Digital wavefront sensor for depth resolved volumetric aberration measurement

The main objective of the proposed research work is to develop and demonstrate a depth resolved digital wavefront sensor (DWFS) based on partial coherent guide star detection that can provide fast, reliable and automatic depth resolved volumetric aberration measurement in a living human eye. In the proposed approach, a physical guide star is created by focusing the light into the retina with low numerical aperture (NA) and the light reflected back, which usually acquires defocus and higher order aberrations due to different depth layers in the retina as well as the eye optics itself, is detected with high NA. The detection is done interferometrically, using swept source optical coherence tomography (SSOCT) technique, which provides point spread functions (PSFs) corresponding to different layers in retina with both amplitude and phase information. This information allows the use of sub-aperture based digital adaptive (DAO) technique to provide depth resolved wavefront aberration measurement. This technique is automatic, non-iterative, does not require the knowledge of system parameters and specimen, and does not require additional hardware such Shack-Hartmann (S-H) WFS sensor, deformable mirrors etc. which makes the system compact and low cost. Also, a novel method of sampling the PSFs is proposed, in which PSFs are translated over the photodiode with pinhole as the laser band-width is swept. This method makes the WFS compatible with high speed swept source lasers working in hundreds of kHz range, and is not limited by the low frame rate and the low quantum efficiency of 2-D CCD/CMOS cameras as core components of current WFSs. The proposed WFS can deliver data at the rate of upto ~86 volumes per second which can accurately and reliably measure the temporal and spatial dynamics of the ocular aberration in 3-D. Measurement and proper understanding of temporal and spatial variation of ocular aberration is critical for the development of customized refractive surgeries, better design of aberrometers and AO systems, especially for the wide field of view imaging, and customized contact lenses. The wavefront sensor will be combined with motion tracked SS OCT system to obtain digital aberration corrected OCT images of living human retina. A combination of OCT and digital WFS can provide cellular level resolution for better visualization of cone photoreceptors, retinal pigment epithelium, retinal nerve fiber layer, retinal vessel wall and lamina cribrosa in 3-D. This can help in the early diagnosis of several eye diseases such as retinal dystrophy, age related macular degeneration, glaucoma, diabetic retinopathy etc. The research will be carried out at the Center of Medical Physics and Biomedical Engineering department of Medical University of Vienna. Prof. Dr. Rainer A. Leitgeb will be the principle investigator of the project and Dr. Abhishek Kumar will be the post-doctoral researcher responsible for building the system and conducting the experiments.

The structures in the human eye that provide a clear image of the environment on our retina are the cornea and the crystalline lens. However, the quality of the image formation is often reduced due to visual defects (aberrations) or lens opacities (cataracts). Aberrations can be measured using a wavefront sensor, e.g. to plan the surgical correction of the cornea (LASIK) precisely to reduce visual defects, or to select an optimal artificial eye lens for cataract operations. In the project, a new method for wavefront measurement was developed, which potentially works with greater precision than conventional methods. This method is based on the principle of optical coherence tomography (OCT), a method which provides a three-dimensional image of the retina without contact or which measures the length of the eye with high accuracy. The length of the eye is an important parameter for the correct selection of the refractive power of the artificial eye lens in cataract operations. The new method for wavefront measurement digitally calculates the aberrations from measured OCT data. It can thus be integrated directly into any OCT platform and thus optimally measures the eye. This means that the correction of visual defects or the selection of lenses can potentially be carried out more precisely, which can ultimately improve the quality of life of patients. On the other hand, the digital calculation of the wavefront also enables these errors in OCT imaging to be corrected in post-processing. This method is called digital aberration correction and allows to vizualise smallest cellular structures of the retina, such as individual photoreceptor cells. Such a procedure is important for the early detection of eye diseases as well as for the general understanding of serious diseases such as age-related macular degeneration (AMD), diabetic retinopathy, or glaucoma. In addition, this method can also support new types of therapy such as cell and gene therapies. These procedures were developed in the frame of the project and must now be validated in follow-up clinical studies.