2009 IEEE Nuclear Science Symposium Conference Record
J01-4
Ultra Precise Timing with SiPM-Based TOF PET Scintillation Detectors Stefan Seifert, Member, IEEE, Ruud Vinke, Member, IEEE, Herman T. van Dam, Herbert Löhner, Member, IEEE , Peter Dendooven, Member, IEEE, Freek J. Beekman, Member, IEEE, Dennis R. Schaart, Member, IEEE
Abstract—The combination of SiPMs with fast and bright scintillators, such as LaBr3:Ce, seems very promising for application in time-of-flight (TOF) PET. We therefore conducted a series of experiments with the goal of obtaining the best possible timing resolution with SiPM-based scintillation detectors in order to establish a bench mark for future experiments with different detector designs. The detectors employed in our measurements consisted of two SiPMs (Hamamatsu MPPCS10362-33-050C), which were directly coupled to small scintillation crystals, viz. LaBr3:Ce and LYSO. An excellent coincidence resolving time (CRT) for 22Na 511 annihilation photons of 99.5 ps ± 0.6 ps FWHM could be achieved at the optimized electronics and digitizer settings with two LaBr3:5%Ce crystals. A CRT of 171.5 ps ± 0.8 ps FWHM was obtained with L(Y)SO crystals. These results compare well to the predictions of a statistical model which was developed to describe the timing performance of SiPM based scintillation detectors.
I. INTRODUCTION
S
Photomultipliers (SiPMs), are a relatively new class of photosensors, with very favorable properties such as high gain, high quantum efficiency, low excess noise, and insensitivity to magnetic fields. This last property makes them very interesting candidates for replacing conventional photomultiplier tubes (PMTs) in medical imaging applications such as single photon emission computed tomography (SPECT) or positron emission tomography (PET), since it would allow for the combination of SPECT/PET and magnetic resonance tomography. The application of SiPMs in PET imposes the additional requirement of excellent timing performance. This is especially true in the light of recent research showing the potentially tremendous improvement in image quality when time of flight (TOF) information can be incorporated in the image reconstruction. What is more is that the image quality ILICON
Manuscript received November 3, 2009. (This work was supported by SenterNovem grant No. IS055019). S. Seifert, H. T. Van Dam, and D. R. Schaart are with Delft University of Technology, Delft, 2629 JB, The Netherlands (corresponding author e-mail:
[email protected]) F. J. Beekman is with Delft University of Technology, Delft, 2629 JB, The Netherlands, and with University Medical Centre Utrecht, Utrecht, 3584 CG, The Netherlands. R. Vinke, P. Dendooven, H. Löhner are with the Kernfysisch Versneller Instituut (KVI), Groningen, 9747 AA, The Netherlands
9781-4244-3962-1/09/$25.00 ©2009 IEEE
keeps improving as the CRT between two detector elements is reduced [1] [2]. It is, however, not trivial to estimate how the timing performance of a SiPM-based scintillation detector would compare to the performance of conventional PMTs. On one hand, the single photon transit time spread of SiPMs is reported to be relatively small (in the order of 100 ps – 200 ps) [3][4], yet, on the other hand, the timing performance is hampered by the large capacitance of these sensors which results in a smaller rise time of the electronic signals making them more susceptible to electronic noise. Therefore, we conducted a series of experiments aimed at the optimization of the CRT of SiPM-based scintillation detectors. Small scintillation crystals were optically coupled to SiPMs (Hamamatsu MPPC-S10362-33-050C). Two different sets of scintillation crystals were investigated: LaBr3:5%Ce and L(Y)SO. Both materials are fast and bright, inorganic scintillators. LaBr3:5%Ce has the higher light yield and the faster decay time (16 ns, 70.000 photons/MeV) [1][2], whilst L(Y)SO exhibits a much faster rise time (0.21 ps compared to 0.93 ps for LaBr3:5%Ce [5] [6]). Furthermore, a statistical model describing the timing performance of SiPM based scintillation detectors was developed. It accounts for the scintillator properties as well as the SiPMs temporal response. II. STATISTICAL TIMING MODEL A model has been developed that describes the timing uncertainty of a timing measurement with SiPM-based scintillation detectors. It is based on the following assumptions: 1. The photon arrival time and the microcell temporal response are statistically independent in the sense that the electronic output signal of a given microcell does not depend on the time ttr when a scintillation photon triggers a discharge. This assumption is fulfilled if the cell had not been triggered within a time rec,full prior to ttr, where rec,full is the time needed for a single cell to fully recharge. During a scintillation pulse this condition is obviously not always met, considering that, even in the case of the slower and less bright L(Y)SO, a 511 keV gamma photon causes about 4700 primary triggers within 40 ns for the MPPC-S10362-33-050C [Herman 1], which has only Ntot = 3600 micro cells while rec,full is
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in the order of 60 ns – 100 ns. Yet for the very first part of the pulse, which is used for the determination of the timestamp, the number of fired cells Nf is still small compared to Ncells (Nf 80 for L(Y)SO and Nf 220 for LaBr3:5%Ce within the first 1 ns of the pulse). Therefore the probability that a cell is triggered multiple times within this first nanosecond is negligible. 2. The electronic output signals of the individually fired cells are statistically independent (and therefore additive). Again, this condition is not fulfilled if a full scintillation pulse is considered, since during a scintillation pulse the voltage-over-breakdown (VVOB) changes due to the SiPM output current that causes a voltage drop over the input resistance of the preamplifier [9][10]. However, since we can limit our considerations to early times in the scintillation pulse, only a relatively small number of microcells is contributing to the signal at this point. This in combination with the relatively slow rise time of the SiPM signal result in a gain change for the next fired cell of only ~0.5 %. 3. The electronic output signal of a single fired cell does not show significant shape variations. This is plausible, since the signals of all cells are shaped by the same shaping circuitry composed of the SiPM capacitance in combination with the preamplifier input impedance, together comprising a low-pass filter, and the finite bandwidth of the preamplifier that also acts as a low-pass filter. Then the electronic output signal of a single fired cell can be modeled as the product of a constant, normalized shape function f(t) and a gain M with a certain variance M. f(t) was modeled as a bi-exponential function with a rise time constant r,SiPM and a decay time constant d,SiPM. This function was convoluted with a Gaussian function in order to take into account the shaping through the different low-pass stages. Furthermore, the probability density function pph(t*) describing the probability for any given primary trigger (i.e., a trigger caused by a scintillation photon) to occur at a time t* is modeled as the convolution of a bi-exponential function and a Gaussian function. The bi-exponential function accounts for the finite rise time constant r,sc and decay time constant d,sc of the scintillation pulse [11] and the Gaussian function introduces an additional transit time spread tr of both the photons within the crystal and the electronic signals within the SiPMs: * ( tˆ − ttr )2 t * − tˆ º t * − t − tˆ ª − τ r,sc + τ d,sc 2 1 τ d,sc τ r,sc * «1 − e » ⋅ e 2σ tr dtˆ (1) ⋅ ⋅³e psps ( t ) = 2 τ d,sc « » σ tr 2π −∞ ¬ ¼ This probability density function can be combined with the above assumptions to calculate the expectation value Vo and
the associated variance var (Vo | t ) of the signal amplitude at a
given time t after the initiation of a scintillation event. At this point the measured variance associated with electronic noise and the contribution of dark counts is added to the calculated signal variance. When a simple leading edge trigger is applied in a timing experiment, the standard deviation t of such a
measurement can be estimated via the projection of the signal standard variation onto the time axis: var (Vo | t )
σt ≈
(2) dVo dt Assuming Gaussian distributions, the CRT obtained from a coincidence timing experiment can be calculated as the quadratic sum of the standard deviations of the individual detectors times a constant factor. For two identical detectors the CRT is given by: CRTFWHM = 8 ⋅ ln(4) ⋅ σ t
(3)
III. MATERIALS AND METHODS A. Experimental Setup The measurements were performed for two sets of identical scintillation crystals, viz. LaBr3:5%Ce (Saint-Gobain BrilLanCe 380®) and L(Y)SO (Crystal Photonics), respectively. The 3 mm x 3 mm x 5 mm large crystals were enclosed
SiPM
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SiPM
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CAEN N568B CAEN V785
Fig. 1:
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Schematic overview of the experimental setup
in a reflective casing made from Spectralon®, a PTFE based material with a reflectivity specified to be better than 98% at 380 nm (the main emission wavelength of LaBr3:5%Ce). The SiPMs (Hamamatsu MPPC-S10362-33-050C) were coupled directly to the crystals using a silicone encapsulation gel (Lightspan LS-3252). The SiPMs have a sensitive area of 3 mm x 3 mm (thus matching the crystals), consisting of an array of 3600 individual self quenched Geiger Mode Avalanche Photodiodes at a 50 m pitch. According to the manufacturer the fill factor equals 61.5%. The photon detection efficiency (PDE) at VVOB = 1.9 V was estimated to be about 27% for the LaBr3:5%Ce and 35% for the L(Y)SO main emission wavelengths, respectively [4][9]. Here it should be noted that these values do not contain contributions from crosstalk or afterpulsing, which otherwise may be as high as
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30% [9][12]. Figure 1 shows a schematic representation of our measurement setup. Two identical SiPM-scintillator detectors were placed on opposing sides of a 22Na positron source. The signals were amplified in two cascaded stages using preamplifiers made in-house. The signals were split after the first amplification stage. For each detector, one signal branch, hereafter referred to as 'energy signal', was fed into a CAEN N568B shaping amplifier (100 ns shaping time, gain x5) and read out by a peak sensitive ADC (CAEN V785). The same signal was also fed into a rise time compensated leading edge discriminator (LeCroy 825) set to accept events above 410 keV only. The discriminator output was used to create a coincidence trigger for two synchronized fast sampling ADCs (Aqiris DC282, sampling rate 8 GS/s, 10 bit resolution, synchronization clock jitter 1 ps), sampling the signals from the secondary amplification stages (henceforth referred to as 'timing signals') of the two detectors. The amplification and ADC range (500 mV) were chosen such that the latter corresponds to only about ~12.5% of the pulse amplitude. In order to ensure proper correlation between the sampled traces and the energy signal, the peak sensing ADC had to be prevented from accepting events that were missed by the sampling ADCs, and vice versa. The first condition was met by using the trigger out of the sampling ADCs as a gate for the peak sensing ADC. In return, the busy signal of the peak sensing ADC was used as a veto for the sampling ADC. All measurements were performed at room temperature and in a dark box. The experiments employing LaBr3:5%Ce were carried out under dry atmosphere (N2 with H2O content 3 ppm), because of the hygroscopicity of the crystals. The bias voltages of the two detectors were optimized by varying the voltage of one detector within a range of 1.4 V to 2.4 V in steps of 0.1 V, while keeping the other at a constant bias. For both SiPMs an optimum timing resolution is reached at VVOB,opt = 1.9 V ± 0.1 V. The results presented in this work were obtained with both detectors operated at VVOB,opt.
TABLE 1: MODEL INPUT PARAMETER ASSOCIATED WITH THE SCINTILLATORS r,sc (ns) d,sc (ns) Mean No. Primary Discharges Energy Resolution (FWHM)
LaBr3:5%Ce
L(Y)SO
0.93 15 9740 3%
0.21 40 4700 8%
Table 2 summarizes the input parameters associated with the SiPMs. The average peak value of the single cell signal Vpeak as well as the gain variation M were determined from the amplitude spectrum of the dark pulses in the timing channel (see Section III. A). The single cell signal decay time d,SiPM was extracted from a fit to the average single cell signal. Since the averaging procedure, however, has a substantial influence on the shape of the single cell signal rising edge, the values for r,SiPM and the Gaussian shaping time constant were obtained by fitting the calculated scintillation pulse expectation value Vo to the average measured signal obtained on the timing channels (see Section III. A). The transient time spread tr was obtained by matching the calculated and measured CRT for LaBr3:5%Ce. This value should in principle contain all statistical fluctuations associated with the signal propagation, i.e. the photon transit time spread within the crystal as well as the contribution of the electronic signal propagation within the SiPMs. Because of the small size of the scintillation crystals, the SiPM single cell signal transit time spread is expected to be the dominant factor in tr. Here it must be noted, that the value quoted in the datasheet of the devices is considerably larger (180 ps) than the value determined in this work [4]. The data presented in this paper, however was recorded at significantly higher VVOB than recommended by the manufacturer (~ 0.5 V larger) and the transit time spread may be reduced significantly as VVOB is increased [7]. TABLE 2: MODEL INPUT PARAMETER ASSOCIATED WITH THE SIPMS Vpeak M d,SiPM r,SiPM shaping time tr 16.3 mV 10% 17.3 ns 1.3 ns 0.3 ns 90 ps
IV. RESULTS AND DISCUSSION A. Model Input parameters All input parameters associated with the scintillation materials are summarized in table 1. The values for r,sc and d,sc were taken from [7] and [8]. The mean number of primary discharges was determined using the SiPM response model reported in [Herman]. The values for the energy resolution are referring to the variation of the number of primary triggers and not to the variation of the measured output charge (which would be considerably higher). These values are based on the intrinsic energy resolution of the scintillators. Changing these values by a factor of 1.5, however, has only a marginal effect on the predicted timing resolution (~ 5 ps in the case of L(Y)SO and ~ 1 ps in the case of LaBr3:5%Ce).
B. Energy Discrimination Figure 2 a) shows typical traces recorded from the energy channels of the two detectors with LaBr3:5%Ce crystals for one coincidence. An energy spectrum obtained from the shaped energy signal is depicted in figure 2 b). The same figure also contains a Gaussian fit to the 511 keV photopeak. The FWHM of the fit was 2.8 % ± 0.5 % for the LaBr3:5%Ce detectors and 5.5 % ± 0.5 % for the L(Y)SO detectors, respectively. These values are not corrected for SiPM nonlinearity and are therefore strongly biased towards too low values. However, for the course of this paper we are only interested in determining whether a given 511 keV--photon deposited its full energy in the crystal. Such a classification
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a) Energy Signal
remainder of this document were obtained by employing a leading edge trigger (LET) to the interpolated traces. The differences between the time stamps of two detectors in coincidence were histogrammed and a Gaussian function was fitted to the resulting timing spectrum (see figure 4). The CRT was extracted as the FWHM of these fits. By repeating this procedure for different combinations of LET thresholds (Vthr), an optimum value of Vthr could be found for each detector. The CRT at optimum trigger settings was found to be 99.5 ps ± 0.6 ps for the LaBr3:5%Ce-SiPM detectors (at Vthr = 129 mV and Vthr = 122 mV for detector 1 and detector 2, respectively) and 171.5 ps ± 0.8 ps (at Vthr = 40 mV and Vthr = 34 for detector 1 and detector 2, respectively).
b) Energy Spectrum 900
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Fig. 2: a) Exemplary traces as recorded from the two energy channels for one coincidence event and b) energy spectrum with Gaussian fit (red line) to the 511 keV photo peak.
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t (ns) Fig. 3: Exemplary traces recorded from the timing channel of a LaBr3:5%Ce-SiPM detector. The solid line depicts the interpolation of the data points with a full cubic spline. The region which was averaged in order to determine the base line is marked in red.
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can be made from the uncorrected spectrum and for the following analysis an energy window corresponding to the full width at tenth maximum of the Gaussian fit was chosen. C. Timing Measurements Time stamps were derived from the digitized timing signals by interpolating each digitized trace with a full cubic spline. This is illustrated for an exemplary trace in figure 3. A baseline was determined for each trace individually as the average signal within 1 ns directly before the onset of the pulse (indicated in red in Figure 3.). This is especially important for measurements at higher bias voltages, where the timing performance is otherwise deteriorated by the influence of dark pulses. Furthermore this procedure drastically reduces the influence of low-frequency electronic noise. Two different time pick off methods were investigated. However, no appreciable difference in the obtained CRT was
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Fig 4: Timing spectra obtained with a) the LaBr3:5%Ce-SiPM detectors and b) the L(Y)SO-SiPM detectors. The red lines indicate Gaussian fits to the data.
At first sight, the fact that the values for Vthr,opt for the L(Y)SO-detectors are more than a factor of 3 smaller than for the LaBr3:5%Ce detectors may be somewhat surprising considering that the overall photoelectron yield is only about a factor of 2 lower. Figure 5 illustrates the different behavior of the two detector pairs in more detail. It shows how the CRT changes as Vthr is varied for both detectors. It can be seen that in the case of L(Y)SO the CRT is not only at much lower values for Vthr, but also it degrades much more rapidly as Vthr is changed for either one of the two detectors. Figure 6 shows the calculated CTRs as a function of Vthr in comparison to the measured data. Both the model predictions and the measured data are given for equal trigger thresholds on both detectors (i.e., corresponding to the diagonal in the planes depicted in figure 5). The two measurements presented for L(Y)SO differ only in the setting of an internal amplifier of the ADC resulting in a smaller dynamic range but also lower digitization noise in measurement 2. The input parameters for the corresponding model predictions are therefore identical except for the electronic noise contribution.
found [13]. The timing measurements presented in the
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V. SUMMARY AND CONCLUSIONS
a) LaBr3:5%Ce
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The results presented in this work demonstrate that excellent coincidence resolving times can be achieved with SiPM based scintillation detectors. After optimizing the bias voltage for optimum timing performance, CRTs of 99.5 ps ± 0.6 ps and of 171.5 ps ± 0.8 ps could be achieved with LaBr3:5%Ce and L(Y)SO crystals, respectively. Furthermore, a statistical model was introduced which predicts the uncertainty in timing experiments with SiPMbased scintillation detectors. The model predictions were found to be in good agreement with the measured data. By varying the input parameters in this model, the limiting factors in the timing performance of the detectors can be identified. The measurements with the L(Y)SO crystals appear to be limited by electronic noise and dark counts. The CRT obtained with the LaBr3:5%Ce detectors, on the other hand, are mostly limited by photon statistics. In this case the largest improvement could be expected if the scintillator rise time, and therefore the initial photon rate, could be improved (e.g. by increasing the Ce concentration [7]).
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V. REFERENCES [1]
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Fig 5: Coincidence resolving time as a function of the leading edge threshold level Vthr for a) LaBr3:5%Ce and b) L(Y)SO. [5]
[6] [7]
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W. W. Moses, “Recent advances and future advances in time-of-flight PET”, Nucl. Instrum. Methods Phys. Res. A, vol. 580, pp. 919-92 (2007) D. J. Kadrmas, M. E. Casey, M. Conti, B. W. Jakoby, C. Lois and D. W. Townsend “Impact of time-of-flight on PET tumor detection” J. Nucl. Med., vol. 50, pp. 1315-1323 (2009) G. Collazuol, G. Ambrosi, M. Boscardin, F. Corsi, G.F. Dalla Betta, A. Del Guerra, et al. “Single photon timing resolution and detection efficiency of the IRST silicon photo-multipliers”, Nucl. Instrum. Methods Phys. Res. A, vol. 581, pp. 461–464 (2007) MPPC data sheet of Hamamatsu Photonics: http://sales.hamamatsu.com/assets/pdf/parts_S/s1036233series_kapd1023e04.pdf G. Bizarri and P. Dorenbos, “Charge carrier and excitation dynamics in LaBr3:Ce3+ scintillators: Experiment and model”, Phys Rev. B, vol. 75, 184302 (2007) J.T.M. de Haas and P. Dorenbos, “Advances in yield calibration of scintillators”, IEEE TNS, vol 55, no 3, pp. 1086- 1092 (2008) J. Glodo, et. al, “Effects of Ce Concentration on Scintillation Properties of LbBr3:Ce, IEEE TNS, vol 52 no 5 (2005), 1805-1808 W. W. Moses, and S. E. Derenzo, “Prospects for Time-of-Flight PET using LSO Scintillator”, IEEE TNS, vol. 46, pp. 474–478 (1999) H.T. van Dam, S. Seifert, R. Vinke, P. Dendooven, H. Löhner, F. J. Beekman, D. R. Schaart, “Silicon Photomultiplier Response Model”, Nuclear Science Symp. Conf. Rec., 2009. NSS '09, Orlando, Fl, (J03-1) H.T. van Dam, et al., to be published Y. Shao, “A new timing model for calculating the intrinsic timing resolution of a scintillator detector”, Phys. Med. Biol., vol. 52, pp. 11031117 (2007) Y. Du, and F. Retière, “Afterpulsing and crosstalk in multi-pixel photoncounters,” Nucl. Instr. and Meth. A, vol. 596, pp. 396-401 (2008) R. Vinke, S. Seifert, D. R. Schaart, F. P. Schreuder, M. R. de Boer, H. T. van Dam, et al. “Optimization of Digital Time Pickoff Methods for LaBr3 –SiPM TOF PET Detectors”, Nuclear Science Symp. Conf. Rec., 2009. NSS '09, Orlando, Fl, (M06 -2)
