Albert Aguilar

Time-to-Digital Converter Based on FPGA With Multiple Channel Capability

This contribution describes an accurate approach implementing a Time-to-Digital Converter using a Field-Programmable Gate Array (FPGA) device. Time differences with a FWHM better than 100 ps for 24 pairs of channels working simultaneously have been achieved. This was possible through the proper management of FPGA internal resources and by an accurate device calibration process minimizing the effect of temperature and voltage fluctuations. The system calibration results and the time differences between multiple channels are presented. The current approach suggests the possibility of carrying out precise Time of Flight (TOF) measurements with, for instance, Positron Emission Tomography (PET) systems.

A PET Design Based on SiPM and Monolithic LYSO Crystals: Performance Evaluation

A new small animal PET based on SiPM and monolithic LYSO crystals has been developed. Eight detector modules form the PET ring, each mounting an array of 12 × 12 SiPMs coupled to a readout providing the summed signals of the pixels on each of the 12 rows and 12 columns of the SiPM array. This design makes it possible to accurately determine the centroid of the scintillation light distribution with about 1.6 mm full width at half maximum (FWHM) resolution without correction for the 1 mm source size, and the photon depth of interaction (DOI) with nearly 2 mm FWHM. This single ring PET system has a homogeneous spatial resolution across the entire 80 mm transaxial field of view (FOV) of about 1 mm FWHM. The noise equivalent count rate (NECR) peak is estimated to occur at around 39.2 MBq with a rate of approximately 82.7 kcps for the mouse-like phantom and 22 kcps at 48.1 MBq for the rat-like phantom. Following the NEMA protocol, the peak absolute sensitivity in the center of the FOV is 2.8% for a 30% peak energy window. A pilot test injecting NaF to a mouse of 20 grams is also presented. Finally, the PET ring has been tested in front of a high field 15.2 T Magnetic Resonance (MR). No significant variation on energy and spatial resolution across the FOV has been observed due to the presence of the magnetic field.

Timing Results Using an FPGA-Based TDC with Large Arrays of 144 SiPMs

Silicon photomultipliers (SiPMs) have become an alternative to traditional tubes due to several features. However, their implementation to form large arrays is still a challenge especially due to their relatively high intrinsic noise, depending on the chosen readout. In this contribution, two modules composed of 12 ×12 SiPMs with an area of roughly 50 mm×50 mm are used in coincidence. Coincidence resolving time (CRT) results with a field-programmable gate array, in combination with a time to digital converter, are shown as a function of both the sensor bias voltage and the digitizer threshold. The dependence of the CRT on the sensor matrix temperature, the amount of SiPM active area and the crystal type is also analyzed. Measurements carried out with a crystal array of 2 mm pixel size and 10 mm height have shown time resolutions for the entire 288 SiPM two-detector set-up as good as 800 ps full width at half maximum (FWHM).

Characterization of protons accelerated from a 3 TW table-top laser system

This project has been funded by Centro para el Desarrollo Tecnologico Industrial (CDTI, Spain) within the INNPRONTA program, grant no. IPT-20111027, by EUROSTARS project E9113, and by the Spanish Ministry for Economy and Competitiveness within the Retos-Colaboracion 2015 initiative, ref. RTC-2015-3278-1.

Detailed requirements for a laser-based proton/ion accelerator for radioisotope production

Background and objectives: Laser-plasma acceleration of protons and ions is often considered a promising technique for compact applications of highly intense beams of multi-MeV particles. A remarkable example is the on-site production of short-lived radioisotopes for medical and preclinical interventions. We study quantitatively the activity of four important PET isotopes which may be obtained by irradiation of suitable target nuclei with laser-accelerated protons and deuterons. These simulations allow for confining the range of useful parameters of a laser-based production system. Methods: We choose a total of ten p- and d-induced reaction channels for the production of F-18, C-11, O-15, and N-13 from suitable target nuclei. We calculate the activity yield as a function of projectile energy starting from the corresponding, known cross sections. In order to simulate typical laser-plasma particle spectra we generate exponentially decaying distributions spread over a wide range, up to a maximum energy between 6 and 16 MeV. From the yield curves and the spectra we obtain the single-shot activation and the total activity after a realistic production time at 100 Hz pulse rate, taking into account saturation effects due to decay during irradiation. Results: We present numerical results for ten reaction channels and six realistic projectile spectra. With single laser shots, the highest activities are generated for O-15 (up to 11 (20) kBq for 16 MeV maximum p (d) energy). After prolonged irradiation at 100 Hz pulse rate, useful quantities of C-11 and O-15 may be obtained from spectra with 10 MeV maximum energies. The production of N-13 and F-18, to the contrary, requires higher energies and/or shot rates. Conclusions: 10 MeV particle energy and 100 Hz pulse rate are realistic benchmarks for a laser-based PET isotope production system. Experimental work to achieve these demanding objectives is in progress.

High-resolution multichannel Time-to-Digital Converter core implemented in FPGA for ToF measurements in SiPM-PET

In this contribution, Coincidence Resolving Time (CRT) results with the developed multichannel FPGA-TDC are showed as a function of different configurations for both, the sensor bias voltage and the digitizer threshold. The dependence of the CRT with the sensor matrix temperature, the amount of SiPM active area and the crystal type are also analyzed. Preliminary measurements carried out with a crystal array of 2 mm pixel size and 10 mm height have shown time resolutions for the entire 144 SiPM two-detectors ensemble as good as 800 ps.

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.

Optimization of a Time-to-Digital Converter and a coincidence map algorithm for TOF-PET applications

This contribution describes the optimization of a multichannel high resolution Time-to-Digital Converter (TDC) in a Field-Programmable Gate Array (FPGA) initially capable of obtaining time resolutions below 100ps for multiple channels. Due to its fast propagation capability it has taken advantage of the FPGA internal carry logic for accurate time measurements. Furthermore, the implementation of the TDC has been performed in different clock regions and tested with different frequencies as well, achieving improvements of up to 50% for a pair of channels. Moreover, since the TDC is potentially going to be used in a trigger system for Positron Emission Tomography (PET), the algorithm for coincidence identification has been subjected to tests in order to estimate the impact on occupied resources and the execution time. This time has been optimized, resulting in speed improvements of up to 20% while preserving occupied resources.

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.