The combination of a high-resolution, efficient and clinic-friendly OCT system with robust image processing techniques represents a significant advance over existing commercially available SD-OCTsystems. We have shown that a commercially available SD-OCT instrument equipped with a broadband light source can provide improved contrast and resolution of outer retinal layers. A local contrast analysis demonstrated an improved contrast of the retinal layers as well as better resolution of the IS/OS layer, as seen by a narrower FWHH when using the broadband light source.
The approach presented here can serve as a model for a more quantitative analysis of OCT images, allowing for meaningful comparisons between subjects, clinics, and OCTsystems. Rather than place qualitative labels on the image and/or devices such as ‘super’, ‘ultra’, ‘high’ or ‘low’, a more quantitative description of image quality is within reach and would go a long way to help clinicians to make decisions about which imaging tool best fits their needs. This is especially relevant as additional technological advances are made in SD-OCT.
Horizontal striations within the IPL were also observed and are believed to represent synaptic sublamination. In most SD-OCT images it is possible to discriminate the nerve fibre layer from the ganglion cell layer and the ganglion cell layer from the IPL. This IPL sublamination has not previously been observed using the standard source or two other commercial SD-OCT systems in our clinic. It would be interesting to examine this lamination in patients with advanced photoreceptor degeneration, as one might be able to visualise downstream disruptions in retinal circuitry known to occur after a loss of photoreceptor input.27
As stated earlier, the axial resolution in OCT is inversely related to the bandwidth of the imaging light source. Thus, an improvement in resolution is expected between the two light sources by a factor of about 3.2. However, it was observed that the FWHH of the ELM decreased by only a factor of 2. Probably the biggest factor preventing the realisation of the full theoretical benefit of the broadband light source is that the human eye is wrought with chromatic aberration. The predicted change in focus for our broadband light source (λmax
=878.4 nm, Δλ
=186.3 nm) is nearly 0.5 D.28
Further improvement in resolution approaching the theoretical improvement could be realised by implementing an achromatising lens to compensate for the eye’s chromatic aberration.29
In addition, the spectrometer in the system is calibrated for the narrow band source and thus the benefit of deeper penetration of the longer wavelengths is not fully captured when using the broadband source.
Last, there are additional advances in clinical OCT imaging on the horizon. Adaptive optics has also been shown to increase the resolution of OCT by correcting for the eye’s monochromatic aberrations,4,5,7,8
making it possible to visualise individual cone photoreceptors in the outer retina. However, recently it has been shown that simply increasing the speed of the line scan camera can mitigate the effects of retinal motion and enable visualisation of the peripheral cone mosaic.12
Systems incorporating both high-speed and adaptive optic correction of the eye’s aberrations would provide even better resolution. Nevertheless, these advances are not currently commercially available in the form of a clinical imaging system. Quantitative analyses such as those described here will be useful in determining the benefit of advanced technology as it becomes available.