Progress in high field MRI at the University of Florida

In this article we report on progress in high magnetic field MRI at the University of Florida in support of our new 750MHz wide bore and 11.7T/40cm MR instruments. The primary emphasis is on the associated rf technology required, particularly high frequency volume and phased array coils. Preliminary imaging results at 750MHz are presented. Our results imply that the pursuit of even higher fields seems warranted. (C) 2002 Elsevier Science B.V. All rights reserved.

Processing of urinary pheromones in Antechinus stuartii (Marsupialia: Dasyuridae): Functional magnetic resonance imaging of the brain

Little is known of the neural mechanisms of marsupial olfaction. However, functional magnetic resonance imaging (fMRI) has made it possible to visualize dynamic brain function in mammals without invasion. In this study, central processing of urinary pheromones was investigated in the brown antechinus, Antechinus stuartii, using fMRI. Images were obtained from 18 subjects (11 males, 7 females) in response to conspecific urinary olfactory stimuli. Significant indiscriminate activation occurred in the accessory olfactory bulb, entorhinal, frontal, and parietal cortices in response to both male and female urine. The paraventricular nucleus of hypothalamus, ventrolateral thalamic nucleus, and medial preoptic area were only activated in response to male urine. Results of this MRI study indicate that projections of accessory olfactory system are activated by chemo-sensory cues. Furthermore, it appears that, based on these experiments, urinary pheromones may act on the hypothalamo-pituitary-adrenocortical axis via the paraventricular nucleus of the hypothalamus and may play an important role in the unique life history pattern of A. stuartii. Finally, this study has demonstrated that fMRI may be a powerful tool for investigations of olfactory processes in mammals.

NMR shim coil design utilising a rapid spherical harmonic calculation method

A rapid spherical harmonic calculation method is used for the design of Nuclear Magnetic Resonance shim coils. The aim is to design each shim such that it generates a field described purely by a single spherical harmonic. By applying simulated annealing techniques, coil arrangements are produced through the optimal positioning of current-carrying circular arc conductors of rectangular cross-section. This involves minimizing the undesirable harmonies in relation to a target harmonic. The design method is flexible enough to be applied for the production of coil arrangements that generate fields consisting significantly of either zonal or tesseral harmonics. Results are presented for several coil designs which generate tesseral harmonics of degree one.

Finite-difference time-domain-based studies of MRI pulsed field gradient-induced eddy currents inside the human body

In modern magnetic resonance imaging (MRI), patients are exposed to strong, rapidly switching magnetic gradient fields that, in extreme cases, may be able to elicit nerve stimulation. This paper presents theoretical investigations into the spatial distribution of induced current inside human tissues caused by pulsed z-gradient fields. A variety of gradient waveforms have been studied. The simulations are based on a new, high-definition, finite-difference time-domain method and a realistic inhomogeneous 10-mm resolution human body model with appropriate tissue parameters. it was found that the eddy current densities are affected not only by the pulse sequences but by many parameters such as the position of the body inside the gradient set, the local biological material properties and the geometry of the body. The discussion contains a comparison of these results with previous results found in the literature. This study and the new methods presented herein will help to further investigate the biological effects caused by the switched gradient fields in a MRI scan. (C) 2002 Wiley Periodicals, Inc.

Cost effectiveness of adding magnetic resonance imaging to the usual management of suspected scaphoid fractures

Objective: To determine the cost effectiveness of a magnetic resonance imaging scan (MRI) within 5 days of injury compared with the usual management of occult scaphoid fracture. Methods: All patients with suspected scaphoid fractures in five hospitals were invited to participate in a randomised controlled trial of usual treatment with or without an MRI scan. Healthcare costs were compared, and a cost effectiveness analysis of the use of MRI in this scenario was performed. Results: Twenty eight of the 37 patients identified were randomised: 17 in the control group, 11 in the MRI group. The groups were similar at baseline and follow up in terms of number of scaphoid fractures, other injuries, pain, and function. Of the patients without fracture, the MRI group had significantly fewer days immobilised: a median of 3.0 (interquartile range 3.0-3.0) v 10.0 (7-12) in the control group (p = 0.006). The MRI group used fewer healthcare units (median 3.0, interquartile range 2.0-4.25) than the control group (5.0, 3.0-6.5) (p = 0.03 for the difference). However, the median cost of health care in the MRI group ($594.35 AUD, $551.35-667.23) was slightly higher than in the control group ($428.15, $124.40-702.65) (p = 0.19 for the difference). The mean incremental cost effectiveness ratio derived from this simulation was that MRI costs $44.37 per day saved from unnecessary immobilisation (95% confidence interval $4.29 to $101.02). An illustrative willingness to pay was calculated using a combination of the trials measure of the subjects' individual productivity losses and the average daily earnings. Conclusions: Use of MRI in the management of occult scaphoid fracture reduces the number of days of unnecessary immobilisation and use of healthcare units. Healthcare costs increased non-significantly in relation to the use of MRI in this setting. However, when productivity losses are considered, MRI may be considered cost effective, depending on the individual case.

Thermal analysis, nuclear magnetic resonance spectroscopy, and impedance spectroscopy of N,N-dimethyl-pyrrolidinium iodide: An ionic solid exhibiting rotator phases

N,N-dimethyl-pyrrolidinium iodide has been investigated using differential scanning calorimetry, nuclear magnetic resonance (NMR) spectroscopy, second moment calculations, and impedance spectroscopy. This pyrrolidinium salt exhibits two solid-solid phase transitions, one at 373 K having an entropy change, Delta S, of 38 J mol(-1) K-1 and one at 478 K having Delta S of 5.7 J mol(-1) K-1. The second moment calculations relate the lower temperature transition to a homogenization of the sample in terms of the mobility of the cations, while the high temperature phase transition is within the temperature region of isotropic tumbling of the cations. At higher temperatures a further decrease in the H-1 NMR linewidth is observed which is suggested to be due to diffusion of the cations. (C) 2005 American Institute of Physics.

Fronto-parietal activation in attention-deficit hyperactivity disorder, combined type: functional magnetic resonance imaging study

A functional magnetic resonance imaging mental rotation paradigm was used to investigate the patterns of activation of fronto-parietal brain areas in male adolescents with attention-deficit hyperactivity disorder, combined type (ADHD-CT) compared with age-, gender-, handedness- and performance IQ-matched healthy controls. The ADHD-CT group had (a) decreased activation of the 'action-attentional' system (including Brodmann's areas (BA) 46, 39,40) and the superior parietal (BA7) and middle frontal (BA10) areas and (b) increased activation of the posterior midline attentional system. These different neuroactivation patterns indicate widespread frontal, striatal and parietal dysfunction in adolescents with ADHD-CT. Declaration of interest None.

Characterization of optically driven fluid stress fields with optical tweezers

We present a controlled stress microviscometer with applications to complex fluids. It generates and measures microscopic fluid velocity fields, based on dual beam optical tweezers. This allows an investigation of bulk viscous properties and local inhomogeneities at the probe particle surface. The accuracy of the method is demonstrated in water. In a complex fluid model (hyaluronic acid), we observe a strong deviation of the flow field from classical behavior. Knowledge of the deviation together with an optical torque measurement is used to determine the bulk viscosity. Furthermore, we model the observed deviation and derive microscopic parameters.

Resonance fluorescence and Autler-Townes spectra of a two-level atom driven by two fields of equal frequencies

We study the effects of driving a two-level atom by two intense field modes that have equal frequencies but are otherwise distinguishable; the intensity of one mode is also assumed to be greater than that of the other. We calculate first the dressed states of the system, and then its resonance fluorescence and Autler-Townes absorption spectra. We find that the energy spectrum of the doubly dressed atom consists of a ladder of doublet continua. These continua manifest themselves in the fluorescence spectrum, where they produce continua at the positions of the Mellow sideband frequencies omega(L)+/-2 Omega of the strong field, and in the Autler-Townes absorption spectrum, which becomes a two-continuum doublet.

A methodology for current density calculations in high-frequency RF resonators

As nuclear magnetic resonance imaging and spectroscopy move inexorably toward higher field-strength magnets in search of improved signal-to-noise ratio, spectral resolution, and spatial resolution, the way in which radiofrequency (RF) probes are designed changes. At higher frequencies, resonant cavities become the favored RF ''coil'' type and may be built using streamline elements to reduce the inductance of the system. In modeling such systems, the quasi-static approach of assuming that current flows evenly in all conductor cross sections and that adjacent conductors do not affect each other becomes less reasonable. The proximity of RF conductors in resonators typically causes RF eddy currents to flow, whereby the current density in each rung is altered by the RF fields generated by nearby conductors. The proper understanding and prediction of how resonators will perform require a model of the current densities flowing in conducting sections, including all RF eddy current effects. Very few models of this type have been presented in the literature. This article presents an overview of one such model and of how it may be applied to a variety of resonators, both shielded and unshielded, circular, and elliptical, in cross section. Results are presented from a shielded head coil operating at 2 tesla. (C) 1997 John Wiley & Sons, Inc.

Conformational dynamics of thyroid hormones by variable temperature nuclear magnetic resonance: The role of side chain rotations and cisoid/transoid interconversions

H-1 NMR spectra of the thyroid hormone thyroxine recorded at low temperature and high field show splitting into two peaks of the resonance due to the H2,6 protons of the inner (tyrosyl) ring. A single resonance is observed in 600 MHz spectra at temperatures above 185 K. An analysis of the line shape as a function of temperature shows that the coalescence phenomenon is due to an exchange process with a barrier of 37 kJ mol(-1). This is identical to the barrier for coalescence of the H2',6' protons of the outer (phenolic) ring reported previously for the thyroid hormones and their analogues. It is proposed that the separate peaks at low temperature are due to resonances for H2,6 in cisoid and transoid conformers which are populated in approximately equal populations. These two peaks are averaged resonances for the individual H2 and H6 protons. Conversion of cisoid to transoid forms can occur via rotation of either the alanyl side chain or the outer ring, from one face of the inner ring to the other. It is proposed that the latter process is the one responsible for the observed coalescence phenomenon. The barrier to rotation of the alanyl side chain is greater than or equal to 37 kJ mol(-1), which is significantly larger than has previously been reported for Csp(2)-Csp(3) bonds in other Ph-CH2-X systems. The recent crystal structure of a hormone agonist bound to the ligand-binding domain of the rat thyroid hormone receptor (Wagner et al. Nature 1995, 378, 690-697) shows the transoid form to be the bound conformation. The significant energy barrier to cisoid/transoid interconversion determined in the current study combined with the tight fit of the hormone to its receptor suggests that interconversion between the forms cannot occur at the receptor site but that selection for the preferred bound form occurs from the 50% population of the transoid form in solution.