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Pushing Cherenkov PET with BGO Via Coincidence Time Resolution

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Pushing Cherenkov PET with BGO Via Coincidence Time Resolution Phys. Med. Biol. 65 (2020) 115004 https://doi.org/10.1088/1361-6560/ab87f9 Physics in Medicine & Biology PAPER Pushing Cherenkov PET with BGO via coincidence time resolution OPEN ACCESS classification and correction RECEIVED 10 January 2020 Nicolaus Kratochwil1,2, Stefan Gundacker1,3, Paul Lecoq1 and Etiennette Auffray1 REVISED 1 23 March 2020 CERN, Esplanade des Particules 1, 1211 Meyrin, Switzerland 2 University of Vienna, Universitaetsring 1, 1010 Vienna, Austria ACCEPTED FOR PUBLICATION 3 UniMIB, Piazza dell’Ateneo Nuovo, 1-20126 Milano, Italy 8 April 2020 E-mail: [email protected] PUBLISHED 2 June 2020 Original Content from Abstract this work may be used under the terms of the Bismuth germanate (BGO) shows good properties for positron emission tomography (PET) Creative Commons Attribution 3.0 licence. applications, but was substituted by the development of faster crystals like lutetium oxyorthosilicate Any further distribution (LSO) for time-of-flight PET (TOF-PET). Recent improvements in silicon photomultipliers of this work must (SiPMs) and fast readout electronics make it possible to access the Cherenkov photon signal maintain attribution to the author(s) and the title produced upon 511 keV interaction, which makes BGO a cost -effective candidate for TOF-PET. of the work, journal citation and DOI. Tails in the time -delay distribution, however, remain a challenge. These are mainly caused by the high statistical fluctuation on the Cherenkov photons detected. To select fast events with a high detected Cherenkov photon number, the signal rise time of the SiPM was used for discrimination. The charge, time delay and signal rise time was measured for two different lengths of BGO crystals coupled to FBK NUV-HD SiPMs and high frequency readout in a coincidence time resolution setup. The recorded events were divided into 5 × 5 categories based on the signal rise time, and time resolutions of 200 ± 3 ps for 2 × 2 × 20 mm3 and 117 ± 3 ps for 2 × 2 × 3 mm3 were measured for the fastest 20% of the events (4% in coincidence). These good timing events can provide additional information for the image reconstruction in order to increase the SNR significantly, without spoiling the detector sensitivity. Putting all photopeak events together and correcting for the time bias introduced by different numbers of Cherenkov photons detected, time resolutions of 259 ± 3 ps for 20 mm long and 151 ± 3 ps for 3 mm long crystals were measured. For a small fraction of events sub-100 ps coincidence time resolution with BGO was reached for a 3 mm short pixel. 1. Introduction In the past, bismuth germanate (Bi4Ge3O12) used to be a good choice for positron emission tomography (PET) in terms of its high density, stopping power Moses (2007), high photo fraction and low production cost. However, due to its slow decay time and low light yield it was substituted by lutetium -based (LSO, LYSO, LGSO) scintillating crystals for time-of-flight PET (TOF-PET) to ensure a fast time resolution for noise equivalent count -rate improvement and, hence, better image quality. Nowadays, the appearance of fast silicon photomultipliers (SiPMs) with good photon detection efficiency (PDE) in the near ultraviolet and low single photon time resolution (SPTR) values combined with fast readout electronics allows to make the best use of the Cherenkov signal produced in BGO for a 511 keV interaction Kwon et al (2016), Piemonte et al (2016), Brunner and Schaart (2017), Cates et al (2018), Gundacker et al (2019), Cates and Levin (2019). The detection of few prompt photons gives a significant improvement in time resolution, and is one possible pathway towards a time resolution of 10 ps for TOF-PET Lecoq (2017), Gundacker et al (2016). BGO has a density of 7.13 g cm−3 Moses (2007), intrinsic light yield of 10.7 photons per keV and a decay time of around 360 ns Gundacker et al (2020), which makes it much slower than LSO:Ce codoped with Ca. The mean Cherenkov photon yield upon 511 keV interaction for BGO is about 17 ± 3 photons between 310–850 nm Gundacker et al (2020). Considering the light transfer efficiency (LTE) from the crystal to the photo detector (LTE = 0.58 for 2 × 2 × 3 mm3 pixel, Teflon wrapped and optically coupled with high © 2020 Institute of Physics and Engineering in Medicine Phys. Med. Biol. 65 (2020) 115004 N Kratochwil et al BGO signal with Cherenkov BGO signal pure scintillation measured rise time with Cherenkov measured rise time pure BGO scintillation 50 mV time delay SiPM signal [mV] single SPAD 10 mV time [ps] Figure 1. (left) Schematics of the CTR-setup: a 22Na source is placed between two crystals coupled to SiPMs. The signal is split for the energy and time information. (right) The principle of the rise time separation: for events where only the BGO scintillation light is detected the signal needs longer to rise up (blue) compared to the signal where Cherenkov photons are detected at the beginning of the signal (red). refractive index materials) and the photo detection efficiency of the detector (39% for the FBK NUV-HD, averaged over Cherenkov emission between 310–850 nm) around four Cherenkov photons (17 × 0.58 × 0.39) are on average detected per 511 keV interaction Gundacker et al (2020). Due to non-isotropic emission from an event to event basis, the number of detected Cherenkov photons underlies high fluctuations Gundacker et al (2020). These fluctuations are responsible for the long tails in the 511 keV coincidence delay histogram, when events with very few or no Cherenkov photons are detected, hence only the BGO scintillation is used to determine the time stamp. To suppress these tails and to better determine the timestamps, a discrimination between events with high numbers of Cherenkov photons and low numbers of Cherenkov photons is necessary. One possible way to carry out the event separation is by employing a double-sided readout, either with two SiPMs Kwon et al (2019) or by combining a SiPM and a micro channel plate photomultiplier to select only fast events. One drawback of this method is the need for an additional readout channel to exploit the presence of Cherenkov photons and, therefore, distinguish between events giving a good or worse time resolution. In this contribution, we introduce a single -sided readout approach to separate events by the use of the signal rise time of the SiPM, which is correlated with the photon density and, therefore, connected to the time resolution of the events. It was already demonstrated that, for a sampling pixel composed of BGO and plastic, the signal rise time allows a separation between BGO and plastic events Turtos et al (2019). 2. Material and methods 2.1. Coincidence time resolution (CTR) setup A 22Na source with an activity of 260 kBq (Nov. 2019) emits two gammas with 511 keV back to back. The gammas are detected in coincidence by two BGO crystals wrapped in Teflon and glued with Meltmount (refractive index n = 1.582) to 4 × 4 mm2 FBK NUV-HD SiPMs with 40 µm2 single photon avalanche diode (SPAD) size. The SiPMs are read out by the high frequency (HF) electronics for the time signal and an analog operational amplifier for the energy signal. The signals are digitized by a LeCroy DDA735Zi oscilloscope with 3.5 GHz bandwidth and a sampling rate of 20 Gs s−1 for four channels. The crystals, SiPMs and the HF ◦ electronics are in a light tight box which is kept stable at 16 C . An illustration of the setup is shown on the left of figure 1. A detailed description of the HF readout is discussed in Gundacker et al (2019). 2.2. Data recording The energy signal of both channels is integrated over a width of 160 ns and stored for offline photopeak selection. For the time signal the leading edge threshold is set on the oscilloscope calculating the signal crossing via interpolation. To find the best configuration, SiPM bias voltage and leading edge threshold scans were performed. The single SPAD amplitude is 44 mV for the best performing SiPM bias of 39 V (10.8 V overvoltage). In addition, the signal rise time of each channel between a 10 mV and 50 mV threshold was recorded and stored. The lower value of the rise time threshold was set to be well above the electronic noise floor, while the upper value was set to be above the signal amplitude of a single SPAD. In total, the charge and signal rise time per channel and the coincidence time delay were stored for each event and analyzed offline. 2 Phys. Med. Biol. 65 (2020) 115004 N Kratochwil et al 2.3. Energy selection The charge-histogram was fitted with a Gaussian function around the photopeak. All events that had an energy lower than 440 keV and higher than 665 keV were rejected. This energy range was taken to be consistent with clinical TOF-PET scanners Kolthammer et al (2014). Using a narrower photopeak selection mainly decreases the number of events without changing the time resolution but for a few picoseconds. The impact on the time resolution for different energy windows is discussed in section 4.1. 2.4. Event identification based on signal rise time The motivation for measuring the signal rise time is that for events with higher numbers of Cherenkov photons the photon density at the very beginning of the scintillation signal is higher, resulting in better time resolution. We call this type of events fast events. In contrast, for pure BGO scintillation, the photon density is lower resulting in a relatively flatter SiPM signal (slow event) and worse time resolution.
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