Corey A. Baron

Oscillating gradient spin‐echo (OGSE) diffusion tensor imaging of the human brain

The dependence of diffusion tensor imaging (DTI) eigenvalues and fractional anisotropy (FA) on short diffusion times was investigated using oscillating gradient spin echo (OGSE) and pulsed gradient spin echo (PGSE) DTI in the human brain in vivo.

The effect of concomitant gradient fields on diffusion tensor imaging.

Concomitant gradient fields are transverse magnetic field components that are necessarily present to satisfy Maxwells 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. Magn Reson Med, 2012.

Amygdala subnuclei response and connectivity during emotional processing.

The involvement of the human amygdala in emotion-related processing has been studied using functional magnetic resonance imaging (fMRI) for many years. However, despite the amygdala being comprised of several subnuclei, most studies investigated the role of the entire amygdala in processing of emotions. Here we combined a novel anatomical tracing protocol with event-related high-resolution fMRI acquisition to study the responsiveness of the amygdala subnuclei to negative emotional stimuli and to examine intra-amygdala functional connectivity. The greatest sensitivity to the negative emotional stimuli was observed in the centromedial amygdala, where the hemodynamic response amplitude elicited by the negative emotional stimuli was greater and peaked later than for neutral stimuli. Connectivity patterns converge with extant findings in animals, such that the centromedial amygdala was more connected with the nuclei of the basal amygdala than with the lateral amygdala. Current findings provide evidence of functional specialization within the human amygdala.

Reduction of Diffusion-Weighted Imaging Contrast of Acute Ischemic Stroke at Short Diffusion Times

Background and Purpose— Diffusion-weighted imaging (DWI) of tissue water is a sensitive and specific indicator of acute brain ischemia, where reductions of the diffusion of tissue water are observed acutely in the stroke lesion core. Although these diffusion changes have been long attributed to cell swelling, the precise nature of the biophysical mechanisms remains uncertain. Methods— The potential cause of diffusion reductions after stroke was investigated using an advanced DWI technique, oscillating gradient spin-echo DWI, that enables much shorter diffusion times and can improve specificity for alterations of structure at the micron level. Results— Diffusion measurements in the white matter lesions of patients with acute ischemic stroke were reduced by only 8% using oscillating gradient spin-echo DWI, in contrast to a 37% decrease using standard DWI. Neurite beading has recently been proposed as a mechanism for the diffusion changes after ischemic stroke with some ex vivo evidence. To explore whether beading could cause such differential results, simulations of beaded cylinders and axonal swelling were performed, yielding good agreement with experiment. Conclusions— Short diffusion times result in dramatically reduced diffusion contrast of human stroke. Simulations implicate a combination of neuronal beading and axonal swelling as the key structural changes leading to the reduced apparent diffusion coefficient after stroke.

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.

Acquisition strategy to reduce cerebrospinal fluid partial volume effects for improved DTI tractography

An acquisition method that does not increase scan time or specific absorption rate is investigated for reducing the deleterious effects of cerebrospinal fluid (CSF) partial volume effects on diffusion tensor imaging (DTI) tractography. It is based on using a shorter repetition time (TR) by means of slice acquisition re‐ordering to reduce the signal of long T1 CSF and a non‐zero minimum diffusion weighting (b‐value) to attenuate rapidly diffusing CSF signal with respect to brain tissue.

Nonrigid motion correction with 3D image-based navigators for coronary MR angiography

To develop a retrospective nonrigid motion‐correction method based on 3D image‐based navigators (iNAVs) for free‐breathing whole‐heart coronary magnetic resonance angiography (MRA).

Isotropic photonic magnetoresistance

The authors demonstrate isotropic photonic magnetoresistive behavior in the far-infrared transmission through ferromagnetic particle collections. Total suppression of the angular anisotropy, a fundamental characteristic of anisotropic magnetoresistance, is achieved in ensembles of highly porous, “cauliflower” shaped ferromagnetic particles. They reveal a morphological origin of the isotropic magnetic behavior using a phenomenological model in conjunction with three-dimensional calculations using Maxwell’s equations.

The effect of a semiconductor-metal interface on localized terahertz plasmons

We demonstrate the terahertz frequency plasmonic characteristics of a metallic/semiconductor interface. The high-frequency transmission of terahertz radiation through dense ensembles of subwavelength sized Cu/CuxO microparticles via near-field coupling of localized plasmons is shown to increase when a CuxO/Au or CuxO/Pt interface is introduced. This finding introduces a previously undocumented characteristic of plasmonic interaction with material interfaces, and lays the physical groundwork for the integration of plasmonic circuits with traditional semiconductor electronics.

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.