C. S. Ferreira

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

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.

Development of breast and tumour models for simulation of novel multimodal PEM-UWB technique for detection and classification of breast tumours

This paper presents an initial study towards the development of a multimodal breast imaging technique that combines Positron Emission Mammography (PEM) and Ultra WideBand (UWB) radar which has the potential to be complementary in breast cancer detection and classification due to the different types of information provided by the two modalities. The first step in evaluating test the potential of the multimodal PEM-UWB technique is to develop realistic numerical breast and tumour models, which can simultaneously be used for both PEM and UWB.

Assessment of DOI resolution in a fully mounted PEM scanner

The ClearPEM scanner is a dedicated system to perform breast imaging with the ability of measuring DOI, allowing an image spatial resolution close to 1.5 mm. The DOI is measured by calculating the amplitude asymmetry between the signals produced by two APD, one in the top and other in the bottom of the crystal. The DOI resolution measured in laboratory tests is 2.0 mm. In this paper we present a method to periodically confirm this value with the scanner fully mounted. The DOI resolution obtained with this method was 2.8 mm, which is consistent with the resolution measured during the laboratory tests.

ClearPEM scanners: Performance results and studies in preclinical environment

The ClearPEM detector is a dual planar Positron Emission Mammography (PEM) scanner that was developed within the framework of the international Crystal Clear Collaboration at CERN. The scanner is being used for preclinical studies and the new phase of the clinical trials will start on the second half of November. A second prototype was build and installed (ClearPEM-Sonic). The main results presented in this paper were obtained with the first prototype. The scanner image performance was evaluated using gelatin phantoms. The methodology included the quantification of the homogeneity (BV) and statistical noise (COV). Lesion detectability was evaluated using parameters such as lesion to background noise (LTBN) and lesion contrast recovery coefficient (CRC). Results indicate BV and COV of the order of 5-9% and 20-24%, respectively. ClearPEM images have been found globally homogeneous showing no artifacts. Small lesion to background ratio (LTB) and lesions of dimensions smaller than 3 mm showed less good contrast, but in most cases we report CRC values between 80% and 100%. The detection of lesions is feasible down to 2-3 mm in diameter.

Scatter and attenuation corrections for a PEM detector using List-Mode OSEM

Emission Tomography imaging techniques are known for its quantitative potential. As in whole-body Positron Emission Tomography (PET), also in breast dedicated Positron Emission Mammography (PEM) systems, this goal can only be achieved if a set of corrections for known physics processes is implemented thoroughly. Corrections for Compton scattered events, photon attenuation and random events are examples of such important data corrections. In this work, we report the performance of scatter and attenuation corrections, specifically implemented for PEM. These correction methods were tested on data from the ClearPEM scanner, a dual-head planar system. The ClearPEM scanner does not incorporate transmission images for attenuation correction. With no anatomical information available, the PEM emission image was used to obtain the acquisitions characteristics needed as input for both the Monte Carlo (MC) based scatter correction method and the attenuation correction algorithm. A MC scatter correction can accurately model the scatter distribution of a PEM acquisition, but removing this estimate from the acquired data can result in a severe bias and increased noise in low counts PEM studies. To avoid such drawbacks, an approach based on a smoothed imaged of the scatter distribution was used. Due to the typically low counts nature of PEM acquisitions, correction procedures were combined with a 3D-OSEM-LM reconstruction, so reconstructed images could be obtained in a short period of time. Results demonstrated that both corrections perform as expected. The presence of scattered events was strongly reduced. Counts loss due to photon attenuation was clearly compensated, along with the recovery of the correct activity distribution.

Image-based data corrections for Positron Emission Mammography

ClearPEM is a rotating, dual planar detector PET system dedicated to breast and axilla imaging that needs data corrections in order to improve quantification and lesion detectability. Since the scanner does not have CT nor transmission sources, we are implementing data corrections that use emission-only data. Also, since in this work we are using an iterative list-mode reconstruction algorithm that projects the data directly through a 3D grid of points, all the corrections are applied in image space. We describe the current implementation and validation of image-based corrections for normalization, scatter, attenuation and randoms. Normalization correction uses sensitivity images generated by backprojecting data from a planar source. Energy-based scatter correction is applied using the Estimation of Trues Method because this method does not require information about attenuating media and takes into account the activity outside the FOV. Attenuation correction is calculated from the segmentation of the breast in the emission images. Randoms correction is performed by subtraction of a smoothed image obtained from the backprojection of the delayed coincidences. The implementation of other corrections is also being pursued in order to obtain fully quantitative images in breast imaging.

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.