R. Moura

Experimental characterization of the Clear-PEM scanner spectrometric performance

In the framework of the Clear-PEM project for the construction of a high-resolution and high-specificity scanner for breast cancer imaging, a Po sitron Emission Mammography tomo- graph has been developed and installed at the Instituto Portuguˆ es de Oncologia do Portohospital. The Clear-PEM scanner is mainly composed by two planar detector heads attached to a robotic arm, trigger/data acquisition electronics system and computing servers. The detector heads hold crystal matrices built from 2 × 2 × 20 mm 3 LYSO:Ce crystals readout by Hamamatsu S8550 APD arrays. The APDs are optically coupled to both ends of the 6144 crystals in order to extract the DOI information for each detected event. Each one of 12288 APDs pixels is read and controlled by Application Specific Integrated Circuits water-cooled by a n external cooling unit. The Clear-PEM frontend boards innovative design results in a unprecedented integration of the crystal matrices, APDs and ASICs, making Clear-PEM the PET scanner with the highest number of APD pixels ever integrated so far. In this paper, the scanners main technical characteristic s, calibration strategies and the first spectrometric performance evaluation in a clinical environment are presented. The first commissioning results show 99.7% active channels, which, after calibration, have inter-pixel and absolute gain distributions with dispersi ons of, respectively, 12.2% and 15.3%, demonstrating that despite the large number of channels, the system is uniform. The mean en- ergy resolution at 511 keV is of 15.9%, with a 8.8% dispersion, and the mean C −1

Clear-PEM: Monte Carlo performance and image reconstruction studies

The Clear-PEM prototype under development aims to improve early stage breast cancer diagnostics. The proposed device is based on cerium doped lutetium crystal matrices developed by the Crystal Clear Collaboration, as well as on modern data acquisition techniques. A series of Monte Carlo studies were performed to evaluate detection sensitivity, background rate and intrinsic spatial resolution in order to optimize the final detector concept. A description of the developed GEANT4 based simulation framework and PEM image reconstruction software is also presented in this paper. First simulation results indicates that Clear-PEM design significantly increases detection sensitivity in comparison with conventional PET cameras for breast cancer diagnostics. Count-rate simulation results are within operation limits for the data acquisition system, able to read 1 MHz event rate, allowing to take full profit of the large detector acceptance.

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.

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.

High-speed Data Acquisition Electronics for a PEM scanner

Positron Emission based mammography systems are expected to constitute a significant breakthrough in the early diagnosis of breast cancer. This may lead to a significant enlargement in the horizon of women life, since breast cancer is considered to be among the most deadly types. With this purpose, a consortium has been built to develop a mammography PET system in the framework of the Clear-PEM Collaboration. The Clear-PEM scanner is a unique Positron Emission Mammography (PEM) scanner using APD-based detectors. It is capable of measuring Depth-of-Interaction (DoI) with a resolution of 2 mm in LYSO:Ce crystals, enabling a spatial resolution of 1.2 mm. An important part of the PEM scanner is a high-speed DAE (Data Acquisition Electronics) system. DAE has been developed to process and acquire data coming from the 12,288 front-end (FE) photo-detector channels. In this paper, an overview of this scanner (system and board-level electronics, mechanical design, service and image reconstruction software) is presented, focusing on the specification and implementation of the DAE which is responsible for real-time retrieving, analyzing, filtering and formatting the data coming from the front-end. The objective is to identify relevant data and after conditioning, according to pre-defined protocols, send the data to an acquisition PC for image reconstruction. Communication between DAE and PC is carried out using the S-Link (proprietary of CERN) protocol. Experimental results of validation and test of the DAE system are presented.

Characterization of avalanche photodiode arrays for the ClearPEM and ClearPEM-Sonic scanners

The ClearPEM scanner is a high-resolution Positron Emission Mammography prototype, developed by the Portuguese PET Consortium in the framework of the Crystal Clear Collaboration at the European Organization for Nuclear Research (CERN). The scanner is based on a novel readout scheme which uses fine-pitch scintillator crystals, avalanche photodiodes, low-noise high-gain integrated electronics and a fast reconfigurable digital data acquisition system. The scanner uses two planar detector heads each composed of 96 detector modules. The detector module is composed of a matrix of 32 identical 2 ? 2 ? 20 mm3 LYSO:Ce scintillator crystals, coupled at both ends to Hamamatsu S8550 APD arrays for Depth-of-Interaction (DOI) measurements. More recently, a new scanner named ClearPEM-Sonic which combines the ClearPEM technology with an Ultrasound apparatus, is being produced. A set of 984 APD arrays used in both scanner assemblies followed a quality control protocol and a characterization procedure. This paper describes the methods used in these measurements and the results obtained with the full APD production batch.

Simple statistical models for relating river discharge with precipitation and air temperature—Case study of River Vouga (Portugal)

Simple statistical models were developed to relate available meteorological data with daily river discharge (RD) for rivers not influenced by melting of ice and snow. In a case study of the Vouga River (Portugal), the RD could be determined by a linear combination of the recent (PR) and non-recent (PNR) atmospheric precipitation history. It was found that a simple linear model including only PR and PNR cannot account for low RD. The model was improved by including non-linear terms of precipitation that accounted for the water loss. Additional improvement of the models was possible by including average monthly air temperature (T). The best model was robust when up to 60% of the original data were randomly removed. The advantage is the simplicity of the models, which take into account only PR, PNR and T. These models can provide a useful tool for RD estimation from current meteorological data.

Development of a high packing fraction detector module with DOI measurement capability for high-resolution PET

The Clear-PEM scanner was developed by the Portuguese PET Consortium under the framework of the Crystal Clear Collaboration for detection of early stage breast cancer. The scanner relies on a modular design in which detection modules are readout by low-noise ASICs that extract the pulse shape information for energy and timing measurements. The developed module has a dual readout, in which avalanche photodiodes arrays are used to extract the light from 20 mm long 2×2 mm2 cross section LYSO:Ce crystals, with a typical depth of interaction (DOI) resolution of 2.2 mm FWHM. The packing fraction of the module is 64%. In order to further improve the detection sensitivity of the scanner, by increasing the packing fraction while maintaining the already achieved DOI resolution, experimental work on the upgrade of the detector module was carried out. The work consisted on the exploration of arrays with 30 mm long crystals as well as the use of crystals with 2.2×2.2 mm2 cross section wrapped in a specular reflector, yielding a larger packing fraction, of about 78%. Energy, DOI resolution and the relative light collection were measured for polished and unpolished crystal arrays. For the polished crystal array, the variation of the photopeak position within the same matrix, was 6% r.m.s. For the 30 mm unpolished crystal array the energy resolution was 14.7% with a DOI resolution of 2.7 mm FWHM. An approach to improve the light collection efficiency is reported in which individual crystals have a mixture of different number of polished and un-polished longitudinal faces. Using crystals with two longitudinal sides polished, leaving the remaining as-cut, allows extracting the DOI coordinate with a resolution of 2.8 mm FWHM, an energy resolution of 16% and the improvement in light collection of about 30%, in respect to the unpolished matrices. Based on these measurements a new set of detector modules were produced and characterized using the Clear-PEM frontend and data acquisition electronics. Preliminary results for the total light collection, light collection asymmetry and energy resolution are reported.