Biomedical Engineering Reference
In-Depth Information
and the two adjacent electrodes are used as reference electrodes. Although these stim-
ulation modes are very promising, so far it has not been possible to show benefits in
terms of sound quality and speech intelligibility in CI users [
1
]. One possible reason
to explain these results is the fact that the amount of spiral ganglion cell survival and
the degree of dendrite degeneration might also contribute to sound perception. For
example, if a narrower field is applied to a region of the cochlea with a low number
of active neurons, the current will have to be increased to reach neighboring neurons
and the spread of excitation will become automatically wider.
FEM simulations provide insight into the coupling between the electrical field
and the neural response and hopefully can give more insight on understanding the
individual characteristics of each cochlear implant for different stimulation modes.
In this paper, we also use a 3D finite element model of the cochlea to further
study the effects of electrical spread of excitation for different electrode positions
and cochlear sizes using different stimulationmodes (monoplar, bipolar and tripolar).
The chapter is organized as follows: first, the methodology followed to build a 3D
model of the cochlea and the FEM to simulate different cochlea geometries, electrode
configurations and electrode positions is presented; next the results on the simulations
are given. Finally, a discussion and conclusions based on the results are given.
2 Methods
In this section we present the methodology to construct the electrical field model
of the cochlea. First, the geometry of the cochlea is constructed; second, material
properties to each domain of the geometry are given; and third, the electrical field is
computed using FEM.
2.1 Geometry
The cochlear geometry was constructed from histological data [
23
] from a single
human cochlea (Fig.
1
). From this data we followed a very similar procedure as
in [
16
] to create the geometry. The cochlear structure was traced using Autodesk
Inventor ©. In a first step, the shapes of the compartments were approximated by
polygons with a relatively low number of key points (Fig.
3
). In a second step the
same structures were repeated in planes every 30
ⓦ
around the vertical axis (Y) fitting
the histological data. As a result, we obtained a cochlea similar to the one presented
in (Fig.
3
).
One of the advantages of the method used to generate the cochlea is that it can
be resized and adapted to individual cochlear shapes. Figure
3
b presents a resized
cochlea adapted to clinical CT scans of the temporal bone before implantation. The
cochlear resizing is achieved by adapting the largest distance from the round window
to the lateral wall (distance A) and the perpendicular distance (B) as defined in [
5
].
