The multimodal AO retinal imager is a powerful platform for the investigation of retinal diseases. The capabilities of this platform include acquisition of AO-corrected SLO and OCT images, wide dynamic range AO-correction with the dual-DM approach, retinal tracking, and auxiliary wide-field LSO imaging. Future studies will fully characterize the depth enhancement with the 1 µm OCT source. In this preliminary report, we demonstrate initial AO performance, SLO imaging of the photoreceptor mosaic and OCT images.
SLO and OCT are neither equivalent nor interchangeable imaging technologies. Consider, for example, the routine ability of AOSLO to resolve photoreceptors (in some instances within <100 µm of the fovea), which most vision researchers and clinicians would agree constitutes a powerful capability for understanding and treating many retinal diseases. Photoreceptors are, after all, the point of optical-neural transduction and thus the starting point of the visual pathway. Recent reports of photoreceptor and RPE cell resolution with OCT [36
], often without AO, may lead some to question whether the AO multimodal approach, with accompanying additional system complexity is justified. While OCT may resolve photoreceptors at some large eccentricities with en-face [36
] or ultra-high speed systems [15
] to partially overcome eye motion artifacts, to date only the AOSLO approach has allowed for relatively rapid mapping of the cone mosaic across large parts of the macula [38
]. The correction of ocular aberrations with AO is essential for either technique to resolve fine scale structures that distinguish coherence artifacts from biological constituents. On the other hand, OCT offers unparalleled axial discrimination of disruption, thinning, or thickening of the retinal layers that are often the first signs of pathology even before changes in ophthalmoscopic appearance. Therefore, correlative multimodal AO systems may be the best approach to examination of disease attributes. We are currently developing automated image acquisition and analysis techniques to address the difficulties that arise from system complexity.
One major advantage of AO-OCT is improved lateral resolution that would allow imaging the 3-D spatial structure of the photoreceptor layers. The 2-D local spatial patterns of cones, etc., are readily seen in small AOSLO image fields. However, small OCT B-scans of 1-2 deg, traversing the cone mosaic along one line, even at high resolution are harder to interpret; it is not necessarily a simple matter to prove that such structure is not speckle. The best way to do this is to produce a 3-D volumetric image and segment out the relevant layers which reveal the mosaic. Unfortunately, in the present system, the swept-source employed, in addition to having considerable intensity noise, is limited to 20 kHz A-line rate. The resulting 3-D frame rate is too slow to capture a local mosaic, due principally to eye motion. Eye tracking can help significantly, but initial tests have shown that stabilization must be to less than one cone diameter (typically <5 µm, to avoid disrupting the pattern beyond recognition) and no tracking methods yet available have that kind of precision. Transversal scanning SLO/OCT methods have shown somewhat better performance, at the price of returning to the time domain and using axial tracking [36
]. High speed SDOCT methods (Basler Sprint, 140 kA-lines/s) have proven much more effective, but met with limited success. At 1070 nm, new MEMS swept-sources sources (Exalos, Axsun) or the FDML [37
] approaches are likely to attain the requisite speed to achieve the full promise of AO SLO/OCT. The important thing to recognize however, is that all of these OCT technologies are amenable to improvement, while the focus of this study was the AO-based multimodal clinical imaging.
While the most desirable clinical scenario is to enable acquisition of all imaging modes simultaneously, one disadvantage of our current approach (i.e., OCT channel using a swept source) is caused by the necessity of acquiring the SLO image at the RS frequency (14.16 kHz) and the OCT image at the swept source frequency (20 kHz). We have therefore configured the multimodal AO system to sequentially acquire images from the SLO and OCT channels in rapid sequence while the LSO, AO, HS-WS, and RT are all running continuously. This was done in a unique configuration whereby the SLO timing board (that drives the scanners) can accept input from either the SLO RS or the OCT swept source. Thus the multiple scanning schemes available for both modes (OCT line and raster, SLO raster, montages, strip scans, etc.) use all the same hardware (scanners, real time processing board) and are set up from an extremely intuitive and flexible user interface.
We developed acompanying post-processing analysis routines for both SLO and OCT images including registration, montage and strip stitching, photoreceptor quantification, and segmentation (retinal layers and drusen) [33
]. Some require limited user input (i.e., are semi-automated) while others operate in a fully automated manner (e.g., photoreceptor counting). With the multimodal image acquisition modes and these analysis tools, it is now possible to fully map retinal layers and critical structures across the macula [38
In the next phase of the research, the system will be installed in an ophthalmology clinic, where it will be validated in studies designed to provide qualitative and quantitative information on the fine structural characteristics of several diseases, including glaucoma, diabetic retinopathy, age-related macular degeneration, and retinitis pigmentosa. We plan to examine vascular and other deeper structures by taking advantage of the enhanced penetration at 1 µm illumination.