Feature of The Week 12/14/14: Penetrate Deeper Using Extended Source OCT
It is an unmet clinical need in ophthalmic imaging to visualize deep tissue structures such as lamina cribrosa, sclera in the optic nerve head. Better penetration requires better sensitivity. The sensitivity of an existing ophthalmic OCT system is mainly limited by the maximum permissible radiant exposure (MPE) imposed by the laser safety standards. OCT devices operating with an extended source can potentially improve imaging sensitivity because higher MPE is allowed for an extended source than that of a point source. Therefore, an extended source OCT technology will effectively enhance the sensitivity thereby increasing the penetration depth.
In this work, we developed a spectrally encoded extended source OCT (SEES-OCT) technique that allows significantly higher MPE than that provided by commercial ophthalmic OCT systems. In a SD-OCT system, a line illumination can be produced by dispersing the sample beam using a prism. The transverse beam distribution is encoded in the wavelength so that transverse resolution is the same as that of a point source system. The prism can be replaced using a mirror which restores the system back to the point scan mode. Imaging was performed using optic nerve head (ONH) tissues from enucleated swine eyes ex vivo and human skin in vivo to demonstrate a sensitivity advantage of 5-dB over the corresponding point source system without significant degradation in spatial resolution. The hardware system of SEES-OCT device is almost the same as the current point source system except for one additional element in the infinity space. Therefore, any existing ophthalmic OCT system can be easily modified so that it can be switched between the point source mode and the extended source mode through a simple switch mechanism. Based on its merits in sensitivity and flexibility, this technique may help visualize deep ocular tissue structures which are at risk of damage in glaucoma such as the lamina cribrosa, the posterior sclera, and the retro-laminar tissues.