G.G. Cook

Effect of 900 MHz Electromagnetic Fields on Nonthermal Induction of Heat-Shock Proteins in Human Leukocytes

Abstract Lim, H. B., Cook, G. G., Barker, A. T. and Coulton, L. A. Effect of 900 MHz Electromagnetic Fields on Nonthermal Induction of Heat-Shock Proteins in Human Leukocytes. Radiat. Res. 163, 45–52 (2005). Despite many studies, the evidence as to whether radiofrequency fields are detrimental to health remains controversial, and the debate continues. Cells respond to some abnormal physiological conditions by producing cytoprotective heat-shock (or stress) proteins. The aim of this study was to determine whether exposure to mobile phone-type radiation causes a nonthermal stress response in human leukocytes. Human peripheral blood was sham-exposed or exposed to 900 MHz fields (continuous-wave or GSM-modulated signal) at three average specific absorption rates (0.4, 2.0 and 3.6 W/kg) for different durations (20 min, 1 h and 4 h) in a calibrated TEM cell placed in an incubator to give well-controlled atmospheric conditions at 37°C and 95% air/5% CO2. Positive (heat-stressed at 42°C) and negative (kept at 37°C) control groups were incubated simultaneously in the same incubator. Heat caused an increase in the number of cells expressing stress proteins (HSP70, HSP27), measured using flow cytometry, and this increase was dependent on time. However, no statistically significant difference was detected in the number of cells expressing stress proteins after RF-field exposure. These results suggest that mobile phone-type radiation is not a stressor of normal human lymphocytes and monocytes, in contrast to mild heating.

Investigating direct detection of axon firing in the adult human optic nerve using MRI.

The aim of this study was to directly detect spectral components of the magnetic fields of ionic currents caused by firing of the axons in the optic nerve in response to visual strobe stimulation. The magnetic field parallel to the main B0 field can potentially alter the local phase and magnitude of the MR signal which can cause signal loss due to intravoxel dephasing. Measured frequency spectra showed evidence of the strobe stimulus localized to regions containing the optic nerve, not thought to be due to motion artifacts, in 30 out of 52 experiments in 5 adult human subjects. The effect was (0.15 +/- 0.05)% of the mean magnitude equilibrium signal from the voxel in the frequency range 0.7-3.3 Hz, corresponding to an estimated field of (1.2 +/- 0.4) nT, at an echo time of TE = 32.4 ms using a 1.5 T MRI scanner. Only 1 of 12 phase image experiments showed effects. These findings provide preliminary evidence for direct detection of axonal firing in the optic nerve.

An Efficient Asymptotic Extraction Approach for the Green's Functions of Conformal Antennas in Multilayered Cylindrical Media

Asymptotic expressions are derived for the dyadic Greens functions of antennas radiating in the presence of a multilayered cylinder, where analytic representation of the asymptotic expansion coefficients eliminates the computational cost of numerical evaluation. As a result, the asymptotic extraction technique has been applied only once for a large summation order. In addition, the Hankel function singularity encountered for source and evaluation points at the same radius has been eliminated using analytical integration.

Comparison of BOLD and direct‐MR neuronal detection (DND) in the human visual cortex at 3T

Direct‐MR neuronal detection (DND) of transient magnetic fields has recently been investigated as a novel imaging alternative to the conventional BOLD functional MRI (fMRI) technique. However, there remain controversial issues and debate surrounding this methodology, and this study attempts clarification by comparing BOLD responses in the human visual system with those of DND. BOLD relies on indirectly measuring blood oxygenation and flow changes as a result of neuronal activity, whereas the putative DND method is based on the hypothesis that the components of the in vivo neuronal magnetic fields, which lie parallel to the B0 field, can potentially modulate the MR signal, thus providing a means of direct detection of nerve impulses. Block paradigms of checkerboard patterns were used for visual stimulation in both DND and BOLD experiments, allowing detection based on different frequency responses. This study shows colocalization of some voxels with slow BOLD responses and putative fast DND responses using General Linear Model (GLM) analysis. Frequency spectra for the activated voxel cluster are also shown for both stimulated and control data. The mean percentage signal change for the DND responses is 0.2%, corresponding to a predicted neuronal field of 0.14 nT. Magn Reson Med 60:1147–1154, 2008.

Modelling of axonal fields in the optic nerve for direct MR detection studies

A number of studies have now shown that direct detection of neuronal firing by MRI may be possible. The optic nerve carries all visual information from the eye to the brain and is a particularly promising target for these measurements. However, it has been assumed that the effects of axonal firing may not be detectable, as a single firing event produces a bipolar waveform of around 1ms duration whose effects should cancel on MR. A simulation of the magnetic modulation which could feasibly be produced by the optic nerve over an extended period and with different firing rates has been developed. The Hodgkin-Huxley equations were calculated for an array of model ganglion cell axons which were assumed to act as voltage to pulse frequency converters. Dependence of the modulating waveform on relative action potential firing start time was investigated. Although the simulated waveforms were bipolar at the beginning, during the period of MR acquisition the different frequencies combine to produce a largely positive waveform. The simulation included only contrast luminance changes and not color, spatial correlations or other more sophisticated processing in the retina. A gradient echo sequence was used at 3T to create images for analysis by the ghost reconstructed alternating current estimation (GRACE) method from phantoms subject to current modulation by the actual modelled axonal waveforms. The optic nerve was also imaged using the same method during visual stimulus by a strobe light in adult human volunteers at 3T. Analysis of digitized video recordings of eye locations during strobe stimulation outside the magnet showed no correlation with the applied strobe frequency over the short duration of the scans. Images of the optic nerves at an echo time of TE=39ms had weak but significant first harmonic ghosts in locations consistent with the applied stimulus as calculated from GRACE theory in just two out of thirteen studies on 10 volunteers and a detection rate of only 15% providing no clear evidence for direct detection in these experiments.

Mapping of periodic waveforms using the ghost reconstructed alternating current estimation (GRACE) magnetic resonance imaging technique

A new method of estimating alternating currents using ghost images created when the magnetic field from a fluctuating current modulates the phase of the magnetic resonance (MR) signal between successive phase‐encode views is described. The method, known as ghost reconstructed alternating current estimation (GRACE), may be useful for directly mapping fields, and hence current impulses produced by neuronal firing events when synchronized periodic modulation can be induced. Images were acquired on a 1.5 T MR system with small oil capsule phantoms and a single wire with an applied alternating current, placed perpendicular to the main field direction. Computer simulations of these experiments yielded ghost images that agreed with experimental results. A simulated ghost image resulting from an evoked neuronal waveform is also discussed. Weak magnetic fields were detected from both sinusoidal and square wave modulations. Magn Reson Med 50:633–637, 2003.

Investigation of axonal magnetic fields in the human corpus callosum using visual stimulation based on MR signal modulation

To investigate the possibility of detecting visually‐evoked axonal currents in the splenium of the human corpus callosum using a 3.0T MRI system.

Investigation of the enhanced efficiencies of small superconducting loop antenna elements

The fields backscattered from arrays of electrically small copper and YBCO superconducting loops are measured and compared with moment method predictions to confirm the gain enhancements and changes in Q factors due to increasing the efficiencies of the antenna elements alone, without the influence of feed and matching networks. Measurements on coplanar and parallel loop arrays show only small increases in the backscattered fields of the YBCO scatterers, with the largest increase in efficiency being for the single loop. The mutual resistance between elements reduces the benefit of adding more elements to a YBCO loop, to a greater extent than when copper elements are added to a copper loop.

An efficient method for attaching thin wire monopoles to surfaces modeled using triangular patch segmentation

A general method is described for attaching thin wires to surfaces segmented with triangular patches which allows significant versatility in their positioning, whilst maximizing computational efficiency by preserving all of the surface segmentation when wires are added or moved, apart from at the connection patch. Variations on the general methodology of single patch subsegmentation are investigated by introduction of convergence metrics and comparisons with controls for input impedances at the monopole bases.

Performance prediction of high Tc superconducting small antennas using a two‐fluid‐moment method model

The radar cross section and Q factors of electrically small dipole and loop antennas made with a YBCO high Tc superconductor are predicted using a two‐fluid‐moment method model, in order to determine the effects of finite conductivity on the performances of such antennas. The results compare the useful operating bandwidths of YBCO antennas exhibiting varying degrees of impurity with their copper counterparts at 77 K, showing a linear relationship between bandwidth and impurity level.