Nicolas Viscogliosi

Performance Evaluation of the LabPET APD-Based Digital PET Scanner

The LabPETTM 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 2x2x10/12 mm3 LYSO and LGSO crystals assembled in phoswich pairs read out by an APD. After digital crystal identification, the average energy resolution is 24 plusmn 6% for LYSO and 25 plusmn 6% for LGSO. The scanner 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 1.35 mm spots and fair identification of 1 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.

The Hardware and Signal Processing Architecture of LabPET™, a Small Animal APD-Based Digital PET Scanner

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- mum, 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 LabPETtrade 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.

System Architecture of the LabPET Small Animal PET Scanner

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 mm2 (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.

The architecture of LabPET/spl trade/, a small animal APD-based digital PET scanner

The highly multiplexed, integrated analog processing front-end of current PET scanners yields high accuracy for timing and crystal identification, but also 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 with one free-running ADC per channel. This approach offers the opportunity to explore new digital signal processing algorithms borrowed to command and control theory, as well as advanced heuristics such as neural networks. The analog front-end consists of a dedicated 0.18-mum, 16-channel CMOS charge sensitive preamplifier. Digitization is performed with off-the-shelf dual 8-bit 45-MHz analog-to-digital converters. Digital processing is concentrated in a dual processor FPGA and a digital signal processor (DSP), which can process the data from up to 64 parallel channels with no 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 auto-regressive algorithms. The entire system is controlled serially through a Firewire link by a graphic user interface. The initial LabPETtrade implementation of the system is a dedicated small animal scanner holding up to 3072 APD channels at an averaged count rate of up to 10 000 events/s each

Real time digital signal processing implementation for an APD-based PET scanner with phoswich detectors

Recent progress in advanced digital signal processing provides an opportunity to expand the computation power required for real time extraction of event characteristics in APD-based positron emission tomography (PET) scanners. These developments are made possible by a highly parallel data acquisition (DAQ) system based on an integrated analog front-end and a high-speed fully digital signal processing section that directly samples the output of each preamplifier with a free-running, off-the-shelf, 45-MHz MAX1193 analog-to-digital converter that feeds the sampled data into a field programmable gate array VirtexII PRO from Xilinx. This FPGA features ~31,000 logic cells and 2 PowerPC processors, which allows up to 64 channels to be processed simultaneously. Each channel has its own digital signal processing chain including a trigger, a baseline restorer and a timestamp algorithm. Various timestamp algorithms have been tested so far, achieving a coincidence timing resolution of 3.2 ns FWHM for APD-LSO and 11.4 ns FWHM for APD-BGO detectors, respectively. Channels are then multiplexed into a TMS320C6414 DSP processor from Texas Instruments for crystal identification by an ARMAX recursive algorithm borrowed from identification and vector quantization theory. The system can sustain an event rate of 10 000 events/s/channel without electronic dead time

Real Time Implementation of a Wiener Filter Based Crystal Identification Algorithm

The recently launched LabPETtrade, 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 for low-energy X-ray photons ( keV) and up to for high energy 511 keV photopeak photons, with a maximum throughput of 10 Mevents/sec when implemented in a field programmable gate array.

Crystal Identification Based on Recursive-Least-Squares and Least-Mean-Squares Auto-Regressive Models for Small Animal 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.

Timing improvement by low-pass filtering and linear interpolation for the LabPET TM scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (≪65 MHz) for reducing 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 both the energy and timing resolution of the LabPETTM scanner. An energy resolution refinement of ˜2 bits can be achieved with such a technique. The better estimation of triggering threshold leads to a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (5-50% 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.

Timing Improvement by Low-Pass Filtering and Linear Interpolation for the LabPET Scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (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 ns for LYSO-LYSO and ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA.

System Integration of the LabPET Small Animal PET Scanner

To address modern molecular imaging requirements, a digital positron emission tomography scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to a LYSO/LGSO phoswich array, the scanner supports up to 3072 channels in a 16.2 cm diameter, 7.5 cm axial field of view with an isotropic 1.2 mm FWHM intrinsic spatial resolution at the center of the FOV. Custom data acquisition boards sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at rates of up to 1250 raw counts per second per mm2 (10k cps/channel). Real time digital signal analysis also filters out events outside the photopeak 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 the prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a computer. The LabPETtrade 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 histograms suitable for fast, automatic channel calibration.