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Electrical conductivities of compact, spongiosum, and bulk layers of the live human skull were determined at varying frequencies and electric fields at room temperature using the four-electrode method. Current, at higher densities that occur in human cranium, was applied and withdrawn over the top and bottom surfaces of each sample and potential drop across different layers was measured. We used a model that considers variations in skull thicknesses to determine the conductivity of the tri-layer skull and its individual anatomical structures. The results indicate that the conductivities of the spongiform (16.2-41.1 milliS/m), the top compact (5.4-7.2 milliS/m) and lower compact (2.8-10.2 milliS/m) layers of the skull have significantly different and inhomogeneous conductivities. The conductivities of the skull layers are frequency dependent in the 10-90 Hz region and are non-ohmic in the 0.45-2.07 A/m2 region. These current densities are much higher than those occurring in human brain.

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A system that simultaneously measures magnetoencephalography (MEG) and nuclear magnetic resonance (NMR) signals from the human brain was designed and fabricated. A superconducting quantum interference device (SQUID) sensor coupled to a gradiometer pickup coil was used to measure the NMR and MEG signals. 1 H NMR spectra with typical Larmor frequencies from 100 -1000 Hz acquired simultaneously with the evoked MEG response from a stimulus to the median nerve are reported. The single SQUID gradiometer was placed approximately over the somatosensory cortex of a human subject to noninvasively record the signals. In this article we describe an instrument for and results from simultaneous recording of an evoked magnetoencephalography (MEG) response and nuclear magnetic resonance (NMR) proton signal from the human brain using a superconducting quantum interference device (SQUID) sensor.MEG is a noninvasive technique that measures magnetic fields at the surface of the head that are the direct consequence of neuronal activity in the living brain (1). MEG requires the use of SQUID sensors to measure the extraordinarily low-level magnetic fields, usually in the range from 10 -15 to 10 -12 T, produced by neuronal activity in the brain. While other functional imaging modalities such as functional MRI depend on the relatively slow and indirect hemodynamic response of the brain, MEG (and electroencephalography, or EEG) can provide measurement of the electromagnetic fields arising from the actual neuronal currents with submillisecond temporal resolution.The inverse problem of source localization from MEG (and EEG) is ill posed. These procedures require models of source current distribution and of the effects of geometry and inhomogeneous conductivity within the head volume conductor. Neural electromagnetic data are often combined with anatomical or functional tomographic data obtained with high-field MRI and functional MRI (fMRI).MEG and MRI data are taken with two completely separate systems because the SQUIDs cannot operate in the large magnetic fields (up to 15 orders of magnitude larger than an MEG signal) required by high-field MRI systems. This necessitates colocalization and data fusion techniques to integrate and superpose the two data sets in an accurate and meaningful way.Recording NMR signals in low fields (B Ͻ 10 -5 T) opens up the possibility of acquiring tomographic images simultaneously with high temporal resolution measurements of MEG, as the magnetic fields required for imaging are now compatible with SQUID sensors. An additional motivation for low-field NMR spectroscopy and MRI is the prospect of reduced system cost and size. For a fixed relative inhomogeneity, broadening of the NMR line scales linearly with the strength of the measurement field, providing very narrow NMR lines (limited by the natural linewidth) at ultralow magnetic fields, and raising the possibility of MRI at lower gradients. Susceptibility artifacts caused by spurious field gradients from the sample, which also broaden resonance lines, are si...

Abstract-A seven-channel system capable of performing both magnetoencephalography (MEG) and ultra-low-field magnetic resonance imaging (ULF MRI) is described. The system consists of seven second-order SQUID gradiometers with 37 mm diameter and 60 mm baseline, having magnetic field resolution of 1.2 -2.8 fT/√Hz. It also includes four sets of coils for 2-D Fourier imaging with pre-polarization. The system's MEG performance was demonstrated by measurements of auditory evoked response. The system was also used to obtain a multi-channel 2-D image of a whole human hand at the measurement field of 46 microtesla with 3 by 3 mm resolution.