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The subject of this article is detection of brain magnetic fields, or magnetoencephalography (MEG). The brain fields are many orders of magnitude smaller than the environmental magnetic noise and their measurement represent a significant metrological challenge. The only detectors capable of resolving such small fields and at the same time handling the large(More)
In order to obtain adequate signal to noise ratio (SNR), stimulus-evoked brain signals are averaged over a large number of trials. However, in certain applications, e.g. fetal magnetoencephalography (MEG), this approach fails due to underlying conditions (inherently small signals, non-stationary/poorly characterized signals, or limited number of trials).(More)
Channel count in modern MEG systems has been steadily increasing, but are more channels necessary? Assuming that the spatial sampling considerations are satisfied, this question can be answered by examining the MEG system's ability to localize and resolve brain sources. For the simple situation where only uncorrelated sensor noise is present, dipole(More)
Fetal magnetoencephalogram (fMEG) is measured in the presence of a large interference from maternal and fetal magnetocardiograms (mMCG and fMCG). This cardiac interference can be successfully removed by orthogonal projection of the corresponding spatial vectors. However, orthogonal projection redistributes the fMEG signal among channels. Such redistribution(More)
Development of the CTF MEG system has been advanced with the introduction of a computer processing cluster between the data acquisition electronics and the host computer. The advent of fast processors, memory, and network interfaces has made this innovation feasible for large data streams at high sampling rates. We have implemented tasks including(More)
The Hilbert phase phi(t) of a signal x(t) exhibits slips when the magnitude of their successive phase difference |phi(t(i+1))-phi(t(i))| exceeds pi. By applying this approach to periodic, uncorrelated, and long-range correlated data, we show that the standard deviation of the time difference between the successive phase slips Deltatau normalized by the(More)
We present a new method for improving the signal-to-noise ratio (SNR) of event-related fetal MEG signals based upon the SAM minimum-variance beamformer. SAM could separate the evoked response source activity from the remaining fMEG signal and interference if the evoked response source coordinates and forward model were know. However, this requires knowledge(More)
It appears to be clear from the results that the third order gradiometer is able to detect small biomagnetic signals from the brain which are related to evoked potentials and spontaneous electrical activity. The instrument operates reasonably well within a noisy environment, however further development is necessary to balance the first gradient. We intend(More)
Fetal brain signals produce weak magnetic fields at the maternal abdominal surface. In the presence of much stronger interference these weak fetal fields are often nearly indistinguishable from noise. Our initial objective was to validate these weak fetal brain fields by demonstrating that they agree with the electromagnetic model of the fetal brain. The(More)
Flash-evoked responses can be recorded from the fetus in utero. However, a standard analysis approach based on orthogonal projection (OP) to attenuate maternal and fetal cardiac signals leads to a spatial redistribution of the signal. This effect prevents the correlation of source location with a known fetal head location in some cases and the(More)