J. P. Rigla

Calibration and Performance Tests of Detectors for Laser-Accelerated Protons

We present the calibration and performance tests carried out with two detectors for intense proton pulses accelerated by lasers. Most of the procedures were realized with proton beams of 0.46-5.60 MeV from a tandem accelerator. One approach made use of radiochromic films, for which we calibrated the relation between optical density and energy deposition over more than three orders of magnitude. The validity of these results and of our analysis algorithms has been confirmed by controlled irradiation of film stacks and reconstruction of the total beam charge for strongly non-uniform beam profiles. For the spectral analysis of protons from repeated laser shots, we have designed an online monitor based on a plastic scintillator. The resulting signal from a photomultiplier directly measured on a fast oscilloscope is especially useful for time-of-flight applications. Variable optical filters allow for suppression of saturation and an extension of the dynamic range. With pulsed proton beams we have tested the detector response to a wide range of beam intensities from single particles to 3 ×105 protons per 100 ns time interval.

3-D photon impact determination using fitting approaches to the Light Distribution

In Positron Emission Tomography (PET) detectors based in monolithic scintillators, the spatial resolution is limited by the accuracy in the determination of the interaction coordinates from the 511 keV photons. When linear algorithms, such as Center of Gravity (CoG) are used a poor estimation of the interaction positions, specially towards the edges is the major limitation in spatial resolution. A novel PET detector block, where complete information of Light Distribution (LD) for each event is available, allows to fit each event to a theoretical model, improving the estimation of the interaction coordinates, and minimizing border effects. In this work, by means of the LD fitting approach, we were able to obtain an average spatial resolution of 1.2 mm in the entire scintillator volume and an average depth of interaction (DOI) resolution of 1.5 mm. Moreover, splitting the data in three DOI regions, we obtained an average spatial resolution of 1.0 mm at the DOI region closer to the photodetectors. Finally, it is remarkable that the implementation of the LD fitting approach is capable of processing up to 50 kcps in a octacore system.

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.

Determination of the Interaction Position of Gamma Photons in Monolithic Scintillators Using Neural Network Fitting

In Positron Emission Tomography (PET) detectors based on monolithic scintillators, the photon interaction position needs to be estimated from the light distribution (LD) on the photodetector pixels. Due to the finite size of the scintillator volume, the symmetry of the LD is truncated everywhere except for the crystal center. This effect produces a poor estimation of the interaction positions towards the edges, an especially critical situation when linear algorithms, such as Center of Gravity (CoG), are used. When all the crystal faces are painted black, except the one in contact with the photodetector, the LD can be assumed to behave as the inverse square law, providing a simple theoretical model. Using this LD model, the interaction coordinates can be determined by means of fitting each event to a theoretical distribution. In that sense, the use of neural networks (NNs) has been shown to be an effective alternative to more traditional fitting techniques as nonlinear least squares (LS). The multilayer perceptron is one type of NN which can model non-linear functions well and can be trained to accurately generalize when presented with new data. In this work we have shown the capability of NNs to approximate the LD and provide the interaction coordinates of γ-photons with two different photodetector setups. One experimental setup was based on analog Silicon Photomultipliers (SiPMs) and a charge division diode network, whereas the second setup was based on digital SiPMs (dSiPMs). In both experiments NNs minimized border effects. Average spatial resolutions of 1.9 ±0.2 mm and 1.7 ±0.2 mm for the entire crystal surface were obtained for the analog and dSiPMs approaches, respectively.

Analysis of the Statistical Moments of the Scintillation Light Distribution With dSiPMs

In γ-ray detectors, monolithic scintillation crystals offer the possibility of preserving the scintillation light distribution especially when painted black. The statistical moments of this distribution provide accurate information on the three photon impact coordinates, including their depth of interaction (DOI). Digital SiPMs (dSiPMs) return digital information based on pixels about the collected light distribution, since the signal is a digital sum of the trigger bins. In this work we present, for the first time, an accurate analysis of the statistical moments of the light distribution using monolithic painted black crystals and state-of-the-art dSiPMs. Two 32.6 ×32.6 mm2 monolithic LYSO crystals covering the entire photodetectors area have been used in coincidence with 10 mm in thickness. The photosensor tiles were kept at a stable temperature of T = 20 °C. Energy resolution of about 18% was reached in relation to the zeroth moment. The first moment, related to the impact position, determined a spatial resolution of about 3 mm near the crystal center, but quadratically degrading towards the crystal borders. The DOI resolution, measured by means of the second moment, was found to be nearing 4 mm in the crystal center region. The third order moment, the so-called skewness, is related to the degree of truncation and once calibrated minimizes the compression effects. A corrected spatial resolution of about 3 mm was then measured for the entire crystal surface. DOI resolution improved at the crystals center, reaching 3.5 mm, but a degradation towards the borders remained due to truncation of the scintillation light distribution.

Design and additive manufacturing of MRI gradient coils

The current manufacturing process of MRI gradient coils is a lengthy process because of material property requirements that address high voltages and currents, and complex 3D geometries (necessary to achieve desired gradient profiles and high magnetic field strengths). To address these requirements we developed software and fast 3D printer technology that automates the design, optimization, and manufacturing of these gradient coils. Our design software applies the principles of 3D printing (rapid prototyping) to control the gradient coil manufacturing process. Our 3D printer is the first printer to combine electrical conductors (e.g. silver) and high-grade electrical insulators (e.g., Kapton) for manufacturing MRI gradient coils. We have applied the additive manufacturing (3D printing) methods to the design and manufacturing of ultra-strong and ultra-fast (rise time ≤ 10 μs) magnetic gradient coils for high-performance magnetic resonance imaging (MRI) systems. Experiments with bi-planar 3D-printed gradient coils installed in a tabletop MRI system (0.34 T) show that we can get images with in-plane resolution of 50 μm and good image signal-to-noise in seconds using fast pulse sequences (fast gradient echo).

Effect of noise in CT image reconstruction using QR-Decomposition algorithm

The QR-Decomposition algorithm for CT 3D image reconstruction uses a linear system of equations to model the CT system response. Linear systems have a condition number that can be used to estimate the image noise. In this work the number of projections and the number of pixels in the detector have been studied to characterize the CT and the linear system of equations. The condition number of the system is estimated for the previous parameters used to generate the CT model with the aim of characterizing how these parameters affect the condition number and therefore bound the image noise level. It is shown that the condition number mainly depends on the size of pixels of the detector rather than the number of projections and this algorithm can be applied to low dose CT 3D image reconstruction without compromising image quality.

Performance evaluation of the dual ring MAMMI breast PET

MAMMI is a dedicated breast positron emission tomograph (PET) based on monolythic LYSO crystals, with a transaxial field of view (FOV) of 170 mm. It has been upgraded by adding a second ring of detectors that extends the axial FOV from 40 mm to 94.4 mm, in order to improve its sensitivity and reduce the acquisition time. In this work we present the performance evaluation of the dual ring MAMMI breast PET and a discussion about the contribution of the addition of a second ring of detectors, the compensation of the detector blur and the increase of the scintillator thickness. Experimental measurements suggested on NEMA NU 4-2008 and NEMA NU 2-2007 have been conveniently adapted to the dimensions of the MAMMI. The addition of the second ring of detectors leads to a rise of the sensitivity from 1.8% to 3.6%. The spatial resolution at one-fourth of the axial FOV (1.5 mm axial, 1.6 mm tangential, 1.7 mm radial) is slightly better than that measured at the axial center (1.9 mm axial, 1.8 mm tangential and radial), because of the 14 mm gap in between detection rings. The results obtained after the evaluation reflect a substantial performance improvement, specially in the absolute sensitivity, because of the changes introduced in the MAMMI PET.

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.