I. Sarasola

PET Demonstrator for a Human Brain Scanner Based on Monolithic Detector Blocks

We have implemented and evaluated a positron emission tomography (PET) demonstrator using two monolithic detector blocks operating in coincidence with dedicated application-specific integrated circuit (ASIC) readout. Each detector is composed of a monolithic lutetium yttrium orthosilicate (LYSO) scintillator coupled to a pair of Hamamatsu S8550-02 APD arrays. The front-end electronics of this demonstrator is based on the VATA240 ASIC readout, which sums the charge provided by each row and column of the APD array. The ASIC has been characterized obtaining a noise per row or column less than 2000 electrons rms with the APD at its inputs and a good linear response in the range from 5 fC to 30 fC. We have acquired energy spectra of 22Na and 137Cs radioactive sources, achieving energy resolutions between 13.2% and 14.1% full width at half maximum (FWHM) at 511 keV. We have estimated the interaction position over the surface of the monolithic blocks using Neural Networks (NN) position determining algorithms, obtaining spatial resolutions at the detector level down to 2.1 mm FWHM. By using this detector technology and electronics we have achieved images of point sources with spatial resolutions as good as 2.1 mm FWHM for filtered back projection (FBP) reconstructions methods with single slice rebinning (SSRB). Based on the results obtained with this demonstrator, we are developing a PET insert for existing magnetic resonance imaging (MRI) equipment, to be installed in a collaborating hospital and used for clinical PET-MRI of the human brain.

Study and optimization of positioning algorithms for monolithic PET detectors blocks

We are developing a PET insert for existing MRI equipment to be used in clinical PET/MR studies of the human brain. The proposed scanner is based on annihilation gamma detection with monolithic blocks of cerium-doped lutetium yttrium orthosilicate (LYSO:Ce) coupled to magnetically-compatible avalanche photodiodes (APD) matrices. The light distribution generated on the LYSO:Ce block provides the impinging position of the 511 keV photons by means of a positioning algorithm. Several positioning methods, from the simplest Anger Logic to more sophisticate supervised-learning Neural Networks (NN), can be implemented to extract the incidence position of gammas directly from the APD signals. Finally, an optimal method based on a two-step Feed-Forward Neural Network has been selected. It allows us to reach a resolution at detector level of 2 mm, and acquire images of point sources using a first BrainPET prototype consisting of two monolithic blocks working in coincidence. Neural networks provide a straightforward positioning of the acquired data once they have been trained, however the training process is usually time-consuming. In order to obtain an efficient positioning method for the complete scanner it was necessary to find a training procedure that reduces the data acquisition and processing time without introducing a noticeable degradation of the spatial resolution. A grouping process and posterior selection of the training data have been done regarding the similitude of the light distribution of events which have one common incident coordinate (transversal or longitudinal). By doing this, the amount of training data can be reduced to about 5% of the initial number with a degradation of spatial resolution lower than 10%.

FlexToT - Current mode ASIC for readout of common cathode SiPM arrays

A front end application specific integrated circuit (ASIC) for the readout of common cathode Silicon Photo-Multipliers arrays is presented with the following features: wide dynamic range, high speed, multi channel, low input impedance current amplifier, low power (≈10mW per channel), common cathode connection, directly coupled input with common mode voltage control and separated timing and charge signal output.A 16 channel prototype with 16 independent outputs for energy and pile-up detection and a single fast timing output is described. The low jitter current mode processing together with a configurable differential current mode logic (CML) output provides a timing signal suitable for Time of Flight (TOF) applications, such as TOF-PET (Positron Emission Tomography). Each channel delivers a digital output of a Time Over Threshold (TOT) type with a pulse width proportional to peak current (charge) input. The current mode input stage features a novel double feedback; a low speed feedback loop keeps input node voltage constant while a higher speed feedback loop keeps input impedance low. Dedicated circuitry allows SiPM high over-voltage operation, thus maximizing Photon Detection Efficiency (PDE) and timing resolution. Design was submitted in June 2012 in Austria Microsystems (AMS) 0.35 μm HBT BiCMOS technology and is under test.

Design and prototyping of a human brain PET scanner based on monolithic scintillators

We are developing a PET insert for existing MRI equipment to be used in clinical PET/MR studies of the human brain. Previous results have demonstrated that our detector concept, based on monolithic scintillator crystals coupled to magnetically-compatible APD matrices with a dedicated ASIC front-end, is suitable for this application. In this work we present the final design of our PET scanner and report on the characterization of a prototype demonstrator used to validate the coincidence processing and data readout architecture.

A novel front-end chip for a human PET scanner based on monolithic detector blocks

We are developing a positron emission tomography (PET) scanner based on avalanche photodiodes (APD), monolithic LYSO:Ce scintillator crystals and a dedicated readout chip. All these components allow operation inside a magnetic resonance imaging (MRI) scanner with the aim of building a PET/MRI hybrid imaging system for clinical human brain studies. Previous work verified the functional performance of our first chip (VATA240) based on a leading edge comparator and the principle of operation of our radiation sensors, which are capable of providing reconstructed images of positron point sources with spatial resolutions of 2.1 mm FWHM. The new VATA241 chip presented in this work has been designed with the aim of reducing the coincidence window of our final PET scanner by implementing an on-chip constant fraction discriminator (CFD), as well as providing a better robustness for its implementation in the full-scale PET scanner. Results from the characterization of the VATA241 chip are presented, together with the first results on coincidence performance, validating the new design for our application.

Performance evaluation of a PET demonstrator for PET-MR imaging based on monolithic LYSO:Ce scintillators

We are developing a positron emission tomography (PET) insert based on avalanche photodiode (APD) arrays and monolithic LYSO:Ce scintillators for human brain functional studies to be used inside a clinical magnetic resonance imaging (MRI) equipment. In a previous work [1], we demonstrated the performance of our detectors by implementing an experimental setup consisting of two monolithic blocks working in coincidence, which were read out by the first version of an application-specific integrated circuit (ASIC), VATA240, followed by external coincidence and digitalization modules. This preliminary demonstrator showed good spatial resolution at detector level on the order of 2.2 mm full-width at half-maximum (FWHM) and good imaging qualities, which achieved reconstructed images of 22Na point sources with spatial resolutions of 2.1 mm FWHM. Nevertheless, we detected image distortions and compressions due to the non-linearities close to the edge of the crystals and the simplicity of that demonstrator with the absence of neighbor blocks [1]. In this work we have implemented a larger scale PET demonstrator, which is based on the new updated ASIC (VATA241) [2] and is formed by two sectors of four monolithic detector blocks placed face-to-face. This new prototype demonstrator has been built for validating the data readout architecture, the coincidence processing implemented in a Xilinx Virtex 5 field programmable gate array (FPGA), as well as the continuous neural networks (NN) training method required to determine the points of entrance over the surface of our monolithic detector blocks.

Evaluation of the FlexToT ASIC on the readout of SiPM matrices and scintillators for PET

We have designed FlexToT, a flexible application specific integrated circuit (ASIC) using time-over-threshold (ToT) techniques for the readout of silicon photomultiplier (SiPM) arrays in positron emission tomography (PET) detectors. The FlexToT ASIC accommodates 16 independent channels in a small die size (2.93 mm by 2.54 mm) with low power consumption (10 mW per channel), providing output signals whose duration is proportional to the current amplitude at its inputs. The threshold, gain and other parameters can be independently set for each channel in a flexible configuration that can be tailored to different applications and devices. Herein we present the results on the evaluation of the FlexToT ASIC operating as front-end electronics for SiPM arrays S11828-3344M from Hamamatsu (Japan) coupled to LYSO:Ce scintillators in a simple PET tomograph demonstrator. Each detector block is composed of a LYSO:Ce matrix coupled to 4 SiPM arrays in a 2×2 mosaic, totaling 64 pixels (8×8) which are read out by 4 FlexToT ASICs. Using this demonstrator we have obtained PET images of 22Na point sources with spatial resolutions better than 2 mm full width at half maximum (FWHM), validating our design of the FlexToT ASIC on the readout of SiPM matrices and scintillators in PET applications.

Evaluation of a PET prototype using LYSO:Ce monolithic detector blocks

We have analyzed the performance of a PET demonstrator formed by two sectors of four monolithic detector blocks placed face-to-face. Both front-end and read-out electronics have been evaluated by means of coincidence measurements using a rotating 22Na source placed at the center of the sectors in order to emulate the behavior of a complete full ring. A continuous training method based on neural network (NN) algorithms has been carried out to determine the entrance points over the surface of the detectors. Reconstructed images from 1 MBq 22Na point source and 22Na Derenzo phantom have been obtained using both filtered back projection (FBP) analytic methods and the OSEM 3D iterative algorithm available in the STIR software package [1]. Preliminary data on image reconstruction from a 22Na point source with ∅ = 0.25 mm show spatial resolutions from 1.7 to 2.1 mm FWHM in the transverse plane. The results confirm the viability of this design for the development of a full-ring brain PET scanner compatible with magnetic resonance imaging for human studies.

Characterization of the VATA241 front-end ASIC for the BrainPET scanner

The BrainPET scanner is a magnetically-compatible human PET insert based on APDs and LYSO:Ce monolithic scintillators, which is currently under development at CIEMAT, intended for the operation inside magnetic resonance imaging (MRI) equipment. An ASIC called VATA241 was specifically designed for the BrainPET project. This ASIC includes a constant fraction discriminator (CFD) in its architecture and presents a better robustness and reliability with respect to the first leading-edge-based ASIC [1], which was successfully utilized in our first BrainPET demonstrator for validating the principle of operation of our detector blocks [2]. In this work we have characterized the VATA241 in terms of noise, time walk, jitter, linearity and operation with detector blocks. Moreover, we have carried out its optimization performance by adjusting the corresponding different bias currents. Based on the characterization carried out by CIEMAT, manufacturer Gamma Medica-Ideas (GM-I) has performed the tuning of the simulation model of this ASIC as well as the optimization of the timing performance for the fabrication of the final VATA241.2 ASIC, which will be used in our full-ring PET scanner.

Performance of the FlexToT v2 ASIC on the readout of different detector designs for PET

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.