C. Solaz

Development of a PET prototype with continuous LYSO crystals and monolithic SiPM matrices

A first prototype of a high resolution small animal PET scanner based on continuous LYSO scintillator crystals and silicon photomultiplier matrices has been developed at IFIC-Valencia, in collaboration with the University of Pisa and INFN Pisa. The prototype consists of two detector heads attached to a rotating stage. Each head is composed of a continuous 12 mm × 12 mm × 5 mm LYSO crystal painted white, coupled to a monolithic SiPM matrix. The matrices, developed at FBK-irst, are composed of 8×8 SiPM elements of 1.5 mm×1.4 mm size in a common substrate. The full characterization of the detector heads with different types of crystals has been carried out, and a method for determining the interaction position of the gamma-rays in the crystals has been successfully employed. Tomographic data have been acquired with the prototype, and images of different source distributions have been reconstructed with the Maximum Likelihood Expectation Maximization algorithm (MLEM). A FWHM close to 1 mm is obtained for one and two point-like sources.

Performance of MACACO Compton telescope for ion-beam therapy monitoring: first test with proton beams

In order to exploit the advantages of ion-beam therapy in a clinical setting, delivery verification techniques are necessary to detect deviations from the planned treatment. Efforts are currently oriented towards the development of devices for real-time range monitoring. Among the different detector concepts proposed, Compton cameras are employed to detect prompt gammas and represent a valid candidate for real-time range verification. We present the first on-beam test of MACACO, a Compton telescope (multi-layer Compton camera) based on lanthanum bromide crystals and silicon photo-multipliers. The Compton telescope was first characterized through measurements and Monte Carlo simulations. The detector linearity was measured employing (22)Na and Am-Be sources, obtaining about 10% deviation from linearity at 3.44 MeV. A spectral image reconstruction algorithm was tested on synthetic data. Point-like sources emitting gamma rays with energy between 2 and 7 MeV were reconstructed with 3-5 mm resolution. The two-layer Compton telescope was employed to measure radiation emitted from a beam of 150 MeV protons impinging on a cylindrical PMMA target. Bragg-peak shifts were achieved via adjustment of the PMMA target location and the resulting measurements used during image reconstruction. Reconstructed Bragg peak profiles proved sufficient to observe peak-location differences within 10 mm demonstrating the potential of the MACACO Compton Telescope as a monitoring device for ion-beam therapy.

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.

Multichannel DAQ system for SiPM matrices

The use of Silicon Photomultiplier (SiPM) arrays requires the use of multichannel data acquisition (DAQ) systems. For this reason a dedicated DAQ system has been developed for the read-out of several SiPM-based detector layers. The frontend of a system is based on the 64-channel ASIC V ATA64HDR16 from Gamma Medica - Ideas. As first application, the DAQ system will be employed in the construction of Compton telescope for dose monitoring in hadron therapy. However the designed system is suitable for any other devices that need to treat large number of SiPM channels. Tests are presented with SiPM matrix of 16 (4×4) elements each coupled to LYSO and LaBr3 continuous crystals. The complete characterization of the DAQ system and the obtained results are presented.

Simulated One Pass Listmode for fully 3D image reconstruction of Compton camera data

Image reconstruction for Compton camera data can be problematic due to the common trade-off between physically realistic models and speed of computation. In this investigation a novel method of system matrix calculation - Simulated One-Pass Listmode (SOPL) - is extended to incorporate Compton camera data. The method reduces the Cone Surface Response for the Compton camera to an ensemble of Siddon-rays and is conducted in two stages. As part of the ENVISION project for monitoring in hadron therapy, a continuous-crystal Lanthanum Bromide Compton camera has been developed and experimental data acquired. Continuous detection geometries are particularly susceptible to variation in both spatial and spectral resolution over the detection volume and so accurate yet flexible models of detection are particularly important. The SOPL-Compton method was applied via the Maximum Likelihood - Expectation Maximization algorithm to experimental data taken using the prototype device. In this investigation, detection modeling using SOPL-Compton in a two interaction Compton camera is validated and the incorporation of a shift-invariant image-space model confirmed as a useful modification to reduce computational expense. Finally experimental data taken using the prototype LaBr3 Compton camera provide confirmation of the SOPL-Compton approach to system modeling. Results indicate a fast, flexible and accurate algorithm that can easily be extended to alternate and novel detection geometries.

An FPGA based DAQ system for the readout of Madeira PET probe

Madeira project aims to significantly improve three-dimensional (3D) nuclear medicine imaging technologies via a compact photon-sensitive probe interfaced to an external conventional PET ring and placed close to the region of interest. The probe consists of several modules densely packed. Each module is made of two high-resistivity silicon detectors of 1 mm thickness and 1040 square pixels of 1 mm2. The detectors are placed back-to-back at a distance of 0.8 mm. The pads are read out with the VATAGP7 chip, a Gamma Medica — Ideas designed application specific integrated circuit (ASIC). A FPGA based DAQ system has been designed and developed at IFIC-Valencia to read out the probe. The system consists of several DAQ boards working in parallel which control the acquisition process. The DAQ board has trigger and coincidence capabilities to be used in coincidence with a conventional PET scanner. This work describes the characteristics of the system and its architecture, and also the future activities.

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.

Data acquisition system for the readout of SiPM arrays

Silicon photomultipliers (SiPM) are becoming more and more popular in different fields of physics. The use of SiPM arrays requires a data acquisition (DAQ) system capable of reading multiple channels for each event. A DAQ system for the control and the readout of 64-channel detectors was designed and fabricated. Significant improvements in performance and new functionalities have been done. The performance of our DAQ system has been tested in a real application: a Compton telescope prototype. Tests, experimental results and future work are described.