Turbo-FLASH based arterial spin labeled perfusion MRI at 7 T.

Motivations of arterial spin labeling (ASL) at ultrahigh magnetic fields include prolonged blood T1 and greater signal-to-noise ratio (SNR). However, increased B0 and B1 inhomogeneities and increased specific absorption ratio (SAR) challenge practical ASL implementations. In this study, Turbo-FLASH (Fast Low Angle Shot) based pulsed and pseudo-continuous ASL sequences were performed at 7T, by taking advantage of the relatively low SAR and short TE of Turbo-FLASH that minimizes susceptibility artifacts. Consistent with theoretical predictions, the experimental data showed that Turbo-FLASH based ASL yielded approximately 4 times SNR gain at 7T compared to 3T. High quality perfusion images were obtained with an in-plane spatial resolution of 0.85×1.7 mm(2). A further functional MRI study of motor cortex activation precisely located the primary motor cortex to the precentral gyrus, with the same high spatial resolution. Finally, functional connectivity between left and right motor cortices as well as supplemental motor area were demonstrated using resting state perfusion images. Turbo-FLASH based ASL is a promising approach for perfusion imaging at 7T, which could provide novel approaches to high spatiotemporal resolution fMRI and to investigate the functional connectivity of brain networks at ultrahigh field

Tackling Challenges in Seebeck Coefficient Measurement of Ultra-High Resistance Samples with an AC Technique

Seebeck coefficient is a widely studied semiconductor property. Conventional Seebeck coefficient measurements are based on DC voltage measurement. Normally this is performed on samples with moderate resistances (e.g., below a few MΩ level). Certain semiconductors are intrinsic and highly resistive. Many examples can be found in optical and photovoltaic materials. The hybrid halide perovskites that have gained extensive attention recently are a good example. Despite great attention from the materials and physics communities, few successful studies exist of the Seebeck coefficient of these compounds, for example CH3NH3PbI3. An AC-technique-based Seebeck coefficient measurement is reported, which makes high-quality Seebeck voltage measurements on samples with resistances up to the 100 GΩ level. This is achieved through a specifically designed setup to enhance sample isolation and increase capacitive impedance. As a demonstration, Seebeck coefficient measurement of a CH3NH3PbI3 thin film is performed at dark, with sample resistance 150 GΩ, and found S = +550 µV K−1. The strategy reported could be applied to the studies of fundamental transport parameters of all intrinsic semiconductors that have not been feasible

Extreme eigenvalues of large-dimensional spiked Fisher matrices with application

Consider two pp-variate populations, not necessarily Gaussian, with covariance matrices Σ1Σ1 and Σ2Σ2, respectively. Let S1S1 and S2S2 be the corresponding sample covariance matrices with degrees of freedom mm and nn. When the difference ΔΔ between Σ1Σ1 and Σ2Σ2 is of small rank compared to p,mp,m and nn, the Fisher matrix S:=S−12S1S:=S2−1S1 is called a spiked Fisher matrix. When p,mp,m and nn grow to infinity proportionally, we establish a phase transition for the extreme eigenvalues of the Fisher matrix: a displacement formula showing that when the eigenvalues of ΔΔ (spikes) are above (or under) a critical value, the associated extreme eigenvalues of SS will converge to some point outside the support of the global limit (LSD) of other eigenvalues (become outliers); otherwise, they will converge to the edge points of the LSD. Furthermore, we derive central limit theorems for those outlier eigenvalues of SS. The limiting distributions are found to be Gaussian if and only if the corresponding population spike eigenvalues in ΔΔ are simple. Two applications are introduced. The first application uses the largest eigenvalue of the Fisher matrix to test the equality between two high-dimensional covariance matrices, and explicit power function is found under the spiked alternative. The second application is in the field of signal detection, where an estimator for the number of signals is proposed while the covariance structure of the noise is arbitrary.published_or_final_versio

Moment approach for singular values distribution of a large auto-covariance matrix

Let (εt)t>0(εt)t>0 be a sequence of independent real random vectors of pp-dimension and let XT=∑s+Tt=s+1εtε∗t−s/TXT=∑t=s+1s+Tεtεt−s∗/T be the lag-ss (ss is a fixed positive integer) auto-covariance matrix of εtεt. Since XTXT is not symmetric, we consider its singular values, which are the square roots of the eigenvalues of XTX∗TXTXT∗. Using the method of moments, we are able to investigate the limiting behaviors of the eigenvalues of XTX∗TXTXT∗ in two aspects. First, we show that the empirical spectral distribution of its eigenvalues converges to a nonrandom limit FF, which is a result previously developed in (J. Multivariate Anal. 137 (2015) 119–140) using the Stieltjes transform method. Second, we establish the convergence of its largest eigenvalue to the right edge of FF.published_or_final_versio

Probing the reactivity of a 2,2′-bipyridyl-3,3′-bis-imine ligand by X-ray crystallography

The reactivity of a Schiff-base bis-imine ligand 3 is probed by X-ray diffraction studies. Its susceptibility to hydrolysis, oxidation and nucleophilic addition reactions is demonstrated by the isolation of the methanol adduct 4 and two diazapene heterocycles 5 and 6. This reactivity is also reflected in the molecular structures of two coordination complexes isolated by the reaction of 3 with MIJhfac)2 salts, to afford [Cu(5)-(hfac)(tfa)] (8) and [Zn(6)(hfac)2] (9)

SpliceNet: recovering splicing isoform-specific differential gene networks from RNA-Seq data of normal and diseased samples

Conventionally, overall gene expressions from microarrays are used to infer gene networks, but it is challenging to account splicing isoforms. High-throughput RNA Sequencing has made splice variant profiling practical. However, its true merit in quantifying splicing isoforms and isoform-specific exon expressions is not well explored in inferring gene networks. This study demonstrates SpliceNet, a method to infer isoform-specific co-expression networks from exon-level RNA-Seq data, using large dimensional trace. It goes beyond differentially expressed genes and infers splicing isoform network changes between normal and diseased samples. It eases the sample size bottleneck; evaluations on simulated data and lung cancer-specific ERBB2 and MAPK signaling pathways, with varying number of samples, evince the merit in handling high exon to sample size ratio datasets. Inferred network rewiring of well established Bcl-x and EGFR centered networks from lung adenocarcinoma expression data is in good agreement with literature. Gene level evaluations demonstrate a substantial performance of SpliceNet over canonical correlation analysis, a method that is currently applied to exon level RNA-Seq data. SpliceNet can also be applied to exon array data. SpliceNet is distributed as an R package available at http://www.jjwanglab.org/SpliceNet.published_or_final_versio

Abnormal audit fees and audit quality: Australian evidence

We examine the association between abnormal audit fees and audit quality using Australian data. We find that audit quality, measured by auditors’ propensity to issue going concern opinions for financially distressed companies, discretionary accruals, and clients’ propensity to meet or beat earnings benchmarks, declines as positive abnormal audit fees increase in magnitude. Our findings support an economic bonding argument that positive abnormal audit fees reflect the extent of economic bonding between the auditor and the client. We also find that negative abnormal audit fees are negatively associated with the absolute value of discretionary accruals. Our findings suggest that academic researchers, practitioners, regulators, and others interested in assessing the effect of auditor remuneration on auditor independence and audit quality should be concerned with audit fees that are above a normal level rather than audit fees that are below a normal level of audit fees. JEL Classification: M42, M4

Monitoring the cementitious materials subjected to sulfate attack with optical fiber excitation Raman spectroscopy

Formation of ettringite and gypsum from sulfate attack together with carbonation and chloride ingress have been considered as the most serious deterioration mechanisms of concrete structures. Although electrical resistance sensors and fiber optic chemical sensors could be used to monitor the latter two mechanisms on site, currently there is no system for monitoring the deterioration mechanisms of sulfate attack. In this paper, a preliminary study was carried out to investigate the feasibility of monitoring sulfate attack with optical fiber excitation Raman spectroscopy through characterizing the ettringite and gypsum formed in deteriorated cementitious materials under an optical fiber excitation + objective collection configuration. Bench-mounted Raman spectroscopy analysis was also conducted to validate the spectrum obtained from the fiber-objective configuration. The results showed that the expected Raman bands of ettringite and gypsum in the sulfate-attacked cement paste can be clearly identified by the optical fiber excitation Raman spectrometer and are in good agreement with those identified from bench-mounted Raman spectrometer. Therefore, based on these preliminary results, it is considered that there is a good potential for developing an optical fiber-based Raman system to monitor the deterioration mechanisms of concrete subjected to sulfate attack in the future