Biomedical Engineering Reference
In-Depth Information
tour map. Otherwise, the interpolation of the value at the interior points of
each triangle can be applied in order to obtain a smoother contour map.
If a source of the magnetic field is an ideal single current dipole, two
extrema, called an inhalant and an exhalant, appear on the field contour
map. When this symmetric pattern as shown in Fig. 3.48a, which is called a
dipole-like distribution, is observed on the field contour map, the location of
the magnetic source can be roughly estimated intuitively.
In the case of Fig. 3.48b, a current dipole can be estimated using the
approximation that the surface of the observation object is flat and that
the sensors are arranged on a plane. The current dipole is located on the
perpendicular bisector to the segment that connects the two peaks at a depth
d
,givenby
L
√
2
,
d
=
(3.100)
where
L
isthedistancebetweenthetwopeaks.Themagnitudeofthedipole
q
is
q
=
6
√
3
πd
2
μ
0
b
max
,
(3.101)
where
μ
0
is permeability of the air and
b
max
is the magnitude of the magnetic
field at the peaks. The direction of the dipole satisfies the right-hand screw
rule.
Vector Map.
A field contour map is the most simple way to visualize the
distribution of the magnetic field, when only the radial component from the
object is considered. However, in the case of three-dimensional vector magne-
tometry, not only the radial component but also the tangential component
should be considered. A vector map is a suitable way to express the distri-
bution of the tangential component of the field. An example of a vector map
is shown in Fig. 3.49.
Extraction of Head and Brain Volume from MRI.
Because of the di-
culty of the MEG inverse problem, it is important to use as much information
as possible to restrict the search only to the solutions that are biologically
consistent [42-44]. One way to achieve this goal is to use magnetic resonance
images (MRI) to obtain information about the head and brain volume. For
example, this MRI information could be used to discard solutions located
outside the brain, to favor certain parts of the brain, or to force the dipole
orientations to follow certain constraints. Also, the 3D representation of the
head and of the brain could also be used as input for realistic modeling of
the forward fields. And, of course, this MRI information can also be used to
construct three-dimensional representations of the head to better visualize
the pattern of the neuromagnetic sources and their evolution with time.
