Biomedical Engineering Reference
In-Depth Information
net impressed current from nearby large pyramidal neurons will
tend to sum up constructively. It is very likely that a large part
of the MEG signal is indeed due to slow PSPs in the apical den-
drites of pyramidal neurons, especially at frequencies well below
100 Hz. For this standard mechanism, typical estimates require
about a million synapses to be simultaneously active to produce
a measurable MEG signal
(6)
. MEG activity at frequencies well
above 100 Hz is likely to be produced by synchronous action
potentials
(14)
.
Although the MEG signal is generated by the collective activity
of a large number of neurons, its strength is extremely weak com-
pared to typical terrestrial magnetic fields. The earth's magnetic
field is about a billion times as strong, while the usual urban envi-
ronment at frequency ranges that overlap the ones of interest in
MEG is still many orders of magnitude higher than the strongest
MEG signal from a normal human brain. A pre-requisite for use-
ful MEG measurements is therefore the availability of sensors that
can detect the weak magnetic fields generated by the brain. Also
required are methods that can exclude the large ambient fields and
tools that can separate out the signal of interest from any remain-
ing interfering signals from the environment and other signals
generated by the body of the subject that are often considerably
stronger than the signal of interest.
The basic MEG measurement relies on the detection of the
electrical current in a small loop of wire, typically about one
centimeter across, induced by the change in the magnetic field
component perpendicular to the loop surface. Measurement of
the induced current determines the value of the change in the
magnetic field. Usually a set of coils is arranged as a gradiome-
ter to emphasize nearby signals from the brain at the expense
of distant sources. The detection of the minute magnetic field
changes outside the head generated by electrical currents in
the brain is measured by coupling the coil or gradiometer to
an extremely sensitive “superconducting quantum interference
device” (SQUID). SQUIDS, as the name implies, rely for their
exquisite sensitivity on superconductivity and together with their
sensing coils must be kept at extremely low temperatures, just a
few degrees above absolute zero. To achieve this, sensing coils and
SQUIDS are kept in a thermos-like container, the Dewar, which
under normal operating conditions is filled with liquid helium. In
modern systems, the bottom of the Dewar is shaped into a helmet
with well over one hundred, nowadays a few hundred, sensing
coils evenly distributed on its inner surface. Just a few millimeters
away, on the other side of the insulating layer, at normal room
temperature, a subject can safely place his/her head inside the
helmet. Each sensing coil samples the local magnetic field and the
full set of sensing coils can be “scanned” a few thousand times
2.2. Recording the
MEG Signal and
Isolating the
Contribution from the
Brain
