Performance Assessment of Pixelated LaBr3 Detector Modules for

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Performance Assessment of Pixelated LaBr3 Detector Modules for Time-of-Flight PET A. Kuhn, S. Surti, Member, IEEE, J. S. Karp, Senior Member, IEEE, G. Muehllehner, Fellow, IEEE, F. M. Newcomer, and R. VanBerg

Abstract—Our recent measurements with pixelated LaBr3 Anger-logic detectors for use in time-of-flight (TOF) PET have demonstrated excellent energy resolution (5.1% at 511 keV) and coincidence time resolution (313 ps full width at half maximum, FWHM) with small prototype configurations [1]. A full size detector module suitable for a whole-body 3D PET scanner has been constructed based on the prototype designs and consists of 1620 4 4 30 mm3 LaBr3 crystals. We have utilized simulations to guide experimental measurements with the goal of optimizing energy and time resolution in evaluating triggering configurations and pulse shaping needed in a full system. Experimental measurements with the detector module indicate energy and time resolution consistent with our earlier prototypes when measured at low count rate. At very high count rate the energy, time and spatial resolution degrade due to pulse pileup. While it is possible to reduce pulse pileup by using smaller photomultiplier tubes (i.e., 39 mm instead of 50 mm diameter), we are trying to limit the total number of PMTs needed for a full-scale PET scanner with a large axial field-of-view. Therefore, we have designed and tested a pulse shaping circuit to improve the detector response and performance at high count rate. Simulations of a complete LaBr3 scanner indicate significant improvements in noise equivalent count rate (NEC) and spatial resolution can be achieved using pulse shaping.

I. INTRODUCTION IME-OF-FLIGHT (TOF) measurement in PET has the potential to significantly improve scanner performance [2]–[4]. However, to realize this improvement requires utilizing a scintillator with high light output, high stopping power, fast decay time, and good linearity [5]–[7]. While scintillator development is on going [8]–[14], the possibility for TOF PET has been proposed for both LSO [15] and lanthanum scintillators [16]. Our current interest is in the LaBr scintillator. LaBr has very high light output ( photons per MeV), fast decay time ( ns) and excellent energy resolution (3.2% at 662 keV) [14] making it a viable detection material for TOF PET. We have recently demonstrated excellent performance of pixelated LaBr detectors in small prototype configurations.

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Manuscript received April 18, 2005; revised February 8, 2006. This work was supported in part by the DOE under Grant DE-FG02-88ER60642, in part by the National Institutes of Health under Grant R33EB001684, and in part by Saint-Gobain Crystals (research agreement). A. Kuhn, S. Surti, and J. S. Karp are with the Department of Radiology, University of Pennsylvania, Philadelphia, PA 19104 USA (e-mail: akuhn@mail. med.upenn.edu; [email protected]; [email protected]). G. Muehllehner is with Philips Medical Systems, Philadelphia, PA 19104 USA (e-mail: [email protected]). F. M. Newcomer and R. VanBerg are with the Department of Physics, University of Pennsylvania, Philadelphia, PA 19104 USA (e-mail: [email protected]. edu; [email protected]). Digital Object Identifier 10.1109/TNS.2006.873708

Fig. 1. (a) Arrangement of PMTs and crystals in the detector module and interaction location of two events (marked with a *). Light from the primary event is spread to the 7 shaded PMTs. The two PMTs with dashed outlines share light from both events. (b) Pulse shapes in the PMTs sharing light from both events. Shaded region indicates the signal added to the primary event.

These detectors consisted of multi-crystal arrays of LaBr (each crystal mm ) coupled to a continuous light guide and hexagonal arrangement of 50 mm photomultiplier tubes (PMTs). We measured an average energy resolution (at 511 keV) of 5.1% full width at half maximum (FWHM) for individual crystals in the detector and a coincidence time resolution of 313 ps FWHM between two LaBr (5% Ce concentration) Anger-logic detectors [1]. We have extended our design to a full size detector module suitable for use in a whole-body 3D PET scanner. Each detector module consists of 1620 individual 4 4 30 mm LaBr crystals (27 transverse by 60 axial crystals with a pitch of 4.3 mm, hermetically sealed and packed in a reflective powder by Saint-Gobain Crystals) coupled to a continuous light guide and hexagonal array of Photonis XP20Y0 PMTs (a fast, 8 stage version of the XP2020). Fig. 1(a) illustrates the hexagonal arrangement of PMTs and pixelated crystals in the detector module. Only a single detector module is shown in the figure. In the full scanner, the top and bottom rows of six axial PMTs would overlap adjacent detector modules (not shown). We have designed the continuous light guide to define light distribution with a width (FWHM) of one PMT diameter, so that an area encompassing a maximum of seven PMTs is illuminated by about 92% of the light from an interaction occurring in a crystal directly above a PMT center. As shown in Fig. 1(a), the light

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from the primary event is distributed to the seven shaded PMTs. These PMTs are then used to decode the events energy, time and position. However, in this design it is possible that the light from two or more events occurring in close proximity, spatial and temporally, will have overlapping regions of light spread. This is illustrated in Fig. 1(a) by the secondary event which would also spread light to the two shaded PMTs with dashed outlines. Fig. 1(b) shows the signals from a PMT sharing the light from both events. Because the pulse from the secondary event occurs within the integration period used to calculate the primary event energy, additional signal is added to the primary event (shown in the shaded region). The effects of pulse pileup can lead to degradation in energy, timing and spatial resolution. These issues are addressed here with respect to the detector module performance. II. DETECTOR MODULE TESTING In order to test energy resolution, coincidence time resolution and relative light output of individual crystals in the module as well as crystal discrimination, a data acquisition system utilizing both a time-to-digital converter (TDC) and charge integrating analog-to-digital converter (ADC) was implemented in a configuration similar to that utilized in reference [1]. A. Energy Resolution at 511 keV The energy resolution of individual crystals in the detector module was measured by applying software gating on event positions to generate a histogram of event energies (obtained by summing the charge in a local group of 7 PMTs) for the individual crystals. This was measured using an integration time of 60 ns in the charge integrating ADC, which is a sufficient period to measure nearly all of the light from LaBr (5% Ce) with an approximate decay time constant of 27 ns. To obtain the energy resolution as a function of event rate in the module, realistic source geometry was chosen which consisted of a 20 cm diameter by 30 cm long cylinder filled with F and water placed 35 cm from the front face of the detector. The average measured energy resolution for a group of 25 crystals near the center of a 7 PMT cluster in the detector module as a function of activity concentration in the cylinder is shown in Fig. 2(a). The measurement error bars represent the standard deviation in the average. Excellent energy resolution of better than 5.5% is achieved for activity concentrations less than 0.5 Ci/cc—well above the clinical range for FDG oncology studies. At very high activity concentration the energy resolution degrades due to pulse pileup in the detector module, as discussed in Section I. In addition to degrading energy resolution the pulse pileup causes a shift in the photopeak location. Fig. 2(b) shows the measured shift in photopeak energy as a function of activity concentration in the cylinder. No shift in the photopeak is measured at activity concentrations less than 0.4 Ci/cc. Fig. 3 shows the energy spectrum representative of activity concentrations up to 0.4 Ci/cc and the spectrum at 0.65 Ci/cc, clearly illustrating the shift in the photopeak. B. Coincidence Time Resolution The coincidence time resolution for crystals in the detector module was measured by utilizing a BC-418 plastic scintillator

Fig. 2. (a) Average measured energy resolution for 25 crystals near the center of a 7 PMT cluster and (b) the shift in the centroid of the photopeak as a function of activity concentration in a 20 cm diameter by 30 cm long uniform cylinder.

Fig. 3. Energy spectrum representative of activity concentrations up to 0.4Ci/cc (solid line) and the spectrum at 0.65  Ci/cc (dashed line).

coupled to a XP2020 PMT to form the start channel to the TDC. With a Ge point source placed between the two detectors and position gating events in software, coincidence time spectra were generated for individual crystals in the detector module. The time resolution was measured for crystals at various locations in the detector module. Fig. 4 illustrates three measured positions in the module. By utilizing the signals from a group of 7 PMTs, light collection is maximized and thus the time resolution is improved. For example, the crystal at position 1 [see

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Table I MEASURED TIME RESOLUTION AND RELATIVE LIGHT COLLECTED USING 1 PMT (WITH SOLID OUTLINE) AND THE SUMMED SIGNAL FROM 7 PMTS AT THE THREE POSITIONS ILLUSTRATED IN FIG. 4. THE LIGHT COLLECTED IS NORMALIZED TO THE CRYSTAL AT POSITION 1 USING THE 7 PMT SUM

Fig. 4. Detector module indicating crystals in three measured positions. Time resolution measurements were made utilizing the signal from (solid outline) a single PMT and using the sum of (solid and dashed outlines) the 7 shaded PMTs.

Fig. 4] distributes all of its light to the region covered by the seven shaded PMTs and only 73% to the PMT directly above it (shown with a solid outline). The corresponding time resolution measured utilizing the signals from the group of 7 PMTs is 295 ps FWHM and 350 ps FWHM using only the single PMT as listed in Table I. By using a local group of seven PMTs to determine the event time, the three measured positions represent the range in light collection for crystals throughout the detector. Therefore, a small variation, on the order of 40 ps, in time resolution is expected over the detector module (measured region is 295–338 ps FWHM). Additionally, the time resolution was measured as a function of event rate. By utilizing additional sources of 511 keV radiation, the event rate in the detector module was increased incrementally. Fig. 5 shows the average measured time resolution for a group of nine crystals positioned above the center of a group of 7 PMTs as a function of the rate of events depositing more than 400 keV in the 7 PMT energy sum. An event rate of 100 thousand counts per second (kcps) corresponds to the rate meaCi/cc in the 20 cm sured at an activity concentration of diameter by 30 cm long cylinder. As the event rate increases the time resolution degrades due to pulse pileup. The dashed curve in Fig. 5 gives the time resolution measured by including 16 neighboring PMT signals in determining the event time, while still using 7 PMTs in energy and position determination. This configuration was investigated to determine the effects of decreasing the number of trigger channels needed for a full scanner (i.e., increasing the number of PMTs per trigger) on the time resolution. However, this further degrades time resolution due to the additional region for spatial event pileup. C. Pulse Shaping We have constructed a preliminary pole-zero cancellation circuit with the goal of reducing the effects of pulse pileup, and thus improving the detector response at very high event rates.

Fig. 5. Measured time resolution as a function of rate of events with energy >400 keV in a 7 PMT cluster. A rate of 100 kcps corresponds to the rate measured with the 20 cm diameter by 30 cm long cylinder at an activity concentration of 0:75 Ci/cc.



A diagram of the circuit is shown in Fig. 6(a). This circuit is similar to the pulse shaping implemented for NaI(Tl) detectors in reference [17], however, our current design does not include any RC-pole shaping in order to preserve the fast leading edge of the LaBr pulse and excellent time resolution. Measured pulse shapes (at 511 keV) from a single LaBr crystal coupled directly to the PMT photocathode with and without pulse shaping are shown in Fig. 6(b). The shaping circuit reduces the pulse width . This reduces the overlap in pulses [shown in Fig. 1(b)] by allowing a shorter integration period to be utilized, and thus reducing pulse pileup. Initial energy resolution measurements have been made with the shaping circuit implemented in a single PMT setup. A degradation in energy resolution was measured, 4.25% at 511 keV with pulse shaping as compared to 3.5% without shaping, for a single 13 mm diameter by 13 mm long cylindrical LaBr crystal directly coupled to a PMT using an integration time of 30 ns and 60 ns, respectively. An accurate measurement of energy resolution incorporating the shaping circuit with a group of PMTs in the detector module requires further development of signal processing electronics that are currently underway. Additionally, it is important that the fast rise of the pulse is preserved with the shaping circuit thus maintaining excellent timing characteristics. A timing resolution measurement utilizing the shaping circuit and signals from a group of 7 PMTs, as described in Section II-B, was performed. Fig. 7 shows the average measured time resolution for a group of nine crystals near the center of a 7 PMT cluster as a function of the rate of events depositing more than 400 keV in the 7 PMT sum. By using

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Fig. 8. Energy resolution simulated for the LaBr scanner (with 50 mm diameter PMTs) as a function of activity concentration in 20 cm diameter by 30 cm long uniform cylinder with and without pulse shaping.

Fig. 6. (a) Diagram of the pulse shaping circuit. (b) Measured pulse shapes (at 511 keV) from a single LaBr crystal coupled directly to a PMT photocathode with and without pulse shaping.

Fig. 9. Scanner energy spectra at low and high event rates. True events are pushed out and additional scattered events are pushed into the fixed energy window at high rate due to pulse pileup.

Fig. 7. Time resolution measured utilizing the shaping circuit as a function of rate of events with energy > 400 keV in 7 PMT s. A rate of 100 kcps corresponds to the rate measured with the 20 cm diameter by 30 cm long cylinder at an activity concentration of 0:75 Ci/cc.



pulse shaping, no degradation in time resolution is measured up kcps. Compare this to Fig. 5, without pulse shaping, to which shows degradation from 295 ps to 320 ps FWHM. Additionally, a time resolution of less than 320 ps FWHM was kcps achieved using pulse shaping at rates as high as (which corresponds to about 1.8 Ci/cc in the 20 cm diameter by 30 cm long cylinder). III. SIMULATED LABR SCANNER PERFORMANCE In order to relate the measured detector module performance to that of the full system, simulations of a full LaBr scanner were performed. The details of the simulation process are similar to those outlined in reference [18]. The scanner simulated

has an axial field-of-view of 25 cm and a diameter of 84 cm (with a 65 cm port). The detector was modeled as a pixelated Anger-logic detector using 4 4 30 mm LaBr crystals with a crystal pitch of 4.3 mm. A fixed energy window of 470 to 665 keV was used to determine valid events, with a coincidence window of 5 ns and no electronics dead time. The system simulation was performed with and without the implementation of pulse shaping (described in Section II-C). Fig. 8 shows the results of scanner energy resolution as a function of activity concentration in a 20 cm diameter by 30 cm long uniform cylinder. As the event rate in the scanner increases, the energy resolution degrades due to pulse pileup. This matches the results measured with the detector module shown in Fig. 2(a). With the implementation of pulse shaping the degradation in energy resolution is reduced. The effects of pulse pileup, on energy resolution and shifting of the photopeak, will degrade scanner performance by causing more true events to fall outside the fixed energy window and additional scattered events to fall inside the window. This is illustrated in Fig. 9 showing the scanner energy spectra representative of low and high event rates. The results of simulated noise equivalent count-rate (NEC), adjusted for the scatter fraction by calculating the net loss of true events and net gain in scatter events with the fixed energy window, is shown in Fig. 10 for four scanner configurations as a function of activity concentration in a 20 cm diameter by 70 cm

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IV. CONCLUSION

Fig. 10. Scatter fraction (SF) adjusted noise equivalent count-rate (NEC) simulated for four scanner configurations as a function of activity concentration in a 20 cm diameter by 70 cm long cylinder. In each configuration the LaBr scanner simulated had an axial field-of-view of 25 cm and a diameter of 84 cm.

Our results indicate that the excellent energy and time resolution we obtained with small LaBr prototype detectors can be extended to a 1620 crystal detector module suitable for use in a whole body 3D TOF PET scanner. Measured energy resolution of better than 5.5% at 511 keV was achieved for activity conCi/cc. Coincidence time resolution of centrations up to better than 310 ps FWHM was measured for event rates well beyond those expected clinically (rates corresponding to activity concentrations up to 1.2 Ci/cc). By utilizing the signal from a hexagonal cluster of 7 PMTs, uniform light collection as well as a small variation in time resolution was measured. A pole-zero cancellation circuit was designed and tested while preserving which reduced the pulse shape width by the measured energy and time resolution of the detector. At high event rate, the measured time resolution was improved by the pulse shaping. Simulations of a full LaBr scanner indicate a significant improvement in NEC and spatial resolution is expected at high event rate using pulse shaping. The excellent energy and time resolution measured with the detector module indicate that a large, continuous, LaBr Angerlogic detector can be used to construct a high performance 3D whole-body PET scanner with the capability of incorporating TOF information. ACKNOWLEDGMENT The authors would like to thank G. Mayers and R. Kulp from the University of Pennsylvania for constructing the pulse shaping circuits and the research members at Saint-Gobain for their continued support. REFERENCES

Fig. 11. Simulated reconstructed spatial resolution for a point source in air as a function of the background activity concentration in the LaBr scanner.

long uniform cylinder. This measure of NEC does not include any potential gain in performance by the incorporation of TOF information. While it is possible to reduce the effect of pulse pileup at high event rates and improve NEC by using smaller PMTs (i.e., 39 mm instead of 50 mm), it is a practical advantage to limit the total number of PMTs needed in a full scanner with a large axial field-of-view. Therefore, by utilizing 50 mm PMTs with pulse % at high event shaping the expected NEC can be increased rates becoming equal to using 39 mm PMTs. Additionally, the spatial resolution for the scanner was calculated for a point source. Fig. 11 shows the reconstructed spatial resolution, as a function of the background activity concentration, calculated with NEMA NU2-2001 analysis [19] and image reconstruction using a 3D-FRP algorithm. Again the use of pulse shaping with 50 mm PMTs improves the predicted spatial resolution to the level achieved with 39 mm PMTs.

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[13] C. W. E. V. Eijk, “New inorganic scintillators- aspects of energy resolution,” Nucl. Instrum. Meth. A., vol. 471, pp. 244–248, 2001. [14] K.S. Shah, J. Glodo, M. Klugerman, W. W. Moses, S. E. Derenzo, and M. J. Weber, “LaBr :Ce Scintillators for gamma ray spectroscopy,” IEEE Trans. Nucl. Sci., vol. 50, no. 14, pp. 2410–2413, Dec. 2003. [15] W. W. Moses and S. E. Derenzo, “Prospects for time-of-flight PET using LSO scintillator,” IEEE Trans. Nucl. Sci., vol. 46, no. 3, pp. 474–478, Jun. 1999. [16] S. Surti, J. S. Karp, G. Muehllehner, and P. S. Raby, “Investigation of lanthanum scintillators for 3-D PET,” IEEE Trans. Nucl. Sci., vol. 50, no. 3, pp. 348–354, Jun. 2003.

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[17] R. Freifelder, J. S. Karp, J. A. Wear, N. S. Lockyer, F. M. Newcomer, S. Surti, and R. V. Berg, “Comparison of multi-pole shaping and delay line clipping pre-amplifiers for position sensitive NaI(Tl) detectors,” IEEE Trans. Nucl. Sci., vol. 45, no. 3, pp. 1138–1143, Jun. 1998. [18] S. Surti, J. S. Karp, and G. Muehllehner, “Image quality assessment of LaBr -based whole-body 3D PET scanners: A Monte Carlo evaluation,” Phys. Med. Biol., vol. 49, pp. 4593–4610, 2004. [19] Performance Measurements of Positron Emission Tomographs. (in NEMA Standards). Rosslyn, VA: National Electrical Manufacturers Association, 2001, Publication NU 2-2001.

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