Biomedical Engineering Reference
In-Depth Information
a second. Each scan delivers an independent measurement of
the instantaneous magnetic field just outside the head. Although
SQUID technology is now mature providing turn-key systems
for whole-head MEG devices, its basic cryogenic requirements
make it very expensive. In recent years, atomic magnetometry,
a method of measuring magnetic fields based on the interaction
of resonant light with atomic vapor, has emerged as a potentially
cheaper alternative to SQUID based sensors
(15)
.
The second requirement, separating the signal of interest
from the larger ambient background and other interfering sig-
nals, is achieved by a combination of passive shielding, use of
gradiometer design either in hardware for the sensing coils cou-
pled to the SQUIDS or in software using additional reference
channels. Other signal processing techniques, e.g. Independent
Component Analysis (ICA)
(16)
coupled to the use of informa-
tion from auxiliary channels like the electrooculogram (EOG) and
electrocardiogram (ECG) can effectively eliminate biological and
other artifacts. The combination of the exquisite SQUID sensi-
tivity with these hardware and software methods allows the mea-
surement of the magnetic field generated by the brain with little
contamination.
In a modern MEG system, typically a few hundred sensing
coils, each coupled to its own SQUID, are housed at the bot-
tom of the helmet-shaped Dewar, distributed so that they capture
evenly the magnetic field just outside the head. The magnetic
field for just one “timeslice” can be mapped by recording the
signal from each sensor independently from, and for all practi-
cal purposes simultaneously with, the signal of each other sensor.
In one second, a few thousand such timeslices can be recorded
so that successive timeslices provide a movie of the instantaneous
change in the magnetic field just outside the head. Since, as we
will shortly describe, the speed of propagation from the genera-
tors to the sensors is the speed of light, the MEG signal change
corresponds to the instantaneous change of the electrical current
density in the brain generated by neuronal activity. The peaks of
the signal generated by the brain are about two orders of magni-
tude higher than the device noise level, so the map of the mag-
netic field not only has exquisite time resolution (a fraction of a
millisecond) but is also a very clean map of the topography of the
magnetic field just outside the head.
3. Forward and
Inverse Problem
The determination of the EEG and MEG signal from the knowl-
edge of the sources, the electrical properties of their biologi-
cal environment and the configuration of the measuring devices
