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Relevant information in positron emission tomography is currently being obtained mostly by analog signal-processing methods. New digital PET scanner architectures are now becoming available, which offer greater flexibility and easier reconfiguration capability as compared to previous PET designs. Moreover, new strategies can be devised to extract more information with better accuracy from the digitized detector signals. Trained artificial neural networks (ANN) have been investigated to improve coincidence timing resolution with different types of Avalanche PhotoDiode (APD)-based detectors. The signal at the output of a charge-sensitive preamplifier was digitized with an off-the-shelf, free-running 100-MHz, 8-bit analog-to-digital converter and time discrimination was performed with ANNs implemented in field-programmable gate array (FPGA). Results show that ANNs can be particularly efficient with slow and low light output scintillators, such as BGO, but less so with faster luminous crystals, such as LSO. In reference to a fast PMT-plastic detector, a time resolution of 6.5 ns was achieved with a BGO--APD detector. With LSO, the ANN was found to be competitive with other digital techniques developed in previous works. ANNs implemented in FPGAs provide a fast and flexible circuit that can be easily reconfigured to accommodate various detectors under different signal/noise conditions. Index Terms--Artificial neural network (ANN), positron emission tomography (PET), timing.

To address modern molecular imaging requirements, a digital positron emission tomography (PET) scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to LYSO/LGSO phoswich detectors, the scanner supports up to 4608 channels in a 16.2 cm diameter, 11.25 cm axial field of view with an isotropic ~ 1.2 mm FWHM intrinsic spatial resolution at the center of the field of view. Custom data acquisition boards preprocess and sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at an average sustained rate of up to 1250 raw counts per second per [mm.sup.2] (10 000 cps/channel). Real time digital signal analysis also filters out events outside the pre-selected energy window with crystal granularity to eliminate Compton events and electronic noise. Retained events are then merged into a single stream through a real-time sorting tree, at which end prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a host computer. The LabPET features four data recording modes, giving the user the choice to retain data for research or to minimize file size for high coincidence count rate and imaging purposes. The electronic system also supports time synchronized data insertion for flags such as vital signs used in gated image reconstruction. Aside from data acquisition, hardware can generate live energy and discrimination spectra suitable for fast, automatic channel calibration. Index Terms--Positron Emission Tomography (PET).

Most Positron Emission Tomography (PET) scanners still partly rely on analog processing to sort out events from the PET detector front-end. Recent all-digital architectures enable the use of more complex algorithms to solve common problems in PET scanners, such as crystal identification and parallax error. Auto-Regressive eXogeneous variable (ARX) algorithms were shown to be among the most powerful methods of crystal identification by Pulse Shape Discrimination (PSD) for parallax mitigation or resolution improvement with phoswich detectors. Although ARX algorithms achieve a nearly 100% discrimination accuracy even in a noisy environment, such methods are computationally expensive and can hardly be implemented in a real time digital PET system. A crystal identification method based on adaptive filter theory using an Auto-Regressive (AR) model is proposed to enable real time crystal identification in a noisy environment. Index Terms--Adaptive filter, auto-regressive (AR) model, crystal identification, PET scanner, real time.

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 Lu1.8Gd0.2SiO_5\!\!: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 (Lu0.4Gd1.6SiO_5\!\!: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. [ABSTRACT FROM PUBLISHER]

Fusion of functional PET or SPECT images and anatomical X-ray CT images would ideally be performed using data recorded concurrently with the same detection system under all three modalities to avoid inaccurate co-registration due to patient movements and normal physiological processes like respiratory motion. One possible approach to achieve this goal is to use an avalanche photodiode (APD) based scintillation detection system. Requirements on scintillators for such an application are high density and high timing resolution for PET, good energy resolution for SPECT, large light output and fast scintillation decay time for counting mode X-ray CT. The purpose of this work is to investigate new Lu1.8 Gd0.2 SiO5 (LGSO) scintillators with improved performance developed purposely for PET applications, but having suitable characteristics for SPECT and CT detection. LGSO with 90% Lu content has high density (7.3 g/cc), large light output (90-120% of NaI), good energy resolution ( ~10 - 12% @ 511 keV), and fast decay time that can be adjusted from 34 to 45 ns by varying Ce concentration. In this work, LGSO with 0.025, 0.15 and 0.75 mol% Ce was assessed with APD readout for PET/SPECT/CT applications by measuring energy spectra at 511 keV (PET), 140 keV (SPECT) and 60 keV (CT), and timing resolution in reference to a fast plastic scintillator. LGSO-APD detectors can be considered as promising candidates for combined multimodality imaging scanners. [ABSTRACT FROM AUTHOR]

Since its discovery in 1993, LGSO has been seen as a promising scintillator for positron emission tomography (PET) imaging. Recently, new high lutetium content Lu1.8Gd0.2SiO5:Ce (LGSO-90%Lu) has been produced with Ce concentration varying from 0.025 to 0.75 mol% Ce, yielding decay times in the range of 34.2 to 44.8 ns. These scintillators could potentially be used as phoswich detectors in PET for improving spatial resolution. To validate this assumption, light output, energy resolution, temperature dependence, timing resolution, and pulse-shape discrimination performance were assessed. The light output of these LGSO scintillators is 3.5 to 4.8 times higher than BGO with avalanche photodiode readout [7.5 to 8 times with photomultiplier tube (PMT)] and increases with Ce concentration. The variation of light output for temperature ranging from 10 to 50 °C is less than 18% with PMT readout. Energy resolution at 662 keV is about 11% (l0%) with avalanche photodiode (APD) (PMT) readout. Timing resolution with APD is ∼1.65 ns for all samples relative to a fast PMT-plastic detector. Pulse-shape discrimination of LGSO-90% Lu with 0.025 mol% Ce ( τ = 34.2 ns) and 0.75 mol% Ce ( τ = 44.8 ns) was successfully achieved in a phoswich assembly, making this new high lutetium content LGSO with adjustable decay time constants a strong candidate for a high-resolution depth-of-interaction detector intended for PET imaging. [ABSTRACT FROM AUTHOR]