Background: High resolution imaging is desirable for advanced study and clinical management of retinal diseases. However, spatial resolution of retinal imaging has been limited due to available numerical aperture and optical aberration of the ocular optics. This study is to develop and validate virtually structured detection (VSD) to surpass diffraction limit for resolution improvement in in vivo retinal imaging of awake human.
Methods: A rapid line scanning laser ophthalmoscope (SLO) was constructed for in vivo retinal imaging. A high speed (25,000 kHz) camera was used for recording the two-dimensional (2D) light reflectance profile, corresponding to each focused line illumination. VSD was implemented to the 2D light reflectance profiles for super-resolution reconstruction. Because each 2D light reflectance profile was recorded within 40 μs, the intra-frame blur due to eye movements can be ignored. Digital registration was implemented to further compensate for inter-frame eye movements, before the VSD processing. Based on digital processing, the modulation transfer function (MTF) of the imaging system was derived for objective identification of the cut-off frequency of ocular optics, which is essential for robust VSD processing to ensure reliable super-resolution imaging. Dynamic motility analysis of the super-resolution images was implemented to further enhance the imaging contrast of retinal rod and cone photoreceptors.
Results: The VSD based super-resolution SLO significantly improved image quality compared with equivalent wide-field imaging. In vivo observation of individual retinal photoreceptors has been demonstrated unambiguously. Dynamic motility analysis of the super-resolution images enhanced the contrast of retinal rod and cone photoreceptors, and revealed sub-cellular structures in cone photoreceptors.
Conclusions: In conjunction with rapid line-scan imaging and digital registration to minimize the effect of eye movements, VSD enabled resolution improvement to observe individual retinal photoreceptors without the involvement of adaptive optics (AO). An objective method has been developed to identify MTF to enable quantitative estimation of the cut-off frequency required for robust VSD processing.
Keywords: Optical imaging; ophthalmology; photoreceptor; retina; scanning laser ophthalmoscopy (SLO); super-resolution imaging.
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