title: A finite element head and neck model as a supportive tool for deformable image registration.
abstract: OBJECTIVE
  A finite element (FE) head and neck model was developed as a tool to aid investigations and development of deformable image registration and patient modeling in radiation oncology. Useful aspects of a FE model for these purposes include ability to produce realistic deformations (similar to those seen in patients over the course of treatment) and a rational means of generating new configurations, e.g., via the application of force and/or displacement boundary conditions.
  
  
  METHODS
  The model was constructed based on a cone-beam computed tomography image of a head and neck cancer patient. The three-node triangular surface meshes created for the bony elements (skull, mandible, and cervical spine) and joint elements were integrated into a skeletal system and combined with the exterior surface. Nodes were additionally created inside the surface structures which were composed of the three-node triangular surface meshes, so that four-node tetrahedral FE elements were created over the whole region of the model. The bony elements were modeled as a homogeneous linear elastic material connected by intervertebral disks. The surrounding tissues were modeled as a homogeneous linear elastic material. Under force or displacement boundary conditions, FE analysis on the model calculates approximate solutions of the displacement vector field.
  
  
  RESULTS
  A FE head and neck model was constructed that skull, mandible, and cervical vertebrae were mechanically connected by disks. The developed FE model is capable of generating realistic deformations that are strain-free for the bony elements and of creating new configurations of the skeletal system with the surrounding tissues reasonably deformed.
  
  
  CONCLUSIONS
  The FE model can generate realistic deformations for skeletal elements. In addition, the model provides a way of evaluating the accuracy of image alignment methods by producing a ground truth deformation and correspondingly simulated images. The ability to combine force and displacement conditions provides flexibility for simulating realistic anatomic configurations.
authors.affiliation: Department of Radiation Oncology, Massachusetts General Hospital, Boston, MA, 02114, USA. jkim115@mgh.harvard.edu.
authors.forename: Jihun
authors.identifier: http://orcid.org/0000-0003-4856-6305
authors.initials: J
authors.lastname: Kim
authors.affiliation: Department of Mechanical Engineering, University of Michigan, Ann Arbor, MI, 48109, USA.
authors.forename: Kazuhiro
authors.initials: K
authors.lastname: Saitou
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 48109, USA.
authors.forename: Martha M
authors.initials: MM
authors.lastname: Matuszak
authors.affiliation: Department of Radiation Oncology, University of Michigan, Ann Arbor, MI, 48109, USA.
authors.forename: James M
authors.initials: JM
authors.lastname: Balter
mesh_terms: D000465:Algorithms; D054893:Cone-Beam Computed Tomography / Q000379:methods*; D020342:Finite Element Analysis; D006257:Head / Q000000981:diagnostic imaging*; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging*; D006801:Humans; D008962:Models, Theoretical; D009333:Neck / Q000000981:diagnostic imaging*
keywords: Deformation; Finite element method; Head and neck; Image registration; Radiation therapy