Illuminating the vitreous for shear wave optical coherence elastography: a feasibility study

Abstract

Measuring the elasticity of the eye in vivo is valuable for diagnosing and monitoring eye-related diseases, in particular, the liquefaction of the vitreous humour which occurs with age and certain pathologies. Elasticity can be determined by elastographic techniques which map mechanical properties with a method of three parts: loading, imaging and reconstructing.We propose a shear wave based optical coherence elastography (OCE) method which generates shear waves with laser radiation, and images the waves with optical coherence tomography (OCT), thus reconstructing the mechanical properties from extraction of the shear wave velocity. A proof-of concept study was conducted to investigate laser radiation as a noncontact and non-invasive loading method to generate waves within the vitreous humour. With an all-optical photoacoustic system, we successfully imaged the ocular interfaces of the bovine eye with particular interest in waves generated at the vitreouslens interface. Wavelengths in the range of 740 nm and 980 nm proved to be suitable in exciting the vitreous, as sufficient energy was transmitted to the vitreous, which successfully enabled wave generation. In contrast, longer wavelengths were highly absorbed by superficial structures before the vitreous. In addition, we developed a method which successfully measures the compressional wave velocity through the vitreous. With measurements of the surface wave velocity the bulk mechanical properties of the vitreous can also be determined. Furthermore, an existing phase-sensitive OCT system was developed into an OCE system with the potential to measure shear waves. Lastly, we investigated the refractive indices, n, of healthy and anoxic multicellular spheroids, grown from HCT116 human colon cancer cells. Knowledge of the refractive index is important in optical imaging modalities, such as confocal microscopy, since refractive index mismatches between sample and immersion liquid results in an undesirable reduction in imaging depth. With the measured refractive indices of the spheroids, two immersion liquids, glycerol (n=1.473) and ScaleView-A2 (n=1.380) were chosen, in which, ScaleView-A2 both preserved spheroid structure and provided a greater imaging depth of at least 400 μm in comparison to depths at around 231 μm with the initial immersion liquid, phosphate-buffered saline, which strongly confirms our refractive index measurements.