Retinal Diagnostic Instruments with Adaptive Optics
Supervisor: [ Dr. Andrey V. Larichev ]

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One of the most popular techniques for clinical investigation of the retina is retinal imaging. It is usually performed by specialized instrument - [ fundus camera ]. The conventional fundus cameras work with an input pupil of about 2mm to reduce the negative influence of eye aberrations. However, using a larger input pupil and introducing adaptive correction of aberrations, it is possible to achieve the resolution of the cell level range. The first results of such adaptive correction were presented by [ Prof. D.Williams and co-workers ]. The information content of fundus images can be further enhanced by spectral-resolved photography called multi- or hyper-spectral imaging.

With a high-resolution spectral imager, a broad range of eye diseases that represent the leading causes of visual impairment and/or blindness can be studied in-vivo. For example, in diabetes, a microvascular disease that causes retinopathy, the proposed device may enable the investigation of detail changes in individual capillary beds that occur early in the disease within the inner retina. At spatial resolutions that will provide unprecedented detail of capillary beds (5-10 microns), one will be able to make measurements of oxyhemoglobin saturation and detect ischemic regions. This resolution is nearly ten times better than can be achieved by existing fundus cameras which can not compensate for the unique aberrations in each patient's eye.

Together with [ Kestrel Corporation ] (Albuquerque, US) our group was involved in the development of the SRSFI (Super Resolution Spectral Fundus Imager) system. The work was supported by NATO SfP Grant #974292.

The basic layout of the system is shown in Figure 1.

The system includes a Shack-Hartmann wavefront sensor, a bimorph adaptive mirror with 18 electrodes, a hi-resolution digital camera (3000x2000 pixels, 12 bit), and a multispectral light source with 8 bands (from 80 to 8nm). A more detailed description of the system can be found in US Patents: US patent #US 6331059 B1 and US patent #US 2002097377 A1.

A snapshot of the system is shown in Figure 2.

The system works with an input pupil of 5mm in diameter and the typical residual error of correction is 0.1mcm (RMS). The spatial resolution of the system is 6mcm on the retina (limited by the CCD sensor). The angular field of view is 15x20 deg. A more detailed technical specification of the device can be found [ here ].

Two SRSFI systems have been built in the framework of the NATO SfP Grant #974292. The systems are almost identical. The first has been installed in Albuquerque (Kestrel Corp.) and the second in Moscow (Medical Physics Department, Faculty of physics, Moscow State University).
An example of a retinal image taken by the SRSFI system is shown in Figure 3 (only a portion of the larger image is presented).

Presently, the second generation of the SRSFI instruments (SRSFI-II) is being developed (see Figure 4). The new instrument has a more advanced wavefront sensor and adaptive optics control with a 77Hz loop rate. The stroke of the bimorph mirror is increased up to 36 microns. A large-format 16 MPix data camera with a high QE sensor from Kodak has been integrated into the system. This makes it possible to increase the spatial resolution by 30% without limiting the angular FOV. A more detailed information about the SRSFI-II can be found [ here ].

An SRSFI-II system is installed in the [ Research Institute of Eye Diseases ], Moscow, Russia, where limited clinical trials of utility of the SRSFI for diagnostics and management of several retinal diseases are being carried out.

The main focus of the current research is now on methods of image analysis and restoration under angular anisoplanatism conditions common for the human eye. Starting from our earlier [ work ] we developed several methods for extending the FOV in human eye adaptive imaging. The scanning reference source employed in the SRSFI-II extends the FOV by 30-50%. This approach is quite similar to the multiple beacons AO. At the same time, wide-angle [ noniterative blind deconvolution ] helps to further extend the system FOV. As a result, the compensated FOV of the SRSFI-II is about 15 deg.

Selected publications:

• A. Larichev, P. Ivanov, I. Irochnikov, V. Shmalhauzen, L.J.Otten, Adaptive system for eye-fundus imaging, Quantum Electronics, 32, №10 (2002)
• A. Larichev, P. Ivanov, I. Irochnikov, S.C. Nemeth, A. Edwards, P. Soliz, High Speed Measurement of Human Eye Aberrations with Shack-Hartman Sensor. [ARVO Abstract], Invest Ophthalmol Vis Sci., 42 (2001) 897
• A.V.Larichev, P.V.Ivanov, I.G.Irochnikov, V.I.Shmal'gauzen, Measurement of eye aberrations in a speckle field, Quantum Electronics, 31 (2001) 1108.
• P. Fournier, G. R. G. Erry, L. J. Otten, A. Larichev, N. Irochnikov, A Next Generation High Resolution Adaptive Optics Fundus Imager. (article)
• P. Fournier, G. R. G. Erry, L. J. Otten, A. Larichev, N. Irochnikov, A Next Generation High Resolution Adaptive Optics Fundus Imager. (presentation)
• A.V. Larichev, J.J. Otten, N.G. Irochnikov, P. Soliz, G.R.G. Erry, V.Y. Panchenko, SuperRez-II adaptive multispectral fundus imager, SPIE, 6138-38 V.1