Combined Depth Imaging using optical coherence tomography as a novel imaging technique to visualize vitreo-retino-choroidal structures

Spectral-domain optical coherence tomography (SD-OCT) is a non-invasive technique to visualize cross-sectional images of vitreo-retinal structures. Image averaging during eyetracking increases the signal-to-noise ratio and improves image quality. The point of maximum sensitivity on SD-OCT (known as the “zero delay line”) is in the vitreous; with increasing depth, the signal is reduced and details of the choroid are reduced.¹

More recently, enhanced depth imaging on SD-OCT (EDI-OCT) has increased the ability to visualize the choroid, capturing images with the choroid close to the zero delay line.² However, with this modality details of the posterior vitreous are reduced. A healthy volunteer was scanned with a SD-OCT device (Spectralis HRA, Heidelberg Engineering). On the conventional SD-OCT B-scan of the macula, the preretinal vitreal pocket was clear. However, the outer choroidal border was not detected (Figure 1,A). The EDI-OCT scan increased the visibility of the choroid borders, but the preretinal vitreal pocket was not seen (Figure 2,B).

Fig. 1.

Multiple acquisition modalities on SD-OCT (Heidelberg Spectralis HRA). A: Conventional linear OCT B-scan allows good visualization of the preretinal vitreal pocket (asterisks) and retinal structures, but the outer choroidal boundary is not visible. B: Linear EDI-OCT B-scan increases visualization of the outer choroidal boundary (arrows), but decreases identification of the posterior vitreal structures. C: Combined conventional OCT and EDI-OCT B-scan yields a single comprehensive image of the posterior structures, including preretinal vitreal pocket (asterisks), retina, and choroid (arrows).

We devised an innovative imaging technique to visualize all posterior structures in a single image, combining the conventional SD-OCT B-scan with the EDI-OCT B-scan. The same volunteer was asked to fixate on the internal fixation centered on the fovea. A 100-frame averaged conventional linear SD-OCT scan in high-resolution mode was then performed. After reaching 50% of the averaging we manually changed to the EDI modality using the dedicated button, and the image was captured after obtaining a good enhancement of the choroid. This allowed the device to create a high-quality combined image of all depths. (Figure 3,C).

This manual technique of Combined Depth Imaging (CDI) on Spectralis HRA yielded a single comprehensive image of the posterior structures without using multiple acquisition modalities. Dedicated built-in software may be useful to achieve this imaging result automatically. Further studies will evaluate the applicability and limitations of this technique.