Hicham Semmaoui

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

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.

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.

Wavelets-Based Crystal Identification of Phoswich Detectors for Small-Animal PET

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 (tr~40 ns) and LGSO (tr~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 phoswich crystal combination readout by an avalanche photodiode.

A Fast Crystal Identification Algorithm Applied to the LabPET™ Phoswich Detectors

Detectors based on LYSO and LGSO scintillators in a phoswich arrangement coupled to an avalanche photodiode are used in the LabPETtrade, 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 was recently proposed for crystal identification of LYSO-LGSO phoswich detectors . 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 for LYSO-LGSO LabPET detectors and 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.

Signal Deconvolution Concept Combined With Cubic Spline Interpolation to Improve Timing With Phoswich PET Detectors

PET imaging scanners based on all-digital architecture offer greater data processing flexibility and the possibility to recalibrate the system with simple software procedures. The LabPET™ scanner, designed at the Université de Sherbrooke, is one such device. It is built around dual LYSO/LGSO scintillators in a phoswich arrangement coupled to Avalanche Photodiodes (APD)and combined with highly parallel readout and processing electronics. This approach enables the implementation of advanced real time digital signal processing methods to compute energy resolution, crystal identification and the arrival time of events. Timing extraction represents the highest challenge because of the low sampling frequency (45 MHz), the quantization error and the presence of a Zero Order Hold (ZOH) in the system. The aim of this paper is to present a method to increase coincidence time accuracy by adequately addressing each of these limitations. The proposed method uses a Deconvolution concept based on adaptive filter theory preceded by Cubic Spline interpolation to improve digital timing performance. The method achieves 4.5 ns, 8.2 ns and 6.5 ns timing resolution with, LYSO-LYSO, LGSO-LGSO and mixed crystals phoswich coincidences, respectively.

Roadmap to fully-digital PET/CT scanners

The ever-increasing needs of molecular imaging now require significant upgrade of conventional PET and CT scanners. Upcoming research protocols ask for low doses, submillimeter resolution, high sensitivity and multimodality. Current scanner technologies are mainly based on analog ASICs having a long design-cycle which hinders rapid scanner improvements and can hardly keep up with the new requirements of biomedical research. With new high-speed processors and configurable electronics, combined with early digitization of the signals from detectors, digital signal processing can flexibly and concurrently deal with many of those requirements. The present paper highlights past, present and foreseen developments in PET/CT signal processing. In particular, different model fits, filtered interpolation and neural networks are compared for timestamping and pulse shape discrimination. Recursive (ARMAX, AR...) and non recursive (Wiener, Fast Fourier transforms, Wavelets...) filtering are compared for crystal identification. Advanced pile-up correction, baseline restoration and energy measurement in photon-counting CT are also discussed. Finally, new techniques dealing with realtime event processing for Compton-scatter LOR computation and alternate random estimation will be briefly introduced. Pros and cons of each method are discussed and the best methods identified for a roadmap to fully digital PET/CT scanning is presented.