Biomedical Engineering Reference
In-Depth Information
was generated by 20 current elements of the model. The thresholds of the
stabilized condition were as follows:
dt
=0
.
5and
θt
=90
◦
. The results of the
estimation were obtained after approximately 100 000 iterations.
Discussion 2.
The assumed volume current source can be described with
the proposed algorithm quite well (Fig. 3.43), although the source is not esti-
mated correctly with the least-square norm only (Fig. 3.45). There are five
pairs of elements facing each other. Such pairs produce a very small magne-
tic field as if no current element existed. In this situation, any motion of the
element increases the value of the evaluation criterion. Thus, the estimated
distribution is not improved in this case. When the evaluation criterion is
the least square norm only, it frequently happens that two current elements
are close together and face each other. The proposed algorithm, however, is
capable of describing a volume current source by introducing current con-
tinuity as a stabilizing factor. The assumption of current continuity shown
in Fig. 3.42 appears to be reasonable, even when this algorithm is applied
to biomagnetic measurements. However, to apply this algorithm to complex
magnetic source distributions, it is necessary to introduce suitable stabilized
conditions which depend on the characteristics of the subject.
3.3.3
Visualization
Field Contour Map.
A contour map is the illustration of the variation
of
b
=
f
(
G
), which is valuable on a surface
G
,bycontoursasaresultof
connection of the points that have the same value of
b
.
Fig. 3.42.
The stabilized condition for optimizing the distribution of current ele-
ments.
a
The situation in which the current flow is continuous and smooth.
b
The
definition of smooth current flow. The distance
d
betweentwoelementsislessthan
the threshold
dt
, and the angle
θ
between them is less than the threshold
θt
.In
this case, the pair is allowed to stay in its present position.
c
The definition of
a nonsmooth current flow. The angle
θ
is larger than
θt
. In this case, the pair is
rejected
