detector, thus the 8 × 4 appearance of the flood histogram did not cause problems in identifying crystal elements. However, extending this phoswich array design for use with arrays in which the phoswich pairs are at the limit of the spatial resolving capability of the photosensor may create challenges in correctly identifying crystals in the detector flood image. Thus this current design may not be suitable for use in conventional block detector designs. Previously, we showed with simulations that a DLO arrangement of phoswich elements could improve axial and transverse
Acknowledgments This work was funded by Discovery Grant 341628 to A. Goertzen from the Natural Science and Engineering Research Council of Canada (NSERC), a Manitoba Health Research Council Postdoctoral Fellowship to J. Thiessen and a University of Manitoba Graduate Student Fellowship to G. Schellenberg. We wish to thank Dr. Jeffrey Martin of the University 127
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of Winnipeg for providing access and assistance with the data acquisition equipment.
[6] M. Bergeron, J. Cadorette, J.F. Beaudoin, M.D. Lepage, G. Robert, V. Selivanov, M.A. Tetrault, N. Viscogliosi, J.P. Norenberg, R. Fontaine, R. Lecomte, Performance Evaluation of the LabPET APD-Based Digital PET Scanner, IEEE Trans. Nucl. Sci. 56 (2009) 10–16. [7] R. Lecomte, Novel detector technology for clinical PET, Eur. J. Nucl. Med. Mol. Imaging 36 (2009) 69–85. [8] M.A. Spurrier, P. Szupryczynski, Y. Kan, A.A. Carey, C.L. Melcher, Effects of Ca2+ Co-doping on the scintillation properties of LSO:Ce, IEEE Trans. Nucl. Sci. 55 (2008) 1178–1182. [9] X. Zhang, C.J. Thompson, J.D. Thiessen, A.L. Goertzen, Simulation studies of a phoswich PET detector design with a two-fold improvement in spatial sampling, in: Nuclear Science Symposium and Medical Imaging Conference, NSS/MIC, 2013 IEEE, 2013, pp. 1-4. [10] L.M. Bollinger, G.E. Thomas, Measurement of the time dependence of scintillation intensity by a delayed-coincidence method, Rev. Sci. Instrum. 32 (1961) 1044–1050. [11] R.A. Mintzer, S.B. Siegel, Design and performance of a new pixelated- LSO/PSPMT gamma-ray detector for high resolution PET imaging, in: 2007 IEEE Nuclear Science Symposium Conference Record, 2007, pp. 3418-3422. [12] Y. Shao, A new timing model for calculating the intrinsic timing resolution of a scintillator detector, Phys. Med. Biol. 52 (2007) 1103–1117. [13] S. Shimizu, C.M. Pepin, M. Bergeron, R. Lecomte, Characteristics of Lu1.8Gd0.2SiO5:Ce (LGSO) for APD-Based PET Detector, IEEE Trans. Nucl. Sci. 57 (2010) 55–62. [14] S. Yamamoto, Basic performance evaluation of a Si-PM array-based LGSO phoswich DOI block detector for a high-resolution small animal PET system, Radiological physics and technology, (2012). [15] C.L. Melcher, Scintillation crystals for PET, J. Nucl. Med. 41 (2000) 1051–1055.
References [1] K. Wienhard, M. Schmand, M.E. Casey, K. Baker, J. Bao, L. Eriksson, W.F. Jones, C. Knoess, M. Lenox, M. Lercher, P. Luk, C. Michel, J.H. Reed, N. Richerzhagen, J. Treffert, S. Vollmar, J.W. Young, W.D. Heiss, R. Nutt, The ECAT HRRT: Performance and first clinical application of the new high resolution research tomograph, IEEE Trans. Nucl. Sci. 49 (2002) 104–110. [2] L. Eriksson, K. Wienhard, M. Eriksson, M.E. Casey, C. Knoess, T. Bruckbauer, J. Hamill, M. Schmand, T. Gremillion, M. Lenox, M. Conti, B. Bendriem, W.D. Heiss, R. Nutt, The ECAT HRRT: NEMA NEC evaluation of the HRRT system, the new highresolution research tomograph, IEEE Trans. Nucl. Sci. 49 (I) (2002) 2085–2088. [3] Y. Wang, J. Seidel, B.M.W. Tsui, J.J. Vaquero, M.G. Pomper, Performance evaluation of the GE Healthcare eXplore VISTA dual-ring small-animal PET scanner, J. Nucl. Med. 47 (2006) 1891–1900. [4] K. Ziemons, E. Auffray, R. Barbier, G. Brandenburg, P. Bruyndonckx, Y. Choi, D. Christ, N. Costes, Y. Declais, O. Devroede, C. Dujardin, A. Fedorovd, U. Heinrichs, M. Korjik, M. Krieguer, C. Kuntner, G. Largeron, C. Lartizien, H. Larue, P. Lecoq, S. Leonard, J. Marteau, C. Morel, J.B. Mosset, C. Parl, C. Pedrini, A.G. Petrosyan, U. Pietrzyk, M. Rey, S. Saladino, D. Sappey-Marinier, L. Simon, M. Streun, S. Tavernier, J.M. Vieira, The ClearPET(TM) project: Development of a 2nd generation highperformance small animal PET scanner, Nucl. Instrum. Methods A 537 (2005) 307– 311. [5] A.L. Goertzen, Q. Bao, M. Bergeron, E. Blankemeyer, S. Blinder, M. Canadas, A.F. Chatziioannou, K. Dinelle, E. Elhami, H.S. Jans, E. Lage, R. Lecomte, V. Sossi, S. Surti, Y.C. Tai, J.J. Vaquero, E. Vicente, D.A. Williams, R. Laforest, NEMA NU 4-2008 comparison of preclinical PET imaging systems, J. Nucl. Med. 53 (2012) 1300–1309.
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Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
A phoswich detector design for improved spatial sampling in PET Jonathan D. Thiessen a,b,c , Merry A. Koschan d , Charles L. Melcher d , Fang Meng d , Graham Schellenberg e , Andrew L. Goertzen c,e, * a b c d e
Imaging Program, Lawson Health Research Institute, London, Ontario, Canada Department of Medical Biophysics, University of Western Ontario, London, Ontario, Canada Department of Radiology, University of Manitoba, Winnipeg, Manitoba, Canada Scintillation Materials Research Center, University of Tennessee, Knoxville, TN, USA Department of Physics & Astronomy, University of Manitoba, Winnipeg, Manitoba, Canada
a r t i c l e
i n f o
Keywords: Positron emission tomography Phoswich detector LSO
a b s t r a c t Block detector designs, utilizing a pixelated scintillator array coupled to a photosensor array in a light-sharing design, are commonly used for positron emission tomography (PET) imaging applications. In practice, the spatial sampling of these designs is limited by the crystal pitch, which must be large enough for individual crystals to be resolved in the detector flood image. Replacing the conventional 2D scintillator array with an array of phoswich elements, each consisting of an optically coupled side-by-side scintillator pair, may improve spatial sampling in one direction of the array without requiring resolving smaller crystal elements. To test the feasibility of this design, a 4 × 4 phoswich array was constructed, with each phoswich element consisting of two optically coupled, 3.17 × 1.58 × 10 mm3 LSO crystals co-doped with cerium and calcium. The amount of calcium doping was varied to create a ‘fast’ LSO crystal with decay time of 32.9 ns and a ‘slow’ LSO crystal with decay time of 41.2 ns. Using a Hamamatsu R8900U-00-C12 position-sensitive photomultiplier tube (PS-PMT) and a CAEN V1720 250 MS/s waveform digitizer, we were able to show effective discrimination of the fast and slow LSO crystals in the phoswich array. Although a side-by-side phoswich array is feasible, reflections at the crystal boundary due to a mismatch between the refractive index of the optical adhesive (𝑛 = 1.5) and LSO (𝑛 = 1.82) caused it to behave optically as an 8 × 4 array rather than a 4 × 4 array. Direct coupling of each phoswich element to individual photodetector elements may be necessary with the current phoswich array design. Alternatively, in order to implement this phoswich design with a conventional light sharing PET block detector, a high refractive index optical adhesive is necessary to closely match the refractive index of LSO. © 2017 Elsevier B.V. All rights reserved.
1. Introduction Phoswich detectors, combining two scintillator materials with different light output properties, have been used successfully in both commercial and research PET systems due to their ability to use scintillator decay time differences to provide an additional binary degree of spatial sampling. In most approaches, the phoswich design is used to provide depth-of-interaction (DOI) information. Successful examples of this design include: (i) the Siemens/CTI High Resolution Research Tomograph (HRRT) [1,2], a dedicated human brain PET system that used a dual layer design with either an LSO/LSO or LSO/GSO phoswich; (ii) the General Electric eXplore VISTA, a small animal PET that used
a dual-layer LYSO/GSO phoswich [3] to avoid severe degradation of spatial resolution due to DOI effects caused by the small 118 mm ring diameter; and (iii) the Raytest ClearPET [4,5], a small animal PET that used a dual-layer LYSO/LuYAP phoswich, where LuYAP has two decay modes (𝑡decay ∼ 20∕250 ns) and LYSO has only one (𝑡decay ∼ 40 ns). An alternative phoswich detector design places the scintillator pairs in a side-by-side configuration rather than a multi-layer configuration. Perhaps the best known application of this detector design in PET is in the LabPET systems from the Université de Sherbrooke group. Their detector consists of a single channel avalanche photodiode (APD) coupled to a LYSO/LGSO phoswich [6]. Two crystals are placed side by side with the APD coupled to the end, effectively reducing the number
* Correspondence to: Department of Radiology, University of Manitoba, 702 John Buhler Research Centre-715 McDermot Ave. Winnipeg, Manitoba, Canada R3E 3P4.
E-mail address: [email protected] (A.L. Goertzen). Abbreviations: PET, positron emission tomography; PS-PMT, position-sensitive photomultiplier tube; DOI, depth-of-interaction; APD, avalanche photodiode; DLO, dual-layer offset; ROI, region of interest https://doi.org/10.1016/j.nima.2017.11.036 Received 24 February 2017; Received in revised form 21 September 2017; Accepted 11 November 2017 Available online 16 November 2017 0168-9002/© 2017 Elsevier B.V. All rights reserved.
J.D. Thiessen et al.
Nuclear Inst. and Methods in Physics Research, A 882 (2018) 124–128
Fig. 1. (A) Schematic diagram of the phoswich scintillator array. A phoswich crystal pair (left) is bonded with an optically transparent adhesive (dashed lines) and assembled into an array (right) with reflective material between phoswich pairs (solid lines), in this diagram creating a 4 × 4 array. (B) Photograph of a 4 × 4 array of this design in which each element is a pair of LSO:Ce,Ca crystals with different Ca concentrations. Each phoswich element is 3.17 ×3.17× 10 mm3 .
this technique, a time-to-amplitude converter was triggered by the rising edge of the scintillation light pulse measured by the ‘‘start PMT’’, and then the arrival times of single photons, correlated with each start signal, at the ‘‘stop PMT’’ were histogrammed to map the scintillation time profile. A least-squares fit of the data to an exponential function was used to calculate the decay time constants. The decay times of the fast and slow scintillators were measured as 32.9 and 41.2 ns, respectively. The crystals were processed by cutting into elements of size 3.17 × 1.58 × 10 mm3 and mechanically polished on all sides. Crystals were assembled into phoswich pairs by bonding the fast/slow crystals using Dymax OP-20 UV curing glue (index of refraction 1.50), creating scintillator elements of size 3.17 × 3.17 × 10 mm3 . A 4 × 4 element array, shown in Fig. 1B, was assembled with the reflector between crystals being a laminate of 11 μm aluminum sandwiched by 3M Enhanced Specular Reflector (ESR). All reflectors and crystals were bonded with Dymax OP-20 UV curing glue. The top surface of the array was covered with bonded ESR. The array processing and assembly was done by Agile Technologies Inc. of Knoxville, TN. The scintillator array was coupled using optical grease to a Hamamatsu R8900U-100-C12 position-sensitive photomultiplier tube (PSPMT) (Hamamatsu Photonics K.K., Hamamatsu City, Japan) with outputs multiplexed to 4 channels using a Siemens Inveon preamplifier board [11]. A 10 μCi 137 Cs sealed source was placed at a suitable distance from the center of the array. A bias voltage of 800 V was provided by a CAEN programmable power supply. The four channels from the PS-PMT and the sum signal were outputted by a custom-made amplifier breakout board (Niagara Engineering, Winnipeg, Canada). The 4 analog output signals from the amplifier (𝑋+ , 𝑋− , 𝑌+ , 𝑌− ) were digitized using a CAEN V1720 waveform digitizer with a 12-bit, 250 MSPS sampling frequency yielding a 4 ns sample time (CAEN, Viareggio, Italy). Data were collected on a PC connected to the CAEN digitizer with a fiber optic link via a PCIe card and consisted of 4 × 106 waveforms. Waveform data were processed using MATLAB R2015a (The Mathworks, Natick, MA). Each waveform is represented by 64 samples with a 4 ns sample time (256 ns long) and a 12-bit scale representing the voltage of the 4 channels from the PS-PMT (Fig. 2A). A baseline signal was calculated as the average of the first 10 samples and subtracted from the signal in each channel. The sum of the signals was calculated (Fig. 2B) and the start of each rise time defined as the point where the summed signal was greater than 10 (ADC units). The expected number of photoelectrons between the time 0 and t is proportional to the integral of the signal as a function of time, 𝑆(𝑡). A bi-exponential timing model was used to solve for both the rise time and decay time in the signal [12]: [ ] 𝑡 𝜏 −𝑡∕ 𝜏1 𝜏2 𝜏 + 𝜏2 −𝑡∕𝜏 𝑒 2 + 1 𝑒 𝜏1 +𝜏2 𝑓 (𝑡) = 𝑆 (𝑡) 𝑑𝑡 = 𝑅 1 − 1 (1) ∫0 𝜏2 𝜏2
of electronics channels in the system by a factor of 2:1. The higher detection efficiency of LYSO compared to LGSO [7] is partially compensated for by having longer LGSO than LYSO crystals, however this complicates reconstruction algorithms that model the system physical response. In recent years there have been significant advances in using codoping approaches to modulate the decay time of LSO scintillator crystals. The best known of these approaches is to use LSO co-doped with cerium (Ce) and calcium (Ca) (LSO:Ce,Ca) with Ca concentrations ranging from 0 to 4% [8], allowing approximately 10 ns variation in the scintillator decay times. In the same paper, the measured light output was reported as 30,900 photons/MeV for 0% Ca concentration and 34,800 photons/MeV for 0.4% Ca concentration [8]. The key benefit of these co-doping approaches is that the attenuation and light output properties of the two materials are similar, thus avoiding the complications of accounting for materials with vastly different properties. We have previously proposed and studied using simulations a phoswich detector design that consists of a 2D array of phoswich crystal pairs [9] as shown in Fig. 1. This detector design resembles conventional 2D scintillator arrays used in PET detector designs, however each crystal element is actually a phoswich pair bonded together with an optically transparent glue. In this design the phoswich discrimination would provide a two-fold improvement in spatial sampling in one dimension of the array without the need to resolve smaller crystals, thus allowing use in conventional block detector designs. In this earlier work, we proposed building dual-layer offset (DLO) detector arrays in which the top and bottom layers could have their phoswich pairs rotated at 90◦ to each other, allowing for improved sampling in both the axial and transverse directions. The simulation studies demonstrated a clear improvement in the ability to resolve small objects using this detector design. In this present work, we present results from a single layer prototype of this phoswich array design. 2. Materials and methods LSO scintillator crystal materials co-doped with cerium (Ce) and calcium (Ca) (LSO:Ce,Ca) were obtained from the Scintillation Materials Research Center at the University of Tennessee in Knoxville [8]. Ca co-doped LSO scintillator crystals were grown in iridium crucibles that were inductively heated to ∼2070 ◦ C in a Cyberstar Oxypuller Czochralski growth station with growth atmosphere of flowing nitrogen to which was added a small fraction of a percent of oxygen. The raw materials used were stoichiometric mixtures of Lu2 O3 , SiO2 , CeO2 , and CaO oxide powders of at least 99.99% purity. In both crystals the cerium concentration in the initial melt was 0.1 atomic percent with respect to lutetium. The fast LSO was co-doped with 0.4 atomic percent Ca while the slow LSO was co-doped with 0.05 atomic percent Ca. The dopant concentrations in the finished crystals will differ from that of the melt due to segregation at the solid–liquid interface during growth. The scintillation time profiles of bulk pieces of LSO:Ce and LSO:Ce,Ca were measured with the time-correlated single photon counting technique that was originally developed by Bollinger and Thomas [10]. In
where 𝜏 1 is the rise time constant, 𝜏 2 is the decay time constant, 𝑅 = 𝜏2
2 𝑎 𝜏 +𝜏 is the total photoelectron yield, and a is a scaling constant. Eq. (1) 1 2 was fit to the integral of each waveform signal from 0 to 160 ns after the initial rise using a non-linear and bounded minimization search. See
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Fig. 2. (A) Example of waveforms captured from a single scintillation event from all 4 channels of the PS-PMT and (B) sum of the 4 channels. (C) The integral of the summed signal starting at the leading edge of the waveform (circles) was fit to a bi-exponential timing model (solid line) in order to determine a scaling factor (𝑅), rise time (𝜏 1 ) and decay time (𝜏 2 ) for the waveform. The decay time was subsequently used to determine the origin of the event, from either fast or slow decaying crystals in each phoswich element. Signal, S(t) has ADC units with a 12-bit scale representing the voltage of the 4 channels from the PS-PMT.
3. Results
Fig. 2C for an example of the integral of the summed signal and fit to this model. The position of each signal was calculated using the maximum value found in each of the 4 waveforms representing the 𝑋+ , 𝑋− , 𝑌+ , 𝑌− outputs on the PS-PMT. The 𝑋 and 𝑌 coordinates were calculated as:
The bi-exponential timing model provided a robust fit to the integral of the signal from each scintillation event (Fig. 2), resulting in equal rise times and scaling constants for both LSO crystals in the phoswich element (Table 1). Similarly, the calculated mean decay times had low variability for both the fast LSO (35.3 ± 0.4 ns) and the slow LSO (41.2 ± 0.7 ns). Decay times showed good agreement with the 32.9 and 41.2 ns values measured at the University of Tennessee. The mean decay time threshold was (37.4 ± 0.5) ns with a range of 36.5–38.3 ns across all of the phoswich pairs. There was ∼15% misidentification of events based on timing discrimination, better than phoswich results reported by Shimizu et al. [13], who reported a 22.4% error for a LGSO/LGSO single crystal phoswich with 5.9 ns 𝛥tdecay . Assuming the second peak of the double Gaussian distribution equals 661.7 keV (gamma ray energy from decay of 137 Cs), the mean energy threshold for fast LSO was 𝐸fast = (500±28) keV with a range of 456–543 keV and for slow LSO was 𝐸slow = (550±14) keV with a range of 515–567 keV. Rejecting events below the energy threshold removed scattering artifacts from the flood histograms and resulted in a cleaner separation of the two scintillators in the phoswich. The energy resolution for the fast and slow LSO was 13.4 ± 0.5% and 14.9 ± 1.0%, respectively. Each phoswich element appeared as two distinct ‘blobs’ in the detector flood images, suggesting that there is significant total internal reflection at the crystal/crystal boundary caused by the lower index of refraction glue that was used to bond the crystal pairs together. Application of both decay time and energy thresholds created reasonably clean flood histograms for both the fast and slow LSO in the phoswich elements (Fig. 3).
𝑋 = (𝑋+ − 𝑋− )∕(𝑋+ + 𝑋− ) 𝑌 = (𝑌+ − 𝑌− )∕(𝑌+ + 𝑌− )
(2)
and binned into a 100 × 100 pixel flood histogram (Fig. 3A), creating a 4 × 4 array of phoswich elements with two distinct blobs per element. A square region of interest (ROI) was defined around each pair of phoswich blobs and the indices of all the contributing signals in the ROI were determined. A histogram of all of the decay times in each pair of phoswich blobs was fit to a double Gaussian distribution. The threshold value, 𝜏2thresh , was determined as the crossing point of the two Gaussian curves, with all decay times shorter than the threshold classified as ‘‘fast’’ and all decay times longer classified as ‘‘slow’’. An example of the decay time histogram with double Gaussian fit for one phoswich element is shown in Fig. 3B. An energy histogram was calculated using the maximum values of the summed signals for each scintillation event in individual fast and slow LSO crystals. A double Gaussian fit was used to identify the 661.7 keV gamma rays photopeak region and the lower energy Compton scatter region. An energy threshold was automatically defined based on the crossing point of the two Gaussian peaks for both the fast (𝐸fast ) and slow (𝐸slow ) LSO (example energy histograms and double Gaussian fit can be seen in Fig. 3C). Only signals with energy greater than 𝐸slow and 𝐸fast were used in the slow and fast decaying flood histograms, respectively. Energy thresholds applied to individual crystals account for small differences in light output from different concentrations of Ca. 126
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Fig. 3. (A) Flood histogram of array acquired on Hamamatsu R8900U-100-C12 PS-PMT with no timing discrimination applied (left panel). Flood histograms were subsequently generated based on events identified as being in the ‘fast’ LSO crystal (𝜏 2 ≤ 𝜏2thresh ) (middle panel) or in the ‘slow’ LSO crystal (𝜏 2 >𝜏2thresh ) (right panel) with events below energy thresholds, 𝐸fast and 𝐸slow , also removed. (B) Example of decay time histogram from a single phoswich pair with decay time threshold, 𝜏2thresh , defined as the crossing point of a double Gaussian fit. (C) Example of energy histograms from the same phoswich pair with energy thresholds, 𝐸fast and 𝐸slow , defined as the crossing point of a double Gaussian fit. Table 1 Phoswich timing parameters determined using a bi-exponential timing model (mean ± standard deviation across 16 phoswich elements). Fast LSO 𝜏2 ≤ 𝜏2thresh and 𝐸 > 𝐸fast
Slow LSO 𝜏2 > 𝜏2thresh and 𝐸 > 𝐸slow
𝑅
𝜏1 (ns)
𝜏2 (ns)
𝑅
𝜏1 (ns)
𝜏2 (ns)
4.043 ± 0.003
4.1 ± 0.2
35.3 ± 0.5
4.093 ± 0.005
4.1 ± 0.1
41.2 ± 0.7
4. Discussion and conclusions
resolution in a PET system [8]. These earlier simulations assumed that light was perfectly shared between the two crystals in the phoswich element. Based on our experimental results, this does not appear to be the case, ruling out the use of this phoswich design for improved spatial sampling in DLO configurations. In the LabPET family of small animal PET systems, it is assumed that light is shared between the two optically coupled crystals [6]. The LabPET benefits from a pseudo one-to-one coupling of crystals to the photodetector elements, an approach that may be necessary for our phoswich array, as opposed to the light sharing approach presented here. Given that optical adhesive tends to have an index of refraction equal to 1.5 and LSO has an index of refraction closer to 1.82 [15], it is not surprising that there is refraction and light tunneling at the interface between the two crystals. In order to facilitate light sharing between a reduced number of photodetector elements for either a single-layer or DLO array, a high refractive index optical adhesive is necessary to closely match the refractive index of LSO.
A 4 × 4 array of phoswich elements was constructed and tested using a Hamamatsu PS-PMT and CAEN waveform digitizer with 4 ns sampling period. We were able to achieve excellent separation of the two LSO crystals with an average decay time difference of 5.9 ns. In the future, since full waveform capture is not practical in a complete PET system, a real-time approach could be adapted from [14], calculating partial and full integrals to determine a crystal identification ratio using an FPGA. Although we successfully separated fast and slow LSO in each phoswich element, the light spread patterns from each half of the phoswich pairs were also clearly distinguished in the full flood histogram. This suggests that, although a side by side phoswich is feasible, reflections at the phoswich pair boundary cause it to behave optically as an 8 × 4 array rather than a 4 × 4 array. For the crystal array tested in this work, the crystal elements were large compared with the spatial resolving capability of the PS-PMT detector, thus the 8 × 4 appearance of the flood histogram did not cause problems in identifying crystal elements. However, extending this phoswich array design for use with arrays in which the phoswich pairs are at the limit of the spatial resolving capability of the photosensor may create challenges in correctly identifying crystals in the detector flood image. Thus this current design may not be suitable for use in conventional block detector designs. Previously, we showed with simulations that a DLO arrangement of phoswich elements could improve axial and transverse
Acknowledgments This work was funded by Discovery Grant 341628 to A. Goertzen from the Natural Science and Engineering Research Council of Canada (NSERC), a Manitoba Health Research Council Postdoctoral Fellowship to J. Thiessen and a University of Manitoba Graduate Student Fellowship to G. Schellenberg. We wish to thank Dr. Jeffrey Martin of the University 127
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of Winnipeg for providing access and assistance with the data acquisition equipment.
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