Biaxial Prestress and Waveguide Effects in Optical-Based Dynamic Elastography of Corneal Phantoms

An ongoing challenge is to increase the scientific knowledge about the relationship between mechanical and structural changes in the cornea, improving those clinically measured optical modifications in pathological conditions and track their development over time. In particular, this is crucial to enhance the detection and management of glaucoma and ectatic corneal disorders such as keratoconus and to comprehend the effects of surgeries like corneal collagen crosslinking (CXL), excimer laser refractive surgery. Hence, advances in this area have the potential to improve diagnosis, enable tailored risk assessment and optimize treatment and surgical planning. Recently, elastography-based imaging methods provided encouraging results, being able to measure non invasively in vivo corneal biomechanical properties. These techniques detect the mechanical wave behavior in soft biological tissues to build back the shear viscoelastic properties that are affected by diseases, injuries or treatments. However, the reconstruction strategies that have been developed so far neglect boundary conditions that characterize the corneal tissue, even if they affect the mechanical wave motion and represent an important confounding factor. To improve current methods, this work collected experimental and computational data using Scanning Laser Doppler Vibrometry (SLDV) as imaging technique and Finite Element Analysis (FEA). The comparison between experiments and simulations helped to better understand the waveguide and prestress effects on complex shear modulus estimates of corneal phantoms. In particular, a new approach called Transformation Acousto Elastography (TAE) was introduced to account for the prestress effect without the need to know its value for the shear modulus calculation. Also, it allows assessing the stress magnitude once the stress-induced deformation and the shear storage modulus are known. Results obtained say that the mechanical properties predictions stay close to each other and within 10% of the expected value taken from the rheological model derived from previous experimental measurements on Ecoflex 00-30. While stress predictions are accurate only if considering an in-plane, biaxial and homogeneous stress field. Given the interesting outcome, this research has been published in the American Society of Mechanical Engineers (ASME) Journal of Engineering and Science in Medical Diagnostics and Therapy, having as authors: Marta Dore, Aime Luna and Dr.Thomas Royston. Some suggestions for the writing of this document have been taken from the above mentioned publication. Elastography-based methods are not yet applied in clinic except for certain liver diseases; but the promising results obtained paved the way to more studies, to iteratively get closer to the real cornea condition proving the efficacy of this method for clinical translation.