Performance of the FlexToT v2 ASIC on the Readout of Different Detector Designs for PET José M. Cela, José M. Fernández-Varea, Lluís Freixas, Lluís Garrido, David Gascón, Ricardo Graciani, Jesús Marín, Gustavo Martínez, Joan Mauricio, Juan C. Oller, José M. Pérez, Member, IEEE, Pedro Rato-Mendes, Member, IEEE, David Sánchez, Andreu Sanuy, Iciar Sarasola, Oscar de la Torre, Oscar Vela Abstract–A new version of the FlexToT application-specific integrated circuit (ASIC) has been designed and fabricated with an extended dynamic range and improved channel uniformity suitable for readout of different detector block designs in time of flight (TOF) positron emission tomography (PET) applications. The performance of the FlexToT v2 ASIC has been evaluated using segmented, monolithic and phoswich scintillator elements and matrices coupled to silicon photmultiplier (SiPM) arrays. The enhanced dynamic range of FlexToT v2 compared to its previous version allows the correct identification of individual crystals in scintillator matrices, both single layer and phoswich. Operation with monolithic scintillators was also demonstrated, with energy resolutions of 18% (FWHM) at 511 keV and reconstructed PET images of point sources yielding spatial resolutions on the order of 2 mm (FWHM). The results show that the FlexToT v2 ASIC is a flexible solution for the front-end readout of different designs of SiPM-based scintillator detectors in TOF-PET applications.
I. INTRODUCTION
P
OSITRON emission tomography (PET) is an imaging technique in constant evolution since the beginning of this century, with important advances such as the introduction of computed tomography (CT) in a combined PET-CT scanner [1], improved sensitivity by using time-of-flight (TOF) information in TOF-PET [2], simultaneous operation inside magnetic resonance (MR) scanners [3] and, more recently, the advent of integrated TOF-PET/MR whole-body systems [4]. These advances were fuelled by developments at the level of scintillators, becoming faster and more efficient, high-gain fast photosensors insensitive to strong magnetic fields such as the silicon photomultiplier (SiPM), and fast readout electronics able to process large amounts of data in short times. The present requirements of having a considerable number of readout channels in a limited space impose the use of application-specific integrated circuits (ASICs), some of which are intended for the readout of scintillator-based detectors in TOF-PET. The FlexToT ASIC was developed with this application in mind, specifically for the readout of
Manuscript received November 20, 2015. This work was supported in part by the Spanish Ministry of Economy and Competitiveness under contract TEC2012-36485. J. M. Cela, L. Freixas, J. Marín, G. Martínez, , J. C. Oller, J. M. Pérez, P. Rato-Mendes and I. Sarasola are with CIEMAT, Avda. Complutense 40, E28040 Madrid, Spain (corresponding author: Pedro Rato-Mendes; phone: +34 91 346 6278; fax: +34 91 346 6275; e-mail:
[email protected]). J. M. Fernández-Varea, L. Garrido, D. Gastón, J. Mauricio, D. Sánchez, A. Sanuy and O. de la Torre are with Institut de Ciències del Cosmos, Universitat de Barcelona, Facultat de Fisica, Martí i Franqués 1, E-08028 Barcelona, Spain.
SiPM arrays coupled to fast scintillators. In this work we report on the performance of the second version of this ASIC on the readout of segmented, phoswich and monolithic scintillator detector block designs suitable for PET imaging. The following sections introduce details on the FlexToT v2 ASIC, the detector designs considered and the PET demonstrator used to obtain tomographic images, concluding with a summary of the results obtained. II. MATERIALS AND METHODS A. The FlexToT v2 ASIC The FlexToT ASIC was developed in collaboration between Universitat de Barcelona (Barcelona, Spain) and CIEMAT (Madrid, Spain), and manufactured in 0.35 micron HBT BiCMOS technology by AMS (Graz, Austria). The ASIC implements independent threshold and bias control for each of its 16 DC-coupled channels, a time-over-threshold (ToT) output for each channels and a common fast trigger (OR of all 16 channels) for timing. The amplitude of the charge pulses at its inputs is encoded as duration of output pulses with linearity better than 5% for input currents ranging from 2.5 mA to 18 mA [5]. The characterization of the first version and an evaluation of its performance on the readout of scintillator matrices for PET imaging has been published, showing a trigger jitter better than 30 ps (rms), coincidence time resolution of 269 ps (FWHM), energy resolution of 9.6% (FWHM) at 511 keV and reconstructed spatial resolution of 1.9 mm (FWHM) for a 0.25 mm diameter 22Na point source [5-7]. The new FlexToT v2 improves upon its predecessor by extending its linear dynamic range down to 0.7 mA and enhancing channel gain uniformity in order to allow operation with monolithic scintillators, furthermore adding a calibration input [8]. The front-end readout and data acquisition system used in this work is similar to the one developed for the first version of FlexToT, the details of which can be found in [6]. B. PET detector blocks Three geometries of detector blocks suitable for PET imaging have been considered: a monolithic LYSO:Ce scintillator with dimensions 27.5 x 27.5 x 10 mm3 wrapped in several sheets of Teflon tape; a single-layer LYSO:Ce matrix with 13 x 13 elements of 2 x 2 x 10 mm3 each separated by 0.1 mm of BaSO4 acting as a diffuse white reflector and optical isolator; and a LYSO:Ce/GSO:Ce phoswich matrix with 13 x 13 elements of 1.45 x 1.45 mm2 cross section and 7 mm
,(((
LYSO:Ce / 8 mm GSO:Ce thicknesses, separated by 0.1 mm of white reflector [9] (Fig. 1). The coupling between the scintillators and the SiPM arrays was done using Saint-Gobain BC-630 Optical Grease; the phoswich matrix was read out from the GSO:Ce layer. Flood images were obtained for the monolithic block and both scintillator matrices using a 1 MBq 22 Na source, and the centroid of the light distribution over all SiPM pixels was calculated for each interaction event (Fig. 2). In the case of the phoswich matrix, polymethylmetacrilate (PMMA) layers of 1, 2, 3 and 4 mm thickness were used as light guides between scintillators and SiPM arrays in order to spread through a larger number of pixels the light channeled in each crystal element. The monolithic scintillator was evaluated for energy resolution and linearity using 1 MBq 22 Na and 370 kBq 137Cs sources. All measurements were performed using 2 x 2 mosaics of Hamamatsu S11828-3344M arrays read out by 4 FlexToT v2 ASIC, totaling 64 channels per detector block.
Fig. 2. Flood images of the detector blocks used in this work. Left to right: monolithic, segmented and phoswich (see text and Fig. 1. for details). Positions of the SiPM pixels are overlaid in the leftmost image.
Fig. 1. Photographs of the PET detector blocks used in this work. Left to right: monolithic LYSO:Ce (27.5 x 27.5 x 10 mm3); segmented LYSO:Ce (13 x 13 elements of 2 x 2 x 10 mm3); and phoswich LYSO:Ce/GSO:Ce (13 x 13 elements of 1.45 x 1.45 x 7+8 mm3).
Fig. 3. The water-cooled PET demonstrator used in this work (left) and an overlay of seven reconstructed images of a 0.25 mm diameter 22Na point source placed at different positions at 2 mm steps (right).
C. PET demonstrator To assess the imaging capabilities of the monolithic scintillators with FlexToT v2 readout we have used a simple PET demonstrator similar to the one used in previous works [6], consisting of a rotating motorized source holder centered between two detector boxes with scintillators standing face to face at a nominal distance of 200 mm (Fig. 3, left). For this occasion the detector boxes were water-cooled at 20 ºC for improved stabilization and better signal-to-noise ratio. Data from a 1 MBq 22Na point source with 0.25 mm diameter were acquired at different positions across the field-of-view (FoV) at 2 mm steps. For each position, tomographic projections were obtained by rotating the source between the stationary detector boxes. Images were reconstructed using single slice rebinning (SSRB) of 3D data into 2D planes, followed by filtered back-projection (FBP) in each plane. The FoV was 32 x 32 mm2 with voxels of (0.5 mm)3. III. RESULTS AND CONCLUSION The performance of the new FlexToT v2 ASIC has been evaluated in the readout of different scintillator detector designs for PET imaging (Fig. 1). Flood images of 22Na sources were obtained with monolithic and both single-layer and phoswich scintillator matrices coupled to SiPM arrays read out by FlexToT v2 ASICs, showing good crystal identification capabilities for different detector geometries (Fig. 2). The extended dynamic range of the FlexToT v2 ASIC
made possible its use in the readout of monolithic scintillators, yielding an energy resolution of 18% (FWHM) at 511 keV with a linearity better than 0.99 up to 1274 keV and PET images of point sources with resolutions on the order of 2 mm (FWHM) (Fig. 3). The overall results indicate that the new FlexToT v2 ASIC provides a flexible solution for the frontend readout of different SiPM-based scintillator detector geometries in time-of-flight PET applications.
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