Richard L. Magin

High-Resolution Microcoil 1H-NMR for Mass-Limited, Nanoliter-Volume Samples

High-resolution, proton nuclear magnetic resonance (NMR) spectra of 5-nanoliter samples have been obtained with much higher mass sensitivity [signal-to-noise ratio (S/N) per micromole] than with traditional methods. Arginine and sucrose show a mean sensitivity enhancement of 130 compared to 278-microliter samples run in a 5-millimeter tube in a conventional, commercial probe. This can reduce data acquisition time by a factor of >16,000 or reduce the needed sample mass by a factor of about 130. A linewidth of 0.6 hertz was achieved on a 300-megahertz spectrometer by matching the magnetic susceptibility of the medium that surrounds the detection cell to that of the copper coil. For sucrose, the limit of detection (defined at S/N = 3) was 19 nanograms (56 picomoles) for a 1-minute data acquisition. This technique should prove useful with mass-limited samples and for use as a detector in capillary separations.

fMRI Studies of Stroop Tasks Reveal Unique Roles of Anterior and Posterior Brain Systems in Attentional Selection

The brains attentional system identifies and selects information that is task-relevant while ignoring information that is task-irrelevant. In two experiments using functional magnetic resonance imaging, we examined the effects of varying task-relevant information compared to task-irrelevant information. In the first experiment, we compared patterns of activation as attentional demands were increased for two Stroop tasks that differed in the task-relevant information, but not the task-irrelevant information: a color-word task and a spatial-word task. Distinct subdivisions of dorsolateral prefrontal cortex and the precuneus became activated for each task, indicating differential sensitivity of these regions to task-relevant information (e.g., spatial information vs. color). In the second experiment, we compared patterns of activation with increased attentional demands for two Stroop tasks that differed in task-irrelevant information, but not task-relevant information: a color-word task and color-object task. Little differentiation in activation for dorsolateral prefrontal and precuneus regions was observed, indicating a relative insensitivity of these regions to task-irrelevant information. However, we observed a differentiation in the pattern of activity for posterior regions. There were unique areas of activation in parietal regions for the color-word task and in occipito-temporal regions for the color-object task. No increase in activation was observed in regions responsible for processing the perceptual attribute of color. The results of this second experiment indicate that attentional selection in tasks such as the Stroop task, which contain multiple potential sources of relevant information (e.g., the word vs. its ink color), acts more by modulating the processing of task-irrelevant information than by modulating processing of task-relevant information.

Fractional Bloch equation with delay

In this paper we investigate a fractional generalization of the Bloch equation that includes both fractional derivatives and time delays. The appearance of the fractional derivative on the left side of the Bloch equation encodes a degree of system memory in the dynamic model for magnetization. The introduction of a time delay on the right side of the equation balances the equation by also adding a degree of system memory on the right side of the equation. The analysis of this system shows different stability behavior for the T1 and the T2 relaxation processes. The T1 decay is stable for the range of delays tested (1-100 @ms), while the T2 relaxation in this model exhibited a critical delay (typically 6 @ms) above which the system was unstable. Delays are expected to appear in NMR systems, in both the system model and in the signal excitation and detection processes. Therefore, by including both the fractional derivative and finite time delays in the Bloch equation, we believe that we have established a more complete and more realistic model for NMR resonance and relaxation.

Focused Array Hyperthermia Applicator: Theory and Experiment

The design and analysis of a focused linear array at 2450 MHz for microwave hyperthermia research is described. The array, which was submerged in deionized water to reduce its size and to provide a better impedance match to a high dielectric medium representative of human tissues, consisted of four titanium dioxide loaded horn antennas with apertures of 2.0 x 1.4 cm and a feed network with weighted phase shifts, Power radiation pattern measurements were made in planes ranging from 7.6 to 10.2 cm from the array to determine the focusing characteristics and beam spot size. Due to high attenuation in the medium, planes beyond 10.2 cm were not considered. The half-power beamwidth (HPBW) measured at the focal point was approximately 1.3 cm. The measured patterns were found to be in close agreement with theoretical predictions.

Miniature magnetic resonance machines

Inventive redesign of smaller RF coils and magnets is the key to down-sized magnetic resonance imagers and spectrometers.

Temperature-dependent permeability of large unilamellar liposomes

The temperature-dependent drug leakage from liposomes composed of dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylglycerol (4:1, by weight) was studied. Experiments were performed in Hepes buffer and 50% fetal calf serum. Large unilamellar liposomes were formed by the reverse phase evaporation process and extruded through a series of polycarbonate membranes with pore sizes of 0.4, 0.2, 0.1 and 0.08 micron. The release of the water soluble radioisotopes cytosine 1-beta-D-[3H]arabinofuranoside and [3H]inulin from the aqueous compartment of these liposomes was measured as a function of time and temperature. Both radioisotopes were released at temperatures near 42 degrees C, the solid-to-liquid-crystalline phase transition temperature of these lipids. The percent drug release decreased as the size of the liposomes was reduced. This effect was more pronounced in Hepes buffer than serum. The release of both radioisotopes was greatest at 40 degrees C in Hepes buffer and at 43 degrees C in 50% fetal calf serum. In addition, the rate of drug release was much faster in serum than in buffer. These results suggest that different drug release processes are occurring in buffer and in serum.

Noninvasive microwave phased arrays for local hyperthermia: A review

Microwave energy has proven useful for treating superficial tumours in the head, neck and chest regions. Currently, multi-element phased arrays are being proposed to upgrade clinical capabilities for localized microwave hyperthermia. When compared with a single radiating element, phased array applicators are expected to provide deeper tissue penetration, reduce undesired heating of normal tissues between the applicator and tumour, and improve local control of the tumour temperature distribution. This paper surveys recent developments in the design and characterization of phased arrays, identifies anatomical and physiological factors that complicate successful clinical treatment and discusses the current state of phased array hardware development for hyperthermia.

NMR microspectroscopy using 100 /spl mu/m planar RF coils fabricated on gallium arsenide substrates

A family of planar gold RF microcoils were fabricated using microlithography on a gallium arsenide substrate. The microcoils were used in /sup 1/H nuclear magnetic resonance (PIMR) spectroscopy experiments at 300 MHz (7.05 T). These new microcoils are a key component in the design of integrated MR coils and amplifiers for NMR microspectroscopy.<<ETX>>

An annular focus ultrasonic lens for local hyperthermia treatment of small tumors

A system for applying local hyperthermia employing ultrasonic transducers with annular focusing lenses gave better temperature uniformity in small fibrosarcomas than conventional unfocused transducers. The lenses were designed for ultrasonic frequencies of 3, 4, 5 and 9 MHz for tumors approximately 6 mm dia. Tests in degassed water indicated that the focusing lenses concentrated approx. 80% of the ultrasonic energy into an annular-shaped focus of 6 mm dia. located at a distance of 3 cm in front of the lens. These transducers were used to heat subcutaneous PARA-7 fibrosarcomas in hamsters. Steady-state temperature measurements indicated temperature gradients within tumors of 0.5-1.0 degree C when transducers employing annular-focused lenses were used, versus gradients of 2.0-2.3 degree C with 10 mm dia. unfocused transducers.

Biological Effects of Long-Duration, High-Field (4 T) MRI on Growth and Development in the Mouse

The effects of long‐duration, high‐field magnetic resonance imaging (MRI) on fetal growth and postnatal development in mice were studied. Seven experimental groups of pregnant ICR mice were exposed for 9 hours on day 9 and/or day 12 post coitus (pc) to magnetic fields (4 T static, 5 T/sec switched gradient, and 0.2 W/kg radiofrequency at 170 MHz) associated with MRI conditions. Two experimental groups (sham and exposure groups) were exposed to a combination of ultrasound (day 9 pc, 3.25 MHz, focused) and MRI‐associated fields (day 12 pc). No statistically significant changes in fetal growth were observed in the animals exposed to only MRI or ultrasound fields. However, in the combined ultrasound and MRI‐exposed group, the fetal weight and crown‐rump length were reduced compared with the sham and cage controls. These results suggest that MRI and ultrasound exposure well in excess of current clinical conditions can exert biological effects if applied at sensitive stages of fetal development. J. Magn. Reson. Imaging 2000;12:140–149.