Learning to see via epiretinal implant stimulationin silicowith model-based deep reinforcement learning.

OBJECTIVE: Diseases such as age-related macular degeneration and retinitis pigmentosa cause the degradation of the photoreceptor layer. One approach to restore vision is to electrically stimulate the surviving retinal ganglion cells with a microelectrode array such as epiretinal implants. Epiretinal implants are known to generate visible anisotropic shapes elongated along the axon fascicles of neighboring retinal ganglion cells. Recent work has demonstrated that to obtain isotropic pixel-like shapes, it is possible to map axon fascicles and avoid stimulating them by inactivating electrodes or lowering stimulation current levels. Avoiding axon fascicule stimulation aims to remove brushstroke-like shapes in favor of a more reduced set of pixel-like shapes. APPROACH: In this study, we propose the use of isotropic and anisotropic shapes to render intelligible images on the retina of a virtual patient in a reinforcement learning environment named rlretina. The environment formalizes the task as using brushstrokes in a stroke-based rendering task. MAIN RESULTS: We train a deep reinforcement learning agent that learns to assemble isotropic and anisotropic shapes to form an image. We investigate which error-based or perception-based metrics are adequate to reward the agent. The agent is trained in a model-based data generation fashion using the psychophysically validated axon map model to render images as perceived by different virtual patients. We show that the agent can generate more intelligible images compared to the naive method in different virtual patients. SIGNIFICANCE: This work shares a new way to address epiretinal stimulation that constitutes a first step towards improving visual acuity in artificially-restored vision using anisotropic phosphenes.

Investigation of a Model-based Time-over-threshold Technique for Phoswich Crystal Discrimination.

The best crystal identification (CI) algorithms proposed so far for phoswich detectors are based on adaptive filtering and pulse shape discrimination (PSD). However, these techniques require free running analog to digital converters, which is no longer possible with the ever increasing pixelization of new detectors. We propose to explore the dual-threshold time-over-threshold (ToT) technique, used to measure events energy and time of occurence, as a more robust solution for crystal identification with broad energy windows in phoswich detectors. In this study, phoswich assemblies made of various combinations of LGSO and LYSO scintillators with decay times in the range 30 to 65 ns were investigated for the LabPET II detection front-end. The electronic readout is based on a 4 × 8 APD array where pixels are individually coupled to charge sensitive preamplifiers followed by first order CR-RC shapers with 75 ns peaking time. Crystal identification data were sorted out based on the measurements of likeliness between acquired signals and a time domain model of the analog front-end. Results demonstrate that crystal identification can be successfully performed using a dual-threshold ToT scheme with a discrimination accuracy of 99.1% for LGSO (30 ns)/LGSO (45 ns), 98.1% for LGSO (65 ns)/LYSO (40 ns) and 92.1% for LYSO (32 ns)/LYSO (47 ns), for an energy window of [350-650] keV. Moreover, the method shows a discrimination accuracy >97% for the two first pairs and ~90% for the last one when using a wide energy window of [250-650] keV.

DOI estimation through signal arrival time distribution: a theoretical description including proof of concept measurements.

The challenge to reach 10 ps coincidence time resolution (CTR) in time-of-flight positron emission tomography (TOF-PET) is triggering major efforts worldwide, but timing improvements of scintillation detectors will remain elusive without depth-of-interaction (DOI) correction in long crystals. Nonetheless, this momentum opportunely brings up the prospect of a fully time-based DOI estimation since fast timing signals intrinsically carry DOI information, even with a traditional single-ended readout. Consequently, extracting features of the detected signal time distribution could uncover the spatial origin of the interaction and in return, provide enhancement on the timing precision of detectors. We demonstrate the validity of a time-based DOI estimation concept in two steps. First, experimental measurements were carried out with current LSO:Ce:Ca crystals coupled to FBK NUV-HD SiPMs read out by fast high-frequency electronics to provide new evidence of a distinct DOI effect on CTR not observable before with slower electronics. Using this detector, a DOI discrimination using a double-threshold scheme on the analog timing signal together with the signal intensity information was also developed without any complex readout or detector modification. As a second step, we explored by simulation the anticipated performance requirements of future detectors to efficiently estimate the DOI and we proposed four estimators that exploit either more generic or more precise features of the DOI-dependent timestamp distribution. A simple estimator using the time difference between two timestamps provided enhanced CTR. Additional improvements were achieved with estimators using multiple timestamps (e.g. kernel density estimation and neural network) converging to the Cramér-Rao lower bound developed in this work for a time-based DOI estimation. This two-step study provides insights on current and future possibilities in exploiting the timing signal features for DOI estimation aiming at ultra-fast CTR while maintaining detection efficiency for TOF PET.

Performance evaluation of the mouse version of the LabPET II PET scanner.

The LabPET II is a new positron emission tomography technology platform designed to achieve submillimetric spatial resolution imaging using fully pixelated avalanche photodiodes-based detectors and highly integrated parallel front-end processing electronics. The detector was designed as a generic building block to develop devices for preclinical imaging of small to mid-sized animals and for clinical imaging of the human brain. The aim of this work is to assess the physical characteristics and imaging performance of the mouse version of LabPET II scanner following the NEMA NU4-2008 standard and using high resolution phantoms and in vivo imaging applications. A reconstructed spatial resolution of 0.78 mm (0.5 µ l) is measured close to the center of the radial field of view. With an energy window of 350 650 keV, the system absolute sensitivity is 1.2% and its maximum noise equivalent count rate reaches 61.1 kcps at 117 MBq. Submillimetric spatial resolution is achieved in a hot spot phantom and tiny bone structures were resolved with unprecedented contrast in the mouse. These results provide convincing evidence of the capabilities of the LabPET II technology for biomolecular imaging in preclinical research.

3D Photon-to-Digital Converter for Radiation Instrumentation: Motivation and Future Works.

Analog and digital SiPMs have revolutionized the field of radiation instrumentation by replacing both avalanche photodiodes and photomultiplier tubes in many applications. However, multiple applications require greater performance than the current SiPMs are capable of, for example timing resolution for time-of-flight positron emission tomography and time-of-flight computed tomography, and mitigation of the large output capacitance of SiPM array for large-scale time projection chambers for liquid argon and liquid xenon experiments. In this contribution, the case will be made that 3D photon-to-digital converters, also known as 3D digital SiPMs, have a potentially superior performance over analog and 2D digital SiPMs. A review of 3D photon-to-digital converters is presented along with various applications where they can make a difference, such as time-of-flight medical imaging systems and low-background experiments in noble liquids. Finally, a review of the key design choices that must be made to obtain an optimized 3D photon-to-digital converter for radiation instrumentation, more specifically the single-photon avalanche diode array, the CMOS technology, the quenching circuit, the time-to-digital converter, the digital signal processing and the system level integration, are discussed in detail.

Laser Driven Miniature Diamond Implant for Wireless Retinal Prostheses.

The design and benchtop operation of a wireless miniature epiretinal stimulator implant is reported. The implant is optically powered and controlled using safe illumination at near-infrared wavelengths. An application-specific integrated circuit (ASIC) hosting a digital control unit is used to control the implant's electrodes. The ASIC is powered using an advanced photovoltaic (PV) cell and programmed using a single photodiode. Diamond packaging technology is utilized to achieve high-density integration of the implant optoelectronic circuitry, as well as individual connections between a stimulator chip and 256 electrodes, within a 4.6 mm × 3.7 mm × 0.9 mm implant package. An ultrahigh efficiency PV cell with a monochromatic power conversion efficiency of 55% is used to power the implant. On-board photodetection circuity with a bandwidth of 3.7 MHz is used for forward data telemetry of stimulation parameters. In comparison to implants which utilize inductively coupled coils, laser power delivery enables a high degree of miniaturization and lower surgical complexity. The device presented combines the benefits of implant miniaturization and a flexible stimulation strategy provided by a dedicated stimulator chip. This development provides a route to fully wireless miniaturized minimally invasive implants with sophisticated functionalities.

Dual-Threshold Time-over-Threshold Nonlinearity Correction for PET Detectors.

The Time-over-Threshold (ToT) analog-to-digital signal processing approach provides a power-efficient and cost-effective technique to extract all relevant information from detectors in high-energy physics and Positron Emission Tomography (PET) imaging. In this work, three calibration methods were investigated to correct the inherent nonlinear response of the ToT data using 1) γ-ray sources of various energies, 2) internal electronic gain variation in the LabPET II ASIC in combination with a single energy γ-ray source, and 3) internal gain variation along with an embedded pulse charge generator in replacement of a γ-ray source. The electronic gain calibration technique was shown to achieve equivalent correction accuracy as the γ-ray sources calibration. Furthermore, this method has the advantage of allowing a faster calibration requiring only one single γ-ray source (e.g., 511 keV) and a quick automated routine to sweep the internal gain. The last technique would be the most convenient method, provided that the signal pulse shape would be similar to the detector signal responding to a typical γ-ray event. Whereas the concept was demonstrated with a step pulse, extensive processing would be required to recover the nonlinearity correction factors for the detector pulse shape. After calibration, the 511-keV energy resolution of typical LabPET II detectors was only slightly degraded, by less than 12% and 8% for methods 1) and 2), respectively, relative to a conventional ADC-based data acquisition system. The feasibility of fast and accurate calibration for the nonlinearity correction of ToT data in PET imaging was demonstrated, making a daily quality control within reach.

A Thermal Model of the LabPET II ASIC.

The LabPET II detection module is the building block of PET scanners for ultra-high-resolution imaging of small to mid-sized animals and the human brain. For optimal performance, it must be operated at a stable temperature. The detection module is composed of four APD-LYSO detector arrays with two flip-chip ASICs mounted on the backside of an interposer generating 550 mW each. Currently, the scanner architecture includes an air cavity around the electronics and smaller cavities close to the detectors. Cooling down the front-end electronics located in these small cavities becomes problematic as the number of modules increases to address the different targeted configurations of the LabPET II scanners from mouse to human brain geometries. A basic knowledge of the heat distribution is necessary to develop an efficient thermal management in all cases. The aim of this work is to build a model of the LabPET II ASIC and associated PCB for enabling heat flow simulations and circumscribe the thermal management requirements. The Flow Simulation module (SolidWorks), was used to build the thermal model. The ASIC and the interconnection with the PCB were reproduced accurately while some adjacent structures were simplified to ease the simulation burden. The model was applied to simulate three different configurations of printed-circuit boards carrying the ASICs and other components where a fan is turned on/off to create a forced airflow. Each simulation was compared to some experimental measurements. A temperature difference of less than 5 degree Celsius between the simulations and experimental measurements is noticed, giving confidence that the thermal model of the ASIC is valid and transferable to different mechanical assemblies.

MRI-compatibility study of a PET-insert based on a low-profile detection front-end with submillimeter spatial resolution.

PURPOSE: The LabPET II detection module is a potential candidate to create an magnetic resonance imaging (MRI) compatible positron emission tomography (PET)-insert with submillimeter spatial resolution for small animal applications. However, the feasibility of such an insert is hampered by the large radial size of the LabPET II front-end electronics and by components containing ferromagnetic materials. In this paper, a new low-profile front-end design based on the LabPET II architecture, called "low-profile detection module," is investigated. MATERIALS AND METHODS: The performance of the low-profile detection module in the presence of MRI-like RF signals and gradient coil pulses was independently examined. The baseline of the analog signal, its RMS noise level, and the energy resolution, determined by a dual time-over-threshold (dTOT) method for each pixel of the new low-profile detection module, was measured in the presence of RF signals at different frequencies equivalent to the Larmor frequency of 3, 7, and 9.4 T MRI. The same parameters were investigated in the presence of a gradient coil switching at frequencies from 10 to 100 kHz. The performance of the low-profile detection module inside a 7 T MRI and its effects on an MR image have also been studied using gradient echo sequences. The same measurements were repeated for the shielded low-profile detection module, inside and outside the MRI. RESULTS: Our results show that pulses in both the kilohertz and megahertz ranges cause up to 50% increase in the noise level of the baseline (DC analog signal at the output of the shaper filter) and up to 17% degradation in TOT energy resolution. By inserting a conducting composite layer as shielding around the low-profile detection module, these degrading effects were avoided. The performance measurement of the low-profile PET detection module inside a 7 T small animal MRI scanner confirmed that the shielded low-profile detection module behavior was similar inside and outside the MRI bore. In addition, gradient echo images of a water-filled phantom without and with the shielded and unshielded low-profile detection modules were acquired. The results demonstrated no evidence of artifacts in the MR image, either due to eddy currents or ferromagnetic materials with the shielded modules. CONCLUSION: A low-profile detection module based on the LabPET II technology was shown to be a viable candidate as a PET-insert for simultaneous PET/MRI applications considering its thin radial size and its EMI immunity due to placing it between two electronic boards. In comparison to the standard LabPET II detection module, it provides better performance in the presence of electromagnetic interferences, but a shielding layer is still required. When properly shielded, the proposed low-profile detection module can be operated inside an MRI without degrading the PET count rate or the MRI performance.

Performance investigation of LabPET II detector technology in an MRI-like environment.

The EMI-compatibility of the LabPET II detection module (DM) to develop a high-resolution simultaneous PET/MRI system is investigated. The experimental set-up evaluates the performance of two LabPET II DMs in close proximity to RF coils excited at three different frequencies mimicking the electromagnetic environments of 3 T, 7 T, and 9.4 T MRI scanners. A gradient coil, with switching frequency from 10 kHz to 100 kHz, also surrounds one of the DMs to investigate the effects of the gradient field on the individual detector performance, such as the baseline of the DC-voltage and noise level along with both the energy and coincidence time resolutions. Measurements demonstrate a position shift of the energy photopeaks (⩽9%) and a slight deterioration of the energy and coincidence time resolutions in the presence of electromagnetic interferences from the gradient and RF coils. The electromagnetic interferences cause an average degradation of up to ~50% of the energy resolution (in time-over-threshold spectra) and up to 18% of the timing resolution. Based on these results, a modified version of the DM, including a composite shielding as well as an improved heat pipe-based cooling mechanism, capable of stabilizing the temperature of the DM at ~40 °C, is proposed and investigated. This shielded version shows no evidence of performance degradation inside an MRI-like environment. The experimental results demonstrate that a properly shielded version of the LabPET II DM is a viable candidate for an MR-compatible PET scanner.

Performance Simulation of an Ultra-High Resolution Brain PET Scanner Using 1.2-mm Pixel Detectors.

The concept of a new ultra-high resolution positron emission tomography (PET) brain scanner featuring truly pixelated detectors based on the LabPET II technology is presented. The aim of this study is to predict the performance of the scanner using GATE simulations. The NEMA procedures for human and small animal PET scanners were used, whenever appropriate, to simulate spatial resolution, scatter fraction, count rate performance and the sensitivity of the proposed system compared to state-of-the-art PET scanners that would currently be the preferred choices for brain imaging, namely the HRRT dedicated brain PET scanner and the Biograph Vision wholebody clinical PET scanner. The imaging performance was also assessed using the NEMA-NU4 image quality phantom, a mini hot spot phantom and a 3-D voxelized brain phantom. A reconstructed nearly isotropic spatial resolution of 1.3 mm FWHM is obtained at 10 mm from the center of the field of view. With an energy window of 250-650 keV, the system absolute sensitivity is estimated at 3.4% and its maximum NECR reaches 16.4 kcps at 12 kBq/cc. The simulation results provide evidence of the promising capabilities of the proposed scanner for ultra-high resolution brain imaging.

Studying the effects of metallic components of PET-insert on PET and MRI performance due to gradient switching.

Inserting positron emission tomography (PET) detection modules inside an MRI bore imposes extra challenges owing to the behavior of metallic materials in a strong magnetic field. The metallic parts even when placed outside an MRI field of view may not only disturb MRI performance, but could also increase temperature and vibrations, leading to premature failure of PET electronics. To investigate the compatibility of detection modules inside 3 T, 7 T and 9.4 T MRI bore, a theoretical study of the metal induced artifacts originating from component materials of electronic circuit is presented. The LabPET II detection module and a modified version of it in which the connector was replaced by ball grid array (BGA) were studied. In addition, the effect of eddy current and the associated heat loss on the PET detection module have been examined using COMSOL Multiphysics® simulations for 10 kHz and 100 kHz gradient switching. Results show that displacement artifacts resulting from the presence of small amounts of ferromagnetic metal and the heating effects of metal due to gradient switching can be compensated by using the slightly modified LabPET II detection module. Thus, the LabPET II system would be MR-compatible with some minor adjustments to operate effectively inside an MRI bore without interfering with its performance.

Real-Time Microfluidic Blood-Counting System for PET and SPECT Preclinical Pharmacokinetic Studies.

UNLABELLED: Small-animal nuclear imaging modalities have become essential tools in the development process of new drugs, diagnostic procedures, and therapies. Quantification of metabolic or physiologic parameters is based on pharmacokinetic modeling of radiotracer biodistribution, which requires the blood input function in addition to tissue images. Such measurements are challenging in small animals because of their small blood volume. In this work, we propose a microfluidic counting system to monitor rodent blood radioactivity in real time, with high efficiency and small detection volume (∼1 µL). METHODS: A microfluidic channel is built directly above unpackaged p-i-n photodiodes to detect ß-particles with maximum efficiency. The device is embedded in a compact system comprising dedicated electronics, shielding, and pumping unit controlled by custom firmware to enable measurements next to small-animal scanners. Data corrections required to use the input function in pharmacokinetic models were established using calibrated solutions of the most common PET and SPECT radiotracers. Sensitivity, dead time, propagation delay, dispersion, background sensitivity, and the effect of sample temperature were characterized. The system was tested for pharmacokinetic studies in mice by quantifying myocardial perfusion and oxygen consumption with (11)C-acetate (PET) and by measuring the arterial input function using (99m)TcO4 (-) (SPECT). RESULTS: Sensitivity for PET isotopes reached 20%-47%, a 2- to 10-fold improvement relative to conventional catheter-based geometries. Furthermore, the system detected (99m)Tc-based SPECT tracers with an efficiency of 4%, an outcome not possible through a catheter. Correction for dead time was found to be unnecessary for small-animal experiments, whereas propagation delay and dispersion within the microfluidic channel were accurately corrected. Background activity and sample temperature were shown to have no influence on measurements. Finally, the system was successfully used in animal studies. CONCLUSION: A fully operational microfluidic blood-counting system for preclinical pharmacokinetic studies was developed. Microfluidics enabled reliable and high-efficiency measurement of the blood concentration of most common PET and SPECT radiotracers with high temporal resolution in small blood volume.

Imaging performance of LabPET APD-based digital PET scanners for pre-clinical research.

The LabPET is an avalanche photodiode (APD) based digital PET scanner with quasi-individual detector read-out and highly parallel electronic architecture for high-performance in vivo molecular imaging of small animals. The scanner is based on LYSO and LGSO scintillation crystals (2×2×12/14 mm3), assembled side-by-side in phoswich pairs read out by an APD. High spatial resolution is achieved through the individual and independent read-out of an individual APD detector for recording impinging annihilation photons. The LabPET exists in three versions, LabPET4 (3.75 cm axial length), LabPET8 (7.5 cm axial length) and LabPET12 (11.4 cm axial length). This paper focuses on the systematic characterization of the three LabPET versions using two different energy window settings to implement a high-efficiency mode (250­650 keV) and a high-resolution mode (350­650 keV) in the most suitable operating conditions. Prior to measurements, a global timing alignment of the scanners and optimization of the APD operating bias have been carried out. Characteristics such as spatial resolution, absolute sensitivity, count rate performance and image quality have been thoroughly investigated following the NEMA NU 4-2008 protocol. Phantom and small animal images were acquired to assess the scanners' suitability for the most demanding imaging tasks in preclinical biomedical research. The three systems achieve the same radial FBP spatial resolution at 5 mm from the field-of-view center: 1.65/3.40 mm (FWHM/FWTM) for an energy threshold of 250 keV and 1.51/2.97 mm for an energy threshold of 350 keV. The absolute sensitivity for an energy window of 250­650 keV is 1.4%/2.6%/4.3% for LabPET4/8/12, respectively. The best count rate performance peaking at 362 kcps is achieved by the LabPET12 with an energy window of 250­650 keV and a mouse phantom (2.5 cm diameter) at an activity of 2.4 MBq ml−1. With the same phantom, the scatter fraction for all scanners is about 17% for an energy threshold of 250 keV and 10% for an energy threshold of 350 keV. The results obtained with two energy window settings confirm the relevance of high-efficiency and high-resolution operating modes to take full advantage of the imaging capabilities of the LabPET scanners for molecular imaging applications.

Fully 3D iterative CT reconstruction using polar coordinates.

PURPOSE: This paper demonstrates the feasibility of fully 3D iterative computed tomography reconstruction of highly resolved fields of view using polar coordinates. METHODS: System matrix is computed using a ray-tracing approach in cylindrical or spherical coordinates. By using polar symmetries inherent to the acquisition geometry, the system matrix size can be reduced by a factor corresponding to the number of acquired projections. Such an important decrease in size allows the system matrix to be precomputed, and loaded all at once into memory prior to reconstruction. By carefully ordering the field of view voxels and the sinogram data, reconstruction speed is also enhanced by a cache-oblivious computer implementation. The reconstruction algorithm is also compatible with the ordered-subsets acceleration method. A final polar-to-Cartesian transformation is applied to the reconstructed image in order to allow proper visualization. RESULTS: The ray-tracing and reconstruction algorithms were implemented in polar representation. Large 3D system matrices were calculated in cylindrical and spherical coordinates, and the performance assessed against Cartesian ray-tracers in terms of speed and memory requirements. Images of analytical phantoms were successfully reconstructed in both cylindrical and spherical coordinates. Fully 3D images of phantoms and small animals were acquired with a Gamma Medica Triumph X-O small animal CT scanner and reconstructed using the manufacturer's software and the proposed polar approach to demonstrate the accuracy and robustness of the later. The noise was found to be reduced while preserving the same level of spatial resolution, without noticeable polar artifacts. CONCLUSIONS: Under a reasonable set of assumptions, the memory size of the system matrix can be reduced by a factor corresponding to the number of projections. Using this strategy, iterative reconstruction from high resolution clinical and preclinical systems can be more easily performed using general-purpose personal computers.

Toward truly combined PET/CT imaging using PET detectors and photon counting CT with iterative reconstruction implementing physical detector response.

PURPOSE: This paper intends to demonstrate the feasibility of truly combined PET/CT imaging and addresses some of the major challenges raised by this dual modality approach. A method is proposed to retrieve maximum accuracy out of limited resolution computed tomography (CT) scans acquired with positron emission tomography (PET) detectors. METHODS: A PET/CT simulator was built using the LabPET™ detectors and front-end electronics. Acquisitions of energy-binned data sets were made using this low spatial resolution CT system in photon counting mode. To overcome the limitations of the filtered back-projection technique, an iterative reconstruction library was developed and tested for the counting mode CT. Construction of the system matrix is based on a preregistered raster scan from which the experimental detector response is obtained. PET data were obtained sequentially with CT in a conventional manner. RESULTS: A meticulous description of the system geometry and misalignment corrections is imperative and was incorporated into the matrix definition to achieve good image quality. Using this method, no sinogram precorrection or interpolation is necessary and measured projections can be used as raw input data for the iterative reconstruction algorithm. Genuine dual modality PET/CT images of phantoms and animals were obtained for the first time using the same detection platform. CONCLUSIONS: CT and fused PET/CT images show that LabPET™ detectors can be successfully used as individual X-ray photon counting devices for low-dose CT imaging of the anatomy in a molecular PET imaging context.

Blood compatible microfluidic system for pharmacokinetic studies in small animals.

New radiotracer developments for nuclear medicine imaging require the analysis of blood as a function of time in small animal models. A microfluidic device was developed to monitor the radioactivity concentration in the blood of rats and mice in real time. The microfluidic technology enables a large capture solid angle and a reduction in the separation distance between the sample and detector, thus increasing the detection efficiency. This in turn allows a reduction of the required detection volume without compromising sensitivity, an important advantage with rodent models having a small total blood volume (a few ml). A robust fabrication process was developed to manufacture the microchannels on top of unpackaged p-i-n photodiodes without altering detector performance. The microchannels were fabricated with KMPR, an epoxy-based photoresist similar to SU-8 but with improved resistance to stress-induced fissuring. Surface passivation of the KMPR enables non-diluted whole blood to flow through the channel for up to 20 min at low speed without clotting. The microfluidic device was embedded in a portable blood counter with dedicated electronics, pumping unit and computer control software for utilisation next to a small animal nuclear imaging scanner. Experimental measurements confirmed model predictions and showed a 4- to 19-fold improvement in detection efficiency over existing catheter-based devices, enabling a commensurate reduction in sampled blood volume. A linear dose-response relationship was demonstrated for radioactivity concentrations typical of experiments with rodents. The system was successfully used to measure the blood input function of rats in real time after radiotracer injection.

A pulse simulator for crystal identification validation of phoswich detectors used in positron emission tomography.

Crystal identification (CI) of phoswich detectors is a technique used in positron emission tomography (PET) for improving spatial resolution through depth-of-interaction determination or higher pixelization. Digital algorithms using advanced digital signal processing techniques currently provide the most powerful approaches for CI of phoswich detectors made of crystals with only slightly different scintillation decay times. Such methods can be implemented in the all-digital architecture of LabPET, a small animal PET scanner developed in Sherbrooke, for fast and accurate real-time CI. In order to validate the new CI algorithms and assess their performance for different front-end electronics, a pulse generator simulator was developed to generate PET signals and investigate the effects of factors such as electronic noise, photon statistics and pulse shaping filter. The pulse generator was validated with LabPET-like pulses and CI results were compared with experimental data. The pulse simulator enables CI algorithms to be validated together with detector performance such as energy and timing resolution at an early stage of scanner design.

An investigation of the physical forces leading to thrombosis disruption by cavitation.

Ultrasound therapy has proven to be an efficient and safe modality for the treatment of acute arterial occlusions, and the use of therapeutic ultrasound for the treatment of thrombosis and vascular diseases holds great promise in overcoming the limitations of other available therapies. Still, there exists little published work that covers the different phenomena that take place in a thorough and comprehensive way. In this paper, we endeavor to address the subject by reviewing work on the physical properties of ultrasound propagation in the blood arteries as it relates to the cavitation of microbubbles, and we compare the impact of the different forces at work for clot disruption. Our conclusion is that the most important effect of ultrasound in the treatment of thrombotic disorders is the liquid-jet impact forces that result from strong bubble collapses in the vicinity of solid boundaries.

Psicofarmacología de la fobia social / Psychopharmacology of social phobia

La fobia social es un trastorno de ansiedad crónico y debilitante, caracterizado por la presencia de síntomas de severa ansiedad ante ciertas situaciones sociales. Este trastorno se complica frecuentemente con abuso o adicción al alcohol y las drogas. El empleo de psicofármacos en el tratamiento de la fobia sociales, ha despertado interés recientemente. Se han observado resultados favorables en tres clases de medicamentos: beta-bloqueadores, inhibidores de la monoamino-oxidasa (IMAO) y benzodiacepinas potentes. En este artículo realizamos una revisión de la literatura existente sobre el uso de fármacos en la fobia social, con recomendaciones sobre su tratamiento