Dielectric Pads Research Articles

Quantitative assessment of the effects of high‐permittivity pads in 7 Tesla MRI of the brain

The use of high-permittivity materials has been shown to be an effective method for increasing transmit and receive sensitivity in areas of low-signal intensity in the brain at high field. Results in this article show that the use of these materials does not increase the intercoil coupling for a phased array receive coil, does not have any detrimental effects on the B(0) homogeneity within the brain, and does not affect the specific absorption rate distribution within the head. Areas of the brain close to the pads exhibit significant increases (>100%) in transmit field efficiency, but areas further away show a less pronounced (~10%) decrease due to the homogenization of the transmit field and the loss introduced by the dielectric pads.

Improvements in high‐field localized MRS of the medial temporal lobe in humans using new deformable high‐dielectric materials

The intrinsic nonuniformities in the transmit radiofrequency field from standard quadrature volume resonators at high field are particularly problematic for localized MRS in areas such as the temporal lobe, where a low signal-to-noise ratio and poor metabolite quantification result from destructive B₁⁺ field interference, in addition to line broadening and signal loss from strong susceptibility gradients. MRS of the temporal lobe has been performed in a number of neurodegenerative diseases at clinical fields, but a relatively low signal-to-noise ratio has prevented the reliable quantification of, for example, glutamate and glutamine, which are thought to play a key role in disease progression. Using a recently developed high-dielectric-constant material placed around the head, localized MRS of the medial temporal lobe using the stimulated echo acquisition mode sequence was acquired at 7 T. The presence of the material increased the signal-to-noise ratio of MRS by a factor of two without significantly reducing the sensitivity in other areas of the brain, as shown by the measured B₁⁺ maps. An increase in the receive sensitivity B₁⁻ was also measured close to the pads. The spectral linewidth of the unsuppressed water peak within the voxel of interest was reduced slightly by the introduction of the dielectric pads (although not to a statistically significant degree), a result confirmed by using a pad composed of lipid. Using LCmodel for quantitative analysis of metabolite concentrations, the increase in signal-to-noise ratio and the slight decrease in spectral linewidth contributed to statistically significant reductions in the Cramer-Rao lower bounds (CRLBs), also allowing the levels of glutamate and glutamine to be quantified with CRLBs below 20%.

Reducing SAR and enhancing cerebral signal‐to‐noise ratio with high permittivity padding at 3 T

Previous works have shown that placement of a high-dielectric pad can improve image intensity in a region adjacent to the pad, or that placement of dielectric pads around a large surface of the head can improve image homogeneity on an entire plane through the head in high-field MRI. Here, experimental results show that use of high-dielectric pads around the human head can reduce the required input radiofrequency power by 50% while enhancing image signal-to-noise ratio by 20-40% throughout the cerebrum at 3 T. Thus, dielectric pads may be used to provide a relatively simple and low-cost method for improving quality and safety of MRI in a variety of applications at 3 T.

MR imaging of the female pelvis at 3 Tesla: Evaluation of image homogeneity using different dielectric pads

To evaluate improvements in image homogeneity in pelvic MR imaging at 3 Tesla (T) using two different dielectric pads. A total of eight healthy females were scanned using a 3T MR scanner equipped with a body-array coil. Axial and sagittal fast spin-echo T2-weighted images (T2WI) (TR/TE = 3200 msec/94 msec), axial fast spin-echo T1-weighted images (T1WI) (TR/TE = 700 msec/11 msec), and sagittal half-Fourier acquisition single-shot turbo spin-echo (HASTE) images (TR/TE = 3000 msec/100 msec) were performed for pelvic imaging. Sequences were repeated with dielectric pads (consisting of either ultrasound [US] gel or water), and without pads. Three or four regions of interest (ROIs) were placed on fatty tissues and the ratio of minimum to maximum signal intensity (RSI) was calculated as a marker of image homogeneity. RSI was significantly higher on T2WI and T1WI when using dielectric pads than when no pad was used. A similar tendency was observed in RSI on HASTE. No significant difference was found between images with US gel pads and those with water pads. Dielectric pads consisting of either US gel or water are effective in improving image homogeneity of the pelvis on 3T MRI.

Optical analogy to electronic quantum corrals.

We describe full multiple-scattering calculations of localized surface photonic states set up by lithographically designed nanostructures made of a finite number of dielectric pads deposited on a planar surface. The method is based on a numerical solution of the dyadic Dyson's equation. When the pads are arranged to form a closed circle, we find field patterns that look like the electronic charge density recently observed above quantum corrals. We propose two experimental techniques that could be used to observe these electromagnetic modes in direct space.

Near-field optical study of mesoscopic Au periodic samples: Effect of the polarization and comparison between different imaging modes

Laboratoire de Microstructures et de Microle´ctronique, 196 Avenue H. Ravera, Boiˆte Postale 107, 92195 Bagneux Cedex, France~Received 1 June 1999!This paper presents a complete study, theoretical as well as experimental, of an electromagnetic fieldscattered by subwavelength metallic pads, allocated in a periodic manner on a silica substrate. The simulationof the far field and near field is obtained with the differential method. When the sample is illuminated in totalinternal reflection, the simulations show that the amplification of the electromagnetic field above the Aumetallic pads depends on different parameters ~wavelength, polarization of light, angle of incidence, and indexof refraction!. In this paper, we only consider the effect of the probe-to-sample distance and of the polarizationof the illuminating light. As the experimental setup, we used the photon scanning tunneling microscope. If wecompare these results with the calculations carried out with the dielectric pads, we show that the amplificationis induced by the dielectric contrast between the metallic structures and their environment. Experimental resultsare presented in two different imaging modes. In the ‘‘constant intensity’’ mode, our experimental results arein excellent agreement with the simulations. Therefore, for a metallic sample analyzed in our experimentalconditions, it validates the assumption that the signal detected is proportional to the square modulus of theelectric field in the absence of the probe. We particularly show that if the polarization and the probe-to-sampledistance are suitably chosen, dramatic localizations of the electromagnetic field are observed. We then presentimages obtained in the so-called ‘‘two-wavelength’’ mode, where two light sources illuminate the sample. Thefirst beam is used for feedback regulation; it consequently allows a control of the tip motion, and permits us todetermine the reference surface. By using such feedback regulation for this first beam, we are able to comparequantitatively the effect of the polarization on the field distribution of the second beam. The results areconfirmed by the corresponding simulations.I. INTRODUCTION

Self-consistent calculation of electrostatic force for two kinds of mesoscopic surface structures

The scanning electrostatic force microscopy (SEFM) can acquire information of surface structures in a non-contact way. We calculate the electrostatic force between the charged tip and polarized surface structure in SEFM in the framework of self-consistent integral equation formalism (SCIEF), incorporating the image method to treat the electrostatic coupling of substrate and tip. We consider two kinds of surface structures, one is the topographic structure on the surface, the other is the dielectric structure embedded in the substrate. The force pattern of the topographic structure shows a protrusion around the surface structure. However, the force pattern displays a hollow around an embedded structure with a dielectric constant less than that of substrate medium. For an embedded structure with a larger dielectric constant, the force pattern exhibits a protrusion, and the force signal is much weaker than that of the topographic structure. Therefore, it is expected that one may identify these surface structures from the pure electrostatic force information in SEFM. The force signal of the densely arranged dielectric pads is simply the superposition of force signal of each pad individually, the interference effect of electric field is not remarkable.

Scanning-electrostatic-force microscopy: Self-consistent method for mesoscopic surface structures

Scanning-electrostatic-force microscopy (SEFM) provides an efficient way to collect information of surface structures. The electrostatic force between the charged tip and the polarized sample cannot be derived analytically for samples with complex profiles and arbitrary dielectric distributions. In this paper we calculate the electrostatic force in the framework of self-consistent integral equation formalism, incorporating the image method to treat the coupling of substrate and tip. The simulation results show that the electrostatic force depends strongly on the bias voltage of tip, tip-sample distance, and dielectricity of the sample and substrate. The scanned force image is in good relation to the topography of the sample. The apparent size of the sample is approximately linear to the probe-sample distance. The force signal of densely arranged cubic dielectric pads is simply the superposition of forces signals of each pad alone. The interference effect of the electric field is not remarkable in SEFM. [S0163-1829(98)06215-8].

Observation of Light Confinement Effects with a Near-Field Optical Microscope.

This Letter reports the experimental observation of light confinement effects by near-field optical microscopy. Depolarization effects giving rise to light confinement close to nanoscopic objects have been unambiguously observed in near-field optical images of subwavelength dielectric pads etched on a flat glass substrate. According to the incident polarization, this phenomenon leads to reverse contrasts in the near-field optical image of the same subwavelength objects.