Simultaneous reconstruction of activity and attenuation for PET/MR

Medical investigations targeting a quantitative analysis of the position emission tomography (PET) images require the incorporation of additional knowledge about the photon attenuation distribution in the patient. Today, energy range adapted attenuation maps derived from computer tomography (CT) scans are used to effectively compensate for image quality degrading effects, such as attenuation and scatter. Replacing CT by magnetic resonance (MR) is considered as the next evolutionary step in the field of hybrid imaging systems. However, unlike CT, MR does not measure the photon attenuation and thus does not provide an easy access to this valuable information. Hence, many research groups currently investigate different technologies for MR-based attenuation correction (MR-AC). Typically, these approaches are based on techniques such as special acquisition sequences (alone or in combination with subsequent image processing), anatomical atlas registration, or pattern recognition techniques using a data base of MR and corresponding CT images. We propose a generic iterative reconstruction approach to simultaneously estimate the local tracer concentration and the attenuation distribution using the segmented MR image as anatomical reference. Instead of applying predefined attenuation values to specific anatomical regions or tissue types, the gamma attenuation at 511 keV is determined from the PET emission data. In particular, our approach uses a maximum-likelihood estimation for the activity and a gradient-ascent based algorithm for the attenuation distribution. The adverse effects of scattered and accidental gamma coincidences on the quantitative accuracy of PET, as well as artifacts caused by the inherent crosstalk between activity and attenuation estimation are efficiently reduced using enhanced decay event localization provided by time-of-flight PET, accurate correction for accidental coincidences, and a reduced number of unknown attenuation coefficients. First results achieved with measured whole body PET data and reference segmentation from CT showed an absolute mean difference of 0.005 cm?1 (< 20%) in the lungs, 0.0009 cm?1 (< 2%) in case of fat, and 0.0015 cm?1 (< 2%) for muscles and blood. The proposed method indicates a robust and reliable alternative to other MR-AC approaches targeting patient specific quantitative analysis in time-of-flight PET/MR.     

Challenges and current methods for attenuation correction in PET/MR

Quantitative PET imaging requires an attenuation map to correct for attenuation. In stand-alone PET or PET/CT, the attenuation map is usually derived from a transmission scan or CT image, respectively. In PET/MR, these methods will most likely not be used. Therefore, attenuation correction has long been regarded as one of the major challenges in the development of PET/MR. In the past few years, much progress has been made in this field. In this review, the challenges faced in attenuation correction for PET/MR are discussed. Different methods have been proposed to overcome these challenges. An overview of the MR-based (template-based and voxel-based), transmission-based and emission-based methods and the results that have been obtained is provided. Although several methods show promising results, no single method fulfils all of the requirements for the ideal attenuation correction method for PET/MR. Therefore, more work is still necessary in this field. To allow implementation in routine clinical practice, extensive evaluation of the proposed methods is necessary to demonstrate robustness and automation.     

Carbon materials in composite bipolar plates for polymer electrolyte membrane fuel cells: A review of the main challenges to improve electrical performance

The technology of polymer electrolyte membrane (PEM) fuel cells is dependent on the performance of bipolar plates. There is a strong relationship between the material used in the manufacturing of the bipolar plate and its final properties. Graphite-polymer composite bipolar plates are well-established commercial products. Several other carbon based fillers are tested. Carbon nanotubes, carbon fibers, carbon black, graphite nanoplatelets and expanded graphite are examples of such materials. Structural characteristics of these particles such as morphology and size have decisive influence on the final properties of bipolar plates. Furthermore, the volumetric fraction of the filler is of prime importance. There is plenty of information on individual aspects of specific composite bipolar plates in the literature. Notwithstanding, the analysis of structure-property relationship of these materials in a comprehensive source is not found. In this paper, relevant topics on the structural aspects of carbon based fillers and how they influence the final electrical performance of composite bipolar plates are discussed. It is intended that this document contribute to the development of new and maximized products to the PEM fuel cell industry.     

Biological oxygen demand dynamics in the lower San Joaquin River, California

Dissolved oxygen less than 5 mg L(-1) (U. S. EPA advisory level) commonly occurs in the lower San Joaquin River (SJR), California. Most acute episodes typically occur in late summer and fall. The oxygen deficit can stress and kill aquatic organisms, often inhibiting the upstream migration of Chinook salmon (Oncorhynchus tshawytscha). This 5 year study examined watershed-scale spatial and temporal biological oxygen demand (BOD) loads and the primary components believed to contribute to BOD: ammonium, algal biomass, nonalgal particulate organic carbon, dissolved organic carbon, and dissolved organic nitrogen. Samples were collected bimonthly at 15 sites during the summers of 2000 and 2001 and from three lower mainstem sites from May 2002 to March 2005. BOD loads showed a downstream increase in parallel with increasing algal biomass loads resulting primarily from in-stream growth. BOD loads from measured tributaries and drains accounted for 28% and 39% of the BOD load at the downstream Vernalis site in 2000 and 2001, respectively. Algal C was positively correlated (r=0.80) with BOD, explaining 64% of BOD variance for data collected from 2001 to 2005. Less than 20% of BOD was found in the dissolved fraction (<0.45 microm). We conclude that algal biomass is the primary contributor to BOD loads in the lower SJR, upstream of Mossdale.     

Personalaufwandsplanung mit Hilfe eines erweiterten linearen Modells

Zusammenfassung Die Personalaufwandsplanung hat für die Unternehmensplanung eines Großunternehmens besondere Bedeutung. Orientierungspunkte dabei sind die gesamtwirtschaftliche Entwicklung und die Absatzmarktlage. Aufgabe ist die Ermittlung des Personalaufwands in Abhängigkeit von Belegschaftszahlen, Arbeitszeitbedarf usw. mit Hilfe eines allgemeinen Matrizenmodells, das auch nichtlineare Komponenten enthält. Das Modell ist routinemäßig im Einsatz.

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Summary Personnel expense planning is of great importance within big enterprise corporate planning. Orientation points are given from total economic development and from the market. The task is to find out personnel expense depending on personnel figures, work time requirement, etc. by implementation of a general matrix model. Non linear components are includet. The model runs in routine.

     

Analyzing SLE 88 memory management security using Interacting State Machines

The Infineon SLE 88 is a smart card processor that offers strong protection mechanisms. One of them is a memory management system typically used for sandboxing application programs dynamically loaded on the chip. High-level (EAL5+) evaluation of the chip requires a formal security model.We formally model the memory management system as an Interacting State Machine and prove, using Isabelle/HOL, that the associated security requirements are met. We demonstrate that our approach enables an adequate level of abstraction, which results in an efficient analysis, and points out potential pitfalls like noninjective address translation.     

Plasmachemische Gasphasenabscheidung und Plasmaätzen bei Atmosphärendruck

Plasma enhanced Chemical Vapour Deposition and plasma etching at atmospheric pressure Plasma processes are applied for a variety of surface modifications. Examples are, e.g. coatings to achieve an improved corrosion and scratch protection, or surface cleaning and texturising. Since these processes, however, usually take place in vacuum, they are unfortunately not applicable for large area industrial use. Plasma enhanced CVD processes at atmospheric pressure enable the deposition of functional coatings on components and semi‐finished parts with in a continuous air‐to‐air process without the use of expensive vacuum systems. By their integration into in‐line production processes the substrate handling and the coating costs are definitely reduced. A thermal plasma source, basing on a linearly extended DC arc discharge at atmospheric pressure, has been tested for the deposition of silicon nitride at substrate temperature of less than 300° in a continuous PECVD process. Furthermore this source has been tested for plasma‐chemical etching and texturising of silicon as well.