Spl Part Research Articles

Flying focal spot (FFS) in cone-beam CT

In the beginning of 2004 medical spiral-CT scanners that acquire up to 64 slices simultaneously became available. Most manufacturers use a straightforward acquisition principle, namely an x-ray focus rotating on a circular path and an opposing cylindrical detector whose rotational center coincides with the x-ray focus. The 64-slice scanner available to us, a Somatom Sensation 64 spiral cone-beam CT scanner (Siemens, Medical Solutions, Forchheim, Germany), makes use of a flying focal spot (FFS) that allows for view-by-view deflections of the focal spot in the rotation direction (/spl alpha/FFS) and in the z-direction (zFFS) with the goal of reducing aliasing artifacts. The FFS feature doubles the sampling density in the radial direction (channel direction, /spl alpha/FFS) and in the longitudinal direction (detector row direction or z-direction, zFFS). The cost of increased radial and azimuthal sampling is a two- or four-fold reduction of azimuthal sampling (angular sampling). To compensate for the potential reduction of azimuthal sampling the scanner simply increases the number of detector read-outs (readings) per rotation by a factor two or four. Then, up to four detector readings contribute to what we define as one view or one projection. A significant reduction of in-plane aliasing and of aliasing in the z-direction can be expected. Especially the latter is of importance to spiral CT scans where aliasing is known to produce so-called windmill artifacts. We have derived and analyzed the optimal focal spot deflection values /spl part//spl alpha/ and /spl part/z as they would ideally occur in our scanner. Based upon these we show how image reconstruction can be performed in general. A simulation study showing reconstructions of mathematical phantoms further provides evidence that image quality can be significantly improved with the FFS. Aliasing artifacts, that manifest as streaks emerging from high-contrast objects, and windmill artifacts are reduced by almost an order of magnitude with the FFS compared to a simulation without FFS. Patient images acquired with our 64-slice cone-beam CT scanner support these results.

Pressure dependence of magnetoresistance for Fe/Cr multilayers

The magnetoresistance (MR) in Fe/Cr magnetic multilayers (MML) has been measured under high pressure up to 2.5 GPa. It is found that the spin-dependent scattering plays an important role in the pressure dependence of MR ratio. In the present work, for [Fe(20/spl Aring/)/Cr(10 /spl Aring/)]/sub 20/ MML with antiferromagnetic (AF) state, the pressure coefficient of saturation field H/sub s/ is (1/H/sub s/)(/spl part/H/sub s///spl part/P)=3.3/spl times/10/sup -2/ GPa/sup -1/ between 0.1 and 2.5 GPa. We found that the (1/|J|)(/spl part/|J|//spl part/P) for AF-Fe/Cr MML with polycrystalline structure is opposite in sign to that with epitaxial one. For [Fe(20 /spl Aring/)/Cr(22/spl Aring/)]/sub 20/ MML with ferromagnetic (F) state, anisotropic magnetoresistance (AMR) decreases with increasing pressure. It is suggested that the anisotropy constant decreases with weakening spin-orbit interaction at high pressure. At high field, the AMR is easily suppressed by applying pressure while the giant magnetoresistance around H/sub s/ increases slightly with increasing pressure for polycrystalline Fe/Cr MML.

High-<tex>$T_c$</tex>Electronic Planar Gradiometer Constructed From Magnetometers on a Chip

We have fabricated a high-T/sub c/ electronic planar gradiometer composed of dc SQUIDs on a 10 mm /spl times/ 10 mm SrTiO/sub 3/ bicrystal substrate. The SQUIDs were patterned in a single-layer YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// thin film which is prepared by pulsed laser deposition. The outputs of the magnetometers are added and subtracted by analog electronic circuits to form the gradiometer. These magnetometers could be used to construct two sets electronic planar first-order gradiometers or an electronic planar second-order gradiometer, which could measure the elements /spl part/B/sub z///spl part/x and /spl part/B/sub z///spl part/y or /spl part//sup 2/B/sub z///spl part/x/spl part/y, respectively. The noise spectra of each magnetometer and a first-order electronic planar gradiometer were measured.

Synthesis and characterization of Cu-doped SrTiO/sub 3/ powders and sol-gel processed buffer layers on IBAD MgO templates

Copper-doped SrTiO/sub 3/ (STO) powders were processed by conventional bulk preparation techniques to establish solubility limits of Cu in STO. The maximum solubility of Cu in STO was identified to be /spl sim/1%. Epitaxial Cu-doped SrTiO/sub 3/ (STO) buffer layers were then prepared by pulsed laser deposition (PLD) and chemical solution deposition (CSD) on ion-beam-assisted deposition (IBAD) magnesium oxide (MgO) flexible metallic textured tapes. Transmission electron microscopy (TEM) analysis of the /spl sim/100 nm thick buffer layer deposited by PLD showed a smooth Cu-doped STO/MgO interface. Films deposited by CSD had an average film thickness of 30 nm and exhibited growth by heterogeneous nucleation with no secondary reaction phases. YBa/sub 2/Cu/sub 3/O/sub 7-/spl part// (YBCO) films were deposited onto the Cu-doped STO buffer layers by PLD.

Microfluidic electrodischarge devices with integrated dispersion optics for spectral analysis of water impurities

This paper reports a microfluidic device that integrates electrical and optical features required for field-portable water-chemistry testing by discharge spectroscopy. The device utilizes a dc-powered spark between a metal anode and a liquid cathode as the spectral source. Impurities are sputtered from the water sample into the microdischarge and characteristic atomic transitions due to them are detected optically. A blazed grating is used as the dispersion element. The device is fabricated from stacked glass layers, and is assembled and used with a charge-coupled device (CCD) sensing element to distinguish atomic spectra. Two structural variations and optical arrangements are reported. Detection of Cr and other chemicals in water samples has been successfully demonstrated with both devices. The angular resolution in terms of angular change per unit variation in wavelength (/spl part//spl theta///spl part//spl lambda/) is experimentally determined to be approximately 0.10 rad//spl mu/m, as opposed to the idealized theoretical estimate of 0.22 rad//spl mu/m. This is because the microdischarge is uncollimated and not a point source. However, this is sufficient angular resolution to allow critical spectra of metal impurities to be distinguished.

Evolution of an Electron Beam With Azimuthal Density Nonuniformity in a Cylindrical Beam Tunnel

We present the results on the deformation of the cross section of an electron beam in a cylindrical beam tunnel with /spl part///spl part/z=0 and homogeneous magnetic field, due to azimuthally nonuniform current density distribution. For this purpose, a new three-dimensional, self-consistent, trajectory code has been used. In addition, a simple theoretical model has been seen to be adequate for the qualitative and quantitative interpretation of the numerical results. It has been found, that the azimuthal nonuniformity causes effects clearly associated with/spl I.oarr/E/spl times//spl I.oarr/B drifts, but in absolute terms these effects are too small to affect the performance of a gyrotron.

Impact of poly-gate depletion on MOS RF linearity

The distortion behavior for thin oxide MOS transistors can be degraded due to polysilicon-gate depletion effects. The nonlinear, bias-dependent gate capacitance for thin oxide MOSFET's results in significant 2nd-order derivatives in gate capacitance, (/spl part//sup 2/C(V/sub gs/)//spl part/V/sub gs//sup 2/), which in turn results in substantial 3rd-order derivative contributions to drain current, (/spl part//sup 3/I/sub ds///spl part/V/sub gs//sup 3/). This may restrict the use of very-thin oxide MOSFET's in RF applications.

The redundancy of source coding with a fidelity criterion .II. Coding at a fixed rate level with unknown statistics

The redundancy problem of universal lossy source coding at a fixed rate level is considered. Under some condition on the single-letter distortion measure, which implies that the cardinality K of the reproduction alphabet is not greater than the cardinality J of the source alphabet, it is shown that the redundancy of universally coding memoryless sources p by nth-order block codes of rate R goes like |(/spl part///spl part/R)d(p,R)|Kln n/2n+o(ln n/n) for all memoryless sources p except a set whose volume goes to 0 as the block length n goes to infinity, where d(p,R) denotes the distortion rate function of p. Specifically, for any sequence {C/sub n/}/sub n=1//sup /spl infin// of block codes, where C/sub n/ is an nth-order block code at the fixed rate R, and any /spl epsiv/>0, the redundancy D/sub n/(C/sub n/,p) of C/sub n/ for p is greater than or equal to |(/spl part///spl part/R)d(p,R)|(K-/spl epsiv/)ln n/2n for all p satisfying some regular conditions except a set whose volume goes to 0 as n/spl rarr//spl infin/. On the other hand, there exists a sequence {C/sub n/}/sub n=1//sup /spl infin// of block codes at the rate R such that for any p satisfying some regular conditions, the super limit of D/sub n/(C/sub n/,p)|(ln n/n) is less than or equal to |(/spl part///spl part/R)d(p,R)|K/2.

Frequency-domain complementary operators for finite-elements simulation

A new mesh-truncation technique is introduced for the frequency-domain (time-harmonic) solution of open-region radiation problems. The technique is based on the complementary operators method (COM), where two independent solutions are averaged to eliminate first-order boundary reflections. The dual complementariness in the frequency domain is achieved by introducing a discrete-domain operator that achieves the objective of /spl part//sub t/ in the original time-domain development of COM.

Metal-bonded Co-ferrite composites for magnetostrictive torque sensor applications

A new class of magnetomechanical sensor materials, Co-ferrite (CoO/spl middot/Fe/sub 2/O/sub 3/) and metal-bonded (Ag, Ni, Co) Co-ferrite composites, has been investigated. These materials exhibited magnetostriction in excess of 200 ppm and high d/sub 33/ (/spl part//spl lambda///spl part/H)/sub /spl sigma// coefficient, 1.3/spl times/10/sup -9/ A/sup -1/ m, at low applied field (<100 kA/m). Selected compositions were formed into test samples in the form of rings brazed to stainless steel through-shafts. Changes of surface axial magnetic field in response to applied torque as high as 64 AN/sup -1/ m/sup -2/ were observed for a demonstration sample of dimensions 25 mm OD, 12.5 mm ID, and 8 mm high. A hysteresis of /spl plusmn/0.5 N/spl middot/m was observed. These materials appear to be promising candidates for torque sensors, and other magnetostrictive sensor and actuator applications.

HTS SQUID magnetometers with intermediate flux transformers

We have modeled and fabricated a novel HTS magnetometer with a d.c. SQUID which couples to an on-chip flux transformer via an intermediate flux transformer. This configuration allows high efficiency coupling between a 65 mm/sup 2/, 28 nH pickup loop and a low inductance (<15 pH) SQUID. The small SQUID inductance is required for low noise and high /spl part/V//spl part//spl Phi/ so that direct readout schemes may be used. The intermediate flux transformer was flip-chipped with a sub-micron separation to the main substrate. The magnetometer makes use of CAM junction technology, double thickness (/spl ap/0.7 /spl mu/m) YBa/sub 2/Cu/sub 3/O/sub 7/ layers, and a groundplaned SQUID slot. The measured responsivity at 77 K was 0.57 mm/sup 2/ (3.6 nT//spl Phi//sub 0/) with /spl part/V//spl part//spl Phi/=98 /spl mu/V//spl Phi//sub 0/ which is close to that predicted by our model. No resonances, which degrade the performance of similarly configured LTS devices, were observed. We attribute this to the use of PrBa/sub 2/Cu/sub 3/O/sub 7/ as the isolation barrier. Excess white noise, probably due to Johnson noise from the PrBa/sub 2/Cu/sub 3/O/sub 7/, limited the sensitivity in our first device to 33 fT//spl radic/Hz at 77 K.

Optimisation of the properties of high-T/sub c/ thick films

High-T/sub c/ thick films of YBa/sub 2/Cu/sub 3/O/sub 7-/spl part// (YBCO) are potentially useful in various applications; for example in practical fault current limiters in the field of power engineering. The material also shows promise in the fabrication of numerous microwave devices. However it is of paramount importance to optimise their microstructure and properties, with particular respect to the critical current density (J/sub c/) and surface resistance (R/sub s/). Doping and barrier layer technology have been used to improve the characteristics of the films. Enhanced current carrying capacity is achieved as a result of doping with Pt and a Ba/sub 4/Cu/sub 1+x/Pt/sub 2-x/O/sub 9-z/ (0412) compound Exploitation of low dielectric loss and economically attractive alumina substrates has also been achieved, by means of barrier layers. The relation between the processing, microstructure and superconducting behaviour is reported.

Wideband compressive receiver based on advanced superconductor and semiconductor circuits

A novel compressive cryoreceiver architecture has been proposed combining analog HTS, cryoelectronic, and advanced high-speed GaAs and high-speed/low-power SOI CMOS semiconductor technologies. The proposed receiver will rival the sensitivity of narrowband receivers while providing unprecedented wideband instantaneous frequency coverage with very small size, weight, and power requirements. Future developments will extend the bandwidth capability. HTS tapped-delay-line chirp filters are the enabling technology for instantaneous bandwidths greater than 1 GHz. The filters support dispersive delays as long as 40 ns and time-bandwidth products in excess of 100 using a bonded/thinned-wafer technique to fabricate YBa/sub 2/Cu/sub 3/O/sub 7-/spl part// stripline structures on 125-/spl mu/m-thick, 5-cm-diam LaAlO/sub 3/ substrates. The filters have produced better than -18-dB error sidelobes in a receiver configuration. Preliminary work toward SOI CMOS receiver ASICs is reported. These ASICs will perform pulse data thinning, and binary integration functions. Requirements for A/D converters are discussed.

Double-sided YBa/sub 2/Cu/sub 3/O/sub 7-∂/ films for planar high-power filters

We have extended the planar DC high oxygen-pressure sputtering process for the successive non-scanning deposition of epitaxial YBa/sub 2/Cu/sub 3/O/sub 7-/spl part// (YBCO) films up to 2 in diameter and 600 nm in thickness. Contamination of the heater-facing side of the LaAlO/sub 3/ substrates could be prevented by use of a thin quartz glass plate. For optimized deposition parameters, inductive measurements indicated comparable and homogeneous quality of both film sides, i.e. T/sub c/>88 K and J/sub c/(77 K) >2 MA/cm/sup 2/. Surface resistance measurements at 19 GHz with a Nb-shielded sapphire resonator resulted in R/sub s/<1 (0.2) m/spl Omega/ at 77 (4) K, which, in some cases, did not show any significant increase up to surface field levels B/sub s/=10 (25) mT. In order to obtain planar microwave filters of small size and high power-handling capability, disc resonators for TM/sub 010/-mode operation at 3.5 and 1.8 GHz were patterned from and 2 diameter double-sided YBCO films. Strongly temperature dependent quality factors Q/sub 0/ as high as 7.2/spl middot/10/sup 4/ (6/spl middot/10/sup 5/) have been achieved at 77 K (30 K) and 3.5 GHz, which are limited by losses in both the superconductor and the dielectric. A circulating power of about 1 kW, corresponding to peak B/sub s/-values above 1 mT, could be achieved without significant Q/sub 0/ degradation. Evidences for field limitations by local defects were found.

Characterization of ramp edge-geometry Ag:YBa/sub 2/Cu/sub 3/O/sub 7-x//PrBa/sub 2/Cu/sub 3/O/sub 7-x//Ag:YBa/sub 2/Cu/sub 3/O/sub 7-x/ junctions and dc SQUIDs

High-temperature superconducting Josephson junctions and dc SQUIDs were fabricated using a ramp edge-geometry SNS configuration. Ag-doped YBa/sub 2/Cu/sub 3/O/sub 7-x/ was used as electrodes and PrBa/sub 2/Cu/sub 3/O/sub 7-x/ as barrier. The I/sub c/R/sub n/ value of the junctions was in excess of 120 /spl mu/V at 75 K. The dc SQUIDs showed a flux modulation depth, /spl part/V//spl part//spl Phi/, over 150 /spl mu/V//spl Phi/, at 75 K. The flux noise of the devices, measured with dc bias currents operated at 75 K, showed a 1/f dependence at low frequency having values below 100 /spl mu//spl Phi//sub 0/Hz/sup - 1/2 / (the best values in the range of 30/spl sim/40 /spl mu//spl Phi//sub 0/Hz/sup - 1/2 /) at 1 Hz and less than 5 /spl mu//spl Phi//sub 0/Hz/sup - 1/2 / in the white noise region. The voltage modulation and magnetic field noise were 37 /spl mu/V and /spl sim/2 pTHz/sup - 1/2 / at 1 kHz for the directly-coupled dc SQUID magnetometers (with a pick-up loop 6 mm/spl times/ 3.5 mm) based on the ramp edge-geometry SNS configuration.

The use of (Double) relaxation oscillation SQUIDs as a sensor

Relaxation Oscillation SQUIDs (ROSs) and Double Relaxation Oscillation SQUIDs (DROSs) are based on relaxation oscillations that are induced in hysteretic dc SQUIDs by an external L-R shunt. The relaxation frequency of a ROS varies with the applied flux /spl Phi/, whereas the output of a DROS is a dc voltage, with a typical flux-to-voltage transfer of /spl part/V//spl part//spl Phi//spl ap/1 mV//spl Phi//sub 0/. The flux-to-frequency response of several ROSs has been measured and compared with theory for frequencies up to 7 GHz. Various DROS designs-a multi-loop direct coupling DROS, a DROS with a washer type signal SQUID and a DROS with gradiometric signal SQUID-will be discussed in this paper. The integration of a DROS with a digital flux locked loop ("Smart DROS") will also be analyzed.

Microstructural changes in TlBa/sub 2/CaCu/sub 2/O/sub 7-∂/ thin films after reducing anneals which enhance critical current density

Microstructural changes in TlBa/sub 2/CaCu/sub 2/O/sub 7-/spl part// (Tl-1212) epitaxial thin films resulting from low oxygen partial pressure furnace anneals at 600/spl deg/C are studied using high resolution transmission electron microscopy (TEM). These post-growth anneals have been shown to significantly raise the superconducting transition temperature from 70 to 90 K, and greatly improve the magnetic flux pinning and the critical current density. Changes occur in both the microstructure and the morphology of the films that correlate with changes in J/sub c/. Plan view TEM and high-resolution cross-sectional TEM analysis of the films before and after anneals demonstrates changes in the lattice fringe image, overall contrast modulation, and nanometer-scale discontinuities.

Pointer adaptation and pruning of min-max fuzzy inference and estimation

A new technique for adaptation of fuzzy membership functions in a fuzzy inference system is proposed. The pointer technique relies upon the isolation of the specific membership functions that contributed to the final decision, followed by the updating of these functions' parameters using steepest descent. The error measure used is thus backpropagated from output to input, through the min and max operators used during the inference stage. This occurs because the operations of min and max are continuous differentiable functions and, therefore, can be placed in a chain of partial derivatives for steepest descent backpropagation adaptation. Interestingly, the partials of min and max act as pointers with the result that only the function that gave rise to the min or max is adapted; the others are not. To illustrate, let /spl alpha/=max [/spl beta//sub 1/, /spl beta//sub 2/, /spl middot//spl middot//spl middot/, /spl beta//sub N/]. Then /spl part//spl alpha///spl part//spl beta//sub n/=1 when /spl beta//sub n/ is the maximum and is otherwise zero. We apply this property to the fine tuning of membership functions of fuzzy min-max decision processes and illustrate with an estimation example. The adaptation process can reveal the need for reducing the number of membership functions. Under the assumption that the inference surface is in some sense smooth, the process of adaptation can reveal overdetermination of the fuzzy system in two ways. First, if two membership functions come sufficiently close to each other, they can be fused into a single membership function. Second, if a membership function becomes too narrow, it can be deleted. In both cases, the number of fuzzy IF-THEN rules is reduced. In certain cases, the overall performance of the fuzzy system can be improved by this adaptive pruning.

The redundancy of source coding with a fidelity criterion. 1. Known statistics

The problem of redundancy of source coding with respect to a fidelity criterion is considered. For any fixed rate R>0 and any memoryless source with finite source and reproduction alphabets and a common distribution p, the nth-order distortion redundancy D/sub n/(R) of fixed-rate coding is defined as the minimum of the difference between the expected distortion per symbol of any block code with length n and rate R and the distortion rate function d(p,R) of the source p. It is demonstrated that for sufficiently large n, D/sub n/(R) is equal to -(/spl part///spl part/R)d(p,R) ln n/2n+o(ln n/n), where (/spl part///spl part/R)d(p,R) is the partial derivative of d(p,R) evaluated at R and assumed to exist. For any fixed distortion level d>0 and any memoryless source p, the nth-order rate redundancy R/sub n/(d) of coding at fixed distortion level d (or by using d-semifaithful codes) is defined as the minimum of the difference between the expected rate per symbol of any d-semifaithful code of length n and the rate-distortion function R(p,d) of p evaluated at d. It is proved that for sufficiently large n, R/sub n/(d) is upper-bounded by ln n/n+o(ln n/n) and lower-bounded by In n/2n+o(In n/n). As a by-product, the lower bound of R/sub n/(d) derived in this paper gives a positive answer to a conjecture proposed by Yu and Speed (1993).

Maximum likelihood estimation of direction of arrival and frequency sweeping rate with linear chirp signals

The paper presents a 1D maximum likelihood (ML) estimation procedure for both the direction of arrival /spl part/ and the frequency sweeping rate /spl alpha/ of a linear chirp signal impinging upon an M-element linear array. The estimates can be obtained by globally maximizing the likelihood function of the observed data on the /spl alpha//spl sim//spl part/ plane. To reduce the heavy 2-D ML computational load, a 1D ML procedure to search for the global maximum along the reduced 1D search space is proposed. >