Rapid microscopic fractional anisotropy imaging via an optimized linear regression formulation.

Water diffusion anisotropy in the human brain is affected by disease, trauma, and development. Microscopic fractional anisotropy (µFA) is a diffusion MRI (dMRI) metric that can quantify water diffusion anisotropy independent of neuron fiber orientation dispersion. However, there are several different techniques to estimate µFA and few have demonstrated full brain imaging capabilities within clinically viable scan times and resolutions. Here, we present an optimized spherical tensor encoding (STE) technique to acquire µFA directly from the 2nd order cumulant expansion of the powder averaged dMRI signal obtained from direct linear regression (i.e. diffusion kurtosis) which requires fewer powder-averaged signals than other STE fitting techniques and can be rapidly computed. We found that the optimal dMRI parameters for white matter µFA imaging were a maximum b-value of 2000 s/mm2 and a ratio of STE to LTE tensor encoded acquisitions of 1.7 for our system specifications. We then compared two implementations of the direct regression approach to the well-established gamma model in 4 healthy volunteers on a 3 Tesla system. One implementation used mean diffusivity (D) obtained from a 2nd order fit of the cumulant expansion, while the other used a linear estimation of D from the low b-values. Both implementations of the direct regression approach showed strong linear correlations with the gamma model (ρ = 0.97 and ρ = 0.90) but mean biases of -0.11 and - 0.02 relative to the gamma model were also observed, respectively. All three µFA measurements showed good test-retest reliability (ρ ≥ 0.79 and bias = 0). To demonstrate the potential scan time advantage of the direct approach, 2 mm isotropic resolution µFA was demonstrated over a 10 cm slab using a subsampled data set with fewer powder-averaged signals that would correspond to a 3.3-min scan. Accordingly, our results introduce an optimization procedure that has enabled nearly full brain µFA in only several minutes.

The effect of concomitant gradient fields on diffusion tensor imaging.

Concomitant gradient fields are transverse magnetic field components that are necessarily present to satisfy Maxwell's equations when magnetic field gradients are utilized in magnetic resonance imaging. They can have deleterious effects that are more prominent at lower static fields and/or higher gradient strengths. In diffusion tensor imaging schemes that employ large gradients that are not symmetric about a refocusing radiofrequency pulse (unlike Stejskal-Tanner, which is symmetric), concomitant fields may cause phase accrual that could corrupt the diffusion measurement. Theory predicting the error from this dephasing is described and experimentally validated for both Reese twice-refocused and split gradient single spin-echo diffusion gradient schemes. Bias in apparent diffusion coefficient values was experimentally found to worsen with distance from isocenter and with increasing duration of gradient asymmetry in both a phantom and in the brain. The amount of error from concomitant gradient fields depends on many variables, including the diffusion gradient pattern, pulse sequence timing, maximum effective gradient amplitude, static magnetic field strength, voxel size, slice distance from isocenter, and partial Fourier fraction. A prospective correction scheme that can reduce concomitant gradient errors is proposed and verified for diffusion imaging.

Active plasmonic devices via electron spin.

A class of active terahertz devices that operate via particle plasmon oscillations is introduced for ensembles consisting of ferromagnetic and dielectric micro-particles. By utilizing an interplay between spin-orbit interaction manifesting as anisotropic magnetoresistance and the optical distance between ferromagnetic particles, a multifaceted paradigm for device design is demonstrated. Here, the phase accumulation of terahertz radiation across the device is actively modulated via the application of an external magnetic field. An active plasmonic directional router and an active plasmonic cylindrical lens are theoretically explored using both an empirical approach and finite-difference time-domain calculations. These findings are experimentally supported.

A plasmonic random composite with atypical refractive index.

We present a material composite consisting of randomly oriented elements governed by non-resonant interactions. By exploiting near-field plasmonic interaction in a dense ensemble of subwavelength-sized dielectric and metallic particles, we reveal that the group refractive index of the composite can be increased to be larger than the effective refractive indices of constituent metallic and dielectric parent composites. These findings introduce a new class of engineered photonic materials having customizable and atypical optical constants.

Effect of physiotherapy on sickness absence in industry: a comparative study.

A cross-sectional comparative study was carried out in two chemical manufacturing plants in order to ascertain the effect of an occupational physiotherapy service on absence attributed to sickness. There was no overall effect on total sickness absence rates in this study, but a possible reduction in short-term sickness absence was noted. Changes in management attitudes to absence attributed to sickness at the comparison site caused a significant reduction in short-term absences. It is concluded that physiotherapy in an occupational setting has little effect on sickness absence compared to management attitudes, but the unquantifiable benefits, such as increased employee mobility, better industrial relations and employee morale may be significant benefits, worthy of further study.