A new approach for simultaneous measurement of ADC and T2 from echoes generated via multiple coherence transfer pathways

The study of rotational and translational diffusion requires the measurement of both T2 and apparent diffusion coefficient (ADC), quantities that are typically measured in separate experiments. The exploitation of echoes generated via multiple coherence transfer pathways offers an opportunity for measuring T2 and ADC values simultaneously in a single experiment. A series of RF pulses can generate multiple echoes via different coherence pathways with each one being uniquely encoded. Here, we demonstrate one pulse sequence that uses an initial theta; RF pulse to generate three coherence orders (C = 0, -1, +1). In the particular version of the method discussed here only two are used (C = 0, +1). Each order is encoded with a different b value from which the ADC is derived. The coherence order echo C = 0 is refocused to quantify T2. The performance of the method--dubbed simultaneous measurement of ADC and relaxation time (SMART)--is demonstrated on a set of samples differing in T2 and ADC achieved by varying the relative volume fractions in mixtures of gadolinium-doped H2O and D2O. The regional SMART derived T2 and ADC agree well with those obtained with conventional double-spin-echo and pulsed gradient spin-echo methods.     

Enhancing the spectral sensitivity of interferometers using slow-light media

We demonstrate experimentally that the spectral sensitivity of an interferometer can be greatly enhanced by introducing a slow-light medium into it. The experimental results agree very well with theoretical predictions that the enhancement factor of the spectral sensitivity is equal to the group index n(g) of the slow-light medium.     

Principal-component-analysis-based estimation of blood flow velocities using optical coherence tomography intensity signals

The intensity signal in optical coherence tomography contains information about the translational velocity of scatterers, and can be used to quantify blood flow. We apply principal component analysis to efficiently extract this information. We also study use of nonuniform temporal sampling of the intensity signal to increase the range of quantifiable flow velocities. We demonstrate this technique in simulation, phantom and in vivo blood flow measurements, and highlight its potential to enable three-dimensional wide-field mapping of blood flow using OCT.     

Measuring spatial coherence by using a mask with multiple apertures

A simple method to measure the complex degree of spatial coherence of a partially coherent quasi-monochromatic light field is presented. The Fourier spectrum of the far-field interferogram generated by a mask with multiple apertures (small circular holes) is analyzed in terms of classes of aperture pairs. A class of aperture pairs is defined as the set of aperture pairs with the same separation vector. The height of the peaks in the magnitude spectrum determines the modulus of the complex degree of spatial coherence and the corresponding value in the phase spectrum determines the phase of the complex degree of spatial coherence. The method is illustrated with experimental results.     

Maximum a posteriori deconvolution of ultrasonic signals using multiple transducers

A new method for deconvolution of ultrasonic pulse-echo measurements employing multiple-transducer setup is proposed in the paper. An optimal way of estimating the material reflection sequence for a linear signal generation model using maximum a posteriori estimation is proposed. The method combines the measurements from a number of transducers covering different frequency bands yielding an optimal estimate of the reflection sequence. The main idea of this approach is to complement the information unavailable from one transducer in some frequency bands with the information from the other transducers. The method is based on the assumption that the measurements are performed using transducers with identical apertures and apodization, which are located exactly at the same position relative to the test object during the measurement. An error analysis presented in the paper proves that when the above assumptions are fulfilled, the proposed method, by utilizing more data for estimation, consistently yields more accurate reflection sequence estimates than the classical Wiener filter. Experimental evidence is presented using both simulated and real ultrasonic data as a verification of the correctness of the multiple-transducer model and the estimation scheme. An illustration of the advantages of the method is also given using real ultrasonic data.     

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.     

Spatial localization in the heteronuclear coherence transfer from scalar order

The spatial localization of the heteronuclear coherence transfer from scalar order was analyzed for a rectangular and an adiabatic remagnetization contact pulses. The transferred coherence amplitudes with and without localization were evaluated for different experimental conditions and an AX-1/2 spin-pair system. The use of this method for¹³C localized spectroscopy is discussed.     

Enhancing the sensitivity of an adaptive holographic interferometer using non-Bragg diffraction orders

The signal-to-noise ratio of the output of an adaptive holographic interferometer (AHI) based on a Bi(12)TiO(20) crystal is investigated. We show experimentally that the sensitivity of an AHI using the non-Bragg orders of diffraction in a thin photorefractive material is more than an order of magnitude greater than that of an AHI employing two-wave mixing in photorefractive volume holograms.     

Enhancing the sensitivity of fiber Mach-Zehnder interferometers using slow and fast light

We demonstrate theoretically and experimentally that the sensitivity of a fiber Mach-Zehnder (M-Z) interferometer can be enhanced by coupling fiber ring resonators with it. The experimental results agree well with theoretical predications that combining slow light with fast light will further increase the sensitivity of a fiber M-Z interferometer.     

Localization of multiple acoustic sources with small arrays using a coherence test

Direction finding of more sources than sensors is appealing in situations with small sensor arrays. Potential applications include surveillance, teleconferencing, and auditory scene analysis for hearing aids. A new technique for time-frequency-sparse sources, such as speech and vehicle sounds, uses a coherence test to identify low-rank time-frequency bins. These low-rank bins are processed in one of two ways: (1) narrowband spatial spectrum estimation at each bin followed by summation of directional spectra across time and frequency or (2) clustering low-rank covariance matrices, averaging covariance matrices within clusters, and narrowband spatial spectrum estimation of each cluster. Experimental results with omnidirectional microphones and colocated directional microphones demonstrate the algorithm's ability to localize 3-5 simultaneous speech sources over 4 s with 2-3 microphones to less than 1 degree of error, and the ability to localize simultaneously two moving military vehicles and small arms gunfire.     

Simultaneous spectral editing for gamma-aminobutyric acid and taurine using double quantum coherence transfer

Conventional double quantum (DQ) editing techniques recover resonances of one metabolite at a time and are thus inefficient for monitoring metabolic changes involving several metabolites. A DQ coherence transfer double editing sequence using a dual-band DQ coherence read pulse is described here. The sequence permits simultaneous spectral editing for two metabolites with similar J coupling constants in a single scan. Simultaneous editing for taurine and gamma-aminobutyric acid (GABA) is demonstrated using solution phantoms and rat brain tissue. Selectivity of the double editing sequence for the target metabolites is as good as that achieved using conventional DQ editing which selects each metabolite individually. With experimental parameters of the double editing sequence chosen to optimize GABA editing, the sensitivity for GABA detection is the same as that with GABA editing only, while the sensitivity for taurine detection is decreased slightly compared to that with taurine editing only.     

Monte carlo simulation of signals from model biological tissues measured by an optical coherence tomograph and an optical coherence Doppler tomograph

The results of Monte Carlo simulation of optical coherence tomograph (OCT) signals from layers of a suspension of erythrocytes and an aqueous solution of Intralipid are presented. It is shown that the rear boundary of a layer of an erythrocyte suspension 0.5 mm thick is distinguished in the OCT signal for all the hematocrits considered (5, 10, and 35%). This is explained by fact that the greatest contribution to the signal is made by low-order scattered photons, which ensures good differentiation of internal inclusions and the rear boundary. In the case of the Intralipid solution, the main contribution is made by multiply scattered photons and the signal from the rear boundary is indistinguishable. Signals of an optical coherence Doppler tomograph (OCDT) from a plane-parallel flow of Intralipid between glass plates are also simulated. The effect of the Intralipid concentration on the velocity profile reconstructed from the OCDT signal is studied. It is shown that an increase in the Intralipid concentration leads to a shift in the maximum of the reconstructed velocity profile and to an stretching of the profile. The reason for these distortions is the contribution to the signal from multiple scattering. OCDT signals from a blood layer immersed in an optical phantom of skin are also simulated, and the distortions of the reconstructed profile are analyzed in relation to the depth of the layer.     

Multifractal analysis of human synchronous 12-lead ECG signals using multiple scale factors

Life is one of the most complex nonlinear systems and heart is the core of this lifecycle system. Electrocardiogram (ECG) signals taken from healthy adult subjects have been found to characterize multifractality. In this paper, multiscale analysis method was introduced to find the most effective parameters for expressing the complex dynamic characteristics during heart electrical activities. We then investigated the multifractal singularity spectrum area of synchronous 12-lead ECG signals from healthy human subjects and those with different clinical syndromes. The spectrum areas have spatial distribution along with scale factors, which is higher in the middle and lower on both sides and is not related to data length. The statistical results suggest the arithmetical mean value of the area of the 12 leads ECG signals is obviously small for myocardial infarction (MI) sufferer and large for healthy young, while the dispersing degree of the area of the 12 leads ECG signals is apparently large for MI sufferer and small for healthy young. As for the other crowds (e.g., the ischemia sufferer), these two values are almost of middle magnitude. Through those individual discrepancies, we can find some effective approaches to distinguish among healthy persons and the heart diseased.     

Slice-selective J-coupled coherence transfer using symmetric linear phase pulses: applications to localized GABA spectroscopy

Symmetric, linear phase, slice-selective RF pulses were analyzed theoretically for performing slice-selective coherence transfer. It was shown using numerical simulations of product operators that, when a prefocusing gradient of the same area as that of the refocusing gradient is added, these pulses become slice-selective universal rotator pulses, therefore, capable of performing slice-selective coherence transfer. As an example, a slice-selective universal rotator pulse based on a seven-lobe hamming-filtered sinc pulse was applied to in vivo single-shot simultaneous spectral editing and spatial localization of neurotransmitter GABA in the human brain.     

利用多角度海底反向散射信号进行地声参数估计

针对现有海底地声参数估计方法的不足,提出了利用相控参量阵浅地层剖面仪接收的多角度海底反向散射信号进行地声参数估计的方法,首先利用正下方和斜入射方向上沉积层上、下表面的差频反向散射信号进行沉积层厚度和声速估计,然后利用正下方沉积层上、下表面两个不同频率的差频信号的反向散射信号估计沉积层衰减系数,最后利用正下方沉积层上表面原频反向散射信号估计沉积层阻抗,计算沉积层密度从而解决和声速的耦合性,通过水池试验验证了该方法的有效性。     

Enhancing the dynamic range of targeted energy transfer in acoustics using several nonlinear membrane absorbers

In order to enhance the robustness and the energy range of efficiency of targeted energy transfer (TET) phenomena in acoustics, we discuss in this paper about the use of multiple nonlinear membrane absorbers (called nonlinear energy sinks or NES) placed in parallel. We show this way, mainly thanks to an experimental set-up with two membranes, that the different absorbers have additional effects that extend the efficiency and the possibilities of observation of TET. More precisely, we present the different behavior of the system under sinusoidal forcing and free oscillations, characterizing the phenomena for all input energies. The frequency responses are also presented, showing successive clipping of the original resonance peak of the system, and strongly modulated regimes (SMR). A model is finally used to generalize these results to more than two NES and to simulate the case of several very similar membranes in parallel which shows how to extend the existence zone of TET.