Spatiotemporal OCT Method Advances Ophthalmological Imaging

Spatio-Temporal Optical Coherence Tomography (STOC-T), an ophthalmological imaging technique originally invented to capture optoretinograms, has been enhanced by its developers at the International Centre for Translational Eye Research (ICTER) to enable views of the retina and choroid at different depths.

STOC-T will make it possible to image the retina in a fraction of a second and record the retina’s entire depth at extremely high resolution. According to the researchers, it will enable the clinician to fully image the eye before the patient has a chance to blink, and to obtain an accurate, detailed image down to the single-cell level.

STOC-T uses light with controlled spatial and temporal coherence and advanced signal processing methods to acquire high-contrast, high-resolution coronal projection images of the retina and choroid at arbitrary depths. It enables uniform, 5-μm resolution imaging of the chorioretinal complex, and full-field detection and digital defocus correction to ensure a greater than 1-mm axial imaging range. Images of the choriocapillaris can be acquired with the system and used to derive quantitative morphometry metrics.

Professor Maciej Wojtkowski, who led the research, said that the team applied known data processing algorithms and developed new ones to handle and process the data sets acquired by STOC-T in order to obtain high-contrast 3D data volumes for the retina in large fields of view. “The technology and algorithms made it possible to image the retina and choroid at high transverse resolution at different depths, making the differentiation of morphology visible for the first time within the Sattler, Haller, and choriocapillaris layers,” he said.

While modern imaging of eye tissues would not be possible without classical OCT, the technique has its drawbacks. Coherent noise can degrade OCT’s image quality. It is challenging to distinguish the morphological elements in the eye using classical OCT techniques.

Image of a selected layer in the human choroid obtained by the new STOC-T method. Courtesy of the International Centre for Translational Eye Research.

Additionally, OCT’s limited axial range precludes full penetration of all layers of the retina and choroid. The limited axial range over which OCT can acquire high-resolution images prevents simultaneous measurement of the retina and the choroid while maintaining adequate lateral resolution. The structural complexity of the choroid, which consists of four layers, exacerbates the imaging challenges.

STOC-T has a longer imaging range than regular scanning OCT, due to the differences in illumination techniques and the ability of STOC-T to computationally correct for aberrations. STOC-T can achieve higher lateral resolution than classical OCT when imaging a human retina, because it is able to use the pupil aperture of the human eye to detect the backscattered signal from the retina. In addition, STOC-T can image the choriocapillaris layer over a larger axial and lateral area than hardware-based adaptive optics (AO)-OCT.

The researchers demonstrated large field-of-view imaging of the choriocapillaris using STOC-T. They showed that retinal images acquired with the STOC-T system maintain a consistent resolution of about 5 μm in all three dimensions, over the entire thickness of about 800 μm, without any mechanical scanning. In addition, they demonstrated that STOC-T can acquire high-contrast volumetric chorioretinal images with reduced scattering effects.

To demonstrate the advantage of the STOC-T method over classical scanning OCT techniques, the researchers imaged a reflective sample with STOC-T and scanning OCT systems. The sample was stepped along the optical axis, and images were acquired at each defocused position. The data was then used to derive depth-of-field (DOF) curves for both techniques.

The researchers found that the DOF for STOC-T was two orders of magnitude broader than the DOF for the scanning OCT. A strongly scattering phantom was also imaged with both systems to illustrate that the broader DOF in STOC-T would also lead to a broader axial imaging range in a real 3D sample.

To show that the STOC-T system improves penetration depth into the choroid, the researchers compared the relative changes in the OCT signal against two commercially available scanning OCT devices with nominal values of lateral resolution of 15 and 20 μm. Despite having nearly three to four times higher lateral resolution, STOC-T was found to have a DOF similar to that of the Triton OCT and significantly greater than that of the Spectralis OCT.

The advancements made to the STOC-T system by the ICTER team enable it to generate high-quality retinal and choroidal images with a simple and robust optical layout, and they make it more suitable to use in a clinical environment. The results of the study demonstrate that STOC-T is capable of in vivo visualization of major retinal and choroidal layers, including that of the choriocapillaris. The researchers believe that STOC tomography has the potential to usher in a new era in the diagnosis of eye diseases, although additional refinements are needed before the system can be routinely disseminated in a clinical setting.

The research was published in iScience (www.doi.org/10.1016/j.isci.2022.105513).