Biomedical Engineering Reference
In-Depth Information
11.2 Subject Specific Orofacial Modeling
One of the basic design decisions that we employed in our approach to orofacial
modeling was to create a workflow for generating subject-specific face, jaw tongue,
skull and hyoid bone models. Subject specificity is important for a number of rea-
sons. Validation of orofacial simulations can be made directly with experimental data
from the same subject to which the model is matched. Also, a number of biomed-
ical applications require patient-specific models. Our approach to subject specific
modeling involves adapting a set of reference models to a specific subject based on
medical imaging data and other clinical measurements. Our subject-specific work-
flow involves two main components: morphology and material properties.
11.2.1 Subject Specific Morphology
Subject specific morphology involves creating a model with anatomical size and
proportions matched to a subject. For whole-body musculoskeletal modeling this
typically involves an overall scaling of a generic model to a specific subject [
1
]. For
our purposes with a face and vocal tract model, we require a more detailed type
of subject-specific morphology, whereby the shapes of individual bones, muscles,
ligaments, and other structures are matched to a subject. This is achieved by adapting
the shape of the model's anatomical structures to medical imaging data of a specific
subject.
Our workflow for a heterogeneous model, such as the face-jaw-tongue system,
involves creating reference models for each model sub-component, adapting the
morphology of each sub-component to fit medical imaging data for a single subject
and then dynamically attaching the sub-components. In this section, we discuss the
reference models for the face, skull, jaw, tongue, and hyoid bone as well as the
adaptation process for morphing the reference models into an integrated subject-
specific model.
11.2.1.1 Reference Face Model
The reference face model was manually built from a CT dataset of a male subject and
has been described in detail elsewhere [
20
]. This FE model is based on a hexahedral
mesh that was carefully constructed to control element quality (such as Jacobian
ratio), midsagittal symmetry, and the density of elements such that more elements
exist in regions of the face that are known to deform to a greater extent (Fig.
11.1
).
The mesh includes three layers of elements from superficial to deep. In total, the
model includes 6342 hexahedral elements. In this reference model, all layers use an
isotropic material, however in the revised model we have implemented an anisotropic
passive material in the most superficial layer representing the epidermis and dermis
(as described below in the
Anisotropic in-vivo measurements
section).
