In vivo super-resolution retinal imaging through virtually structured detection

Virtually structured detection enables super-resolution ophthalmoscopy of rod and cone photoreceptors in human retina

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.

Super-resolution ophthalmoscopy: Virtually structured detection for resolution improvement in retinal imaging

Quantitative retinal imaging is essential for advanced study and clinical management of eye diseases. However, spatial resolution of retinal imaging has been limited due to available numerical aperture and optical aberration of the ocular optics. Structured illumination microscopy has been established to break the diffraction-limit resolution in conventional light microscopy. However, practical implementation of structured illumination microscopy for in vivo ophthalmoscopy of the retina is challenging due to inevitable eye movements that can produce phase artifacts. Recently, we have demonstrated the feasibility of using virtually structured detection as one alternative to structured illumination microscopy for super-resolution imaging. By providing the flexibility of digital compensation of eye movements, the virtually structured detection provides a feasible, phase-artifact-free strategy to achieve super-resolution ophthalmoscopy. In this article, we summarize the technical rationale of virtually structured detection, and its implementations for super-resolution imaging of freshly isolated retinas, intact animals, and awake human subjects.

Functional optical coherence tomography of retinal photoreceptors

IMPACT STATEMENT

Retinal photoreceptors are the primary target of age-related macular degeneration (AMD) which is the leading cause of severe vision loss and legal blindness. An objective method for functional assessment of photoreceptor physiology can benefit early detection and better treatment evaluation of AMD and other eye diseases that are known to cause photoreceptor dysfunctions. This article summarizes in vitro study of IOS mechanisms and in vivo demonstration of IOS imaging of intact animals. Further development of the functional IOS imaging may provide a revolutionary solution to achieve objective assessment of human photoreceptors.

In vivo super-resolution imaging of transient retinal phototropism evoked by oblique light stimulation

Rod-dominated transient retinal phototropism (TRP) has been observed in freshly isolated retinas, promising a noninvasive biomarker for objective assessment of retinal physiology. However, in vivo mapping of TRP is challenging due to its subcellular signal magnitude and fast time course. We report here a virtually structured detection-based super-resolution ophthalmoscope to achieve subcellular spatial resolution and millisecond temporal resolution for in vivo imaging of TRP. Spatiotemporal properties of in vivo TRP were characterized corresponding to variable light intensity stimuli, confirming that TRP is tightly correlated with early stages of phototransduction.

In vivo observation of transient photoreceptor movement correlated with oblique light stimulation

Rod-dominated transient retinal phototropism (TRP) has been observed in freshly isolated retinas, promising a noninvasive biomarker for high resolution assessment of retinal physiology. However, in vivo mapping of TRP is challenging due to its fast time course and sub-cellular signal magnitude. By developing a line-scanning and virtually structured detection based super-resolution ophthalmoscope, we report here in vivo observation of TRP in frog retina. In vivo characterization of TRP time course and magnitude were implemented by using variable light stimulus intensities.