Identifying potential imaging markers for diffusion property changes in a mouse model of amyotrophic lateral sclerosis: Application of the continuous time random walk model to ultrahigh b-value diffusion-weighted MR images of spinal cord tissue.

Diffusion MRI (dMRI) explores tissue microstructures by analyzing diffusion-weighted signal decay measured at different b-values. While relatively low b-values are used for most dMRI models, high b-value diffusion-weighted imaging (DWI) techniques have gained interest given that the non-Gaussian water diffusion behavior observed at high b-values can yield potentially valuable information. In this study, we investigated anomalous diffusion behaviors associated with degeneration of spinal cord tissue using a continuous time random walk (CTRW) model for DWI data acquired across an extensive range of ultrahigh b-values. The diffusion data were acquired in situ from the lumbar level of spinal cords of wild-type and age-matched transgenic SOD1G93A mice, a well-established animal model of amyotrophic lateral sclerosis (ALS) featuring progressive degeneration of axonal tracts in this tissue. Based on the diffusion decay behaviors at low and ultrahigh b-values, we applied the CTRW model using various combinations of b-values and compared diffusion metrics calculated from the CTRW model between the experimental groups. We found that diffusion-weighted signal decay curves measured with ultrahigh b-values (up to 858,022 s/mm2 in this study) were well represented by the CTRW model. The anomalous diffusion coefficient obtained from lumbar spinal cords was significantly higher in SOD1G93A mice compared with control mice (14.7 × 10-5 ± 5.54 × 10-5  vs. 7.87 × 10-5 ± 2.48 × 10-5  mm2 /s, p = 0.01). We believe this is the first study to illustrate the efficacy of the CTRW model for analyzing anomalous diffusion regimes at ultrahigh b-values. The CTRW modeling of ultrahigh b-value dMRI can potentially present a novel approach for noninvasively evaluating alterations in spinal cord tissue associated with ALS pathology.

Reconfigurable beam-steerable leaky-wave antenna loaded with metamaterial apertures using liquid crystal-based delay lines.

An original liquid crystal (LC)-based substrate integrated waveguide (SIW) leaky-wave antenna is proposed. Inside the SIW, there is an embedded stripline sandwiched between an LC pool and another dielectric slab. The antenna couples the guided quasi-TEM mode into free space through a periodic set of complementary electric inductive-capacitive (cELC) resonators. Simulation results show that the antenna performs fixed-frequency continuous beam steering of 52° from backward -28° to forward 24° at 25.85 GHz. This relatively wide beam scan angle is achieved by tuning the LC permittivity through an applied quasi-DC bias voltage to the stripline. Simulation results show that the antenna has high realized gain through the entire scanning range (less than 1 dB degradation), relatively wide bandwidth, and good tolerance to frequency drift and fabrication errors.

A fast, analytically based method to optimize local transmit efficiency for a transmit array.

PURPOSE: To develop an analytically based algorithm for rapid optimization of the local radiofrequency magnetic (B1+) field intensity for a given radiofrequency power through a transmit array. The analytical nature of the method will yield insight to optimization requirements and provides a valuable reference for numerically based searches. METHODS: With the knowledge of the B1+ field distribution generated by each single coil of the array, both the phases and the amplitudes of each coil current are optimized to maximize the magnitude of the B1+ field in a specific location of the body per unit of power transmitted through the array and, consequently, minimizing the whole body specific absorption rate for a given pulse sequence. RESULTS: Simulations considering the human body show that the proposed method can reduce the whole-body specific absorption rate for a given B1+ magnitude at the location of interest by a factor of about 6.3 compared to the classic birdcage current configuration, and by a factor of 3.2 compared to phase-only shimming in a case with significant coupling between the elements of the array. CONCLUSION: The proposed method can rapidly provide valuable information pertinent to the optimization of field distributions from transmit arrays.

An approach to rapid calculation of temperature change in tissue using spatial filters to approximate effects of thermal conduction.

We present an approach to performing rapid calculations of temperature within tissue by interleaving, at regular time intervals, 1) an analytical solution to the Pennes (or other desired) bioheat equation excluding the term for thermal conduction and 2) application of a spatial filter to approximate the effects of thermal conduction. Here, the basic approach is presented with attention to filter design. The method is applied to a few different cases relevant to magnetic resonance imaging, and results are compared to those from a full finite-difference (FD) implementation of the Pennes bioheat equation. It is seen that results of the proposed method are in reasonable agreement with those of the FD approach, with about 15% difference in the calculated maximum temperature increase, but are calculated in a fraction of the time, requiring less than 2% of the calculation time for the FD approach in the cases evaluated.

Superhydrophobic and conductive carbon nanofiber/PTFE composite coatings for EMI shielding.

This paper presents a solvent-based, mild method to prepare superhydrophobic, carbon nanofiber/PTFE-filled polymer composite coatings with high electrical conductivity and reports the first data on the effectiveness of such coatings as electromagnetic interference (EMI) shielding materials. The coatings are fabricated by spraying dispersions of carbon nanofibers and sub-micron PTFE particles in a polymer blend solution of poly(vinyledene fluoride) and poly(methyl methacrylate) on cellulosic substrates. Upon drying, coatings display static water contact angles as high as 158° (superhydrophobic) and droplet roll-off angles of 10° indicating self-cleaning ability along with high electrical conductivities (up to 309 S/m). 100 µm-thick coatings are characterized in terms of their EMI shielding effectiveness in the X-band (8.2-12.4 GHz). Results show up to 25 dB of shielding effectiveness, which changed little with frequency at a fixed composition, thus indicating the potential of these coatings for EMI shielding applications and other technologies requiring both extreme liquid repellency and high electrical conductivity.