A method to improve light collection efficiency of gamma-ray imaging detectors by using retroreflector arrays has been tested, simulations of the behaviour of the scintillation light illuminating the retroreflector surface have been made. Measurements including retroreflector arrays in the setup have also been taken. For the measurements, positron emission tomography (PET) detectors with continuous scintillation crystals have been used. Each detector module consists of a continuous LSO-scintillator of dimensions 49x49x10 mm(3) and a H8500 position-sensitive photo-multiplier (PSPMT) from Hamamatsu. By using a continuous scintillation crystal, the scintillation light distribution has not been destroyed and the energy, the centroids along the x- and y-direction and the depth of interaction (DOI) can be estimated. Simulations have also been run taking into account the use of continuous scintillation crystals. Due to the geometry of the continuous scintillation crystals in comparison with pixelated crystals, a good light collection efficiency is necessary to correctly reconstruct the impact point of the gamma-ray. The aim of this study is to investigate whether micro-machine retro-reflectors improve light yield without misestimation of the impact point. The results shows an improvement on the energy and centroid resolutions without worsening the depth of interaction resolution. Therefore it can be concluded that using retroreflector arrays at the entrance side of the scintillation crystal improves light collection efficiency without worsening the impact point estimation.

In this paper we describe the overall design of a PET-MR system for head imaging within the framework of the DREAM Project as well as the first detector module tests. The PET system design consists of 4 rings of 16 detector modules each and it is expected to be integrated in a head dedicated radio frequency coil of an MR scanner.The PET modules are based on monolithic LYSO crystals coupled by means of optical devices to an array of 256 Silicon Photomultipliers. These types of crystals allow to preserve the scintillation light distribution and, thus, to recover the exact photon impact position with the proper characterization of such a distribution. Every module contains 4 Application Specific Integrated Circuits (ASICs) which return detailed information of several light statistical momenta. The preliminary tests carried out on this design and controlled by means of ASICs have shown promising results towards the suitability of hybrid PET-MR systems. (C) 2012 Elsevier B.V. All rights reserved.

Improving timing resolution in positron emission tomography (PET), thus having fine time information of the detected pulses, is important to increase the reconstructed images signal to noise ratio (SNR) [1]. In the present work, an integrated circuit topology for time extraction of the incoming pulses is evaluated. An accurate simulation including the detector physics and the electronics with different configurations has been developed. The selected architecture is intended for a PET system based on a continuous scintillation crystal attached to a SiPM array. The integrated circuit extracts the time stamp from the first few photons generated when the gamma-ray interacts with the scintillator, thus obtaining the best time resolution. To get the time stamp from the detected pulses, a time to digital converter (TDC) array based architecture has been proposed as in [2] or [3]. The TDC input stage uses a current comparator to transform the analog signal into a digital signal. Individually configurable trigger levels allow us to avoid false triggers due to signal noise. Using a TDC per SiPM configuration results in a very area consuming integrated circuit. One solution to this problem is to join several SiPM outputs to one TDC. This reduces the number of TDCs but, on the other hand, the first photons will be more difficult to be detected. For this reason, it is important to simulate how the time resolution is degraded when the number of TDCs is reduced. Following this criteria, the best configuration will be selected considering the trade-off between achievable time resolution and the cost per chip. A simulation is presented that uses Geant4 for simulation of the physics process and, for the electronic blocks, spice and Matlab. The Geant4 stage simulates the gamma-ray interaction with the scintillator, the photon shower generation and the first stages of the SiPM. The electronics simulation includes an electrical model of the SiPM array and all the integrated circuitry that generates the time stamps. Time resolution results are analyzed using Matlab. The goal is to analyze the best resolution achievable with the SiPM and its degradation due to different circuitry configurations.

AMIC architecture has been introduced in previous works in order to provide a generic and expandable solution for implementing large number of outputs SiPM array/PMT detectors. The underlying idea in AMIC architecture is to calculate the moments of the detected light distribution in an analog fashion. These moments provide information about energy, x/y position, etc. of the light distribution of the detected event. Moreover this means that a small set of signals contains most of the information of the event, thus reducing the number of channels to be acquired.This paper introduces a new front-end device AMIC2GR which implements the AMIC architecture improving the features of the former integrated devices. Higher bandwidth and filtering coefficient precision along with a lower noise allow to apply some detector enhancements. Inhomogeneity among detector elements throughout the array can be reduced. Depth of interaction measurements can be obtained from the light distribution analysis. Also a common trigger signal can be obtained for the whole detector array. Finally AMIC2GR preamplifier stage close to SiPM output signals optimizes signal to noise ratio, which allows to reduce SiPM gain by using lower operating voltages thus reducing dark noise.

The center of gravity algorithm leads to strong artifacts for gamma-ray imaging detectors that are based on monolithic scintillation crystals and position sensitive photo-detectors. This is a consequence of using the centroids as position estimates. The fact that charge division circuits can also be used to compute the standard deviation of the scintillation light distribution opens a way out of this drawback. We studied the feasibility of maximum likelihood estimation for computing the true gamma-ray photo-conversion position from the centroids and the standard deviation of the light distribution. The method was evaluated on a test detector that consists of the position sensitive photomultiplier tube H8500 and a monolithic LSO crystal (42mm×42mm×10mm). Spatial resolution was measured for the centroids and the maximum likelihood estimates. The results suggest that the maximum likelihood positioning is feasible and partially removes the strong artifacts of the center of gravity algorithm.

Intrinsic parameters in scintillation crystals play a crucial role in the measured spatial and energy resolutions. In many cases the measurement of these intrinsic parameters is difficult to be carried out or even there are parameters that need a specific setup to be measured directly, e.g. the intrinsic light yield and the absorption and scattering coefficients. These uncertainties in the parametrization of the characteristics of the scintillation crystals make it quite difficult to evaluate the quality and characteristics of the scintillation crystals and their impact on the resolutions. Results of measurements of depth of interaction and its resolutions for three different scintillation crystals (one LSO and LYSO of two different providers) are presented. We have noticed that for one of the LYSO a worse depth of interaction resolution (ΔDOI) was obtained when compared to the other both crystals. Measurements of the transmittance of all these crystals allow us to obtain the scattering coefficient (αscatt) for wavelengths of λ=0.40–0.50 [μm].We found that different values of the scattering coefficient for the different crystals are consistent with the measured resolutions for the depth of interaction and prove the strong impact of the αscatt for each crystal on the ΔDOI.

A novel design for an inexpensive depth of interaction capable detector for γ-ray imaging has been developed. The design takes advantage of the strong correlation between the width of the scintillation light distribution in monolithic crystals and the interaction depth of γ-rays. We present in this work an inexpensive modification of the commonly used charge dividing circuits which enables the instantaneous and simultaneous computation of the second order moment of light distribution. This measure provides a good estimate for the depth of interaction and does not affect the determination of the position centroids and the energy release of γ-ray impact. The method has been tested with a detector consisting of a monolithic LSO block sized 42×42×10mm3 and a position-sensitive photomultiplier tube H8500 from Hamamatsu. The mean spatial resolution of the detector was found to be 3.4mm for the position centroids and 4.9mm for the DOI. The best spatial resolutions were observed at the center of the detector and yielded 1.4mm for the position centroids and 1.9mm for the DOI.

We report on the design of a small animal PET scanner based on continuous LYSO crystals and position sensitive photomultiplier tubes (PSPMTs), together with the first results from the calibration. The scanner consists of eight compact modules forming an octagon and leaving a port of 110 mm aperture. Each module is made out of a continuous LYSO crystal and a PSPMT, and contains its associated electronics together with its power supply. For each module, five signals are read, summarizing all the information coming out from its 64 anode pads. Therefore, for the whole scanner only 40 signals are digitized. A calibration procedure has been implemented, measuring a spatial resolution of approximately 1.5 mm at the center of the field of view and an energy resolution of 18%. The sensitivity of the system at the center of the field of view, using only 4 modules, is observed to be of about 1%.

We studied the possibility of correcting edge artifacts caused by the center of gravity positioning algorithm (COG) within thick continuous scintillation crystals for /spl gamma/-ray imaging detectors. An enhanced charge division readout (CDR) together with multi-channel position sensitive photo-multiplier tubes allows the measurement of the 2nd moment of the scintillation light distribution within continuous and large-sized crystals. Using the 2nd moment together with the centroids provided by the CDR, the standard deviation /spl sigma/ of the light distribution can be computed, providing a good estimator for the depth of interaction (DOI). We measured the /spl sigma/ resolution at 81 positions of a test-detector's sensitive area and found an average resolution of 6/spl plusmn/4.0 mm. At the central region of 20/spl times/20 mm/sup 2/ the performance of our method was with 3/spl plusmn/0.8 mm /spl sigma/ resolution much better. The lowest observed value was 1.8 mm. The 0th, 1st/sub X/, 1st/sub Y/ and 2nd moments of the light distribution were also measured for the 81 positions and compared to predictions obtained by a model for the light distribution. Analytic studies show that the use of the first low order moments probably allows significant reduction of the DOI-caused border artifacts of the COG algorithm.