Biomedical Engineering Reference
In-Depth Information
Chapter 1
Introduction to Electromagnetic
Brain Imaging
1.1 Functional Brain Imaging and Bioelectromagnetic
Measurements
Noninvasive functional brain imaging has made a tremendous impact in improving
our understanding of the human brain. Functional magnetic resonance imaging
(fMRI) has been the predominant modality for imaging the functioning brain since the
middle of the 1990s. fMRI measures changes in blood oxygenation-level-dependent
(BOLD) signals caused by neuronal activation. It is a noninvasive method that allows
for whole-brainmeasurement and the examination of activity in deep brain structures.
However, fMRI lacks the temporal resolution required to image the dynamic
and oscillatory spatiotemporal patterns associated with activities in a brain. This is
because the BOLD signal, which is only an indirect measure of neural activity, is
fundamentally limited by the rate of oxygen consumption and subsequent blood flow.
Furthermore, since the BOLD signal is only an approximate and indirect measure
of neuronal activity, it might not accurately reflect true neuronal processes. Hence,
to observe neurophysiological processes more directly within relevant timescales,
imaging techniques that have both high temporal and adequate spatial resolution
are needed.
Neurons in the brain function electrically, as well as chemically. Therefore, their
activity generates associated electric and magnetic fields that can be detected outside
the head. Electromagnetic brain imaging is a term intended to encompass noninva-
sive techniques which probe brain electromagnetic activity and properties. Two tech-
niques currently exist for detecting electrical brain activities: electroencephalography
(EEG) and magnetoencephalography (MEG). EEG measures the electric potential
by means of electrodes placed on the scalp, and MEG measures magnetic fields by
means of sensors placed near the head.
In contrast to fMRI, both MEG and EEG directly measure electromagnetic fields
emanating from the brain with excellent temporal resolution of less than 1ms,
and allow the study of neural oscillatory processes over a wide frequency range
(1-600Hz). Because of such high temporal resolution, MEG and EEG can provide
