Ultra-widefield optical coherence tomography of the human retina

Optical coherence tomography (OCT) has become a standard of care in ophthalmology since it was first demonstrated over 20 years ago. Images acquired using commercial OCT systems have been reported to have a maximum imaging length of only 9 mm, which is equivalent to an internal-field of view of 45◦. This narrow-field limitation has been acceptable because of the high level of clinical data available in the macular region and the relative simplicity of imaging this area of the retina. Commercially-available scanning laser ophthalmoloscope (SLO) systems such as the Optos 200Tx are capable of providing a fundus image with an internal-field of view of 200◦. These systems have shown that significant early disease markers can be found earlier by investigating the retinal periphery. It is therefore clinically desirable to merge the 3D measurement of OCT with the ultra widefield capability of the Optos SLO to allow clinicians to investigate the underlying morphology and progression of disease in the retinal periphery. To meet this clinical need, an ultra-widefield SD:OCT prototype system has been developed using the Optos ellipsoidal-mirror architecture. Modifications were made to the standard SD:OCT system to compensate for off-axis defocus, varying optical path difference, the changing corneal birefringence and the limitations inherent for the Optos ellipsoidal mirrors. The optical performance of the ultra-widefield SD:OCT system was verified using a novel wide-field phantom eye (WPE). The WPE was designed to measure; the transverse and axial point-spread function, field of view, imaging range, sensitivity roll-off, dispersion and measurement accuracy of the ultra-widefield SD:OCT system in both the posterior and peripheral segments of the retina. From these modifications, commercially-viable, ultra-widefield, SD:OCT has been demonstrated and verified using the WPE. In addition, we report the use of the WPE to compare both the imaging performance and measurement accuracy of the following ophthalmic instruments: the Optos 200Tx, Heidelberg Spectralis, Zeiss FF4, Optovue iVue, Zeiss Cirrus and Optos OCT/SLO. The WPE was successful in extracting image performance metrics for imaging feature sizes above 20 μm; however, targets fabricated using 3D-printing will require either a further advancement of the technology or hybridising with higher-precision structures to measure axial and transverse resolution