In vivo cellular resolution human retinal imaging is relatively new, and holds promise for scientific and clinical applications. In most cases, in vivo cellular resolution refers only to successful imaging of the cone photoreceptor mosaic, not the hundreds of millions of other retinal cells. Thus, while progress in retinal imaging has been impressive over the past decade, many challenges lie ahead.
All three retinal imaging modalities used in today’s ophthalmic clinics, i.e., the fundus camera [1
], scanning laser ophthalmoscope (SLO) [2
], and optical coherence tomography (OCT) [3
], have been combined successfully with adaptive optics (AO) for cellular resolution imaging. Historically, the first AO implementation was with a fundus camera [4
]. After a few years, an AO-SLO [5
] was introduced, followed shortly by AO-OCT [6
Because cone photoreceptor density changes with retinal eccentricity (decreasing with the radial distance from the fovea) [9
], one can observe cones at higher eccentricities even using retinal instruments with relatively low lateral resolution [10
]. Additionally, recent improvements in acquisition speed and sensitivity of research-grade OCT instruments now permits clear and reliable imaging of the cone mosaic in young healthy volunteers without AO [11
]. However, as predicted by diffraction theory and ocular aberrations in the normal healthy human population [13
], AO with wavefront correction is required [14
] to allow cellular resolution imaging near the fovea. The need for AO becomes evident if one increases the size of the imaging aperture over 2 mm at the eye’s pupil [15
The main difference between AO-fundus and AO-SLO instruments is full field vs. raster image acquisition schemes and the optional confocal detection scheme of the latter. Both systems can be used to detect scattered as well as fluorescent photons from the sample. In contrast, standard OCT, due to its coherent detection nature, can only detect elastic back-scattered photons. This difference has many implications and explains why the fundus camera and/or SLO can be seen as complimentary modalities to OCT. Despite limitations of OCT (e.g., inability to detect fluorescent photons) recent progress in acquisition speed and sensitivity allowed by Fourier domain Fd-OCT [16
] has already revolutionized clinical diagnostics and monitoring of retinal diseases [18
]. This is because OCT offers sufficient axial resolution (few µm), independent from lateral resolution, for in vivo
visualization and characterization of all the main cellular layers in the human retina [21
Interestingly the confocal detection nature of SLO and standard OCT makes the combination of these modalities rather straightforward for multimodal retinal imaging systems. For example, some recently introduced state-of-the-art clinical Fd-OCT systems have a built-in SLO to be used as a large field-of-view (FOV) finder. Some of these clinical systems even support an SLO for fundus autofluorescense (AF) as well as fluorescein angiograpy (FA) and indocyanine green angiography (ICGA) imaging. In parallel with these developments, several laboratories have actively explored the possibility of multimodal imaging with SLO and OCT. A recent review by Podelanu and Rosen [23
] includes a detailed summary of the main developments in that area.
There are generally two main approaches for combing SLO and OCT modalities: one implements transverse scanning time-domain (Td)-OCT and the second uses Fd-OCT. Both approaches have been combined with adaptive optics to achieve cellular resolution retinal imaging. Merino et al. [24
] described AO SLO/OCT with Td-OCT using T-scan acquisition, and this was followed by Pircher et al. [25
] showing increased acquisition speed using a similar configuration. Recently the combination of AO SLO/OCT with OCT based on A-scan acquisition has been presented for both Fd-OCT variations: spectral OCT [26
] and swept source OCT [27
]. In the first approach (transverse scanning time-domain OCT), due to identical data acquisition mode (T-scans), all the scanning and adaptive optics components can be shared while in the second approach some additional components are needed to allow multiplexing of OCT and SLO signals.
In this manuscript we describe an AO system that combines an SLO with spectrometer-based Fourier-domain OCT to allow simultaneous data acquisition with two modalities. In contrast to the design for combined SLO and Fd-OCT presented by Mujat et al [27
], here SLO and OCT horizontal scanning is decoupled. Detailed information about instrument design and its performance are provided. This includes the implementation of an ALPAO novel membrane magnetic deformable mirror with increased stroke and actuator count used as single wavefront corrector. We also discuss laser safety levels for this multimodal system. Finally images of the retina acquired in vivo
with this multimodal system are presented.