Measurement of in vivo multi-component T2 relaxation times for brain tissue using multi-slice T2 prep at 1.5 and 3 T

The objective of this study was to implement a clinically relevant multi-slice multi-echo imaging sequence in order to quantify multi-component T2 relaxation times for normal volunteers at both 1.5 and 3 T. Multi-echo data were fitted using a nonnegative least square algorithm. Twelve echo data with nonlinear echo sampling were acquired using a receive-only eight-channel phased array coil and volume head coil for phantoms and normal volunteers, and compared to 32-echo data with linear echo sampling. It was observed that the performance of the 180 degrees refocusing trains was more spatially uniform for the receive-only eight-channel phased array coil than for the head coil, particularly at 3 T. The phantom study showed that the estimated T2 relaxation times were accurate and reproducible for both single- and multi-slice acquisition from a commercial phantom with known T2 relaxation times. Short T2 components (T2 <50 ms) were mainly observed within the white matter for normal volunteers, and the fraction of short T2 water components (i.e., myelin water) was 7-12% of total water. It was observed that the calculated myelin water fraction map from the nonlinearly sampled 12-echo data was comparable with that from the linearly sampled 32-echo data. Quantification of T2 relaxation times from multi-slice images was accomplished with a clinically acceptable scan times (16 min) for normal volunteers by using a nonselective T2 prep imaging sequence. The use of the eight-channel head coil involved more accurate quantification of T2 relaxation times particularly when the number of echoes was limited.     

Numerical discrimination of intrachannel cross-phase modulation and intrachannel four-wave mixing and their respective effect on 40 Gbit/s transmissions

We propose a numerical method to accurately discriminate the influence of the different intrachannel nonlinear effects occurring in 40 Gbit/s optical transmissions, following an analogy with methods used to discriminate WDM interchannel effects. In contrast to other studies showing the predominance of intrachannel cross-phase modulation when low-dispersion fibers are used, in our study intrachannel four-wave mixing is the most penalizing effect in all investigated cases.     

Universal relations for identical bosons from three-body physics

Systems consisting of identical bosons with a large scattering length satisfy universal relations determined by 2-body physics that are similar to those for fermions with two spin states. They require the momentum distribution to have a large-momentum 1/k(4) tail and the radio-frequency transition rate to have a high-frequency 1/ω(3/2) tail, both of which are proportional to the 2-body contact. Identical bosons also satisfy additional universal relations that are determined by 3-body physics and involve the 3-body contact, which measures the probability of 3 particles being very close together. The coefficients of the 3-body contact in the 1/k(5) tail of the momentum distribution and in the 1/ω(2) tail of the radio-frequency transition rate are log-periodic functions of k and ω that depend on the Efimov parameter.     

Bending insensitivity of fiber Bragg gratings in suspended-core optical fibers

This Letter presents simulation and experimental results that explore bending insensitivity of fiber Bragg gratings in suspended-core optical fibers. The implementation of thin silica bridge in the fibers enhances index contrast of the fiber core and reduces bending-induced strain transfer to the fiber core. This fiber design lead to a reduction of over 7 times in strain-induced fiber Bragg grating resonant peak shifts in the suspended-core fiber compared with that in standard telecommunication fiber, and an 0.14 dB bending loss at a bending radius of 6.35 mm.     

Nanosecond dynamics of ferroelectric/dielectric superlattices

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.     

Temporal cleaning of a high-energy fiber-based ultrafast laser using cross-polarized wave generation

We report the use of cross-polarized wave generation to perform both pulse shortening and temporal cleaning of a high-energy ytterbium-doped fiber-based femtosecond laser system. The nonlinear processes allow both a highly efficient nonlinear conversion of 20% and a large compression ratio of 3.5, with inherently improved coherent and incoherent contrasts. This results in the generation of 37 μJ, 115 fs pulses at a repetition rate of 100 kHz with high temporal quality.     

A computational strategy for multiscale systems with applications to Lorenz 96 model

Numerical schemes for systems with multiple spatio-temporal scales are investigated. The multiscale schemes use asymptotic results for this type of systems which guarantee the existence of an effective dynamics for some suitably defined modes varying slowly on the largest scales. The multiscale schemes are analyzed in general, then illustrated on a specific example of a moderately large deterministic system displaying chaotic behavior due to Lorenz. Issues like consistency, accuracy, and efficiency are discussed in detail. The role of possible hidden slow variables as well as additional effects arising on the diffusive time-scale are also investigated. As a byproduct we obtain a rather complete characterization of the effective dynamics in Lorenz model.     

Efficient Stark deceleration of cold polar molecules

Stark deceleration has been utilized for slowing and trapping several species of neutral, ground-state polar molecules generated in a supersonic beam expansion. Due to the finite physical dimension of the electrode array and practical limitations of the applicable electric fields, only molecules within a specific range of velocities and positions can be efficiently slowed and trapped. These constraints result in a restricted phase space acceptance of the decelerator in directions both transverse and parallel to the molecular beam axis; hence, careful modeling is required for understanding and achieving efficient Stark decelerator operation. We present work on slowing of the hydroxyl radical (OH) elucidating the physics controlling the evolution of the molecular phase space packets both with experimental results and model calculations. From these results we deduce experimental conditions necessary for efficient operation of a Stark decelerator.     

A three-dimensional model of vocal fold abduction/adduction

A three-dimensional biomechanical model of tissue deformation was developed to simulate dynamic vocal fold abduction and adduction. The model was made of 1721 nearly incompressible finite elements. The cricoarytenoid joint was modeled as a rocking-sliding motion, similar to two concentric cylinders. The vocal ligament and the thyroarytenoid muscle's fiber characteristics were implemented as a fiber-gel composite made of an isotropic ground substance imbedded with fibers. These fibers had contractile and/or passive nonlinear stress-strain characteristics. The verification of the model was made by comparing the range and speed of motion to published vocal fold kinematic data. The model simulated abduction to a maximum glottal angle of about 31 degrees. Using the posterior-cricoarytenoid muscle, the model produced an angular abduction speed of 405 degrees per second. The system mechanics seemed to favor abduction over adduction in both peak speed and response time, even when all intrinsic muscle properties were kept identical. The model also verified the notion that the vocalis and muscularis portions of the thyroarytenoid muscle play significantly different roles in posturing, with the muscularis portion having the larger effect on arytenoid movement. Other insights into the mechanisms of abduction/adduction were given.     

Endemic infrared divergences in QED3 at finite temperature

We demonstrate that massless QED in three dimensions contains endemic infrared divergences. It is argued that these divergences do not affect observables; furthermore, it is possible to choose a gauge that renders the theory finite.