Ane Etxebeste

3D position determination in monolithic crystals coupled to SiPMs for PET

The interest in using continuous monolithic crystals in positron emission tomography (PET) has grown in the last years. Coupled to silicon photomultipliers (SiPMs), the detector can combine high sensitivity and high resolution, the two main factors to be maximized in a positron emission tomograph. In this work, the position determination capability of a detector comprised of a [Formula: see text] mm(3) LYSO crystal coupled to an [Formula: see text]-pixel array of SiPMs is evaluated. The 3D interaction position of γ-rays is estimated using an analytical model of the light distribution including reflections on the facets of the crystal. Monte Carlo simulations have been performed to evaluate different crystal reflectors and geometries. The method has been characterized and applied to different cases. Intrinsic resolution obtained with the position estimation method used in this work, applied to experimental data, achieves sub-millimetre resolution values. Average resolution over the detector surface for 5 mm thick crystal is  ∼0.9 mm FWHM and  ∼1.2 mm FWHM for 10 mm thick crystal. Depth of interaction resolution is close to 2 mm FWHM in both cases, while the FWTM is  ∼5.3 mm for 5 mm thick crystal and  ∼9.6 mm for 10 mm thick crystal.

First Images of a Three-Layer Compton Telescope Prototype for Treatment Monitoring in Hadron Therapy.

A Compton telescope for dose monitoring in hadron therapy is under development at IFIC. The system consists of three layers of LaBr3 crystals coupled to silicon photomultiplier arrays. 22Na sources have been successfully imaged reconstructing the data with an ML-EM code. Calibration and temperature stabilization are necessary for the prototype operation at low coincidence rates. A spatial resolution of 7.8 mm FWHM has been obtained in the first imaging tests.

submitter : Performance evaluation of MACACO: a multilayer Compton camera

Compton imaging devices have been proposed and studied for a wide range of applications. We have developed a Compton camera prototype which can be operated with two or three detector layers based on monolithic lanthanum bromide ([Formula: see text]) crystals coupled to silicon photomultipliers (SiPMs), to be used for proton range verification in hadron therapy. In this work, we present the results obtained with our prototype in laboratory tests with radioactive sources and in simulation studies. Images of a [Formula: see text]Na and an [Formula: see text]Y radioactive sources have been successfully reconstructed. The full width half maximum of the reconstructed images is below 4 mm for a [Formula: see text]Na source at a distance of 5 cm.

Performance of VATA64HDR16 ASIC for medical physics applications based on continuous crystals and SiPMs

Detectors based on Silicon Photomultipliers (SiPMs) coupled to continuous crystals are being tested in medical physics applications due to their potential high resolution and sensitivity. To cope with the high granularity required for a very good spatial resolution, SiPM matrices with a large amount of elements are needed. To be able to read the information coming from each individual channel, dedicated ASICs are employed. The VATA64HDR16 ASIC is a 64-channel, charge-sensitive amplifier that converts the collected charge into a proportional current or voltage signal. A complete assessment of the suitability of that ASIC for medical physics applications based on continuous crystals and SiPMs has been carried out. The input charge range is linear from 0−2 pC up to 55 pC. The energy resolution obtained at 511 keV is 10% FWHM with a LaBr3 crystal and 16% FWHM with a LYSO crystal. A coincidence timing resolution of 24 ns FWHM is obtained with two LYSO crystals.

Study and comparison of different sensitivity models for a two-plane Compton camera

Given the strong variations in the sensitivity of Compton cameras for the detection of events originating from different points in the field of view (FoV), sensitivity correction is often necessary in Compton image reconstruction. Several approaches for the calculation of the sensitivity matrix have been proposed in the literature. While most of these models are easily implemented and can be useful in many cases, they usually assume high angular coverage over the scattered photon, which is not the case for our prototype. In this work, we have derived an analytical model that allows us to calculate a detailed sensitivity matrix, which has been compared to other sensitivity models in the literature. Specifically, the proposed model describes the probability of measuring a useful event in a two-plane Compton camera, including the most relevant physical processes involved. The model has been used to obtain an expression for the system and sensitivity matrices for iterative image reconstruction. These matrices have been validated taking Monte Carlo simulations as a reference. In order to study the impact of the sensitivity, images reconstructed with our sensitivity model and with other models have been compared. Images have been reconstructed from several simulated sources, including point-like sources and extended distributions of activity, and also from experimental data measured with 22Na sources. Results show that our sensitivity model is the best suited for our prototype. Although other models in the literature perform successfully in many scenarios, they are not applicable in all the geometrical configurations of interest for our system. In general, our model allows to effectively recover the intensity of point-like sources at different positions in the FoV and to reconstruct regions of homogeneous activity with minimal variance. Moreover, it can be employed for all Compton camera configurations, including those with low angular coverage over the scatterer.

Simulation study of sensitivity and resolution for a small animal PET ring based on continuous crystals

Continuous scintillator crystals can offer several advantages over pixellated crystals. Apart from an increase of the active volume, spatial resolution is not limited to the crystal size and Depth of interaction (DoI) can be estimated from the light distribution. Continuous crystals coupled to finely segmented photodetectors can improve simultaneously spatial resolution and sensitivity, the main two factors to maximize in Positron Emission Tomography (PET). The main drawback is the difficulty in the position estimation within the crystal; thus, accurate position estimation algorithms are needed. However, if the standard cuboid crystals are used in the full ring geometry, there will be large wedge-shaped gaps between detector heads. For maximizing the sensitivity and avoiding data truncation, a scanner based on tapered continuous crystals is considered. In this work, a basic comparison between two scanners of the same dimensions based on tapered and cuboid crystals was performed. The study is focused on the quantification of the increase on sensitivity while first preliminary results were obtained in terms of image quality and spatial resolution, An increase of 14.08% on the average sensitivity was obtained for tapered crystals. Image reconstruction was implemented with Filtered Back-Projection (FBP) method using the ideal position interaction of the annihilation photon within the crystal given by the simulations. An improvement on the recovery of homogeneous regions was observed due to less data truncation using tapered crystal.

Characterization and simulation results of a two/three-layer compton telescope with LaBr 3 and SiPMs

A three layer Compton telescope for dose monitoring in hadron therapy has been developed at IFIC. It consists of three detector layers and each layer is made of a continuous LaBr3 crystal coupled to four silicon photomultiplier (SiPMs) arrays. A dedicated image reconstruction method has also been developed. The telescope has been characterized and tested with radioactive sources and in-beam. Simulations with GATE have been carried out in order to optimize the detector geometry and response. Ongoing work is mainly focused on performance optimization and on the simultaneous acquisition and processing of two- and three-layer interaction events.

Electronics upgrade and crystal geometry optimization for a sub-millimeter small animal PET based on continuous crystals and SiPMs

A prototype of a high resolution small animal PET scanner with continuous LYSO crystals coupled to SiPM matrices has been developed. Images of point-like sources in different positions of the FOV have been obtained with a FWHM better than 1 mm. In the near future, a full-ring PET will be developed. Due to the increase in the number of detector heads, a new Data Acquisition (DAQ) system is needed. First tests with the VATA64HDR16 ASIC from Gamma Medica - IDEAS have been carried out. Also, the use of tapered crystals is being considered for reduction of the gaps between detectors and therefore increase the full-ring sensitivity.

Studies for performance improvement of a small animal PET prototype based on continuous LYSO crystals and SiPM matrices

A prototype of a high resolution small animal PET scanner with continuous LYSO crystals coupled to SiPM matrices has been developed. Initial tomographic images of point-like sources have been obtained with a spatial resolution of 0.7±0.2 mm FWHM using crystals with 5 mm and 10 mm thickness. The next step in the prototype development is to improve the scanner performance. From the instrumentation point of view, a slight misalignment of the detector heads has been identified as well as a malfunctioning in the ADC that leads to a degraded time resolution. In addition, a GEANT4 simulation of one detector head has been implemented including generation and transportation of optical photons for a better understanding of its response and to predict its performance in different conditions. Squared and tapered crystals geometries have been simulated. Simulated data have been compared to experimental data, showing good agreement.