Jean-Daniel Leroux

Architecture of a dual-modality, high-resolution, fully digital positron emission tomography/computed tomography (PET/CT) scanner for small animal imaging

Contemporary positron emission tomography (PET) scanners are commonly implemented with very large scale integration analog front-end electronics to reduce power consumption, space, noise, and cost. Analog processing yields excellent results in dedicated applications, but offers little flexibility for sophisticated signal processing or for more accurate measurements with newer, fast scintillation crystals. Design goals of the new Sherbrooke PET/computed tomography (CT) scanner are: 1) to achieve 1 mm resolution in both emission (PET) and transmission (CT) imaging using the same detector channels; 2) to be able to count and discriminate individual X-ray photons in CT mode. These requirements can be better met by sampling the analog signal from each individual detector channel as early as possible, using off-the-shelf, 8-b, 100-MHz, high-speed analog-to-digital converters (ADC) and digital processing in field programmable gate arrays (FPGAs). The core of the processing units consists of Xilinx SpartanIIe that can hold up to 16 individual channels. The initial architecture is designed for 1024 channels, but modularity allows extending the system up to 10 K channels or more. This parallel architecture supports count rates in excess of a million hits/s/scintillator in CT mode and up to 100 K events/s/scintillator in PET mode, with a coincidence time window of less than 10 ns full-width at half-maximum.

Time determination of BGO-APD detectors by digital signal processing for positron emission tomography

Coincidence timing resolution in Positron Emission Tomography (PET) can be improved by replacing fast analog pulse shaping and Constant Fraction Discriminator (CFD) with fully digital signal processing. This can be achieved by digitizing the signal from individual detectors using 100-MHz, 8-bit Analog-to-Digital converters (ADC) and by processing the data on-the-fly in Field Programmable Gate Arrays (FPGA). Various digital filters and baseline restorers were implemented and combined with numerical least mean square fit to the data to extract the time of interaction and the energy deposited in BGO-APD detectors. An intrinsic time resolution of 7.2 ns was obtained with digital techniques. However, it is shown that bias in the timestamp estimation can be introduced by digital time discrimination techniques, which could affect the ability of digital methods to accurately estimate random event rates by the delayed time window method. Accordingly, the coincidence FWHM metric should not be the only figure of merit when comparing digital and analog time discrimination strategies.

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

Time discrimination techniques using artificial neural networks for positron emission tomography

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.

FPGA/DSP-based coincidence unit and data acquisition system for the Sherbrooke animal PET scanner

The use of field programmable gate arrays (FPGA) is a natural step in the evolution of modern PET scanners to improve system flexibility, increase design speed, and reduce electronic circuit size and cost. Hybrid high-performance programmable digital interfaces integrating an on-board FPGA and digital signal processor (DSP) were used to rebuild the coincidence units and data acquisition system of the Sherbrooke animal PET scanner. Logic was implemented within the FPGA to process singles and coincidence events, both trues and randoms. Coincidence validation is realized directly in the FPGA by an AND gate of the time signals generated by the scanner CF triggers. Twelve coincidence units from eight groups of detectors were equally divided between four FPGA/DSP modules to evenly distribute the event rate from the detectors. The event data is streamingly transmitted to the DSP for real-time histogramming into host memory or list mode data storage on disk. Event rates in excess of 10/sup 7//second can be achieved, which allows ultra-fast calibration and single-photon transmission measurements to be performed. The whole system occupies three full-length PCI slots in a standard PC computer.

Fast, accurate and versatile Monte Carlo method for computing system matrix

A new methodology is proposed to mitigate the high computation cost required to derive accurate Monte Carlo (MC) based system matrix for tomographic image reconstruction. The strategy consists of taking advantage of the symmetries between the lines of response to increase the statistics of data collected for the determination of the system matrix coefficients. By using the rotation and axial symmetries of a cylindrical camera, the number of MC generated events can be reduced substantially without affecting the matrix coefficient accuracy. Moreover, using the GATE simulator list-mode saving capabilities for storing coincidence events, single events and/or single hits with all their relevant information, the MC simulation can be performed only once and system matrices for different system configurations be derived from the same simulation. Using for example Positron emission tomography (PET), the processing of the collected MC data can be fine tuned to the imaging system characteristics by setting accordingly the time and energy blurring, the detector efficiencies, the coincidence time window width and the energy thresholds. The system matrix can also include or exclude PET events like scatters and randoms. Furthermore, the system matrix can be computed for different image grids and basis functions without requiring a new MC simulation to be performed.

Sensitivity Increase Through a Neural Network Method for LOR Recovery of ICS Triple Coincidences in High-Resolution Pixelated- Detectors PET Scanners

Scanner sensitivity is often critical in high-resolution Positron Emission Tomography (PET) dedicated to molecular imaging. In neighboring pixelated detectors with individual readout, sensitivity decreases because of multiple coincidences produced by Compton scattering. Correct analysis of those coincidences would enable a substantial sensitivity increase. However, including scattering byproducts in the image often lead to image quality degradation because of inaccurate Line-of-Response (LOR) assessment. In such scanners, to support high count rates, multiple coincidences are usually discarded when image degradation is not acceptable, or blindly accepted for a low computational burden. This paper presents a new, real-time capable method that includes Inter-Crystal Scatter (ICS) triple coincidences in the image without significant quality degradation. The method computes the LOR using a neural network fed by preprocessed raw data. As a proof of principle, this paper analyzes the simplest ICS scenario, triple coincidences where one photoelectric 511-keV event coincides with two more whose energy sum is also 511 keV. The paper visits the algorithm structure, presents Monte Carlo assessment with the LabPET model, and displays images reconstructed from real data. With an energy window of 360-660 keV and a singles energy threshold of 125 keV, the inclusion of triple coincidences yielded a sensitivity increase of 54%, a resolution degradation similar to that of other sensitivity-increasing methods, and only a slight contrast degradation for real LabPET data, with potential for numerous further improvements.

Performance evaluation of the LabPET12, a large axial FOV APD-based digital PET scanner

The LabPET12 is the latest version of LabPET¿ avalanche photodiode (APD)-based digital scanners, having an 11.4 cm axial field of view and 9216/4608 crystals/electronic readout channels to achieve high sensitivity and count rate performance, while preserving the same very high spatial resolution. Its performance characteristics have been thoroughly investigated largely based on the new NEMA NU4 - 2008 small animal PET standard. Average energy resolution is 20±3% for LYSO and 19±3% for LGSO. After timing alignment, the overall FWHM timing resolution is 7.1/8.3/9.2 ns for LYSO/LYSO, mixed and LGSO/LGSO coincidences. Imaging performance was assessed using various energy windows with lower threshold from 100 to 350 keV. Sensitivity varied from 8 to 2.8%, between 100 and 350 keV. With the lower threshold set at 200 keV, peak NEC count rates were 449 kcps at 66 MBq and 179 kcps at 81 MBq for the mouse and rat NEMA phantoms, respectively. Two acquisition modes were investigated, a high sensitivity and high resolution mode. Whole-body static images of mice were obtained with Na18F. In conclusion, the LabPET12 provides excellent dynamic performance with shorter imaging time or lower injected radioactivity while achieving the same exquisite spatial resolution as the LabPET4 and LabPET8 scanners.

Accelerated iterative image reconstruction methods based on block-circulant system matrix derived from a cylindrical image representation

Iterative image reconstruction methods based on an accurate and fully three-dimensional (3D) system probability matrix are well-known to provide images of higher quality. However, the size of the system matrix and the computation burden often make such methods impractical. To address this problem, we proposed to use a cylindrical image representation that preserves both in-plane and axial symmetries between the tubes of response for a given camera, leading to a system matrix having a block-circulant structure. For 3D image reconstruction, such a system matrix can be structured into a block-circulant matrix where blocks are themselves block-circulant. By storing only non-redundant parts of the block-circulant matrix, memory requirements can be reduced by a factor equivalent to the total number of system symmetries. The block-circulant system matrix can be stored in the Fourier domain representation to accelerate the forward and back projection steps of the iterative image reconstruction methods. When represented in the Fourier domain, the system matrix sparsity is reduced compared to the spatial domain representation, but some null values are still preserved.

Performance evaluation of a dual-crystal APD-based detector modules for positron emission tomography

Positron Emission Tomography (PET) scanners dedicated to small animal studies have seen a swift development in recent years. Higher spatial resolution, greater sensitivity and faster scanning procedures are the leading factors driving further improvements. The new LabPETTM system is a second-generation APD-based animal PET scanner that combines avalanche photodiode (APD) technology with a highly integrated, fully digital, parallel electronic architecture. This work reports on the performance characteristics of the LabPET quad detector module, which consists of LYSO/LGSO phoswich assemblies individually coupled to reach-through APDs. Individual crystals 2×2×~10 mm3 in size are optically coupled in pair along one long side to form the phoswich detectors. Although the LYSO and LGSO photopeaks partially overlap, the good energy resolution and decay time difference allow for efficient crystal identification by pulse-shape discrimination. Conventional analog discrimination techniques result in significant misidentification, but advanced digital signal processing methods make it possible to circumvent this limitation, achieving virtually error-free decoding. Timing resolution results of 3.4 ns and 4.5 ns FWHM have been obtained for LYSO and LGSO, respectively, using analog CFD techniques. However, test bench measurements with digital techniques have shown that resolutions in the range of 2 to 4 ns FWHM can be achieved.