High-Res In-Vivo Retinal Imaging with FF-OCT
Optica Publishing Group
Abstract
Time-domain full-field OCT (FF-OCT) represents an imaging modality capable of recording high-speed en-face sections of a sample at a given depth. One of the biggest challenges to transfer this technique to image in-vivo human retina is the presence of continuous involuntary head and eye axial motion during image acquisition. In this paper, we demonstrate a solution to this problem by implementing an optical stabilization in an FF-OCT system. This was made possible by combining an FF-OCT system, an SD-OCT system, and a high-speed voice-coil translation stage. B-scans generated by the SD-OCT were used to measure the retina axial position and to drive the position of the high-speed voice coil translation stage, where the FF-OCT reference arm is mounted. Closed-loop optical stabilization reduced the RMS error by a factor of 7, significantly increasing the FF-OCT image acquisition efficiency. By these means, we demonstrate the capacity of the FF-OCT to resolve cone mosaic as close as 1.5o from the fovea center with high consistency and without using adaptive optics.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Introduction
Optical Coherence Tomography (OCT) has become a gold standard for living human retinal imaging in the clinical environment [1,2]. Owing to its unprecedented axial resolution, OCT enables the diagnosis of retinal disorders at the earliest stages and monitoring of the progression of retinal diseases. While physicians are capable of interpreting tomographic retinal cross-sections from OCT, there is nevertheless a demand for en-face views. Although OCT can produce en-face retinal images from 3-D stacks, it presents a low lateral resolution compared with Adaptive Optics (AO) ophthalmoscopes [3]. By correcting static and dynamic ocular aberrations [4,5], AO can explore the full aperture of the eye’s pupil to achieve micrometer lateral resolution. To be able to combine both high axial resolution from OCT, and the high lateral resolution from AO, a great effort was made to develop AO-OCT systems [6,7]. However, the commercialization and the clinical deployment of AO-OCT are still challenging, mainly because of cost, optical complexity, size and heavy image post-processing linked to the use of AO [6].
Another modality of OCT is the time-domain Full-Field OCT (FF-OCT) [8] (henceforth, we refer to this imaging modality simply as FF-OCT). Contrary to conventional OCT, which scans an illumination spot across a tissue surface and records at each spot the depth structure, FF-OCT uses a spatially incoherent light source, high-speed megapixel camera and time-domain phase modulation to acquire en-face sections of the sample at a given depth. One attractive point of this technique for retinal imaging, is the fact that FF-OCT presents a low sensitivity to low-order ocular aberrations (i.e. defocus and astigmatism), due to the use of the spatially incoherent light source [9,10]. This interesting feature was highlighted in [11], where cone mosaic at 6𝑜� from the foveal center of a healthy subject (without prescription eyeglasses) was resolved without using AO. This first image was made possible by the combination of FF-OCT and a Spectral-Domain OCT (SD-OCT). Through the images generated by the SD-OCT both retinal cross-section and the FF-OCT reference mirror were visible, and a manual match, just before the acquisition, was possible. Nevertheless, this strategy faces significant challenges in providing consistent and reproducible images, mainly due to involuntary axial head and eye motion, which can achieve a median maximum axial speed in the order of 1mm/s [12]. As a consequence, a considerable amount of images has to be acquired to increase the chances of matching the optical path of both arms for a retinal layer of interest.
Hence, to be able to record efficiently and consistently en-face sections of the retina in-vivo, an axial optical stabilization strategy is necessary. An optical stabilization of axial motion applied in time-domain FF-OCT for retinal imaging was first proposed by Miller et al., combining the FF-OCT with an auxiliary time-domain 1-D OCT and a voice coil translation stage in the reference arm [13]. However, the time-domain 1-D OCT used for axial motion tracking was too slow to work in-vivo (20 A-scans per second). Later, Pircher et al. [14] combined a transverse scanning OCT system (which directly extracts en-face images) with a high-speed Fourier domain 1-D OCT, used to track the cornea apex signal, and a rapid scanning optical delay line montage for real-time optical stabilization. Although the achieved performance was satisfactory, because of the scanning illumination/detection scheme of the proposed OCT, en-face images were acquired with a low frame rate, limited FOV and suffered from motion artifacts and distortion due to fixational eye movements [15]. Therefore, continuous involuntary head and eye motion remains one of the biggest challenges in time-domain FF-OCT when imaging the living human retina.
In this paper, we present an FF-OCT retinal imaging system with optical stabilization of axial motion. This system was made possible by combining the FF-OCT system with an SD-OCT. The idea is to use B-scans generated by the SD-OCT to measure the retinal axial position and to drive a high-speed voice-coil translation stage, where the reference arm of the FF-OCT is mounted, correcting for the eye motion. By these means, we demonstrate the capacity of the FF-OCT to resolve cone mosaics as close as 1.5𝑜� from the fovea center with high consistency and efficiency, without using adaptive optics.
