Crystal scatter effects in a large-area dual-panel Positron Emission Mammography system.

Positron Emission Mammography (PEM) is a valuable molecular imaging technique for breast studies using pharmaceuticals labeled with positron emitters and dual-panel detectors. PEM scanners normally use large scintillation crystals coupled to sensitive photodetectors. Multiple interactions of the 511 keV annihilation photons in the crystals can result in event mispositioning leading to a negative impact in radiopharmaceutical uptake quantification. In this work, we report the study of crystal scatter effects of a large-area dual-panel PEM system designed with either monolithic or pixelated lutetium yttrium orthosilicate (LYSO) crystals using the Monte Carlo simulation platform GATE. The results show that only a relatively small fraction of coincidences (~20%) arise from events where both coincidence photons undergo single interactions (mostly through photoelectric absorption) in the crystals. Most of the coincidences are events where at least one of the annihilation photons undergoes a chain of Compton scatterings: approximately 79% end up in photoelectric absorption while the rest (<1%) escape the detector. Mean positioning errors, calculated as the distance between first hit and energy weighted (assigned) positions of interaction, were 1.70 mm and 1.92 mm for the monolithic and pixelated crystals, respectively. Reconstructed spatial resolution quantification with a miniDerenzo phantom and a list mode iterative reconstruction algorithm shows that, for both crystal types, 2 mm diameter hot rods were resolved, indicating a relatively small effect in spatial resolution. A drastic reduction in peak-to-valley ratios for the same hot-rod diameters was observed, up to a factor of 14 for the monolithic crystals and 7.5 for the pixelated ones.

Localization in Two-Dimensional Quasicrystalline Lattices.

We investigate the emergence of localization in a weakly interacting Bose gas confined in quasicrystalline lattices with three different rotational symmetries: five, eight, and twelve. The analysis, performed at a mean field level and from which localization is detected, relies on the study of two observables: the inverse participation ratio (IPR) and the Shannon entropy in the coordinate space. Those physical quantities were determined from a robust statistical study for the stationary density profiles of the interacting condensate. Localization was identified for each lattice type as a function of the potential depth. Our analysis revealed a range of the potential depths for which the condensate density becomes localized, from partially at random lattice sites to fully in a single site. We found that localization in the case of five-fold rotational symmetry appears for (6ER,9ER), while it occurs in the interval (12ER,15ER) for octagonal and dodecagonal symmetries.

Energy spectra due to the intrinsic radiation of LYSO/LSO scintillators for a wide range of crystal sizes.

PURPOSE: Most detectors in current positron emission tomography (PET) scanners and prototypes use lutetium oxyorthosilicate (LSO) or lutetium yttrium oxyorthosilicate (LYSO) scintillators. The aim of this work is to provide a complete set of background energy spectra, due to the scintillator intrinsic radioactivity, for a wide range of crystal sizes. METHODS: An analytical model, developed and validated in a previous work, was used to obtain the background energy spectra of square base cuboids of different dimensions. The model uses the photon absorption probabilities of the three gamma rays (88, 202, and 307 keV) emitted following the beta decay of 176 Lu to 176 Hf excited states. These probabilities were obtained for each crystal size considered in this work from Monte Carlo simulations using the PENELOPE code. The probabilities are then used to normalize and shift the beta spectrum to the corresponding energy value of the simultaneous detection of one, two, or three gamma rays in the scintillator. The simulated cuboids had side lengths of 5, 10, 20, 30, 40, 50, and 60 mm and crystal thickness T = 2.5, 5, 10, 15, and 20 mm. From these results a complete set of energy spectra, including intermediate dimensions, were obtained. In addition, LYSO and LSO were compared in terms of their analytical background energy spectra for two crystal sizes. The analytical spectra were convolved using a variable Gaussian kernel to account for the energy resolution of a typical detector. A parameterization of the photon absorption probabilities for each gamma ray energy as a function of the cuboid volume to surface area ratio was obtained. RESULTS: A data set of L(Y)SO background energy spectra was obtained and is available for the reader as 2D histograms. The model accurately predicts the structure of the energy spectra including the relative peak and valley intensities. The data allow visualizing how the structure evolves with increasing crystal length and thickness. Lutetium yttrium oxyorthosilicate and LSO present very similar background energy spectra for the range of sizes studied in this work and therefore the data generated can be confidently used for both scintillator materials. The filtered spectra showed a variable shift in the main peaks, depending on crystal size, alerting that to achieve a correct detector calibration using the background spectrum is not straight forward and requires precise data analysis and measurements. In addition, we found that square base L(Y)SO cuboids with same volume to surface area ratio have background spectra with the same structure. CONCLUSIONS: We present the energy spectra of L(Y)SO crystal of different sizes which will be very useful for industry and research groups developing and simulating detectors for positron imaging applications in terms of calibration, quality assurance, crystal maps, detector fine gain tuning, background reduction and other applications using the long-lived 176 Lu source. We analyzed the data produced in this work and found that crystal cuboids with equal volume to surface area ratio produce the same background energy spectra, a conclusion that simplifies its calculation and clarifies why the same energy spectrum is observed under different experimental setups.

Experimental validation of the ANTS2 code for modelling optical photon transport in monolithic LYSO crystals.

In this work the scintillation energy spectra originating from the background radioactivity from polished monolithic lutetium yttrium oxyorthosilicate coupled to position-sensitive silicon photomultipliers (SiPM) was studied using the open source Monte Carlo simulation package ANTS2. Two crystal sizes, fully and partially covering the photosensor area, three surface crystal wrappings (black, specular or diffuse) and the full signal formation process in the photosensor were considered. The simulation results were validated with experimental data acquired under the same geometric and detector operating conditions. In all cases ANTS2 simulated spectra have very good agreement with experimental results, reproducing the expected shape, with correct onset and end at 88 and 1190 keV, respectively, as well as sharp edges at the reference energies of 88, 88 + 202, 88 + 307 and 88 + 202 + 307 keV. The normalized root-mean square error between simulated and measured spectra varied between 4.3% and 10.4%.

A dedicated phantom design for positron emission mammography performance evaluation.

A standard protocol for performance evaluation of positron emission mammography (PEM) systems has not yet been established. In this work we propose a methodology based on the design of specific phantoms for this imaging modality with component dimensions in accordance with typical breast lesion sizes together with the adaptation of current international protocols designed for clinical and preclinical positron emission tomographs (PET) systems. This methodology was used to evaluate the performance of the Flex Solo II PEM scanner in terms of spatial resolution, uniformity and contrast lesion detectability, recovery coefficients and spill-over ratios. Positron range effects were studied with 18F and 68Ga, which have very different energy spectra. Our results indicate that in-plane spatial resolution of the system is around 3.0 mm and 4.4 mm for 18F and 68Ga, respectively. Lesion detectability tests with sphere diameters between 4 and 10 mm confirmed that the PEM system can resolve all the spheres (hot or cold). Percent contrast values for 18F lie between 6%-38% and 34%-51% for hot- and cold- spheres, respectively; the corresponding intervals for 68Ga are lower, 4%-25% and 32%-44%. Regarding uniformity quantification, the system shows percentage standard deviations within 4.9%-5.7%, while the percent background variability measurements ranged between 6.7% and 10.9% for both radionuclides. Recovery coefficients measured with hot rod diameters between 1.5 and 9 mm, have values between 0.2-1.05 and 0.17-0.69 for 18F and 68Ga, respectively. Spill-over ratios have large values (0.22 in average) for both radionuclides. Our results indicate that the phantoms and the methodology developed in this work can serve as the basis for establishing an image quality protocol for the systematic evaluation of PEM systems, with a potential extension for performance evaluation of dedicated breastPET scanners.

Coincidence energy spectra due to the intrinsic radioactivity of LYSO scintillation crystals.

BACKGROUND: Lutetium oxyorthosilicate or lutetium yttrium oxyorthosilicate (LYSO) scintillation crystals used in most current PET scanner detectors contain 176Lu, which decays by beta emission to excited states of 176Hf accompanied by the emission of prompt gamma rays or internal conversion electrons. This intrinsic radioactivity can be self-detected in singles mode as a constant background signal that has an energy spectrum whose structure has been explained previously. In this work, we studied the energy spectrum due to the intrinsic radioactivity of LYSO scintillation crystals of two opposing detectors working in coincidence mode. The investigation included experimental data, Monte Carlo simulations and an analytical model. RESULTS: The structure of the energy spectrum was completely understood and is the result of the self-detection of beta particles from 176Lu in one crystal and the detection of one or more prompt gamma rays detected in coincidence by the opposing crystal. The most probable coincidence detection involves the gamma rays of 202 and 307 keV, which result in two narrow photopeaks, superimposed on a continuous energy distribution due to the beta particle energy deposition. The relative intensities of the gamma ray peaks depend on crystal size and detector separation distance, as is explained by the analytical model and verified through the Monte Carlo simulations and experiments. CONCLUSIONS: The analytical model used in this work accurately explains the general features of the coincidence energy spectrum due to the presence of 176Lu in the scintillation crystals, as observed experimentally and with Monte Carlo simulations. This work will be useful to those research studies aimed at using the intrinsic radioactivity of LYSO crystals for transmission scans and detector calibration in coincidence mode.

Investigación preclínica por microPET en la UNAM / Pre-clinical research at UNAM using microPET

La tomografía por emisión de positrones (PET) es una técnica de imágenes de medicina nuclear ya establecida en México, fundamental en el diagnóstico y seguimiento clínico de enfermedades oncológicas, neurológicas y cardiológicas. Esta modalidad de imagenología molecular está basada en la administración de cantidades muy pequeñas de fármacos marcados con emisores de positrones y en la subsecuente detección de radiación con el fin de obtener imágenes tomográficas que reflejan la distribución del radiofármaco en el paciente. El desarrollo de nuevos radiofármacos para PET requiere de un método para verificar que éstos siguen las rutas metabólicas de interés, que su vida media biológica es suficiente para la realización de un estudio, que no tienen efectos adversos y que es viable para estudios en pacientes. El desarrollo de equipos de microtomografía por emisión de positrones (microPET), dedicados a estudiar animales de laboratorio, ha permitido realizar estas pruebas antes de su aplicación clínica. Además, el microPET es una herramienta de gran utilidad en la investigación preclínica de diversas enfermedades, en el desarrollo de tratamientos innovadores que permite el seguimiento no invasivo en modelos animales. En la Unidad PET/CT-Ciclotrón de la Facultad de Medicina de la UNAM, se cuenta desde hace unos años con un equipo microPET para investigación. En este trabajo se muestran algunos resultados de los estudios que se realizan con mayor frecuencia con el microPET utilizando los radiofármacos de mayor uso en el medio clínico y se muestra la utilidad que puede tener en diversos proyectos de investigación.

Positron emission tomography (PET) is a nuclear medicine imaging technique well established in Mexico, essential for the clinical diagnosis and follow-up of oncological, neurological and cardiac pathologies. This molecular imaging modality is based on the administration of small amounts of drugs labeled with a positron emitting radionuclides and the subsequent radiation detection to obtain tomographic images which reflect the distribution of the radiopharmaceutical in the patient. The development of new radiopharmaceuticals for PET requires a method to verify that they follow the expected metabolic pathways, that they have a long-enough biological half-life for imaging studies, that they have no side effects and that it is viable for use in patients. The development of positron emission microtomography (microPET) systems to be used in small laboratory animale has allowed researchers to perform these tests on radiopharmaceuticals before being used in the clinic. In addition, microPET is a useful tool in preclinical research of different diseases in the development of innovating non-invasive treatments allowing to follow up animal models. At the PET/CT-Ciclotron Unit, Facultad de Medicina, UNAM, a microPET system has been available in the last few years for research purposes. In this work, examples of frequent imaging studies performed with the microPET and in-the-clinic commonly-used radiopharmaceuticals, as well the use it may have in different research projects are shown here.