Correlations Between Mechanical and Structural Anisotropy: A Foundation for Non-Invasively Assessing Radiation Injury
Y Feng*, R Okamoto, D Du, S Mutic, G Genin, P Bayly, Y Hu, Washington University in St Louis, St Louis, MOSU-E-J-201 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose: Tour long-term goal is to develop techniques for evaluating radiation injury based upon mechanical changes to tissue. Here, we study the mechanical response of white matter ex vivo using dynamic shear testing (DST) and indentation testing in an animal model, with the goals of establishing in normal (non-radiation-injured) tissue (1) a baseline degree of mechanical anisotropy, and (2) correlations between mechanical and structural anisotropy.
Methods: Fresh whole brains were acquired from local slaughter house. White matter samples were dissected and from the central corpus callosum region, while gray matter samples were acquired from temporal region. Dynamic shear testing and indentation testing were applied to both white and gray matter samples. White matter samples were tested in shear with axonal fibers aligned or perpendicular to the shear force direction, and in indentation with the fiber axis parallel or perpendicular to the long side of the indenter head. Gray matter samples were tested in an arbitrary direction and rotated 90 degrees for the second test, for each testing procedure. Shear modulus and indentation stiffness were calculated for each sample.
Results: The storage and loss moduli of the white matter samples were significantly larger when the samples were tested with primary axonal fibers aligned with the shearing direction. The indentation stiffness was also significantly higher when indented perpendicular to the fiber direction. No significant differences were observed for gray matter samples for both testing procedures.
Conclusion: The results confirm that white matter exhibits larger moduli when stretched or sheared along the fiber direction; mechanical anisotropy correlates with structural anisotropy. The mechanical isotropy observed in gray matter is consistent with the structural isotropy. These results will be useful for investigations of changes in mechanical properties after radiation therapy in cerebral tumor patients, and for numerical modeling of surgery and traumatic brain injury (TBI).