J. Barrio

Characterization of a PET detector head based on continuous LYSO crystals and monolithic, 64-pixel silicon photomultiplier matrices

The characterization of a PET detector head based on continuous LYSO crystals and silicon photomultiplier (SiPM) arrays as photodetectors has been carried out for its use in the development of a small animal PET prototype. The detector heads are composed of a continuous crystal and a SiPM matrix with 64 pixels in a common substrate, fabricated specifically for this project. Three crystals of 12 mm × 12 mm × 5 mm size with different types of painting have been tested: white, black and black on the sides but white on the back of the crystal. The best energy resolution, obtained with the white crystal, is 16% FWHM. The detector response is linear up to 1275 keV. Tests with different position determination algorithms have been carried out with the three crystals. The spatial resolution obtained with the center of gravity algorithm is around 0.9 mm FWHM for the three crystals. As expected, the use of this algorithm results in the displacement of the reconstructed position toward the center of the crystal, more pronounced in the case of the white crystal. A maximum likelihood algorithm has been tested that can reconstruct correctly the interaction position of the photons also in the case of the white crystal.

Simulations of the 4DMPET SiPM-based PET module

The 4DMPET project aims to develop PET detector modules compatible with operations inside a MRI scanner. PET/MRI hybrid imaging will improve the diagnostic capabilities in soft tissues like the human brain, providing functional and morphological information within the same device. The block detector, designed to provide a measurement of the Depth of Interaction and the Time Of Flight, will feature Silicon Photomultipliers (SiPM) coupled to LYSO scintillator slabs. The Monte Carlo simulations presented here are validated by the comparison with experimental data obtained with 4 × 4 and 8 × 8 SiPM matrices coupled to a LYSO scintillator. The proposed block layout is based on a 48 mm × 48 mm × 10 mm LYSO slab, read out by two 16 × 16 identical SiPM matrices on both sides. The preliminary results of the simulation of the proposed detector are presented, showing good performance in terms of energy, spatial and time resolution.

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.

A Compton imaging algorithm for on-line monitoring in hadron therapy

Hadron therapy, a subject of study by the ENVISION project, promises to provide enhanced accuracy in the treatment of cancer. The Bragg-peak, characteristic of the hadron-beam structure provides larger dose to the tumor while being able to spare surrounding tissue - even tissues in the beam-path, beyond the tumor-site. However, increased dose gradients require more precise treatment, as small beam misalignment can result in dose to healthy, often delicate, surrounding tissue. The requirement for accuracy necessitates imaging during therapy, yet the lack of a transmitted beam makes this difficult. The particulate beam interacts with the target material producing neutrons, positron emitting isotopes and a broad spectra of gamma radiation. Photons from positron-annihilation allow in-beam PET to provide on-line measurements of dose deposition during therapy. However, ib-PET suffers from low statistics and lost projections due to low sensitivity and detector constraints respectively. Instead, Compton imaging of gamma radiation is proposed to provide on-line monitoring for hadron therapy. Compton imaging suffers similarly from low statistics, especially, as is the case here, when incident energy is unknown. To surmount this problem, a method of Compton image reconstruction is proposed and tested using simulated data, which reconstructs incident energy along with the spatial variation in emission density. Through incident energy estimation, a larger range of measurements are available for image-reconstruction - greatly increasing the sensitivity of the system. It is shown in this preliminary study that, even with few statistics, a reasonable estimate of the beam path is calculable.

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.

Position reconstruction in detectors based on continuous crystals coupled to silicon photomultiplier arrays

Sensitivity represents one of the major limitations for high resolution systems used in emission imaging for nuclear medicine. Currently, detectors are based on pixelated scintillators to detect ionizing radiation. There is a recent growing interest in continuous scintillators due to their increased sensitivity by eliminating insensitive areas in the detector crystals and reduced cost. To use such crystal an accurate position estimation algorithm is required to determine the location where photons interact, as opposed to pixelated scintillators, where the position is given by the crystal where the interaction took place. Additionally, including the depth where the photon interacted (DoI) inside the scintillator in the reconstruction algorithm, can be used to mitigate parallax effects. In this work we investigate the feasibility of using an existing analytical position estimation method applied to two different detectors designed to be used in two different imaging modalities: Positron Emission Tomography (PET) and Compton imaging. An LYSO crystal coupled to a 8×8 SiPM array, as one of the detector heads for the PET prototype, and a LaBr3 crystal coupled to a 4×4 SiPM array, as part of a Compton telescope comprised of several stacked detectors, is used for each application. Results show that submillimetric resolution is measured with both detectors in most of the studied positions, near the centre and close to the edges, in Monte Carlo simulations and experimentally. The measured FWHM for the PET detector is 0.6 mm while the FWHM measured for the detector used in the Compton camera is ∼0.7 mm. DoI measurements were taken only in simulations of the PET detector, where submillimetric bias was achieved and ∼1.6 mm FWHM was measured.

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.

Characterization of a detector head based on continuous LaBr 3 crystals and SiPM arrays for dose monitoring in hadron therapy

In hadron therapy, prompt gamma rays are emitted during the treatment from the excited nuclei in the tissue, and can be employed for dose monitoring. A Compton telescope is under development within the ENVISION project for imaging the prompt gamma rays. The telescope will consist of several detector layers, each composed of a continuous LaBr3 crystal coupled to silicon photomultiplier (SiPM) arrays. A first detector made of a 16×18×5 mm3 LaBr3 crystal coupled to a SiPM array has been characterized in order to evaluate its performance for the proposed application.