Andrea Gonzalez-Montoro

Performance study of a PET scanner based on monolithic scintillators for different DoI-dependent methods

One of the technical objectives of the MindView project is developing a brain-dedicated PET insert based on monolithic scintillation crystals. It will be inserted in MRI systems with the purpose to obtain simultaneous PET and MRI brain images. High sensitivity, high image quality performance and accurate detection of the Depth-of-Interaction (DoI) of the 511keV photons are required. We have developed a DoI estimation method, dedicated to monolithic scintillators, allowing continuous DoI estimation and a DoI-dependent algorithm for the estimation of the photon planar impact position, able to improve the single module imaging capabilities. In this work, through experimental measurements, the proposed methods have been used for the estimation of the impact positions within the monolithic crystal block. We have evaluated the PET system performance following the NEMA NU 4-2008 protocol by reconstructing the images using the STIR 3D platform. The results obtained with two different methods, providing discrete and continuous DoI information, are compared with those obtained from an algorithm without DoI capabilities and with the ideal response of the detector. The proposed DoI-dependent imaging methods show clear improvements in the spatial resolution (FWHM) of reconstructed images, allowing to obtain values from 2mm (at the center FoV) to 3mm (at the FoV edges).

A brain PET insert MR compatible: Final design and first results

A whole-body PET device is sometimes not suitable for brain studies because the achieved image resolution is typically not sufficient to investigate small size structures. Thus, a dedicated brain PET insert system with high performance would overcome such limitations. Moreover, these functional studies lack of anatomical information. It is shown elsewhere the convenience of simultaneously acquisition of PET and MR data. In this work we show the final design and first pilot evaluation tests of a novel brain PET insert. Each detector block is based on a monolithic scintillation crystal, an array of SiPMs and a readout allowing characterizing the scintillation light distribution in the X and Y detector axes. The scintillators have a parallelepiped geometry with dimensions of 50×50×20 mm3. Their lateral walls are black painted and with the entrance face coupled to a retroreflector optical layer. We have determined an average (XYZ) detector spatial resolution through the FWHM of 1.2 mm (whole scintillator volume). The DOI resolution was measured with lateral incidence experiments and found to be about 3.5 mm, also on average for all photons depth of interactions and crystal positions. Thanks to the retroreflector, the energy resolution improves when compared to a case with all surfaces black painted, resulting on an average value of 13%. The tomographic reconstruction of the data was evaluated using different algorithms, including analytical (FBP STIR-3D), iterative (MLEM and List Mode OS) and a novel method that provides images by directly tracing the measured LORs. The minimum pixel/voxel sizes that were tried are 0.8/0.4 mm, 1.0/0.5 mm and 0.16/0.16 mm, respectively. All methods made it possible to show the PET system capabilities to resolve 1.6 mm rods in a Derenzo-like phantom filled with 150 uCi and scanned for 20 minutes. Pilot tests of the PET insert inside a clinical 3T MR showed a good system performance for most of the sequences typically used for brain imaging.

Performance of large BGO arrays coupled to SiPM photosensors — Continued study

Recently, several studies have been carried in order to determine potential capabilities of BGO scintillator crystals coupled to SiPM photosensor devices for PET and SPECT applications. The prior studies have been typically done on small size BGO samples. Despite the fact that timing capabilities for devices based on BGO are worse than those based in commonly used fast scintillators, such as LYSO and LSO, the price of BGO material is considerably lower and, thus, BGO could be an option for systems where the required scintillator volume is significantly higher. In this report we present results of further studies using 12×12 SiPM arrays of the 3 mm C-Series SensL sensors, placed at a pitch of 4.2 mm, and coupled to the readout recording the 12 row and 12 column SiPM array signals. Two types of pixellated BGO crystals were tested: an array of 10×10 elements with 2.5 mm pitch and 10 mm thickness and an array of 30×30 pixels with 1.67 mm pitch but only 3 mm thick. A staggered depth-of-interaction (DOI) configuration was also evaluated using two layers of 2.5 mm pitch BGO pixels, with 10×10 (top) and 11×11 (bottom) elements with a total thickness of 20 mm. An energy resolution as good as 12% FWHM has been obtained. Our new results confirm that either single layer crystal arrays with pitch values as low as 1.67 mm or two staggered layers of 2.5 mm pitch could be well suited for PET applications, especially for large systems or low cost dedicated PET systems.

Feasibility study of a gradient coil for a dedicated and portable single-sided MRI system

Magnetic Resonance Imaging (MRI) is a widely used technique to obtain images in different applications based on the nuclear magnetic renonance (NMR) phenomenon. Gradient coils are the responsible components for encoding the volume of interest (VOI). Linearity, inductance and resistance are taken in account to perform the gradient coil design. In this work, EM and thermal gradient coil properties are studied and two cooling system are presented to cool them. Finally, the gradient coils are tested in a biplanar permanent magnet system and a 2D phantom image is obtained.

A direct image reconstruction algorithm for PET scanners based on monolithic crystals

In this work, a direct image reconstruction algorithm for PET is proposed. This method follows naturally from the concept of LOR, and easily admits depth of interaction and time of flight information. In addition, this algorithm does not require the computation of sinograms or a system matrix. Thus, it is not restricted in the formation of high pixel density images and allows the image formation in real-time (simultaneously to the data acquisition process). One of the key advantages of this algorithm is the preservation of high frequencies, producing accurate images that can be generated in high resolution, because of the lack of space restrictions imposed by the intermediate generation of sinograms, aside of the native support of real-time image reconstruction. In this work, a C++ implementation of the described algorithm has been developed. Image reconstructions of simulated PET events according to the MindView (a brain dedicated PET insert) scanner geometry and real data measured with the first one ring prototype system have been performed. Results show capability of correct image formation, either with simulated and real data.

Pilot tests of a PET detector using the TOF-PET ASIC based on monolithic crystals and SiPMs

In this work we show pilot tests of PET detector blocks using the TOF-PET ASIC, coupled to SiPM detector arrays and different crystal configurations. We have characterized the main ASIC features running calibration processes to compensate the time dispersion among the different ASIC/SiPM paths as well as for the time walk on the arrival of optical photons. The aim of this work is to use of LYSO monolithic crystals and explore their photon Depth of Interaction (DOI) capabilities, keeping good energy and spatial resolutions. First tests have been carried out with crystal arrays. Here we made it possible to reach a coincidence resolving times (CRT) of 370 ps FWHM, with energy resolutions better than 20% and resolving well 2 mm sized crystal elements. When using monolithic crystals, a single-pixel LYSO reference crystal helped to explore the CRT performance. We studied different strategies to provide the best timestamp determination in the monolithic scintillator. Times around 1 ns FWHM have been achieved in these pilot studies. In terms of spatial and energy resolution, values of about 3 mm and better than 30% were found, respectively. We have also demonstrated the capability of this system (monolithic and ASIC) to return accurate DOI information.

A PET detector ring with homogenous spatial resolution in the presence of a magnetic field

This works shows the feasibility study performed with a new small animal PET design based on SiPM arrays in front of a high field MR scanner. The PET ring is made out of 8 detector blocks each containing a single monolithic 50×50×10 mm3 LYSO block and coupled to an array of 12×12 SiPMs. The PET system performance has been evaluated both alone and in front of the MR with about 400-500 mT magnetic field. We evaluated two important parameters such as the energy and the spatial resolution. We did not observe, on overage, any energy resolution degradation when the system was inside the residual magnetic field. A slight shift to lower energies of about 7 keV on average was determined. The use of monolithic blocks and the high readout granularity has allowed us to explore novel approaches to precisely determine the 511 keV photon depth of interaction (DOIs). An accurate DOI resolution permitted to efficiently correct the parallax error for all events within the FOV, even those near the edges, resulting on a homogenous spatial resolution across the entire FOV. The spatial resolution performance did not degrade in the presence of the residual magnetic field.

Highly improved operation of monolithic BGO-PET blocks

In PET scanners both scintillation crystals and photosensors are key components defining the systems performance and cost. Original PET systems used BGO or NaI(Tl) scintillators but achieved limited performance due to its slow decay and relatively low light output. Moreover, NaI(Tl) has low stopping power for 511 keV annihilation photons. In this study we report the possibility to reintroduce BGO crystals, and in particular in the form of monolithic blocks, especially suitable for low-dose large-size PET scanners, offering significantly improved sensitivity at a highly reduced cost compared to LYSO type fast scintillators. We have studied the performance of a monolithic BGO block as large as 50 × 50 × 15 mm3 with black-painted lateral walls to reduce lights spread, enabling accurate photon depth of interaction (DOI) measurements. A directional optical layer, called retro-reflector, was coupled to the entrance face bouncing back the scintillation light in the direction of the emission source and, therefore, adding to the light signal while preserving the narrow light cone distribution. Four configurations namely 12 × 12 and 16 × 16 SiPM arrays (3 mm × 3 mm each) as photosensors, with or without a nanopattern treatment at the crystal exit face, have been studied. This structure consisted of a thin layer of a specific high refractive index material shaped with a periodic nanopattern, increasing the scintillation light extraction. The readout returned information for each SiPM row and column, characterizing the X-Y light distribution projections. We have studied the detector spatial resolution using collimated 22Na sources at normal incidence. The DOI resolution was evaluated using collimated gamma beams with lateral incidence. The overall best detector performance was obtained for the 16× 16 SiPM array offering higher readout granularity. We have determined the spatial resolution for 3 separated DOI layers, obtaining the best results for the DOI region near to the photosensor.

PETIROC2 based readout electronics optimization for Gamma Cameras and PET detectors

Developing front-end electronics to improve charge detection and time resolution in gamma-ray detectors is one of the main tasks to improve performance in new multimodal imaging systems that merge information of Magnetic Resonance Imaging and Gamma Camera or PET tomographs.The aim of this work is to study the behaviour and to optimize the performance of an ASIC for PET and Gamma Camera applications based on SiPMs detectors. PETIROC2 is a commercial ASIC developed by Weeroc to provide accurate charge and time coincidence resolutions. It has 32 analog input channels that are independently managed. Each channel is divided into two signals, one for time stamping using a TDC and another for charge measurement. In this work, PETIROC2 is evaluated in an experimental setup composed of two pixelated LYSO crystals based detectors, each coupled to a Hamamatsu 4×4 SiPM array. Both detectors are working in coincidence with a separation distance between them that can be modified.In the present work, an energy resolution of 13.6% FWHM and a time coincidence resolution of 815 ps FWHM have been obtained. These results will be useful to optimize and improve PETIROC2 based PET and Gamma Camera systems.

Pilot tests of a PET insert based on monolithic crystals in a 7T MR

We designed a novel PET insert based on monolithic LYSO crystals. From our first evaluation, we can conclude that sub-millimeter detector spatial resolution, combined with accurate photon DOI determination, make it possible to acquire high resolution reconstructed images. This enables us to combine simultaneously high resolution and sensitivity PET with high field preclinical MRI to extract simultaneously complex data capable to provide both anatomical and molecular information and to dynamically follow non-invasively animal models of different pathologies with no compromise in performance of each imaging modality. Each detector head (8 in total) mounts a 50×50×10 mm3 LYSO block coupled to a high density array of SiPMs. The readout used on each detector returns the X and Y projections of the scintillation light distribution. The PET geometry defines an axial and transaxial FOV of 46 mm (1 ring system) and 80 mm, respectively. The PET is surrounded of carbon fiber to avoid RF field interferences into the PET electronics without generating eddy currents affecting the MRI integrity. Current results show a PET resolution nearing 750 um (using Derenzo-like phantoms) simultaneously working with the MRI. In-vivo pilot tests of the PET insert system at the KU-Leuven (Belgium) inside a 7 T Bruker MRI equipped with a BGA 20S-HP gradient coil have shown the capability of both PET and MRI to simultaneously work providing high resolution PET and MRI images.