The transverse electromagnetic (TEM) resonator design (1,5-7) as an RF coil has received heightened attention as a superior replacement for the standard birdcage coil (4) in high-field 4.7-9.4 T MRI applications. It has been demonstrated (1-3) that at the corresponding operating frequencies of 200 and 400 MHz, the TEM resonator can achieve better field homogeneity and a higher quality factor than an equivalent birdcage coil, resulting in improved image quality.In our opinion, the primary difference between a TEM resonator and a birdcage coil is the cylindrical shield, which functions as an active element of the system, providing a return path for the currents in the inner conductors. In a birdcage coil, the shield is a separate entity, disconnected from the inner elements, only reflecting the fields inside the coil to prevent excessive radiation losses. Because of this shield design, the TEM resonator behaves like a longitudinal multiconductor transmission line (MTL), capable of supporting standing waves that occur at high frequencies. Unlike a birdcage coil, the TEM resonator's inner conductors do not possess connections to their closest neighbors, but instead connect directly to the shield through capacitive elements. Resonance mode separation is accomplished though mutual coupling between the inductive inner conductors. Since all the conductors connect to the shield with tunable capacitive elements, the field distribution can be adjusted to achieve the best homogeneity.The first descriptions of TEM resonator structures appeared in patents (5,6). Subsequently, several works (1,8 -11) have been published on the subject of modeling the TEM resonator. The models are typically based on transmission line concepts. Vaughan et al. (1) derived an estimate for the resonance frequency by treating the resonator as a section of coaxial transmission line terminated by capacitors. Tropp (8) developed a lumped circuit model for a TEM resonator, wherein the inner conductors are treated as inductors with mutual coupling and the terminating elements are capacitors. The model predicts all resonant modes of the TEM resonator and shows good agreement with experimental data. However, the measurements are conducted at a relatively low frequency of 143 MHz. Rö schmann (9) and Chingas et al. (10) developed TEM resonator models based on simplified coupled transmission line equations. These models are accurate at predicting particular resonance frequencies, but require complete symmetry of the structure and its terminating elements.Baertlein et al. (11) developed a resonator model based on MTL theory, which included accurate calculations of per-unit-length parameter matrices, resonance frequencies, and field distributions inside the coil. The TEM coil can be modeled under linear or quadrature drive conditions. The resonance frequency predictions compared well with measurements and a full-wave finite-difference time domain (FDTD) model, developed separately in Ref.12. The transmission line model is suitable for studying the resonance beh...
Non-acoustic speech sensors have a long history of clinical applications but have only recently been applied to the problem of measuring speech signals in the presence of strong background noise. These sensors typically provide measurements of one or more aspects of the speech production process, such as glottal activity, as a proxy for the actual speech and tend to be highly immune to acoustic noise. In this paper, a new non-acoustic speech sensor based on a tuned electromagnetic resonator collar is proposed. The collar is designed to be worn around the talker's neck and is sensitive to small changes in the dielectric properties of the glottis as well as subglottal and supraglottal systems that result from voiced speech. Unlike the majority of previously developed non-acoustic speech sensors, the proposed sensor does not require skin contact or precise alignment to effectively measure glottal activity. This paper develops the sensor concept and provides analytical, simulated and experimental results that demonstrate the potential of the new speech sensor.
This study describes a whole-body, non-contact electromagnetic stimulation device based on the concept of a conventional MRI Radio Frequency (RF) resonating coil, but at a much lower resonant frequency (100-150 kHz), with a field modulation option (0.5-100 Hz) and with an input power of up to 3 kW. Its unique features include a high electric field level within the biological tissue due to the resonance effect and a low power dissipation level, or a low Specific Absorption Rate (SAR), in the body itself. Because of its large resonator volume together with non-contact coupling, the subject may be located anywhere within the coil over a longer period at moderate and safe electric field levels. The electric field effect does not depend on body position within the resonator. However, field penetration is deep anywhere within the body, including the extremities where muscles, bones, and peripheral tissues are mostly affected. A potential clinical application of this device is treatment of chronic pain. Substantial attention is paid to device safety; this includes both AC power safety and exposure of human subjects to electromagnetic fields. In the former case, we employ inductive coupling which eliminates a direct current path from AC power to the coil. Our design enhances overall device safety at any power level, even when operated under higher-power conditions. Human exposure to electromagnetic fields within the coil is evaluated by performing modeling with two independent numerical methods and with an anatomically realistic multi-tissue human phantom. We show that SAR levels within the body correspond to International Electrotechnical Commission (IEC) safety standards when the input power level of the amplifier driver does not exceed 3 kW. We also show that electric field levels generally comply with International Commission on Non-Ionizing Radiation Protection safety standards if the input power level does not exceed 1.5 kW.
This paper presents a new apparatus developed for non-destructive evaluation (NDE) of green-state powder metal compacts. A green-state compact is an intermediate step in the powder metallurgy (PM) manufacturing process, which is produced when a metal powder-lubricant mixture is compacted in a press. This compact is subsequently sintered in a furnace to produce the finished product. Non-destructive material testing is most cost effective in the green state because early flaw detection permits early intervention in the manufacturing cycle and thus avoids scrapping large numbers of parts. Unfortunately, traditional NDE methods have largely been unsuccessful when applied to green-state PM compacts. A new instrumentation approach has been developed, whereby direct currents are injected into the green-state compact and an array of spring-loaded needle contacts records the voltage distributions on the surface. The voltage distribution is processed to identify potentially dangerous surface and sub-surface flaws. This paper presents the custom-designed hardware and software developed for current injection, voltage acquisition, pre-amplification and flaw detection. In addition, the testing algorithm and measurement results are discussed. The success of flaw detection using the apparatus is established by using controlled samples, which are PM compacts with dielectric inclusions inserted.
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