Brian A. Baertlein

Effect of RF coil excitation on field inhomogeneity at ultra high fields: A field optimized TEM resonator

In this work, computational methods were utilized to optimize the field produced by the transverse electromagnetic (TEM) resonator in the presence of the human head at 8 Tesla. Optimization was achieved through the use of the classical finite difference time domain (FDTD) method and a TEM resonator loaded with an anatomically detailed human head model with a resolution of 2 mm x 2 mm x 2 mm. The head model was developed from 3D MR images. To account for the electromagnetic interactions between the coil and the tissue, the coil and the head were treated as a single system at all the steps of the model including, numerical tuning and excitation. In addition to 2, 3, 4, 6, and 10-port excitations, an antenna array concept was utilized by driving all the possible ports (24) of a 24-strut TEM resonator. The results show that significant improvement in the circularly polarized component of the transverse magnetic field could be obtained when using multiple ports and variable phase and fixed magnitude, or variable phase and variable magnitude excitations.

Dielectric resonances and B1 field inhomogeneity in UHFMRI: computational analysis and experimental findings

B(1) Field inhomogeneity and the relative effects of dielectric resonances are analyzed within the context of ultra high field MRI. This is accomplished by calculating the electromagnetic fields inside spherical phantoms and within a human head model in the presence and absence of an RF coil. These calculations are then compared to gradient echo and RARE images, respectively. For the spherical phantoms, plane incident wave analyses are initially presented followed by full wave finite difference time domain (FDTD) calculations. The FDTD methods are then utilized to examine the electromagnetic interactions between the TEM resonator and an anatomically detailed human head model. The results at 340 MHz reveal that dielectric resonances are most strongly excited in objects similar in size to the human head when the conducting medium has a high dielectric constant and a low conductivity. It is concluded that in clinical UFHMRI, the most important determinants of B(1) field homogeneity consist of 1) the RF coil design, 2) the interaction between the RF coil, the excitation source and the sample, and finally 3) the geometry and electrical properties of the sample.

Application of finite difference time domain method for the design of birdcage RF head coils using multi-port excitations

A three-dimensional finite difference time domain model was developed where the high pass birdcage coil and the imaged object are analysed as a single unit. A study was performed comparing linear, conventional quadrature, and four-port excitation at 64 MHz and 200 MHz for different coil loadings, namely muscle phantoms and an anatomically detailed human head model. A phase array concept was utilized to excite the birdcage coil in four ports. Two phase conditions were analyzed, the simple fixed phase and the variable phase. At 200 MHz, compared to the conventional quadrature drive, the four-port drive reduces the effects of the tissue-coil interactions leading to more uniform currents on the coil legs and consequently to a better B(1) field homogeneity. Also at 200 MHz, driving the coil in four ports provides an SAR distribution with peak values that are significantly less than those with linear or quadrature excitations.

Dielectric resonance phenomena in ultra high field MRI.

PURPOSE Dielectric resonances have previously been advanced as a significant cause of image degradation at higher fields. In this work, a study of dielectric resonances in ultra high field MRI is presented to explore the real importance of dielectric resonances in the human brain in this setting. METHOD Gradient-recalled echo images were acquired using a transverse electromagnetic resonator at 1.5, 4.7, and 8 T. Images were obtained from the human head and from phantoms filled with pure water, saline, and mineral oil. In addition, an exact theoretical analysis of dielectric resonances is presented for a spherical phantom and for a model of the human head. RESULTS Theoretical results demonstrate that distilled water can sustain dielectric resonances in head-sized spheres near 200 and 360 MHz, but the presence of significant conductivity suppresses these resonances. These findings are confirmed experimentally with proton images of water and saline (0.05 and 0.125 M NaCl). For lossy phantoms, coupling between the source and phantom overwhelms the dielectric resonance. Because of their low relative permittivity, mineral oil phantoms with 20 cm diameter do not exhibit dielectric resonances below approximately 900 MHz. Significant dielectric resonances were not observed in human head images obtained at 1.5, 4.7, and 8 T.

Computational analysis of the high pass birdcage resonator: finite difference time domain simulations for high-field MRI.

In this work, a finite difference time domain (FDTD) algorithm is validated at 1.5 tesla using the standard GE Signa quadrature head coil and a muscle phantom. The electrical characteristics of the birdcage head coil are then calculated for the linear and quadrature cases. Unlike previous computational analysis which assume idealized currents on the end rings and the struts of the resonator, a complete computational analysis is provided. This treatment considers the coupling between the resonator and the sample and includes a real coil excitation, a complete current derivation, and a thorough description of both B(1) fields and RF radiation. With this improvement, electromagnetic phenomena such as radiation, standing wave currents on the wires, and field inhomogeneities due to interactions between the coil and the load inside the coil are observed. At 200 MHz, it is demonstrated that this particular coil does not work well due to radiation and non-uniformities on the struts of the device. Also, at this frequency magnetic field inhomogeneities become large when the coil is loaded with a phantom.

Subspace decomposition technique to improve GPR imaging of antipersonnel mines

Ground-reflected clutter is often a performance-limiting factor in ground-penetrating radar detection of near-surface targets including anti-personnel mines. When a down-looking antenna is scanned across the surface this reflection produces a strong band in the image, which obscures shallow targets. Imperfections in the system impulse response can produce similar bands. Radar images of buried targets can be degraded by these forms of clutter.

Effects of thin metal outer case and top air gap on thermal IR images of buried antitank and antipersonnel land mines

A numerical simulation is carried out to study the effect of the thin metal outer case of an antitank mine and the top air gap of an antipersonnel mine on the passive infrared imaging signature. In addition, an antipersonnel surface mine is also analyzed in the present investigation to show its effect on the soil thermal content. The effect of short- and long-wavelength radiation as well as the convective heat transfer is incorporated in this analysis. The temporal development of the temperature distribution over a diurnal cycle is presented for both buried mines. The results show that the thin metal outer case of a buried antitank mine and the top air space of a buried antipersonnel mine have a pronounced effect on the depthwise temperature through the soil. Also, the results show that both buried mines have a noticeable effect on the intensity of the landmine signature on the soil-top surface over a diurnal cycle. A nonexisting mine signature on the soil-top surface is established for an antitank mine with a thin metal outer case. An almost nonexistent signature is also in evidence for the antipersonnel mine with or without an air gap. The results of the present investigation show that the thermal signature of a surface mine produces much larger temperature extremes than the thermal signature of a buried mine. These results play an important role in producing more effective techniques for mine imaging detection.

Numerical simulation of thermal signatures of buried mines over a diurnal cycle

3D thermal and radiometric models have been developed to study the passive IR signature of a land mine buried under a rough soil surface. A finite element model is used to describe the thermal phenomena, including temporal variations, the spatial structure of the signature, and environmental effects. The Crank-Nicholson algorithm is used for time-stepping the simulation. The mine and the surroundings are approximated by pentahedral elements having linear interpolation functions. The FEM grid for the soil includes a random rough surface having a normal probability density and specified covariance function. The mine is modeled as a homogeneous body of deterministic shape having the thermal properties of TNT. Natural solar insolation and the effects of convective heat transfer are represented by linearized boundary conditions. The behavior over a periodic diurnal cycle is studied by running the simulation to steady state. Finite element solutions for the thermal emissions are combined with reflected radiometric components to predict the signatures seen by an IR camera. Numerical simulations are presented for a representative target, a 25 cm anti-tank mine simulant developed by the US Army. The temporal evolution of the temperature distribution and IR signature are presented for both smooth and rough surfaces.

Authors' reply [to comments on "Theoretical model for an MRI radio frequency resonator"]

The authors thank the commentors for their remarks (see De Zanche et al., ibid., vol.49, p.494, 2002) and for the opportunity to clarify and correct some statements in the subject paper (see ibid., vol.47, p.535, 2000). The principal issue raised by De Zanche deals with a remark that appears the in original paper which seems to suggest that maxima of |E| and maxima of |H| are coincident in a transverse electromagnetic (TEM) resonator. The analysis presented in the original makes it clear that this cannot occur. The periodic nature of the TEM resonator fields is relevant to high-field coil design.

Comparison of predetection and postdetection fusion for mine detection

We present and compare methods for pre-detection and post- detection fusion of multi-sensor data. This study emphasis methods suitable for data that are non-commensurate and sampled at non-coincident points. Decision-level fusion is most convenient for such data, but this approach is sub- optimal in principle, since targets not detected by all sensor will not achieve the maximum benefits of fusion. A novel feature-level fusion algorithm for these conditions is described. The optimal forms of both decision-level and feature-level fusion are described, and some approximations are reviewed. Preliminary result for these two fusion techniques are presented for experimental data acquired by a metal detector, a ground-penetrating radar, and an IR camera.