Development and characterization of a [formula omitted]E-TOF detector prototype for the FOOT experiment

There has recently been renewed interest in time-of-flight (TOF) PET due to the availability of fast scintillators that also have high light output and high stopping power, as well as cost-effective fast photomultiplier tubes and stable electronics. Early results with these TOF-PET systems have shown both an improved contrast/noise trade-off and faster convergence compared with reconstructions without TOF information. Simulations have predicted further improvement in imaging performance with better timing resolution. A prototype whole-body PET scanner incorporating Ce-doped LaBr <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> crystals and specialized timing circuitry to take advantage of the scintillator’s fast timing characteristics has recently been completed. The intrinsic performance of the scanner has been measured. The average energy resolution over all crystals is 6.5%, and the system timing resolution is 375 ps. The scatter fraction for 20-, 27-, and 35-cm diameter cylinders is 21, 27, and 32%, respectively, for a 485-keV lower energy threshold. The average spatial resolution is 5.8 mm at 1 cm and 6.5 mm at 10 cm. Resolution modeling has been incorporated into the list-mode TOF iterative algorithm. Simulation studies were carried out to measure the relative impact of timing resolution, energy resolution and lower energy threshold, and spatial resolution modeling on TOF-PET imaging performance as characterized by the contrast/noise trade-off. It was found that improved timing resolution leads to faster, more uniform convergence with less variability in quantification as a function of either radial position or local activity environment. Better energy resolution allows for the use of a tighter energy window, which leads to fewer accepted scatter events and improved quantitative accuracy. Resolution modeling improves contrast recovery at the cost of slower convergence; further work is needed to define an accurate model of the point spread function for the LaBr <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> system. The superior timing and energy resolutions appear to mitigate the loss of spatial resolution that arises from the lower stopping power of the crystal.

Using Bismuth Germanate (BGO) scintillators coupled to general purpose analog Silicon Photomultipliers (SiPM) we have developed methods to improve the coincidence timing resolution of PET detectors with BGO in order to reach a timing resolution that can be used for time of flight (TOF) PET detection. TOF measurements have already been acquired using LYSO crystals, but the low cost of BGO along with its optical properties and favorable energy resolution make BGO an attractive option to reduce the overall cost of PET scanners without sacrificing image quality. Using an analog SiPM with a pre-amplifier at a higher voltage improves energy and timing resolution for BGO coincidence detection, but also introduces a much higher baseline noise level, which can obscure the start of the waveform. We have found this noise level can be significantly reduced by cooling the detectors to values below -30°C, allowing us to perform leading edge discrimination (LED) timing at much lower voltages. On average cooling the detectors to this point improves their timing resolution by around 300ps, and as much as 600ps. Using this setup our values are approaching the values necessary for time of flight PET detection. In this paper we will discuss the experimental setup in further depth and share promising initial timing and energy resolution results from our findings.

The Drift Chamber detector of the FOOT experiment: Performance analysis and external calibration

Our recent measurements with pixelated LaBr/sub 3/ 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 FWHM) with small prototype configurations. 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/spl times/4/spl times/30 mm/sup 3/ LaBr/sub 3/ 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 PMTs (i.e., 39 mm instead of 50 mm), 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 implemented a pulse shaping circuit to improve the detector response and performance at high count rate. Simulations of a complete LaBr/sub 3/ scanner indicate significant improvements in NEC and spatial resolution can be achieved using pulse shaping.

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket mission which will study particle acceleration and coronal heating on the Sun through unprecedented high-resolution imaging in the hard X-ray energy band (5-15 keV). Energy release occurring in the quiet region of the Sun may potentially play an important role in the coronal heating mechanism. With a combination of high-resolution focusing X-ray optics and fine-pitch imaging sensors, FOXSI will achieve superior sensitivity; two orders of magnitude better than that of the RHESSI satellite. FOXSI requires the spectral capability down to 5 keV, which requires a development of a new ASIC and detector with a better energy resolution. We plan to use a Double-sided Si Strip Detector (DSSD) with a low-noise front-end ASIC as the FOXSI focal plane detector, which will fulfill the scientific requirements on the spatial resolution, energy resolution, lower threshold energy and time resolution. We have designed and fabricated a DSSD with a thickness of 500 ¿m and a dimension of 9.6 mm×9.6 mm, containing 128 strips separated by a pitch of 75 ¿m, which corresponds to 8 arcsec at the focal length of 2 m. The DSSD was successfully operated in a laboratory experiment. Under a temperature of -20°C and a bias voltage of 250 V, we obtained spectra from both sides of the electrodes. The energy resolution was measured to be 980 eV and 2.4 keV for the p-side and n-side at 14 keV gamma-ray, sufficient for the FOXSI mission requirement.

A universal test platform for research and development of planar radiation detectors for use in medicine has been created having the possibility of obtaining complete information about the characteristics of the detector before creating a detecting module with specialized electronics. 5 jobs were created with the ability to perform the following work: measurement of the electrophysical characteristics of individual detectors and selected detection elements of the matrices based on the station with manual movement of microprobes; determination of the yield of suitable detector elements of matrices using a test station with automated microprobe movement and automated collection and processing of electrophysical measurement data; measurement of the energy resolution (ER) of individual silicon detectors, “scintillator-silicon photosensor” type detectors and measurement of the ER of selected matrix elements before creating a detection module with specialized electronics.

We are currently developing a small animal positron emission tomography (PET) scanner with a design goal of 1 microlitre (1 mm3) image resolution. The detectors consist of a 12 × 12 array of 1 × 1 × 10 mm lutetium oxyorthosilicate (LSO) scintillator crystals coupled to a 64-channel photomultiplier tube (PMT) via 5 cm long optical fibre bundles. The optical fibre connection allows a high detector packing fraction despite the dead space surrounding the active region of the PMT. Optical fibre bundles made from different types of glass were tested for light transmission, and also their effects on crystal identification and energy resolution, and compared to direct coupling of the LSO arrays to the PMTs. We also investigated the effects of extramural absorber (EMA) in the fibre bundles. Based on these results, fibre bundles manufactured from F2 glass were selected. We built three pairs of prototype detectors (directly coupled LSO array, fibre bundle without EMA and fibre bundle with EMA) and measured flood histograms, energy resolution, intrinsic spatial resolution and timing resolution. The results demonstrated an intrinsic spatial resolution (FWHM) of 1.12 mm (directly coupled), 1.23 mm (fibre bundle without EMA coupling) and 1.27 mm (fibre bundle with EMA coupling) using an approximately 500 μm diameter Na-22 point source. Using a 330 μm outer diameter steel needle line source filled with F-18, spatial resolution for the detector with the EMA optical fibre bundle improved to 1.05 mm. The respective timing and energy FWHM values were 1.96 ns, 21% (directly coupled), 2.20 ns, 23% (fibre bundle without EMA) and 2.99 ns, 30% (fibre bundle with EMA). The peak-to-valley ratio in the flood histograms was better with EMA (5:1) compared to the optical fibre bundle without EMA (2.5:1), due to the decreased optical cross-talk. In comparison to the detectors used in our current generation microPET scanner, these detectors substantially improve on the spatial resolution, preserve the timing resolution and provide adequate energy resolution for a modern high-resolution animal PET tomograph.

A novel energy resolved neutron imaging detector based on a time stamping optical camera for the CSNS

Time-of-flight (TOF) information can improve the imaging quality of positron emission tomography (PET) camera by calculating the original positron-electron annihilation position. In this work, we measured the preliminary TOF resolution of non-TOF-optimized LYSO arrays for TOF PET using our PMT-Quadrant-Sharing (PQS) technology which has higher light collection efficiency (no external light guide). We investigated digital signal processing methods that may achieve better time resolution. Digital oscilloscope Tektronix DPO7054 was used to capture the waveforms. Three timing methods, Linear Fitting (LF) to the pulse rising time, Constant Fraction Discriminator (CFD) and Leading edge Discriminator (LD), were studied. A new Time-Energy (T-E) correction using Multivariate Least Mean Square Fit was applied to minimize time walk caused by the variation of signal shape. The time resolution as a function of the depth of interaction was also studied. The average block-to-block coincident timing resolution of 13×13 (4×4×20 mm3 detector pixel) array with fast PMT R9779 (51-mm diameter) was measured to be 432 ps (FWHM). Another 13×13 crystal array with ultrahigh spatial resolution 1.4×1.4×10 mm3 detector pixels coupling to small 19-mm diameter poor-timing XP1912 PMTs was also tested and achieved 551 ps. This study shows that our regular PQS detector blocks (without optimization for TOF) have good TOF time resolution suitable for TOF PET systems, in addition to higher spatial resolution and reducing PMT cost.

The high time resolution detectors are relevant in those experiments or simulations were the particles to detect, have a very short time of flight (TOF), and due this it´s required that the detections times are ranged between ns. &amp; ps.Using Geant4 software, it was made thirty simulations of coupled detectors to plastic scintillators with two silicon photomultipliers (SiPMs) located on the scintillator’s central sides. To characterize the time resolution, it was required to quantify the optical photons that reach the Score in a certain time, which are generated by muons on the surface of the plastic scintillator. Different configurations of muon beams were simulated at energy of 1 GeV, to interact with the configuration of the scintillator material of its corresponding arrangement. The simulations were made varying three parameters: the scintillator material “BC404 &amp; BC422”, its size, and the location of the radiation source. Fifteen simulations correspond to BC404 material &amp; fifteen simulations to BC422 material respectively. The first five simulations consisted in varying the scintillator’s volumetric size and collocate the muons beam guided randomly distributed over it, the next five simulations differentiate from setting up a directly centered beam, and the last five simulations for guide the beam on the left lower corner of each scintillator.The best time resolution achieved was σ= 8.67 +/− 0.26 ps., reported by the detector with BC422 scintillator material which has a volume of 20x20x3 mm3.

The scintillation light distribution produced by photodetectors in positron emission tomography (PET) provides the depth of interaction (DOI) information required for high-resolution imaging. The goal of positioning techniques is to reverse the photodetector signal’s pattern map to the coordinates of the incident photon energy position. By considering the DOI information, monolithic crystals offer good spatial, energy, and timing resolution along with high sensitivity. In this work, a supervised deep neural network was used for the approximation of DOI and to assess through Monte Carlo (MC) simulations the performance on a small-animal PET scanner consisting of ten 50 × 50 × 10 mm3 continuous Lutetium-Yttrium Oxyorthosilicate doped with Cerium (LYSO: Ce) crystals and 12 × 12 silicon photomultiplier (SiPM) arrays. The scintillation position was predicted by a multilayer perceptron neural network with 256 units and 4 layers whose inputs were the number of fired pixels on the SiPM plane and the total deposited energy. A GEANT4 MC code was used to generate training and test datasets by altering the photons’ incident position, energy, and direction, as well as readout of the photodetector output. The calculated spatial resolutions in the X-Y plane and along the Z-axis were 0.96 and 1.02 mm, respectively. Our results demonstrated that using a multilayer perceptron (MLP)-based positioning algorithm in the detector modules, constituting the PET scanner, enhances the spatial resolution by approximately 18% while the absolute sensitivity remains constant. The proposed algorithm proved its ability to predict the DOI for depth under 7 mm with an error below 8.7%.

Silicon drift photodetectors for scintillation readout in medical imaging

Simultaneous PET/MRI combines two powerful and complementary modalities, providing multiparametric information to assess the anatomic as well as biochemical basis of disease. However, the high cost of the current commercial integrated PET/MRI systems limit the long-term potential, accessibility, and availability of PET/MRI. A portable PET insert provides a cost- effective alternative to achieve simultaneous PET/MRI for the sites that are already equipped with MR systems. Therefore, we are developing a radiofrequency (RF)-penetrable MR-compatible time-of-flight (TOF)-PET insert that can acquire simultaneous PET/MR data using the MR systems built-in body coil for RF transmission. In this study, we characterized performance of two detector modules in the MR bore under four different MRI conditions: B 0 only, echo planar imaging (EPI), gradient echo (GRE), and fast spin echo (FSE), each running for 15 minutes. The one-hour study results for coincidence timing resolution, energy resolution, coincidence count rate and ASIC chip temperature of two detector modules demonstrate the PET performance stability as well as the MR compatibility of the PET detectors under different MRI sequences. The global coincidence timing and energy resolution achieved was 264.21 ± 4.82 ps FWHM and 13.66 ± 0.08 % FWHM, respectively.

Lutetium oxide <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{Lu}_{2} \mathrm{O}_{3})$</tex>, known for its high density <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(9.4 ~\mathrm{g} / \text{cm}^{3})$</tex>, is a promising scintillation host material for detecting annihilation photons (511 keV) in positron emission tomography (PET) systems. However, the practical applications of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Lu}_{2} \mathrm{O}_{3}$</tex>-based scintillators have been limited by challenges in crystal growth, crystal brightness and low light yield. Recent advancements in ceramic processing techniques have enabled the fabrication of transparent <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Lu}_{2} \mathrm{O}_{3}$</tex> ceramics, while their brightness has been enhanced through controlled doping strategies. In this study, we evaluate the performance of a newly developed <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Lu}_{2} \mathrm{O}_{3}$</tex>-based ceramic scintillator for PET applications. The new ceramic scintillator, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\text{Lu}, \mathrm{Y})_{2} \mathrm{O}_{3}$</tex>: La, is a modified version of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Lu}_{2} \mathrm{O}_{3}$</tex>-based material mixed with yttrium (Y) and doped with lanthanum (La). Produced ceramic disks were cut into small pieces of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3 \times 3 \times 5 ~\text{mm}^{3}$</tex> in size. The performance of these samples, including decay time, energy resolution, and coincidence timing resolution (CTR), was evaluated and compared with that of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{Lu}_{2} \mathrm{O}_{3}: \text{Yb}$</tex> samples of the same size. The decay time was evaluated by measuring the waveforms from the sample, which was mounted on an R9800 photomultiplier and irradiated with 511 keV photons from a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">${ }^{22} \text{Na}$</tex> source. The energy and timing resolutions were measured with the sample mounted on a silicon photomultiplier (SiPM) and placed in coincidence with a reference LYSO <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(3 \times 3 \times 5 ~\text{mm}^{3})$</tex> detector. The developed ceramic scintillators exhibited a multi-exponential decay profile, characterized by a fast component ranging from 29.5 to 52.5 ns and a slower component between 231 and 837 ns, attributed to variations in Y and La concentrations across five samples. The best energy resolution of 15.3% at 511 keV and CTR of 202 ps full width at half maximum (FWHM) were observed in the sample with the slowest decay component. The developed (Lu, Y)<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf>: La composition demonstrated promising potential for time-of-flight (TOF) PET applications.

Digital Light Processing (DLP) 3D printing technique can be a powerful tool to fabricate plastic scintillator with a geometrically desired shape in innovatively fast time. Plastic scintillator with the size of 30 mm × 30 mm × 10 mm was fabricated by using the plastic resin and the DLP 3D printer (ASIGA, Pico2HD). The characteristics of decay time, energy resolution, intrinsic detection efficiency were analyzed and compared between the fabricated 3D printing plastic scintillator and a commercial plastic scintillator BC408 (Saint-Gobain Crystal). Decay time profile of the tested plastic scintillators was measured for 137Cs Compton maximum electron 477 keV by using a modified time correlated single photon counting (TCSPC) setup. The time profile was fitted by reconvolution function, and each decay time component and contribution was analyzed. For energy resolution of plastic scintillator, the Gaussian spectrum for 137Cs Compton maximum electron 477 keV was selectively measured by using the γ-γ coincidence experimental setup. As a result, it was confirmed that the 3D printing plastic scintillator showed average decay time 15.6 ns and energy resolution 15.4%. These characteristics demonstrates the feasibility of 3D printing plastic scintillator as a radiation detector.