Near infrared adaptive optics flood illumination retinal angiography

Abstract

Image-based angiography is a well-adapted technique to characterize vasculature, and has been used in retinal neurovascular studies. Because the microvasculature is of particular interest, being the site of exchange between blood and tissue, a high spatio-temporal resolution is required, implying the use of adaptive optics ophthalmoscopes with a high frame rate. Creating the opportunity for decoupled stimulation and imaging of the retina makes the use of near infrared (NIR) imaging light desirable, while the need for a large field of view and a lack of distortion implies the use of a flood illumination-based setup. However, flood-illumination NIR video sequences of erythrocytes, or red blood cells (RBC), have a limited contrast compared to scanning systems and visible light. As a result, they cannot be processed via existing image-based angiography methods. We have therefore developed a new computational method relying on a spatio-temporal filtering of the sequence to isolate blood flow from noise in low-contrast sequences. Applying this computational approach enabled us to perform angiography with an adaptive optics flood illumination ophthalmoscope (AO-FIO) using NIR light, both in bright-field and dark-field modalities. Finally, we demonstrate the capabilities of our system to differentiate blood flow velocity on a retinal capillary network in vivo.

Adaptive optics imaging of the retinal microvasculature

“…A programmable aperture can be used to explore structure oriented in arbitrary directions . Recently developed darkfield methods may be able to reveal similar details for flood‐based illumination geometries …”

Coherence gate shaping for wide field high-resolution in vivo retinal imaging with full-field OCT

“…Although FF-OCT axial resolution may be slightly poorer than AO-OCT [10], it has advantages of larger FOV, higher-frame rate and lower complexity and size, and is still able to resolve the photoreceptor interfaces such as IS/OS junction and cone outer segment tips. Moreover, the high temporal resolution of FF-OCT may play an important role to characterize the temporal behavior of retinal capillaries from different plexus [36,37] and the temporal dynamics of subcellular structures [27]. Finally, although FF-OCT is only weakly sensitive to low-order ocular aberrations in terms of resolution, aberrations still reduce the FF-OCT signal level [38].…”

Optical Incoherence Tomography: a method to generate tomographic retinal cross-sections with non-interferometric adaptive optics ophthalmoscopes

“…Multiply scattered light (or nonconfocal) imaging modalities such as dark-field, offset aperture and split detection, widely applied in AO-SLO [5], and recently introduced for AO-FIO [10,43], provide excellent contrast for blood vessels, mural cells [8,10] and translucent retinal structures [6,7], which are poorly or not visualized in back-scattered light imaging systems. Unlike OCT, in which the coherent detection limits the use of multiply scattered light, we showed that OIT can make use of incoherent light, generating, for the first time to our knowledge, a retinal cross-sectional view using multiply-scattered light, here in split-detection mode (Figs.…”

“…for different focal planes, and the assessment of image quality after acquisition, to select the best image stack, are mandatory, time-consuming steps which are not always compatible with the clinical environment. To avoid these drawbacks, a focus-guidance tool becomes essential, especially to reveal hypo-reflective or transparent structures such as cone photoreceptor inner segments (IS) [6], retinal ganglion cells (RGC) [7], perfusion in microvasculature [8,10] or those masked by neighboring structures of high reflectivity such as retinal pigment epithelium (RPE) lying beneath photoreceptors [9].…”