Page 239 - Computational Modeling in Biomedical Engineering and Medical Physics

228 Computational Modeling in Biomedical Engineering and Medical Physics

Figure 7.4 Assessment of operational characteristics for three magnetic applicator coils (Morega,
2000). (A) Concentrated coil. (B) Double coil (figure of eight shape). (C) Quadruple coil (flower-like
shape). AF, Activating function.

The superiority of the quadruple coil is evident, for the following criteria: the best
26 2
focalization F 5 6.25 3 10 m , the same as for the double coil but half compared
with the single coil, and the highest efficiency, with ER 5 2.08, compared with 1
for the other two configurations. The negative peak of AF is located at the cross-
section of its symmetry axes (x 5 0, y 5 0 in the considered coordinate system) that
ensures more precision and safety to the procedure. It is expected that quadruple coils
with an eccentric position of turns could better concentrate AF at the intersection of
their axes of symmetry, as Zhang and Edrich (1996) illustrates with eccentric, yet con-
centrated coils. They also suggest a different problem that occurs in MS: the uneven
distribution of AF in the depth of the tissue (z-axis direction). For horizontal turns,
the peak values of AF are maxim at the skin surface and decrease (approximately