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The LabPET[TM] is a fully digital avalanche photodiode (APD) based PET scanner designed for state-of-the-art molecular and genomic imaging of small animals. Two versions of the scanner were evaluated, having 3.75 (LabPET4) and 7.5 cm axial FOV (LabPET8). The detectors are made of 2 x 2 x 12/14 [mm.sup.3] LYSO and LGSO crystals assembled in phoswich pairs read out by an APD. After digital crystal identification, the average energy resolution is 24 [+ or -] 6% for LYSO and 25 [+ or -] 6% for LGSO. The scanner's overall timing resolution is 6.6 ns for LYSO/LYSO and 10.7 ns for LGSO/LGSO coincidences after coarse timing alignment. The FBP reconstructed tangential/radial resolution is 1.3/1.4 mm FWHM (2.5/2.4 mm FWTM) at the FOV center and remains below 2.1 mm FWHM (3.6 mm FWTM) within the central 4-cm diameter FOV. MLEM reconstruction of a micro resolution phantom provided clear separation of the 1.0 mm spots. With an energy window of 250-650 keV, the sensitivity is 1.1% for LabPET4 and 2.1% for LabPET8. The imaging capabilities of the scanner are demonstrated with in vivo images of rats and mice. Index Terms--Avalanche photodiodes, performance evaluation, PET instrumentation, positron emission tomography.

The highly multiplexed analog processing front-end of current Positron Emission Tomography (PET) scanners yields high accuracy for timing but adds significant dead time and offers little flexibility for improvement. A new fully digital APD-based scanner architecture is proposed wherein nuclear pulses are sampled directly at the output of the Charge Sensitive Preamplifier (CSP) with one free-running ADC per channel. This approach offers the opportunity to explore new digital signal processing algorithms borrowed from other fields like command and control theory, as well as advanced heuristics such as neural networks. The analog front-end consists of a dedicated 0.18-[micro]m, 16-channel CMOS charge sensitive preamplifier. Digitization is performed with off-the-shelf dual 8-bit analog-to-digital converters running at 45-MSPS. Digital processing is shared between a FPGA and a Digital Signal Processor (DSP), which can process the data from up to 64 parallel channels without dead time. The FPGA deals with the initial signal analysis for energy measurement and time stamping, while crystal identification is deferred to the DSP running computation-intensive recursive algorithms. The entire system is controlled serially through a Firewire link by a Graphic User Interface. The initial LabPET implementation of the system is a dedicated small animal scanner holding up to 4608 APD channels at an averaged count rate of up to 10 000 events/s each. Index Terms--Data acquisition systems, digital signal processing, positron emission tomography (PET).

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.

Detectors based on LYSO and LGSO scintillators in a phoswich arrangement coupled to an avalanche photodiode are used in the LabPET[TM], an all-digital positron emission tomography (PET) scanner for small animal imaging developed in Sherbrooke. A Wiener filter based crystal identification (CI) algorithm achieving excellent discrimination accuracy (~98%) was recently proposed for crystal identification of LYSO-LGSO phoswich detectors [1]. This algorithm was based on estimating parameters describing the scintillation decay time constant and the light yield of events sampled at 45 MSPS. The CI process was performed by applying a threshold on the scintillation decay parameter of events. The light yield was not considered in the CI process even if it should be. We propose a 2-fold faster CI approach which takes both the scintillation decay and light yield coefficients of each crystal into consideration. The new algorithm uses the previous Wiener filter based algorithm as a calibration process in order to evaluate the model of each individual crystal. The DAQ chain model as a priori knowledge is then incorporated into the model of each crystal and the output signal is estimated. The CI is performed by evaluating a single parameter representing the percentage contribution of each crystal characteristics in the event signal. The CI algorithm demonstrated a discrimination rate accuracy > 98.5% for LYSO-LGSO LabPET detectors and > 99% for LSO-GSO crystals in phoswich arrangement for 511 keV events. Although a calibration is required, the real-time implementation of the new CI algorithm needs 2 times less direct operations. An FPGA clocked at 400 MHz can process up to 25 M events/sec with such an algorithm. Index Terms--Crystal identification, digital signal processing, phoswich, positron emission tomography (PET), Wiener filter.

The recently launched LabPET[TM], a small animal Avalanche PhotoDiode (APD)-based PET scanner with quasi-individual readout and massively parallel processing, makes it possible to acquire real-time information necessary for Positron Emission Tomography (PET) image reconstruction. Since each APD is coupled to an LYSO/LGSO phoswich scintillator pair, an efficient crystal identification algorithm must be developed to sustain real-time crystal feature extraction in high PET count rate. Furthermore, a less application specific algorithm is needed to easily expand its use to a large range of crystal materials. For these reasons, a new ultra-fast crystal identification algorithm based on a Wiener filter is proposed. This optimum filter instantly recovers crystal parameters by minimizing a linear cost function. A one-dimension projection based discrimination is used to identify the scintillating crystal. The algorithm achieves a discrimination rate of ~88% for low-energy X-ray photons (~60 keV) and up to ~100% for high energy 511 keV photopeak photons, with a maximum throughput of 10 Mevents/sec when implemented in a field programmable gate array. Index Terms--Crystal identification, positron emission tomography, Wiener filter.

Abstract--The advent of new all-digital electronic architectures in PET scanners enables the development and investigation of novel crystal identification algorithms for phoswich detectors used for parallax mitigation or higher detector pixelization. The high flexibility and real-time signal processing capability of FPGA/DSP-based digital electronics, such as the one developed for the LabPET scanner, provide an excellent platform to test enhanced digital methods. A novel approach based on the wavelet analysis theory has been investigated for crystal identification in phoswich detectors with crystals having similar scintillation characteristics such as LYSO ([t.sub.r] ~40 ns) and LGSO ([t.sub.r] ~65 ns). The proposed algorithm uses Stationary Wavelet Transform to clean the digitized signal and Discrete Wavelet Transform for crystal identification. Such a process can achieve a successful discrimination rate of ~95% for PET events measured with an LYSO-LGSO phaswich crystal combination readout by an avalanche photodiode. Index Terms--Crystal identification, LabPET scanner, phoswich detector, small-animal PET, wavelet transform.

Recent developments in detectors and electronics enable both positron emission tomography (PET) and X-ray computed tomography (CT) data to be acquired concurrently using the same detection front-end for dual-modality PET/CT imaging. Moreover, it would potentially allow substantial reduction of cost and housing size, in addition to facilitating image fusion. However, the lower energy signals (~ 60 keV versus 511 keV) and higher photon flux per pixel (> 1 Mcps versus 10 kcps) in CT relative to PET cause significant pile-up and dead-time in CT data acquired in photon counting mode. A digital signal processing method was developed and implemented to improve processing of detector signals sampled at low frequency (~ 45 MHz) in presence of pile-up. The method consists in digitally subtracting the detector impulse response at the output of the preamplifier to restore the signal baseline for more accurate energy estimation. When compared to a fixed threshold counting technique, the proposed method features better noise immunity, higher energy resolution and 50% higher rates measured at an estimated true rate of 2.75 Mcps, making CT integration within modern digital PET hardware feasible. Index Terms--Computed tomography, photon-counting, positron emission tomography/computed tomography (PET/CT).

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (< 65 MHz) to reduce power consumption. Timestamps must then be interpolated between samples to achieve adequate time resolution for coincidence detection of annihilation radiation. A low-pass filter based interpolation algorithm adding up to 31 samples between original samples was designed to improve timing resolution of the LabPET scanner. A 2-bit refinement in the determination of the pulse maximum amplitude leads to a better estimation of the triggering threshold, which in turn enables a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (15%-50% of maximum value). With the trigger threshold set at 20%, coincidence time resolution of ~ 5.0 ns for LYSO-LYSO and ~ 9.6 ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA. Index Terms--Digital signal processing, filtered interpolation, position emission tomography (PET), timestamp generation.