Two dimensional elasticity mapping of partially cross-linked rabbit corneas using optical coherence elastography Jiasong Li 1, Manmohan Singh 1, Srilatha.

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Two dimensional elasticity mapping of partially cross-linked rabbit corneas using optical coherence elastography Jiasong Li 1, Manmohan Singh 1, Srilatha Vantipalli 2, Zhaolong Han 1, Michael D. Twa 2, and Kirill V. Larin 1,3 1 Department of Biomedical Engineering, University of Houston 2 Departmen of Optometry, University of Houston 3 Department of Molecular Physiology and Biophysics, Baylor college of Medicine SFM 2014

1.Biomechanical properties of cornea Effects of corneal disease Tracking of healing process of cornea Detection of corneal pathologies such as keratoconus 2.General methods to quantify stiffness of cornea Induce a displacement and measure the corneal response 3.Phase resolved method for quantifying surface response Gelatin phantom of different concentrations results Ex vivo results of 2-D spatial mapping of the biomechanical properties of partially CXL rabbit cornea. Overview

Diagnosis Tracking of healing process UV-induced CXL is emerging clinical treatment method: increase the stiffness & reduce further deformation. Outcomes of refractive surgeries such as micro incisions, corneal transplants, etc., Accurate measurements of IOP Corneal Hysteresis M. Tanter, et al., IEEE Trans Med Imaging, 28 (12), (2009) Keratoconus, myopia, iatrogenic keratoectasia: Change of corneal curvature Induces changes in strain distribution stiffness shear modulus bending Why do we have to study biomechanical properties of cornea

Induce a displacement and measure the corneal response 1.Mechanical stimulus 2.Ultrasound 3.Laser pulse 4.Air-pulse 1.MRI 2.B-mode Ultrasound 3.Supersonic shear imaging 4.Acoustic radiation force imaging 5.Brillouin microscopy 6.Electro optical systems (ORA & CorVis) 7.Optical methods including OCT Require tissue displacement on the order of mm. Brillouin microscopy can provide high resolution three dimensional maps of tissue elasticity. However, correlating elasticity parameters such as Young’s modulus via Brillouin microscopy is still an open question. The Ocular Response Analyzer (ORA) and CorVis are commercially available clinical instruments. However, both of ORA and CorVis require a large displacement of the corneal surface. The predictability of these systems are still under investigation. M. Tanter, et al., IEEE Trans Med Imaging, 28 (12), (2009) G. Scarcelli et al., Opt Express, 20(8), (2012). General methods

Induce a displacement and measure the corneal response 1.MRI 2.B-mode Ultrasound 3.Supersonic shear imaging 4.Acoustic radiation force imaging 5.Brillouin microscopy 6.Electro optical systems (ORA & CorVis) 7.Optical methods including OCT General methods

J Li et al., J. Biomed. Opt. 18(12), (Oct 02, 2013). S Wang et al., Laser Physics Letters. 07/2013; 10(7): S Wang et al., Optics Letters. Vol. 39, Issue 1, pp (2014). Our previous study

J Li et al., J. Biomed. Opt. 18(12), (Oct 02, 2013). S Wang et al., Optics Letters. Vol. 39, Issue 1, pp (2014). Center wavelength = 1310nm Bandwidth = 150nm Axial/transverse resolution = 10µm/7.9µm in air Stability = 16 mrad (~ 3nm) System Setup

D. Alonso-Caneiro, et al., Optics Express, 19, (2011). C. Dorronsoro et al., Biomedical Optics Express, 3, (2012). Method

Air-pulse characterization S Wang et al., Optics Letters. Vol. 39, Issue 1, pp (2014).

Gelatin Phantom Result Relaxation rate of 16% gelatin phantom with various excitation air puff pressure J. Li, et al., Laser Physics Letters, Vol. 11, doi: / /11/6/ Displacement increases with the increase of the air-pulse pressure Relaxation rate does not change. Gelatin phantom in higher concentration has faster relaxation rate. Gelatin phantom in lower concentration has larger displacement Relaxation rate of 8%, 12% and 16% gelatin phantom with 1.5 Pa air puff excitation

Gelatin Phantom Result Relaxation rate of 16% gelatin phantom with various excitation air puff pressure Relaxation rate of 8%, 12% and 16% gelatin phantom with 1.5 Pa air puff excitation J. Li, et al., Laser Physics Letters, Vol. 11, doi: / /11/6/ Relaxation rate of gelatin phantoms in various concentrations

Rabbit Cornea preparation and experimental procedure Central brown circle indicates the masked (untreated) area. Yellow points indicate measurement locations. Large blue area shows the extent of the CXL treatment.

Rabbit cornea results (a)3D OCT image. (b) Sample signal. (c) Spatial map of the relaxation rates

Rabbit cornea results The ratio of relaxation rate of the CXL to UT region of 4 samples. It clearly indicates that the CXL and UT region can be differentiated by this method.

Analysis: Experimental: Relaxation rate depends on both elastic and viscous properties of tissue. The influence of viscosity was not considered here, additional studies are needed to investigate the influence of viscosity on the relaxation rate. Sophisticated viscoelastic model is required to translate the observed relaxation rate into mechanical property of tissue, such as elastic and viscous moduli. The air-pulse is shorter than 1 ms, which causes a dynamic response of the tissue and this response could significantly affect the relaxation rate observed in the experiment. Better control the incident angle and distance between the air-pulse port and the surface of the cornea. This will result in a smaller variance of the data and provide more accurate relaxation rate measurements and calculations. Future Work

We have demonstrated that the PhS-SSOCE can be used to monitor and assess the relaxation rate of surface displacement that is induced by a focused air puff system in the tissue-mimicking phantoms and rabbit corneas of different conditions ex vivo. The phantom results clearly show that the stimulation pressure does not change the relaxation rate in the same sample, and the relaxation rates are higher in stiffer gelatin phantoms (higher gelatin concentration). The rabbit cornea results clearly show that the relaxation rate of the cornea is ~1.4 times greater in CXL region. Therefore, with further development, this noncontact optical method can be used to study the stiffness of delicate tissues such as cornea. Conclusions

This study was supported by NIH grant 1R01EY and P30EY Dr. Kirill Larin Associate Professor, UH Dr. Michael Twa Associate Professor, UH Zhaolong Han Manmohan Singh Acknowledgments Srilatha Vantipalli