Depth-adaptive regularized reconstruction for reflection diffuse optical tomography

We study reflection diffuse optical tomography using two-dimensional (2D) continuous-wave source-detector arrays on the surface of semi-infinite medium, aiming at imaging the perfusion and the hemoglobin oxygen saturation variation of human cerebral cortex with brain activation. We had previously formulated the inverse problem with Moore-Penrose inversion. When we use simple regularization in this inverse problem, the reconstruction sensitivity decreases markedly with the depth so that the signal in the deep range may be masked by an unwanted signal in the shallow range. In this paper, we propose a depth-adaptive regularized reconstruction, in which we assign a smaller regularization parameter with the depth. We demonstrate improvement of the three-dimensional (3D) reconstruction uniformity using the proposed scheme.     

On the role of group I elements in ZnO

Diffusion of Li, Na and K into single-crystal substrates of ZnO was performed. We compare the results with ZnO epitaxial films doped with the respective elements during growth. The diffused and in-situ doped layers were studied using mass spectroscopy and low temperature photoluminescence spectroscopy. Li and Na are known to produce deep acceptor centers which give rise to shallow donor to deep acceptor recombinations in the visible spectral region. We will demonstrate that shallow acceptors are also introduced, having binding energies around 300 meV. A donor–acceptor pair recombination (zero phonon line at 3.05 eV) with LO phonon replica is observed. We further investigated bulk ZnO crystals which contained the deep Li acceptor by thermal treatments under H₂ atmospheres. With increasing annealing temperature shallow donors are introduced as monitored by EPR while the EPR signal of the neutral Li acceptors decreases. Quite unexpectedly, the shallow Li acceptor centre which is not present in the as-grown state is also created. PACS 71.55.Gs; 78.55.Et; 76.30.Da     

自由多子尾区俘获动力学及多子俘获截面的测量

本文讨论了自由多子尾被半导体深中心俘获的动力学。指出,当DLTS响应区被局限在尾区端部时,DLTS信号强度与多子脉冲宽度成正比。它可用于多子俘获截面(包括浅杂质浓度较高及多子俘获截面较大的情况)的测量。 关键词：     

Temporal coherence of sound transmissions in deep water revisited

This paper examines the signal coherence loss due to internal waves in deep water in terms of the signal coherence time and compare to data reported in the literature over the past 35 years. The coherence time of the early raylike arrivals was previously modeled by Munk and Zachariasen ["Sound propagation through a fluctuating stratified ocean: Theory and observation," J. Acoust. Soc. Am. 59, 818-838 (1976)] using the supereikonal approximation and by Dashen et al. ["Path-integral treatment of acoustic mutual coherence functions for arrays in a sound channel," J. Acoust. Soc. Am. 77, 1716-1722 (1985)] using the path integral approach; a -1 [corrected] power frequency dependence and a -1/2 [corrected] power range dependence were predicted. Recent data in shallow water in downward refractive environments with internal waves suggested that the signal coherence time of the mode arrivals follows a -3/2 power frequency dependence and a -1/2 power range dependence. Since the temporal coherence of the acoustic signal is related to the temporal coherence of the internal waves, based on the observation that the (linear) internal waves in deep and shallow waters have a similar frequency spectrum, it is argued that the modelike arrivals in deep water should exhibit a similar frequency dependence in deep and shallow waters. This argument is supported by a brute-force application of the path integral to mode arrivals based on the WKB relation between the ray and mode. It is found that the data are consistent with the -3/2 power frequency dependence but more data are needed to further test the hypothesis.     

Alignment of low‐dose X‐ray fluorescence tomography images using differential phase contrast

X‐ray fluorescence nanotomography provides unprecedented sensitivity for studies of trace metal distributions in whole biological cells. Dose fractionation, in which one acquires very low dose individual projections and then obtains high statistics reconstructions as signal from a voxel is brought together (Hegerl & Hoppe, 1976), requires accurate alignment of these individual projections so as to correct for rotation stage runout. It is shown here that differential phase contrast at 10.2 keV beam energy offers the potential for accurate cross‐correlation alignment of successive projections, by demonstrating that successive low dose, 3 ms per pixel, images acquired at the same specimen position and rotation angle have a narrower and smoother cross‐correlation function (1.5 pixels FWHM at 300 nm pixel size) than that obtained from zinc fluorescence images (25 pixels FWHM). The differential phase contrast alignment resolution is thus well below the 700 nm × 500 nm beam spot size used in this demonstration, so that dose fractionation should be possible for reduced‐dose, more rapidly acquired, fluorescence nanotomography experiments.     

Frequency discrimination in quiet and in noise for signals with triangular spectral envelopes

The just-noticeable-difference in frequency (jndf) for complex signals with triangular spectral envelopes is found to depend on the envelope slope. For shallow slopes (less than 140 dB/oct), jndf increases with decreasing slope. Addition of noise also impairs frequency discrimination within a region of about 20 dB above masked threshold. This is found for both maskers used: a wideband noise and a narrow-band masker which is below the signal in frequency. When wideband noise is used, frequency discrimination of complex signals with shallow slopes deteriorates more rapidly with decreasing signal-to-noise ratio than it does when the signals have steep spectral slopes.     

Size optimization of a quad-cell alignment detector for short range free space optical interconnects

In this paper we consider geometry optimization of a quad-cell alignment detector for short range free space optical interconnects. The alignment technique uses alignment beams and quad detectors to detect different misalignment errors. We show that the size of the quad detector cells can be used as a design parameter to optimize the sensitivity of the alignment system. The sensitivity analysis and error signal generation are presented assuming standard Gaussian beam models. Results on optimum sensitivity for the case of transverse and angular misalignments are presented and discussed.     

Letters to the editor

The damage formed by self-implantation in Cu and Al at two temperatures, where vacancies are considered to be either immobile or mobile, but where interstitials are always highly mobile, is studied before and after annealing, using dechanneling methods. Two different regions of damage are distinguished: a surface region coinciding with the projected range of the implanted self-ions, and a region extending up to 10 times beyond this range.

On the basis of differences on the formation of the deep and the shallow damage as a function of implantation temperature and a different behaviour of annealing it is concluded that the two regions contain different types of damage.

A mechanism is proposed that explains the observed damage distributions qualitatively. In this model the damage in the surface region of copper is assumed to consist of mainly clusters of vacancies. In the deep region of copper and aluminium it is assumed that the damage consists of clusters or loops formed by interstitials. Due to insufficient knowledge of the parameters involved a quantitative test is as yet impossible.     

A novel local PCA-based method for detecting activation signals in fMRI.

A novel local principal component analysis (LPCA) technique is presented for activation signal detection in functional magnetic resonance imaging (fMRI) without explicit knowledge about the shape of the model activation signal. Unlike the traditional PCA methods, our LPCA algorithm is based on a measure of separation between two clusters formed by the signal segments in active periods and inactive periods, which is computed in an eigen-subspace. In addition, we only applied PCA to the temporal sequence of each individual voxel instead of applying PCA to the fMRI data set. In our algorithm, we first applied a linear regression procedure to alleviate the baseline drift artifact. Then, the baseline-corrected temporal signals were partitioned into active and inactive segments according to the paradigm used for the fMRI data acquisition. Principal components were computed from all these segments for each voxel by PCA. By projecting the segments of each voxel onto a linear subspace formed by the corresponding most dominant principal components, two separate clusters were formed from active and inactive segments. An activation measure was defined based on the degree of separation between these two clusters in the projection space. We show experimental results on the activation signal detection from various sets of fMRI data with different types of stimulation by using the proposed LPCA algorithm and the standard t-test method for comparison. Our experiments indicate that the LPCA algorithm in general provides substantial signal-to-noise ratio improvement over the t-test method.     

Current-induced magnetic resonance phase imaging

Electric current-induced phase alternations have been imaged by fast magnetic resonance image (MRI) technology. We measured the magnetic resonance phase images induced by pulsed current stimulation from a phantom and detected its sensitivity. The pulsed current-induced phase image demonstrated the feasibility to detect phase changes of the proton magnetic resonance signal that could mimic neuronal firing. At the present experimental setting, a magnetic field strength change of 1.7 +/- 0.3 nT can be detected. We also calculated the averaged value of the magnetic flux density BT parallel to B0 produced by electric current I inside the voxel as a function of the wire position. The results of the calculation were consistent with our observation that for the same experimental setting the current-induced phase change could vary with location of the wire inside the voxel. We discuss our findings in terms of possible direct MRI detection of neuronal activity.     

深度学习在超声检测缺陷识别中的应用与发展*

深度学习（Deep Learning）是目前最强大的机器学习算法之一,其中卷积神经网络（Convolutional Neural Network, CNN）模型具有自动学习特征的能力,在图像处理领域较其他深度学习模型有较大的性能优势。本文先简述了深度学习的发展史,然后综述了深度学习在超声检测缺陷识别中的应用与发展,从早期浅层神经网络到现在深度学习的应用现状,并借鉴医学影像识别和射线图像识别领域的方法,分析了卷积神经网络对超声图像缺陷识别的适用性。最后,探讨归纳了目前在超声检测图像识别中使用CNN存在的一些问题,及其主要应对策略的研究方向。     

Compensation of susceptibility-induced signal loss in echo-planar imaging for functional applications

Functional magnetic resonance imaging favors the use of multi-slice gradient-recalled echo-planar imaging due to its short image acquisition times, whole brain coverage and sensitivity to BOLD contrast. However, despite its advantages, gradient-recalled echo-planar imaging also is sensitive to static magnetic field gradients arising primarily from air-tissue interfaces. This can lead to image artifacts such as voxel shifts and complete signal loss. A method to recover signal loss by adjusting the refocusing gradient amplitude in the slice-select direction, preferably axially, is proposed. This method is implemented as an automated computer algorithm that partitions echo-planar images into regions of recoverable signal intensities using a histogram analysis and determines each region's proper refocusing gradient amplitude. As an example, different refocusing gradient amplitudes are interleaved in a fMRI acquisition to maximize the signal to noise ratio and obtain functional activation in normal and dropout regions. The effectiveness of this method is demonstrated by recovering signal voids in the orbitofrontal cortex, parahippocampal/amygdala region, and inferior visual association cortex near the cerebellum.     

Acoustic calibration of the signal propagation path in the shallow water

The propagation of low-frequency pulsed frequency-modulated signals in the shallow water (80m deep) is analyzed. The characteristics of the first five modes in the frequency range from 25 to 155 Hz are found by the correlation analysis of signal spectra. The group velocities and mode attenuation coefficients are used to reconstruct the acoustic characteristics of the sea bottom. A model of the sea bottom is developed, which allows one to formulate the model of transfer function along the signal propagation path.     

Characterization of a 3D MEMS fabricated micro-solenoid at 9.4 T

We present for the first time a complete characterization of a micro-solenoid for high resolution MR imaging of mass- and volume-limited samples based on three-dimensional B(0), B(1) per unit current (B(1)(unit)) and SNR maps. The micro-solenoids are fabricated using a fully micro-electromechanical systems (MEMS) compatible process in conjunction with an automatic wire-bonder. We present 15 μm isotropic resolution 3D B(0) maps performed using the phase difference method. The resulting B(0) variation in the range of [-0.07 ppm to -0.157 ppm] around the coil center, compares favorably with the 0.5 ppm limit accepted for MR microscopy. 3D B(1)(unit) maps of 40 μm isotropic voxel size were acquired according to the extended multi flip angle (ExMFA) method. The results demonstrate that the characterized microcoil provides a high and uniform sensitivity distribution around its center (B(1)(unit) = 3.4 mT/A ± 3.86%) which is in agreement with the corresponding 1D theoretical data computed along the coil axis. The 3D SNR maps reveal a rather uniform signal distribution around the coil center with a mean value of 53.69 ± 19%, in good agreement with the analytical 1D data along coil axis in the axial slice. Finally, we prove the microcoil capabilities for MR microscopy by imaging Eremosphaera viridis cells with 18 μm isotropic resolution.     

Application of Bessel beams to 2D microfabrication

Fs laser-based two-photon polymerisation (2PP) has been widely reported as a means of directly writing three-dimensional nanoscale structures. Usually the voxel of a high numerical aperture microscope objective is scanned through the resin to build up the required model. In the case of high aspect ratio two-dimensional structures, such as cell scaffolds, repeated scanning is required to build up the height.The voxel shape can be substantially elongated by the inclusion of an axicon lens in the laser beam line. In this report we describe the use of a Bessel beam produced in the region beyond the focus of an objective lens when the beam has been modified in this way.A Ti:sapphire laser was used to write a range of 2D square cell structures in a Zr-loaded sol-gel system. The process was characterised, in terms of the dimensions of the polymerised Bessel region, for different processing conditions. Examples of the structures are also described.     

Asymmetry of masking between noise and iterated rippled noise: evidence for time-interval processing in the auditory system.

This study describes the masking asymmetry between noise and iterated rippled noise (IRN) as a function of spectral region and the IRN delay. Masking asymmetry refers to the fact that noise masks IRN much more effectively than IRN masks noise, even when the stimuli occupy the same spectral region. Detection thresholds for IRN masked by noise and for noise masked by IRN were measured with an adaptive two-alternative, forced choice (2AFC) procedure with signal level as the adaptive parameter. Masker level was randomly varied within a 10-dB range in order to reduce the salience of loudness as a cue for detection. The stimuli were filtered into frequency bands, 2.2-kHz wide, with lower cutoff frequencies ranging from 0.8 to 6.4 kHz. IRN was generated with 16 iterations and with varying delays. The reciprocal of the delay was 16, 32, 64, or 128 Hz. When the reciprocal of the IRN delay was within the pitch range, i.e., above 30 Hz, there was a substantial masking asymmetry between IRN and noise for all filter cutoff frequencies; threshold for IRN masked by noise was about 10 dB larger than threshold for noise masked by IRN. For the 16-Hz IRN, the masking asymmetry decreased progressively with increasing filter cutoff frequency, from about 9 dB for the lowest cutoff frequency to less than 1 dB for the highest cutoff frequency. This suggests that masking asymmetry may be determined by different cues for delays within and below the pitch range. The fact that masking asymmetry exists for conditions that combine very long IRN delays with very high filter cutoff frequencies means that it is unlikely that models based on the excitation patterns of the stimuli would be successful in explaining the threshold data. A range of time-domain models of auditory processing that focus on the time intervals in phase-locked neural activity patterns is reviewed. Most of these models were successful in accounting for the basic masking asymmetry between IRN and noise for conditions within the pitch range, and one of the models produced an exceptionally good fit to the data.     

On the growth of masking asymmetry with stimulus intensity

Masking asymmetry was investigated over a wide range of stimulus intensities for two signal frequencies, fo = 1.0 and 4.0 kHz, using both fixed-masker and fixed-signal paradigms. The masker was a notched noise with the upper and lower edges of the notch, fu and fl, respectively, placed asymmetrically about fo. For various notch widths, the asymmetry of masking was measured as the difference between the masked threshold obtained when fl was nearer fo and that obtained when fu was nearer fo. For maskers with wide notches, (fu - fl)/fo greater than 0.15, masking asymmetry changed with stimulus level; at the highest level, masked threshold was greatest when fl was nearer fo, and, at the lowest level the asymmetry reversed slightly for fo = 1.0 kHz so that masked threshold was actually greater when fu was nearer fo. Nonparallel growth of masking functions reveal changes in masking asymmetry with signal level as well as with masker level. It is concluded that the nonlinear growth of masking with level is due primarily to changes in the auditory filter, rather than changes in the detector following the filter.     

一种基于简正波模态消频散变换的声源距离深度估计方法

针对浅海环境中传播的低频宽带水声脉冲信号,基于简正波水平波数差和波导不变量之间的关系,本文提出了一种利用距离-频散参数二维平面聚焦测距与匹配模态能量定深的目标声源定位方法.首先,通过将由频散参数和波导不变量表示的前几阶模态相速度与由环境模型计算的相速度进行对比分析,从而估计出前几阶模态的频散参数和环境的波导不变量.其次,利用估计出的频散参数值和波导不变量对接收信号进行消频散变换处理,只有当接收信号的距离参数等于目标声源距离时,各号简正波的幅度均达到最大值,在距离-频散参数二维平面上,出现声压聚焦的现象,利用此现象可以估计目标声源的距离.不仅如此,消频散变换后的接收信号,前几阶模态在时域上明显地分离开来,可以准确地估计出前几阶模态的能量,采用多模态能量匹配的方式,可以估计出目标声源的深度.最后,通过对仿真和冬季获得的气枪信号数据处理结果验证了本文方法的有效性.     

深紫外分光光度检测系统的稳定性及灵敏度研究

现有的国标光度法无法直接测定流程工业中连续反应单元生产过程的污染物,主要原因是氧气在深紫外区对紫外光的吸收干扰了紫外分光光度计对目标物质的检测,导致检测结果存在一定程度偏差。因此,解决这一问题的关键核心是稳定获取深紫外区不同特征波长物质的高灵敏光度信息。在紫外分光光度计基础上加装氮气输配系统,同时设计了自动进样流通池及进样托盘以实现检测间隙自动进样功能,减少检测间隙氮气消耗。为提高仪器稳定性,分别精准控制通入仪器内部光学系统区、样品室和数据接收区三个腔体的氮气流量,数值分别为6,2和3L·min^-1,使仪器基线平直度平均值由0.108降低至0.010,较空气条件削减了90.7%。通过对比空气与氮气两种气氛下直接测定SO₄^2-的吸光度、灵敏度、灵敏度变化量和线性范围的差异,发现氮气气氛下检测结果的吸光度和灵敏度在光程b=1～100mm范围内均有提升,灵敏度变化量随b=1mm时的10.42%增大至b=100mm时30.65%,线性范围却随光程的增加由0.09g·L^-1缩短至0.03g·L^-1<...     

Frequency invariability of acoustic field and passive source range estimation in shallow water

The frequency invariability of the warped modal signal and the warped signal autocorrelation function in shallow water is discussed.A method is proposed for passive source-range estimation based on the frequency invariability and warping transform of signal autocorrelation function received by a single hydrophone in a range-independent or weak range-dependent shallow water environment.In the method,a guided source with a known range is employed to provide the crucial and relative invariant scaled features.The experimental data in shallow water with an iso-speed profile and a fluctuated thermocline are used to verify this approach.The relative errors of the source range estimation are basically less than 10%.