V. Herrero

Dependency of Energy-, Position- and Depth of Interaction Resolution on Scintillation Crystal Coating and Geometry

Options for optimizing the energy and spatial resolution of gamma-ray imaging detectors based on thick, monolithic crystals shaped like flat-topped pyramids were studied. Monte Carlo simulations were made of the scintillation light transport for evaluating the effect of four parameters on the energy resolution, the spatial resolutions, and the depth of interaction (DOI) resolution of the gamma-ray imaging detector. These four parameters are: the reflectivity of the surface coating; the scatter mean free path; the absorption mean free path of the scintillation light; and the angle that defines the inclination of the sides of the pyramidal frustum. In real detectors, the values for the mean free paths for optical photons are normally not known. We estimated these by comparing MC simulations of detector resolutions to measurements for three gamma-ray imaging detectors with LYSO and LSO from different suppliers and with different surface coatings and geometries. The gamma-ray imaging detector measures the energy, centroids, and depth of interaction of the gamma-ray. DOI enhanced charge dividing readouts were used to measure the depth of interaction.

DOI measurement with monolithic scintillation crystals: A primary performance evaluation

We report a first assessment of image quality enhancement achieved by the implementation of depth of interaction detection with monolithic crystals. The method of interaction depth measurement is based on analogue computation of the standard deviation with an enhanced charge divider readout. This technique of depth of interaction detection was developed in order to provide fast and determination of this parameter at a reasonable increase of detector cost. The detector consists of an large-sized monolithic scintillator coupled to a position sensitive photomultiplier tube. A special design feature is the flat-topped pyramidal shape of the crystal. This reduces image compression near the edges of the scintillator. We studied the image enhancement qualitatively with a FDG filled hot spot phantom and quantitatively by displacing a single point source along a radial axis. An important uniformity improvement was observed for the reconstructed image of the hot spot phantom when depth of interaction correction was applied. A moderate improvement of the spatial resolution was observed when reconstructing the images of the point source with depth of interaction correction.

Design and Calibration of a Small Animal Pet Scanner Based on Continuous LYSO Crystals and PSPMTs

We report on the design of a small animal PET scanner based on continuous LYSO crystals and position sensitive photomultiplier tubes (PSPMTs), together with the first results from the calibration. The scanner consists of eight compact modules forming an octagon and leaving a port of 110 mm aperture. Each module is made out of a continuous LYSO crystal and a PSPMT, and contains its associated electronics together with its power supply. For each module, five signals are read, summarizing all the information coming out from its 64 anode pads. Therefore, for the whole scanner only 40 signals are digitized. A calibration procedure has been implemented, measuring a spatial resolution of approximately 1.5 mm at the center of the field of view and an energy resolution of 18%. The sensitivity of the system at the center of the field of view, using only 4 modules, is observed to be of about 1%.

A mixed hardware-software approach to flexible Artificial Neural Network training on FPGA

FPGAs offer a promising platform for the implementation of Artificial Neural Networks (ANNs) and their training, combining the use of custom optimized hardware with low cost and fast development time. However, purely hardware realizations tend to focus on throughput, resorting to restrictions on applicable network topology or low-precision data representation, whereas flexible solutions allowing a wide variation of network parameters and training algorithms are usually restricted to software implementations. This paper proposes a mixed approach, introducing a system-on-chip (SoC) implementation where computations are carried out by a high efficiency neural coprocessor with a large number of parallel processing elements. System flexibility is provided by on-chip software control and the use of floating-point arithmetic, and network parallelism is exploited through replicated logic and application-specific coprocessor architecture, leading to fast training time. Performance results and design limitations and trade-offs are discussed.

Improved Digital Pulse Height Estimation for PET Detectors Using LMS Adaptive Filters

Digital positron emission tomography (PET) systems enable us to implement sophisticated algorithms for real time signal processing. Usually, energy and position are encoded on the amplitude of the digitized signals. Although a lot of work has been made for spectroscopy applications, digital pulse height estimation in PET detectors presents important challenges mainly due to the fast response of state-of-the-art detectors. In this paper we propose a digital filtering technique for improving amplitude estimation in order to achieve high spatial and energy resolution with moderate conversion rates (80 MSPS). Our approach is based on the trapezoidal shaping of the incoming pulses by using adaptive filtering based on Least-Mean-Squares (LMS) algorithm for modelling detector and front-end electronics response. Thus, the filtered signals are well suited both for pile-up recovery and accurate pulse height computation. The filter can be easily implemented on programmable logic devices and integrated on the data acquisition chain for real-time applications.

Accurate Simulation Testbench for Nuclear Imaging Systems

Current testbenches for nuclear imaging devices aim to simulate only a single stage of the system at a time. This approach is useful in early design stages where accuracy is not necessary. However, it would be desirable that different tools could be combined to achieve more detailed simulations. In this work, we present a high precision testbench that has been developed to test nuclear imaging systems. Its accuracy lies in the possibility of linking different simulation tools using the right one for each part of the system. High energy events are simulated using Geant4 (High Energy Simulator). Analog and digital electronics are verified using Cadence Spectre and ModelSim. This testbench structure allows testing any physical topology, scintillation crystals, photomultiplier tubes (PMTs), avalanche photodiodes (APDs), with any kind of ASIC, discrete analog and digital electronics, thus reducing the prototyping and design time. New system developments can be easily verified using behavioral and circuital description models for analog and digital electronics. Finally, a dual-head continuous LSO scintillation crystal positron emission tomography (PET) system has been used as an example for evaluation of the testbench.

On the Implementation of a Margolus Neighborhood Cellular Automata on FPGA

Margolus neighborhood is the easiest form of designing Cellular Automata Rules with features such as invertibility or particle conserving. In this paper we propose two different implementations of systems based on this neighborhood: The first one corresponds to a classical RAM-based implementation, while the second, based on concurrent cells, is useful for smaller systems in which time is a critical parameter. This implementation has the feature that the evolution of all the cells in the design is performed in the same clock cycle.

Lifting folded pipelined discrete wavelet packet transform architecture

The present article describes a new high-efficient architecture for 1-D discrete wavelet packet transform (DWPT) base on lifting, folded and pipeline techniques, which makes possible to expand three completes levels. An architecture for a CDF(2,2) wavelet base is proposed. We have designed a filter bank using a lifting factorization for these coefficients and we have used an extension of the recursive pyramid algorithm (RPA) to obtain the three complete levels. We have pipelined our architecture to reach a maximally fast structure with only one logic operator in the critical path. Moreover, our architecture performances 75 % of hardware utilization for a DWPT realization. A comparative is presented between our DWPT architecture with others DWPT architectures. Our proposal lifting pipelined DWPT architecture is a maximally fast structure with only one logic operator in the critical path. Others DWPT architectures are based on memory access, that implies lower operation frequency and higher power consumption as our architecture.

Demonstration of Single-Barium-Ion Sensitivity for Neutrinoless Double-Beta Decay Using Single-Molecule Fluorescence Imaging

A new method to tag the barium daughter in the double-beta decay of ^{136}Xe is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (Ba^{++}) resolution at a transparent scanning surface is demonstrated. A single-step photobleach confirms the single ion interpretation. Individual ions are localized with superresolution (∼2  nm), and detected with a statistical significance of 12.9σ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double-beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.

Digital delay line shaping-zero crossing algorithm for timestamp extraction in PET

Timing resolution is a key parameter in PET scanners in order to reduce random events. However, fully digital DAQ systems working in free-running sampling mode with conversion rates ≪ 100 MHz have to rely on interpolation or model fitting techniques in order to achieve a tight coincidence window. This paper presents a simple but accurate method for on-line timestamp extraction in PET detectors. The proposed algorithm is based on a simple bipolar shaping using digital delays and a subsequent zero-crossing detection by linear interpolation. It can be easily integrated in an FPGA and it operates synchronously with the data processing chain. Experimental results show that this method produces a gaussian time difference spectrum with good timing resolution of σ = 2.20 ns (5.17 ns FWHM) obtained with a fairly moderate conversion rate of 40 MHz.