Francis Loignon-Houle

LabPET II, an APD-based Detector Module with PET and Counting CT Imaging Capabilities

Computed tomography (CT) is currently the standard modality to provide anatomical reference for positron emission tomography (PET) in molecular imaging applications. Since both PET and CT rely on detecting radiation to generate images, using the same detection system for data acquisition is a compelling idea even though merging PET and CT hardware imposes stringent requirements on detectors. These requirements include large signal dynamic range with high signal-to-noise ratio for good energy resolution in PET and energy-resolved photon-counting CT, high pixelization for suitable spatial resolution in CT, and high count rate capability for reasonable CT acquisition time. To meet these criteria, the avalanche photodiode (APD)-based LabPET II module is proposed as the building block for a truly combined PET/CT scanner. The module is made of two monolithic 4×8 APD pixel arrays mounted side-by-side on a custom ceramic holder. Individual APD pixels have an active area of 1.1×1.1 mm2 at a 1.2 mm pitch. The APD arrays are coupled to a 12-mm high, 8 ×8 LYSO scintillator array made of 1.12 ×1.12 mm2 pixels also at a pitch of 1.2 mm to ensure direct one-to-one coupling to individual APD pixels. The scintillator array was designed with unbound specular reflective material between pixels to maximize the difference between refractive indices and enhance total internal reflection at the crystal side surfaces for better light collection, and the APD quantum efficiency was improved to ~ 60% at 420 nm to optimize intrinsic detector performance. Mean energy resolution was 20 ±1% at 511 keV and 41±4% at 60 keV. The measured intrinsic spatial and time resolution for PET were respectively 0.81 ±0.04 mm FWHM/1.57 ±0.04 mm FWTM and 3.6±0.3 ns FWHM with an energy threshold of 400 keV. Initial phantom images obtained using a CT test bench demonstrated excellent contrast linearity as a function of material density. With a magnification factor of 2, a CT spatial resolution of 0.66 mm FWHM/1.2 mm FWTM, corresponding to 1.18 lp/mm at MTF10%/0.67 lp/mm at MTF50%, was measured, allowing 0.75 mm air holes in an Ultra-Micro Hot Spot resolution phantom to be clearly distinguished.

Simulation of signal losses in highly pixelated scintillator arrays read out by discrete photodetectors

The performance of scintillation detectors used in Positron Emission Tomography imaging strongly depends on the scintillation light transport from the crystal to the photodetector. In highly pixelated scintillator arrays with individual pixels approaching millimetric cross section, the loss of signal is compounded with crosstalk effects, squandering valuable signal to adjacent pixels, and with light absorption in lateral faces adhesive materials and imperfect reflectors. The purpose of this simulation study is to uncover processes responsible for light losses in scintillator arrays. Four sources of losses through crosstalk between pixels were identified, namely 1) escaping photoelectrons to other pixels after photoelectric interactions, 2) X-ray fluorescence and Auger emission, 3) reflector transparency to scintillation light, and 4) light leakage to other crystals due to adhesive material between reflectors and scintillators in which optical photons can propagate to other crystals. An important source of signal loss and energy resolution degradation was found to be related to the transmittance of the adhesive material used to bond reflectors to scintillators. Moreover, the angular distribution of scintillation photons impinging on the detection face was assessed in order to weigh the proportion of trapped photons through total internal reflection due to the refractive index difference between scintillators and optical coupling medium.

Improved LabPET Detectors Using

The scintillator is one of the key building blocks that critically determine the physical performance of PET detectors. The quest for scintillation crystals with improved characteristics has been crucial in designing scanners with superior imaging performance. Recently, it was shown that the decay time constant of high lutetium content Lu1.8Gd0.2SiO5: Ce (LGSO) scintillators can be adjusted by varying the cerium concentration from 0.025 mol% to 0.75 mol%, thus providing interesting characteristics for phoswich detectors. The high light output (90%-120% NaI) and the improved spectral match of these scintillators with avalanche photodiode (APD) readout promise superior energy and timing resolutions. Moreover, their improved mechanical properties, as compared to conventional LGSO ( Lu0.4Gd1.6SiO5: Ce), make block array manufacturing readily feasible. To verify these assumptions, new phoswich block arrays made of LGSO-90%Lu with low and high mol% Ce concentrations were fabricated and assembled into modules dedicated to the LabPET scanner. Typical crystal decay time constants were 31 ns and 47 ns, respectively. Phoswich crystal identification performed using a digital pulse shape discrimination algorithm yielded an average 8% error. At 511 keV, an energy resolution of 17-21% was obtained, while coincidence timing resolution between 4.6 ns and 5.2 ns was achieved. The characteristics of this new LGSO-based phoswich detector module are expected to improve the LabPET scanner performance. The higher stopping power would increase the detection efficiency. The better timing resolution would also allow the use of a narrower coincidence window, thus minimizing the random event rate. Altogether, these two improvements will significantly enhance the noise equivalent count rate performance of an all LGSO-based LabPET scanner.

{\rm Lu}_{1.8}{\rm Gd}_{0.2}{\rm SiO}_{5}\!\!:{\rm Ce}

High lutetium content LGSO [Lu1.8Gd0.2SiO5:Ce] with decay time constants in the range 30-45 ns were introduced by Hitachi Chemical Ltd. (Japan) around 2006. At first an experimental development, mass production has now been established and mature 90%Lu LGSO scintillators with stable characteristics have become available. The aim of this work was to assess the scintillation characteristics of the current production of 90%Lu LGSO with different decay times using avalanche photodiode (APD) readout. Samples from the most recent production of Fast (~30 ns), Standard (~40 ns) and Slow (~47 ns) 90%Lu LGSO (manufactured in 2013), extracted from top, middle and bottom positions into the ingots, have been studied. The decay time and central emission wavelength results demonstrate good agreement for samples of the same ingot, irrespective of the position where the sample was obtained. The photoelectron yields of the most recent Slow, Standard and Fast LGSO scintillators show good uniformity. Their average photoelectron yields are respectively 34 000 ± 1 000 phe/MeV, 31 200 ± 500 phe/MeV and 28 000 ± 2 000 phe/MeV without any definite trend or bias with respect to position in ingots. The average energy resolution of Slow is 10 ± 2% whereas Standard and Fast LGSO samples have identical energy resolution of 11 ± 3%. The light yield non-proportionality as a function of irradiation energy was investigated. At 60 keV, the departure from linearity is 16 ± 4% for Slow and Standard LGSO and 15 ± 1% for Fast LGSO.

(LGSO) Scintillator Blocks

Individually coupled scintillation detectors used in positron emission tomography (PET) imaging suffer from important signal losses due to the suboptimal light collection from crystals. As only a fraction of the light is generally extracted from long and thin scintillators, it is important to identify and understand the predominant causes of signal loss in order to eventually recover it. This simulation study investigates the multiple factors affecting the light transport in high-aspect ratio LYSO scintillators wrapped in specular reflectors through a full factorial design. By exploring various combinations of crystal geometry, readout conditions and wrapping conditions, it was found that an optimum light output can only be achieved through a careful selection of highly reflective material along with high-transmittance optical adhesive used to bond the reflector. Decreasing the adhesive thickness was also found to have a positive outcome in most explored configurations, however to a much lesser extent. Suboptimal reflectivity and adhesive transmittance also lead to an asymmetric light output distribution dependent on the depth of interaction of the radiation, potentially degrading energy resolution. By identifying the factors causing the most significant scintillation light losses through a factorial design, the most promising detector configurations have been identified in the quest for optimal light collection from scintillators.

Scintillation characteristics of 90%Lu LGSO with different decay times

High lutetium content LGSO [Lu1.8Gd0.2SiO5:Ce] scintillators with decay time constants in the range 30-45 ns were introduced some ten years ago. At first an experimental development, mass production was gradually established and mature 90%Lu LGSO scintillators with stable characteristics have become available. The aim of this work was to assess the scintillation characteristics of recent 90%Lu LGSO production with different decay times using avalanche photodiode (APD) readout. Samples of Fast (~ 30 ns), Standard (~ 40 ns) and Slow (~ 47 ns) 90%Lu LGSO, extracted from top, middle and bottom positions into ingots, have been investigated. The decay time and central emission wavelength were found to be stable for samples from the same ingot, irrespective of the position where the sample was obtained. The photoelectron yields of Slow, Standard and Fast LGSO scintillators generally showed good uniformity. Their average photoelectron yields at 511 keV were respectively 34 000 ± 1 000 phe/MeV, 31 200 ± 500 phe/MeV and 28 000 ± 2 000 phe/MeV without any definite trend or bias with respect to position in ingots. The combination of photon yield and decay time results in high initial emission rate and a subnanosecond time resolution performance of 914 ± 18 ps for the three categories of decay times. The average energy resolution at 511 keV of Slow LGSO was 10 ± 2% whereas Standard and Fast LGSO samples had identical energy resolution of 11 ± 3%. The light yield non-proportionality as a function of irradiation energy was also investigated. At 60 keV, the departure from linearity reached 19 ± 2%.

Simulation of scintillation light output in LYSO scintillators through a full factorial design

Purpose: LabPET II is a new generation APD-based PET scanner designed to achieve sub-mm spatial resolution using truly pixelated detectors and highly integrated parallel front-end processing electronics. Methods: The basic element uses a 4×8 array of 1.12×1.12 mm2 Lu1.9Y0.1SiO5:Ce (LYSO) scintillator pixels with one-to-one coupling to a 4×8 pixelated monolithic APD array mounted on a ceramic carrier. Four detector arrays are mounted on a daughter board carrying two flip-chip, 64-channel, mixed-signal, application-specific integrated circuits (ASIC) on the backside interfacing to two detector arrays each. Fully parallel signal processing was implemented in silico by encoding time and energy information using a dual-threshold Time-over-Threshold (ToT) scheme. The self-contained 128-channel detector module was designed as a generic component for ultra-high resolution PET imaging of small to medium-size animals. Results: Energy and timing performance were optimized by carefully setting ToT thresholds to minimize the noise/slope ratio. ToT spectra clearly show resolved 511 keV photopeak and Compton edge with ToT resolution well below 10%. After correction for nonlinear ToT response, energy resolution is typically 24±2% FWHM. Coincidence time resolution between opposing 128-channel modules is below 4 ns FWHM. Initial imaging results demonstrate that 0.8 mm hot spots of a Derenzo phantom can be resolved. Conclusion: A new generation PET scanner featuring truly pixelated detectors was developed and shown to achieve a spatial resolution approaching the physical limit of PET. Future plans are to integrate a small-bore dedicated mouse version of the scanner within a PET/CT platform.