Fully three-dimensional reconstruction for a PET camera with retractable septa

A fully 3-D reconstruction algorithm has been developed to reconstruct data from a 16 ring PET camera (a Siemens/CTI 953B) with automatically retractable septa. The tomograph is able to acquire coincidences between any pair of detector rings and septa retraction increases the total system count rate by a factor of 7.8 (including scatter) and 4.7 (scatter subtracted) for a uniform, 20 cm diameter cylinder. The reconstruction algorithm is based on 3-D filtered backprojection, expressed in a form suitable for the multi-angle sinogram data. Sinograms which are not measured due to the truncated cylindrical geometry of the tomograph, but which are required for a spatially invariant response function, are obtained by forward projection. After filtering, the complete set of sinograms is backprojected into a 3-D volume of 128x128x31 voxels using a voxel-driven procedure. The algorithm has been validated with simulation, and tested with both phantom and clinical data from the 953B.     

Bandwidth and dispersion characteristics of a new rectangularwaveguide with two L-shaped septa

A new rectangular waveguide with two L-shaped septa has been added to the family of septa-loaded broad-band waveguides. Its modal characteristics and dispersion in the inhomogeneously loaded version have been theoretically determined. The cutoff frequency and the bandwidth of the lowest TE mode are also compared with the experimentally measured values obtained with a test cavity     

Electrodynamic analysis of eigenmodes in a rectangular waveguide with two L-shaped septa

A technique of electrodynamic calculation and computer-aided simulation of the structure of the electromagnetic fields of TE and TM waves in a rectangular waveguide with two L-shaped ridges and a lossless air filling is presented. The problem is solved by means of the partial domain method with allowance for the edge singularity of the electromagnetic field. Cutoff wave numbers, the structure of the electromagnetic fields of various TE — and TM-wave modes in the cutoff regime, the characteristic impedance, and the breakdown power for the fundamental waveguide mode are calculated.     

A method for determination of the timing stability of PET scanners

We report on the timing resolution and stability of the MicroPET R4 PET scanner. Its detectors have energy resolutions in the range of 25% and previously reported timing resolutions 3.2 ns. Our preliminary evaluation of this instrument showed that artefact-free normalization sinograms could only be obtained with a timing window of 10 ns or more in spite of a timing resolution of 3.2 ns. This instrument uses high-speed electronics albeit with 2-ns timing clock. We performed sham transmission scans with nothing in the field of view, and a range of timing windows from 2 to 14 ns and used a 14-ns timing blank scan to generate effective attenuation sinograms as a function of timing window. These showed trues count-rates which fit well to a ERF (tau) function. However, the effective attenuation value, which should be 1.0, changes from block to block and becomes very high in some blocks (> 3.5 at 6 ns) suggesting the need for timing alignment. A method was devised to measure the timing stability to a precision better than the timing bin width of 2 ns.     

Optimization of noise equivalent count rate performance for a partially collimated PET scanner by varying the number of septa

We present a simulation study of the global count-rate performance of a positron emission tomography (PET) scanner with different levels of partial collimation to maximize the noise equivalent count rate for whole-body PET imaging. We achieve partial collimation by removing different numbers of septal rings from the standard 2-D septa set for the GE Advance PET scanner. System behavior is studied with a photon tracking simulation package, which we modify to enable the production of random coincidences. The simulations are validated with measured data taken in 2-D and fully 3-D acquisition mode on a GE Advance system using the National Electrical Manufacturers Association NU-2 count-rate phantom with two sets of annular sleeves to expand the diameter to 27 and 35 cm. For all diameters and in 2-D and fully 3-D mode, there is good agreement between measurements and simulations. All studies use the three phantom diameters to evaluate the effect of patient thickness for each amount of collimation. Optimized system parameters, such as maximum ring difference for single slice rebinning, are determined for the five partially collimated systems considered. The resulting global count rates for true, scattered, and random coincidences, the noise equivalent count (NEC) rates, and the scatter fractions for different levels of collimation are compared along with the results from the conventional 2-D and fully 3-D modes. Improved statistical data quality relative to both 2-D and fully 3-D data is found with the partially collimated systems, particularly when one-half or two-thirds of the septal rings are removed. An increase in NEC rates of as much as 50% is found for clinically relevant activities between 5-10 mCi (184-370 MBq). Scatter fractions for the partially collimated systems are intermediate between the 2-D and fully 3-D numbers. Many factors that affect image quality have not been considered in this paper. However, the significant increase in statistical data quality warrants further investigation of the impact of partial collimation on clinical whole-body PET imaging.     

Noise equivalent count measurements in a neuro-PET scanner with retractable septa

The noise-equivalent count-rate (NEC) performance of a neuro-positron emission tomography (PET) scanner has been determined with and without interplane septa on uniform cylindrical phantoms of differing radii and in human studies to assess the optimum count rate conditions that realize the maximum gain. In the brain, the effective gain in NEC performance for three-dimensions (3-D) ranges from >5 at low count rates to approximately 3.3 at 200 kcps (equivalent to 37 kcps in 2-D). The gains of the 3-D method assessed by this analysis are significant, and are shown to be highly dependent on count rate and object dimensions.     

Modeling and incorporation of system response functions in 3-D whole body PET

Appropriate application of spatially variant system models can correct for degraded resolution response and mispositioning errors. This paper explores the detector blurring component of the system model for a whole body positron emission tomography (PET) system and extends this factor into a more general system response function to account for other system effects including the influence of Fourier rebinning (FORE). We model the system response function as a three-dimensional (3-D) function that blurs in the radial and axial dimension and is spatially variant in radial location. This function is derived from Monte Carlo simulations and incorporates inter-crystal scatter, crystal penetration, and the blurring due to the FORE algorithm. The improved system model is applied in a modified ordered subsets expectation maximization (OSEM) algorithm to reconstruct images from rebinned, fully 3-D PET data. The proposed method effectively removes the spatial variance in the resolution response, as shown in simulations of point sources. Furthermore, simulation and measured studies show the proposed method improves quantitative accuracy with a reduction in tumor bias compared to conventional OSEM on the order of 10%-30% depending on tumor size and smoothing parameter.     

The dielectric response function of systems with reduced dimensionality

The dielectric response function of the electron gas in a system with reduced dimensionality within the RPA and the Hubbard approximation, are given by using a linear response theory in which the conditions of a multisubband spectrum and finite spatial localization of the wavefunction in one, two or three dimensions, proper of such systems, are taken into account. The final expressions for the dielectric response function is given in each case and its general properties are analyzed. These results are applied to the calculations of the intrasubband and the intersubband polarizabilities in a quantum well (QW) and a quantum well wire (QWW). For the QW, the analysis of plasmons in a semiconductor based and a metal-based QW are given and compared with other theoretical calculations. In the metallic QW, exchange provokes the stabilization of the modes with respect to the RPA plasmons. For the QWW, the exchange modifies the polarizability in both the intrasubband and the intersubband transitions. The main arguments to perform the analysis to achieve the dielectric response function for the quantum dot (QD) in the above both approximations and for the Singwi-Tosi-Lang-Sjölander (STLS) approximation (which takes account of exchange and correlation) for the QD, the QW and the QWW are outlined.     

Design of quasi-elliptic function filters with a dual-passband response

Compact miniaturized hairpin resonators are used to design cross-coupled filters with a dual-passband response of elliptic function type. The hairpin resonators are treated as a stepped impedance resonator (SIR) in analysis. In a filter, two different hairpin resonators are used to establish proper couplings required for the two design frequencies. Two filters are fabricated and measured. The results show a good agreement with the simulation.     

Absorption-amplification response with or without spontaneously generated coherence effect in a four-level atomic system

We discuss and analyze the absorption properties of a weak probe field in a typical four-level atomic system in the presence of a spontaneously generated coherence (SGC) term. The influences of the SGC and a coherent pump field on the probe absorption-amplification are investigated. The results show that the absorption of such a weak probe field can be dramatically enhanced due to the SGC effect. At the same time, the probe-absorption profile exhibits a two-peak structure and the probe-absorption peak gradually decreases as the pump intensity increases. On the contrary, the amplification of such a weak probe field near the line center of the probe transition can be achieved by adjusting the coherent pump field intensity in the absence of the SGC effect.     

Synthesis of an in-line prototype filter with two transmission zeros without cross couplings

This letter describes a method for synthesizing a general in-line prototype filter presenting one or two transmission zeros without using cross couplings; the basic topology is constituted by resonators coupled through inverters, with input and/or output taken at the second and/or last but one resonators. In this case, the first and last resonators, tuned at frequencies in the stopband, produce two zeros in the transfer function. It is shown that, even if it is not possible to exactly synthesize this structure with extracted poles techniques, an approximated design procedure can be derived; the procedure is based on the synthesis of a low pass prototype with multiple couplings starting from source and/or load, followed by suitable network transformations. This design technique can be easily implemented in case of classical combline or interdigital filters.     

Assessment of autonomic response by broad-band respiration

We present a technique for introducing broad-band respiratory perturbations so that the response characteristics of the autonomic nervous system can be determined noninvasively over a wide range of physiologically relevant frequencies. A subject's respiratory bandwidth was broadened by breathing on cue to a sequence of audible tones spaced by Poisson intervals. The transfer function between the respiratory input and the resulting instantaneous heart rate was then computed using spectral analysis techniques. Results using this method are comparable to those found using traditional techniques, but are obtained with an economy of data collection.     

Wavelength-time codes with maximum cross-correlation function of two for multicode-keying optical CDMA

In this paper, a new family of wavelength-time codes with expanded code cardinality and the maximum cross-correlation function of 2 (i.e., /spl lambda//sub c/=2) is constructed and analyzed. One application of the large code cardinality of our /spl lambda//sub c/=2 codes is multicode-keying wavelength-time optical code division multiple access (O-CDMA), in which each user is allocated with multiple code matrices, instead of just one code matrix in the conventional ON-OFF keying (OOK) O-CDMA. System throughput is increased because a lower baud rate O-CDMA system can be used to support higher bit-rate transmission since each code matrix is used to represent a "symbol" of several data bits. User code confidentiality is improved because of symbol transmission. The performances of two multicode-keying O-CDMA schemes with the new /spl lambda//sub c/=2 wavelength-time codes are also analyzed. The results in this paper show that there is a tradeoff between the performance and the number of code matrices per user.     

新型激光扫描仪RF620及其应用

RIFFEK公司的RF620系列二维激光位移传感器（二维轮廓扫描仪）问世以来,以其极高的性价比久负盛名。该激光轮廓扫描仪主要应用于物体轮廓、二维尺寸、二维位移的非接触测量与检验,以及目标物体的分拣。本文介绍了此系列扫描仪的原理、接口、配置及软件开发工具。     

Studies of the angular correlation function of scattering by randomrough surfaces with and without a buried object

The discrimination of the scattered wave from an object buried in shallow ground from that of the rough surface is a difficult task with present ground penetrating radar (GPR) systems. Recently, a new approach for this classical problem has been proposed and its effectiveness has been verified. This new method is based on the angular correlation function (ACF) of the scattered wave observed at two or more different incident and scattered angle combinations. It has been shown that the angular memory signatures of rough surfaces are substantially different from those of typical man-made targets and by choosing the appropriate incident and scattered angles, the surface scattering can be minimized whereas the scattering from the target is almost unchanged. The authors present detailed numerical studies of the ACF of the scattered wave from rough surfaces with and without a buried object. To obtain the ACF, the three averaging methods: realization, frequency and angular averaging, are tested numerically. It is shown that a single random rough surface of moderate extent can exhibit memory effect by using frequency averaging. Frequency averaging with a wide bandwidth is also effective for suppressing fluctuation in ACF and is most useful for practical applications. Numerical simulations indicate that even when the ratio of scattered intensities with and without the buried object is close to unity, the corresponding ratio of ACF magnitude can be more than 10 dB. Thus, using the ACF is superior to using the radar cross section (RCS) in the detection of buried objects     

The transfer function and impulse response of photorefractivetwo-beam coupling

The transfer function and impulse response of photorefractive two-beam coupling are derived in the undepleted pump approximation. For sufficiently strong coupling ΓL, the impulse response features a broad delayed output pulse. In the limits of negligible and strong absorption αL, this coupling threshold reads ΓL_thr=4 and Γ_thr=2α, respectively. The time delay and pulse height are functions of the coupling ΓL, the photorefractive time constant τ, and the effective absorption αL. Experiments on a BaTiO₃ crystal measuring the absoluted square of its transfer function and the impulse response are used to determine the coupling and time constant     

Dielectric response function of breakdown plasma in silicon-controlled rectifiers

The real and imaginary parts of the dielectric constant of the plasma in silicon-controlled rectifier was studied as functions of frequency from 0.3 to 30 MHz for different injection levels of the current. The relaxation time was estimated to vary from 40 to 25 ns from lower injection levels to higher injection levels. The results also exhibit a nonlinear dependence of damping constant and plasma frequency on the injection level.     

Optical spectral inversion without frequency shift by four-wavemixing using two pumps with orthogonal polarization

We propose a method to obtain spectral inversion of a signal without shifting its frequency. We use four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) with two pumps of orthogonal polarization. A conjugate field is generated with a polarization orthogonal to that of the incoming signal and is selected at the amplifier output by a polarizer. Using a commercially available pig-tailed SOA of 1.5-mm length, we obtain after the polarizer 21.5 dB of intensity ratio between conjugate and amplified incoming signal     

The potential role of thermal infrared multispectral scanners in geological remote sensing

Thermal infrared scanners with multiple channels in the 8-14-µm wavelength region are important for geological remote sensing because this spectral region contains important compositional information about silicate rocks and minerals that cannot be duplicated by remote sensors operating elsewhere in the electromagnetic spectrum. Emittance minima in this spectral region, caused by interatomic oscillations, occur at different wavelengths depending on silicate rock type. It has been demonstrated that an image constructed from a signal that is proportional to a ratio of radiances in two thermal scanner channels can be used to map compositional (chemical and mineral) variations in silicates, while suppressing temperature variations across a scanned scene. Theoretical studies indicate that future infrared scanners with eight to twelve channels in the 8-14-µm region might be used to produce an image that could be simply level-sliced (divided into discrete gray levels) to map silicate rocks according to traditional rock classification charts. This is a field in which sensor technology is still the limiting factor. However, improvements in extrinsic sensor properties, especially an increase in the number of spatially coregistered detector elements in a single dewar, are more important than improvements in intrinsic properties, such as detectivity.     

On variations of queue response for inputs with the same mean andautocorrelation function

This paper explores the variations in mean queue length for stationary arrival processes with the same mean and autocorrelation functions, or equivalently, the same mean and power spectrum. Three types of processes, namely, two-state Markov-modulated Poisson processes, periodic-sequence modulated Poisson processes and processes generated by randomly filtering a white noise process, are investigated. Results show that the mean queue length can vary substantially for the first type of process, and can vary moderately for the second type of process, as the parameters of the processes are varied, subject to a specified mean and autocorrelation function. However, the mean queue lengths for the third type of arrival processes are determined by the input mean and autocorrelation functions. The results suggest that the queueing performance can be hard to predict from spectral data alone when the power in low frequencies is large