ABSTRACT:A new design approach for the construction of gradient coils for magnetic resonance imaging is presented. The theoretical formulation involves a constraint cost function between the desired field in a particular region of interest in space and an almost arbitrarily defined surface that carries the current configuration based on Biot-Savart's integral equation. An appropriate weight function in conjunction with linear approximation functions permits the transformation of the problem formulation into a linear matrix equation whose solution yields discrete current elements in terms of magnitude and direction within a specified coil surface. Numerical predictions and comparisons with practical measurements for the G x , G y , G z gradient coils underscore the success of this approach in terms of achieving highly linear fields while maintaining low parasitic fields and low inductances.

Summary:Purpose: Functional magnetic resonance imaging (fMRI) was used to identify areas of brain activation during absence seizures in an awake animal model. Methods: Blood-oxygenation-level-dependent (BOLD) fMRI in the brain was measured by using T 2 * -weighted echo planar imaging at 4.7 Tesla. BOLD imaging was performed before, during, and after absence seizure induction by using γ -butyrolactone (GBL; 200 mg/kg, intraperitoneal).Results: The corticothalamic circuitry, critical for spike-wave discharge (SWD) formation in absence seizure, showed robust BOLD signal changes after GBL administration, consistent with EEG recordings in the same animals. Predominantly positive BOLD changes occurred in the thalamus. Sensory and parietal cortices showed mixed positive and negative BOLD changes, whereas temporal and motor cortices showed only negative BOLD changes.Conclusions: With the BOLD fMRI technique, we demonstrated signal changes in brain areas that have been shown, with electrophysiology experiments, to be important for generating and maintaining the SWDs that characterize absence seizures. These results corroborate previous findings from lesion and electrophysiological experiments and show the technical feasibility of noninvasively imaging absence seizures in fully conscious rodents. Key Words: Absence seizure-fMRIEpilepsy-Imaging-GHB.Typical absence seizures consist of multiple, brief (≤20 s) impairments of consciousness with characteristic bilaterally synchronous 3-Hz spike-wave discharges (SWDs) on electroencephalography (EEG). A typical absence seizure is manifested behaviorally as a "staring spell" and can be accompanied by atonic postures such as drooping of the head and/or automatisms such as lip smacking. The incidence of absence seizures in the United States is 1.9 to 8 per 100,000, usually occurring in children between the ages of 4 years and adolescence, with girls affected twice as often as boys (1).Early studies on cats showed that electrical stimulation to the midline and intralaminar nuclei of the thalamus at a frequency of 3 Hz produced SWDs on EEG (2). This finding was corroborated after the discovery of identical 3-Hz SWDs in children with deep recording electrodes Accepted May 7, 2003. Address correspondence and reprint requests to Dr. J.R. Tenney at University of Massachusetts Medical School, CCNI Building, 55 Lake Avenue North, Worcester MA 01655, U.S.A. E-mail: jeffrey. tenney@umassmed.edu in the thalamus (3). These early seminal studies pointed to the thalamus as the initiation site of SWDs, and recent work with rodent models has shown that corticothalamic interactions are involved in the pathogenesis of absence seizures.γ -Hydroxybutyric acid (GHB) is a naturally occurring metabolite of the inhibitory neurotransmitter γ -aminobutyric acid (GABA). Normal rats given an intraperitoneal (i.p.) injection of GHB will show all the behavioral and electrophysiological signs of generalized absence seizure (4). Synchronized bursting activity from recording electrodes in the somatosensory ventroba...

Purpose: To evaluate brain activity associated with sexual arousal, fully conscious male marmoset monkeys were imaged during presentation of odors that naturally elicit high levels of sexual activity and sexual motivation. Material and Methods:Male monkeys were lightly anesthetized, secured in a head and body restrainer with a built-in birdcage resonator and positioned in a 9.4-Tesla spectrometer. When fully conscious, monkeys were presented with the odors of a novel receptive female or an ovariectomized monkey. Both odors were presented during an imaging trial and the presentation of odors was counterbalanced. Significant changes in both positive and negative BOLD signal were mapped and averaged.Results: Periovulatory odors significantly increased positive BOLD signal in several cortical areas: the striatum, hippocampus, septum, periaqueductal gray, and cerebellum, in comparison with odors from ovariectomized monkeys. Conversely, negative BOLD signal was significantly increased in the temporal cortex, cingulate cortex, putamen, hippocampus, substantia nigra, medial preoptic area, and cerebellum with presentation of odors from ovariectomized marmosets as compared to periovulatory odors. A common neural circuit comprising the temporal and cingulate cortices, putamen, hippocampus, medial preoptic area, and cerebellum shared both the positive BOLD response to periovulatory odors and the negative BOLD response to odors of ovariectomized females. Conclusion:These data suggest the odor-driven enhancement and suppression of sexual arousal affect neuronal activity in many of the same general brain areas. These areas included not only those associated with sexual activity, but also areas involved in emotional processing and reward.

Olfactory cues can elicit intense emotional responses. This study used fMRI in male common marmoset monkeys to identify brain areas associated with sexual arousal in response to odors of ovulating female monkeys. Under light anesthesia, monkeys were secured in a specially designed restrainer and positioned in a 9.4 T magnetic resonance spectrometer. When fully conscious, they were presented with the scents of both ovariectomized and ovulating monkeys. The sexually arousing odors of the ovulating monkeys enhanced signal intensity in the preoptic area and anterior hypothalamus compared to the odors of ovariectomized monkeys. These data corroborate previous findings in monkeys based on invasive electrical lesion and stimulation techniques and demonstrate the feasibility of using non-invasive functional imaging on fully conscious common marmosets to study cue-elicited emotional responses.

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...