John V. M. McGinley

Electromagnet arrangements for producing a magnetic field of high homogenity

A coil arrangement for producing a magnetic field of high homogeneity, such as is required in magnetic resonance imaging, comprising a single pair of identical annular coils (17A, 17B) disposed coaxially in spaced relationship and a pair of annular members (19A, 19B) of ferromagnetic material disposed coaxially with the coils, symmetrically with respect to the plane which perpendicularly intersects the axis of the coils centrally between them. The coils and ferromagnetic members have dimensions and are relatively positioned so that, with the coils carrying equal energizing currents, the more significant spherical harmonic coefficients are eliminated.

Supercapacitor Energy Storage for Magnetic Resonance Imaging Systems

Magnetic resonance imaging (MRI) involves very short pulses of very high current. Substantial savings in the high cost of MRI installations may be realized by employing suitable electrical energy storage, for which supercapacitors are strong candidates in view of high specific power and long cycle life. A key question is whether the well-known capacitance degradation with increased frequency is compatible with the complex and highly variable duty cycles of various MRI sequences. Compatibility of the supercapacitor voltage range with the MRI system must also be considered. We present a detailed analysis of power duty profiles in MRI, using actual imaging sequences, that has not been reported previously. We also propose and validate a simplified supercapacitor model that can accurately simulate its performance in the MRI system, involving pulses that are several orders of magnitude shorter than those considered previously. Results of equivalent experiments involving lithium-ion iron phosphate ( LiFePO4) batteries are also reported. Finally, we present a detailed analysis of the overall energy storage performance in a realistic neurological examination. The study is based on a specific system of our own design, and we fully disclose its relevant parameters, so that the results would be of direct practical value to the wider community, including developers of MRI.

Design and Initial Evaluation of a Low-Cost 3-Tesla Research System for Combined Optical and Functional MR Imaging With Interventional Capability

A 3‐Tesla research system has been developed for functional and interventional magnetic resonance imaging (MRI) procedures on animal models based on a low field niche spectrometer. Use of two stages of fourth harmonic frequency multiplication has allowed us to produce a high‐frequency spectrometer with good frequency stability based on a low‐frequency direct digital synthesizer. The system has been designed with the ability to introduce interventional tools such as biopsy needles, radiofrequency (RF) electrodes, and fiber optics for optical spectroscopy and thermal ablation as well as drug infusions to allow function to be studied in the presence of external challenges. Full MR‐compatible physiologic support capability allows animals to be maintained in a stable condition over extended periods of study. Functional MR images have been acquired by using gradient echoes (TR/TE = 40/12 msec) from the rat whisker barrel cortex using electrical stimulation (5‐V, 1.5‐mA, 1‐msec pulses at 5 Hz via two needle electrodes inserted into the rat whisker pad). Initial results using respiratory gas challenges of 100% N2, 100% O2, and 10% CO2 have shown excellent agreement between single wavelength (633 nm) optical and functional MR time series with subsecond time resolution. The 1‐mm copper electrodes for interventional radiofrequency ablation procedures were easily visualized in the superior colliculus by using gradient echo sequences. This novel, low‐cost, high field system appears to be a useful research tool for functional and interventional studies of rat brain and allows concurrent optical spectroscopy. J. Magn. Reson. Imaging 2001;13:87–92.

Tunable dye laser spectroscopy of atomic calcium: collisional redistribution of radiation

By tuning a pulsed dye laser through the resonance line of calcium at 422.7 nm the authors have investigated the spectral redistribution of laser radiation by calcium vapour. Two main components were resolved in the side emission from an oven containing calcium vapour and argon buffer gas. These are produced by Rayleigh scattering and by collision-induced fluorescence. The intensity of the Rayleigh component was found to vary as Delta -2 where Delta = omega 21- omega L is the laser detuning from resonance. In contrast the intensity of the collision-induced fluorescence showed a markedly asymmetric dependence on detuning. Detailed studies of the collision-induced fluorescence have enabled the complete line shape of the calcium resonance line perturbed by argon to be determined. The interpretation of this data in terms of a frequency-dependent collision rate, gamma E( Delta ), is discussed. This paper also reports calculations of theoretical line profiles using the unified Franck-Condon theory and trial interatomic difference potentials. The comparison of the experimental and theoretical line profiles enables the range of validity of the unified Franck-Condon theory to be assessed and the form of the optimum Ca-Ar difference potential to be selected.

Numerical Study of Quench Protection Schemes for a

Thermal stability and protection in the event of quench are key issues in the design of superconducting magnets. Quench development and propagation strongly depend on the conductor characteristics and the magnet configuration. An adequate quench protection method must maintain both the peak temperature and the peak voltage during the event within acceptable limits. This paper presents quench modeling and evaluation of candidate protection schemes for a superconducting coil based on a magnesium diboride (MgB2) wire, designed for use in a new cryogen-free magnetic resonance imaging scanner. The wire properties are different, and the current density is significantly higher from those previously reported. In contrast to previous studies, it is concluded that the coil cannot be considered self-protecting and that protection using external resistance provides a practically acceptable solution.

\hbox{MgB}_{2}

Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p 1P1, have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p2 1S0, 4p2 1D2 and 4s6s 1S0 levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.

Superconducting Magnet

A novel design of open permanent magnet is presented, in which the magnetic field is oriented parallel to the planes of its poles. The paper describes the methods whereby such a magnet can be designed with a field homogeneity suitable for Magnetic Resonance Imaging (MRI). Its primary purpose is to take advantage of the Magic Angle effect in MRI of human extremities, particularly the knee joint, by being capable of rotating the direction of the main magnetic field B0 about two orthogonal axes around a stationary subject and achieve all possible angulations. The magnet comprises a parallel pair of identical profiled arrays of permanent magnets backed by a flat steel yoke such that access in lateral directions is practical. The paper describes the detailed optimization procedure from a target 150mm DSV to the achievement of a measured uniform field over a 130mm DSV. Actual performance data of the manufactured magnet, including shimming and a sample image, is presented. The overall magnet system mounting mechanism is presented, including two orthogonal axes of rotation of the magnet about its isocentre.