Pedro Lousã

The Clear-PEM Electronics System

The Clear-PEM detector system is a compact positron emission mammography scanner with about 12000 channels aiming at high sensitivity and good spatial resolution. Front-end, Trigger, and Data Acquisition electronics are crucial components of this system. The on-detector front-end is implemented as a data-driven synchronous system that identifies and selects the analog signals whose energy is above a predefined threshold. The off-detector trigger logic uses digitized front-end data streams to compute pulse amplitudes and timing. Based on this information it generates a coincidence trigger signal that is used to initiate the conditioning and transfer of the relevant data to the data acquisition computer. To minimize dead-time, the data acquisition electronics makes extensive use of pipeline processing structures and derandomizer memories with multievent capacity. The system operates at 100-MHz clock frequency, and is capable of sustaining a data acquisition rate of 1 million events per second with an efficiency above 95%, at a total single photon background rate of 10 MHz. The basic component of the front-end system is a low-noise amplifier-multiplexer chip presently under development. The off-detector system is designed around a dual-bus crate backplane for fast intercommunication between the system boards. The trigger and data acquisition logic is implemented in large FPGAs with 4 million gates. Monte Carlo simulation results evaluating the trigger performance, as well as results of hardware simulations are presented, showing the correctness of the design and the implementation approach

Design and test issues of a FPGA based data acquisition system for medical imaging using PEM

The main aspects of the design and test (D&T) of a reconfigurable architecture for the data acquisition electronics (DAE) system of the clear-PEM detector are presented in this paper. The application focuses medical imaging using a compact PEM (positron emission mammography) detector with 12288 channels, targeting high sensitivity and spatial resolution. The DAE system processes data that comes from a front-end (FE) electronics that identifies the relevant data and transfers it to a PC for image processing. The design is supported in a novel D&T methodology, in which hierarchy, modularity and parallelism are extensively exploited to improve design and testability features. Parameterization has also been used to improve design flexibility. Nominal frequency is 100 MHz. The DAE must respond to a data acquisition rate of 1 million relevant events (coincidences) per second, under a total single photon background rate in the detector of 10 MHz. Trigger and data acquisition logic is implemented in eight 4-million, one 2-million and one 1-million gate FPGAs (Xilinx Virtex II). Functional built-in self test (BIST) and debug features are incorporated in the design to allow on-board FPGA testing and self-testing during product lifetime

Design and test issues of an FPGA based data acquisition system for medical imaging using PEM

The main aspects of the design and test (D&T) of a reconfigurable architecture for the Data Acquisition Electronics (DAE) system of the Clear-PEM detector are presented in this paper. The application focuses medical imaging using a compact PEM (Positron Emission Mammography) detector with 12288 channels, targeting high sensitivity and spatial resolution. The DAE system processes data frames that come from a front-end (FE) electronics, identifies the relevant data and transfers it to a PC for image processing. The design is supported in a novel D&T methodology, in which hierarchy, modularity and parallelism are extensively exploited to improve design and testability features. Parameterization has also been used to improve design flexibility. Nominal frequency is 100 MHz. The DAE must respond to a data acquisition rate of 1 million relevant events (coincidences) per second, under a total single photon background rate in the detector of 10 MHz. Trigger and data acquisition logic is implemented in eight 4-million, one 2-million and one 1-million gate FPGAs (Xilinx Virtex II). Functional Built-In Self Test (BIST) and Debug features are incorporated in the design to allow on-board FPGA testing and self-testing during product lifetime.

Performance simulation studies of the clear-PEM DAQ/trigger system

The clear-PEM detector is a positron emission mammography scanner based on high-granularity avalanche photodiodes readout with 12 288 channels. The front-end sub-system is instrumented with low-noise 192:2 channel amplifier-multiplexer ASICs and free-running sampling ADCs. The off-detector trigger, implemented in a FPGA based architecture, computes the pulses amplitude and timing required for coincidence validation from the front-end data streams. A high-level C++ simulation tool was developed for data acquisition performance analysis and validated at bit level against FPGA VHDL testbenches. In this work, simulation studies concerning the performance of the on-line/off-line energy and time extraction algorithms and the foreseen detector energy and time resolution are presented. Time calibration, trigger efficiency and ghosting are also discussed

Built-in Clock Domain Crossing (CDC) test and diagnosis in GALS systems

The purpose of this paper is to present a novel built-in Clock Domain Crossing (CDC) test and diagnosis methodology for Globally Asynchronous, Locally Synchronous (GALS) systems. The methodology allows design and prototype validation, low maintenance and repair costs, and production / lifetime at-speed test. Moreover, high resolution diagnosis is obtained, to identify which device(s) and/or communication channel(s) is (are) faulty. This is not trivial in GALS systems, for which the CDC issue is challenging. The underlying principle of the proposed methodology is to embed a CDC test and diagnosis (CDC T&D) structure in each locally synchronous domain. Complete device-to-device communication channels are tested, including transceivers, buses, and board connectors. Identical test patterns (generated to detect static (stuck-at, shorts and open faults) and dynamic (crosstalk) faults) are used in each FPGA. The proposed CDC T&D methodology is validated in a case study, the acquisition electronics of a complex multi-board, multibus, multi-FPGA (nine Xilinx™ xc2v4000-4bf957) system. Test and validation results are presented.

An overview of the Clear-PEM breast imaging scanner

We present an overview of the Clear-PEM breast imaging scanner. Clear-PEM is a unique dual-head Positron Emission Mammography scanner using APD-based detector modules that are capable of measuring depth-of-interaction (DOI) with a resolution of 2 mm in 20 mm long LYSO:Ce crystals. Such capability leads to an image spatial resolution of 1.2 mm and a high efficiency, foreseeing the detection of 3 mm breast lesions in less than 7 minutes exams. The full system comprises 192 detector modules in a total of 6144 LYSO:Ce crystals and 384 32-pixel APD arrays readout by ASICs with 192 input channels that represents an unprecedented level of integration in PET systems. Throughout the project and besides the detector module, we had developed dedicated Frontend and Data Acquisition electronics, the mechanical design and construction of the detector heads and the robotic gantry, as well as all the software that include calibration (energy, time and DOI), normalization and image reconstruction algorithms. In this work we will discuss the developments and present the commissioning results of the detector before the beginning of the clinical trials program, scheduled for the end of the present year.

Characterization of the Clear-PEM breast imaging scanner performance

We present results on the characterization of the Clear-PEM breast imaging scanner. Clear-PEM is a dual-head Positron Emission Mammography scanner using APD-based detector modules that are capable of measuring depth-of-interaction (DOI) with a resolution of 2 mm in LYSO:Ce crystals. The full system comprises 192 detector modules in a total of 6144 LYSO:Ce crystals and 384 32-pixel APD arrays readout by ASICs with 192 input channels, which represents an unprecedented level of integration in APD-based PET systems. The system includes Frontend and Data Acquisition electronics and a robotic gantry for detector placement and rotation. The software implements calibration (energy, time and DOI), normalization and image reconstruction algorithms. In this work, the scanner main technical characteristics, calibration strategies and the spectrometric performance in a clinical environment are presented. Images obtained with point sources and extended uniform sources are also presented. The first commissioning results show 99.7% active channels. After calibration, the dispersion of the channels absolute gain is 15.3%, which demonstrate that despite the large number of channels the system is rather uniform. The mean energy resolution at 511 keV is 15.9% for all channels, and the mean DOI constant is 5.9%/mm, which is consistent with a 2 mm DOI resolution, or better. The coincidence time resolution at 511 keV, for a energy window between 400 and 600 keV, is 5.2 ns FWHM. The image resolution measured with point sources was found to be of the order of 1.3 mm FWHM. The DOI capability was found to have a strong impact on the image sharpness. Images of extended uniform 68Ge sources, corrected for sensitivity and for the artifacts due detector dead spaces, have good uniformity. First clinical breast images are presented.

Experimental validation and performance analysis of the clear-PEM data acquisition electronics

Obtaining images with high resolution and contrast from short exams is crucial for the viability of Positron Emission Mammography as an early breast cancer detection technique. The Clear-PEM detector is a Positron Emission Mammography scanner, developed by the Portuguese Consortium in the framework of the Crystal Clear Collaboration at CERN, based on high-granularity avalanche photodiodes readout with 12288 channels, coupled to pixilated 2x2x20 mm3 LYSO:Ce crystals in a double readout configuration. The scanner features a high bandwidth three-level acquisition system with negligible dead time in order to minimize exam time. The frontend is instrumented with low-noise amplifier/discriminator/multiplexer ASICs (L0 trigger) and free-running ADCs while the off-ddetector electronics, implemented in FPGAs, computes the trigger primitives from the pulse amplitude and timing. With this information the readout electronics selects interesting photoelectric events and groups multi-hit events due to in-Compton scatter into possible coincidences. After a first-level trigger (L1) is generated, filtered data is read out over a high speed data link into a second-level (L2) software trigger which reprocesses the selected events in temporal coincidence with more accurate energy and time extraction algorithms. The DOI coordinate and the re-validated energy and time allow to achieve an improved reconstruction of in-detector Compton scattering and rejection of random and scattered coincidences. On this paper, we report on the experimental validation, characterization and optimization of the L1 data acquisition electronics, readout link and L2 software trigger and present performance results. We discuss the validation of the associated operation software.

Performance Simulation Studies of the Clear-PEM DAQ/Trigger System

The Clear-PEM detector is a positron emission mammography scanner based on a high-granularity avalanche photodiode readout with 12 288 channels. The front-end sub-system is instrumented with low-noise 192:2 channel amplifier-multiplexer ASICs and free-running sampling ADCs. The off-detector trigger, implemented in a FPGA based architecture, computes the pulses amplitude and timing required for coincidence validation from the front-end data streams. A high-level C++ simulation tool was developed for data acquisition performance analysis and validated at bit-level against FPGA VHDL testbenches. In this work, simulation studies concerning the performance of the on-line/off-line energy and time extraction algorithms and the foreseen detector energy and time resolution are presented. Time calibration and trigger efficiency are also discussed

First experimental results with the Clear-PEM detector

First experimental results of the imaging system Clear-PEM for positron emission mammography, under development within the framework of the Crystal Clear Collaboration at CERN, are presented. The quality control procedures of crystal pixels, APD arrays and assembled detector modules are described. The detector module performance was characterized in detail. Results on measurements of light yield, energy resolution, depth-of-interaction and inter-channel cross-talk are discussed. The status of the development of the front-end electronics and of the data acquisition boards is reported.