Enzyme activities are well established biomarkers of many pathologies. Imaging enzyme activity directly in vivo may help to gain insight into the pathogenesis of various diseases but remains extremely challenging. In this communication, we report the use of EPR imaging (EPRI) in combination with a specially designed paramagnetic enzymatic substrate to map alkaline phosphatase activity with a high selectivity, thereby demonstrating the potential of EPRI to map enzyme activity.
The advent of hybrid scanners, combining complementary modalities, has revolutionized the application of advanced imaging technology to clinical practice and biomedical research. In this project, we investigated the melding of two complementary, functional imaging methods: positron emission tomography (PET) and electron paramagnetic resonance imaging (EPRI). PET radiotracers can provide important information about cellular parameters, such as glucose metabolism. While EPR probes can provide assessment of tissue microenvironment, measuring oxygenation and pH, for example. Therefore, a combined PET/EPRI scanner promises to provide new insights not attainable with current imagers by simultaneous acquisition of multiple components of tissue microenvironments. To explore the simultaneous acquisition of PET and EPR images, a prototype system was created by combining two existing scanners. Specifically, a silicon photomultiplier (SiPM)-based PET scanner ring designed as a portable scanner was combined with an EPRI scanner designed for the imaging of small animals. The ability of the system to obtain simultaneous images was assessed with a small phantom consisting of four cylinders containing both a PET tracer and EPR spin probe. The resulting images demonstrated the ability to obtain contemporaneous PET and EPR images without cross-modality interference. Given the promising results from this initial investigation, the next step in this project is the construction of the next generation pre-clinical PET/EPRI scanner for multi-parametric assessment of physiologically-important parameters of tissue microenvironments.
<div class="section abstract"><div class="htmlview paragraph">The control and design optimization of a Free Piston Engine Generator (FPEG) has been found to be difficult as each independent variable changes the piston dynamics with respect to time. These dynamics, in turn, alter the generator and engine response to other governing variables. As a result, the FPEG system requires an energy balance control algorithm such that the cumulative energy delivered by the engine is equal to the cumulative energy taken by the generator for stable operation. The main objective of this control algorithm is to match the power generated by the engine to the power demanded by the generator. In a conventional crankshaft engine, this energy balance control is similar to the use of a governor and a flywheel to control the rotational speed. In general, if the generator consumes more energy in a cycle than the engine provides, the system moves towards a stall. If the generator consumes less energy, then the effective stroke, compression ratio and maximum translator velocity must rise steadily from cycle-to-cycle until the heat transfer losses stop the increase. Moreover, when stiff springs are added to the FPEG system, the dynamics becomes more sinusoidal and more consistent with increasing spring stiffness. To understand the behavior of proposed control and cycle-to-cycle variations, a comprehensive FPEG numerical model with a 1 kW target electric power was developed in MATLAB<sup>®</sup>/Simulink. An FPEG system corresponding to that numerical model has been operated in the laboratory. This MATLAB<sup>®</sup>/Simulink numerical model has been used to examine the sensitivity of FPEG dynamics and performance parameters to the changes in design and operating inputs. A difficulty during the modeling is associated with the cycle-to-cycle energy balance, and this difficulty is also reflected in the real-world FPEG control. Therefore, the authors have devised a control strategy similar to the real world intended control methodology. In this numerical model, two different feedback control methodologies were implemented and investigated. These control methodologies were applied to regulate the generator load with selected control or input variables, namely peak pressure, mid-stroke piston velocity, trapped compression ratio and dead center set points. The controllers with optimized coefficients demonstrated the feasibility of energy balance management during the transient operation. Based on the simulation results, the controllers with compression ratio, peak pressure and dead center clearance set points as control variables demonstrated stable FPEG operation whereas the mid-stroke velocity failed to achieve the steady-state operation due to deviation in the piston dynamics. The simulation results from this study will be used as the pathway for improving and optimizing the experimental FPEG design.</div></div>
Free Piston Linear Engines and Alternators (FPLEA) may be designed following several different baseline configurations. Common designs include a translator that carries permanent magnets, with either one piston attached to one end of the translator, or a piston at each end of the translator. The single cylinder engine requires a reversing force from a spring so that it can operate whereas the dual cylinder version can operate without a spring, but inclusion of a stiff springs would serve to raise operating frequency. Higher spring constants drive higher frequencies and also reduce the variability of the FPLEA compression ratio. The major component choices include the use of one or two cylinders, a spring constant, a bore and a stroke, and volumetric heat release. For design, the alternator is a component with translating mass that depends by design on the frequency, stroke and electrical power. The alternator demand must be matched to the engine power or the operating condition will change for the next cycle. Though there are many different FPLEA configurations, the performance comparisons of several baseline configurations have not been completely explored. A MATLAB®/Simulink numerical model with translator rod dynamics and in-cylinder thermodynamics is employed to predict the overall performance and efficiency of diesel-fueled FPLEA. This allowed comparisons of different FPLEA configurations for a variety of design variables. First, a two-cylinder FPLEA design is considered where the spring constant is varied, changing the frequency of operation and the motion of the translator. The simulation results show that without springs the motion is far from sinusoidal, and low in frequency and power, whereas the presence of stiff springs in the system strongly dictates nearly sinusoidal motion and high power at high frequency. Further, Fourier coefficients are used to characterize the motion of springs for different configurations. Effects of other parameters such as stroke and bore are also examined. Comparison is also performed for competing designs with the same power, but with one or two cylinders. The results provide a basis for selecting major design parameters before proceeding with a detailed design.
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