No abstract

MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

A growing literature documents the relationship between tongue strength and oral phase swallowing function. Objective measures of strength have been recommended as more valid and reliable than subjective measures for the assessment of tongue function, yet subjective measures remain the more commonly used clinical method for assessing tongue strength. This study assessed the relationships among subjective and objective measures of tongue strength and oral phase swallowing impairments. Both subjective and objective measures of tongue strength were observed to be good predictors of the presence of oral phase swallowing impairments. The specific oral phase swallowing functions of bolus manipulation, mastication, and clearance were moderately correlated with subjective ratings of tongue strength. Experienced and inexperienced raters appeared to judge tongue strength differently, with the ratings of experienced raters being more predictive of swallowing function.

The brainstem nucleus tractus solitarii (NTS) is the principal site of synaptic contact for visceral afferent fibres of the vagus nerve. Input to the NTS is arranged viscerotopically, with fibres carrying gustatory information terminating primarily in the rostral NTS and respiratory, cardiovascular and gastrointestinal afferents terminating primarily in the intermediate and caudal NTS regions. Central control of visceral function is generally thought to involve both reflex activation of local circuitry in the NTS and reciprocal connections with other brain areas implicated in visceral system regulation. The latter connections suggest a role for the NTS in relaying visceral information to other brain regions. The resulting integrated information is eventually transmitted by neurones in the dorsal motor nucleus of the vagus (DMNX) to their target organs.

Wavelength effects influence radiofrequency (RF) power deposition distributions and limit magnetic resonance (MR) medical applications at very high magnetic fields. The power depositions in spherical saline gel phantoms were deduced from proton resonance shift thermal maps at both 1.5 T and 3.0 T over a range of conductivities. Phase differences before and after RF heating were measured for both a quadrature head coil and a circular surface coil. A long echo time (TE) pulse sequence with a 3D phase unwrap algorithm provided increased thermal sensitivity. The measured thermal maps agreed with a model of eddy-current heating by circularly polarized oscillating RF fields in a conducting dielectric sphere. At 3.0 T, thermal maps were acquired with a <0.32°C temperature rise at 4 W. Proton resonance shift thermal maps provided a measure of hot spots in very-high-field MR imaging (MRI), in which both the phase sensitivity and signalto-noise ratio (SNR) were increased. The method provides a means of studying the heat distribution generated by RF coils excited by clinical pulse sequences. MRI thermal mapping provides a means of estimating the radiofrequency (RF) power deposition (1). As the resonance frequency increases, effects from the finite electromagnetic wavelength increasingly influence the power distribution. Measuring the nonuniform power deposition in very-high-field magnetic resonance (MR) systems is an important step in establishing a safe operating environment for fast imaging sequences (2,3). RF fields in the head during a high-field MR exam exhibit wave phenomena (4 -11) similar to those of RF fields interacting with a dielectric sphere-a problem that has been discussed in a number of textbooks (4 -7). The distribution of heat absorbed in a conducting dielectric sphere, and the RF field pattern both depend on the diameter relative to the radiation wavelength. An oscillating magnetic field induces eddy currents that act to reduce the field inside the sphere, as well as displacement currents from the dielectric properties that enhance the field in the center (7). Early low-field MRI systems had a long wavelength compared to the head size. It was argued that eddy-current screening would limit the application of high-field systems (8); however, the first 1.5-T imaging system built in our laboratory had a uniform RF field profile that balanced conductivity screening and dielectric field focusing. At higher field strengths, finite wavelength effects become the dominant phenomenon, and the RF field is observed to peak in the center of the head (9 -11). Tissue conductivity leads to Joule heating, which imposes an imaging limitation described by the specific absorption rate (SAR) that increases with RF frequency. The RF power absorbed in tissue raises the temperature depending on both the tissue conductivity and the mean square electric field induced by the circularly polarized magnetic field. For safety reasons, it is important to establish methods to accurately predict heating during an MR examination. Numerical...

No abstract

No abstract

To explore the physiological characteristics of central projections of gastric afferent fibers, single-unit activity was recorded extracellularly from neurons in the brain stem during phasic distension of the stomach in cats anesthetized by halothane and nitrous oxide. The brain stem units were identified on the basis of discharge patterns that were phase locked to distension or relaxation of the stomach. Based on stereotaxic coordinates, the units responding to the distension stimulus were located in the region of nucleus and tractus solitarius and extended ventrally toward nucleus ambiguus. Both phasically and tonically discharging units were identified, which exhibited either an excitatory or inhibitory response during distension of the stomach at pressures of 6-8 cmH2O. Units excited by distension increased their firing rate or began to discharge near the peak of distension at rates of 4-6 Hz. Other units responded only during the phase when the distending pressure was decreasing. Sectioning the vagi abolished the response of these brain stem units to gastric distension. The origin of the stimulus that initiated the unit response was localized in the fundus and body of the stomach in many instances. The results indicate that neuronal activity in discrete areas of the brain stem of the cat is altered in response to phasic gastric distension and that this response is vagally mediated.