Phase-Sensitive Optical Coherence Microscopy (OCM) - Biomedical Optical Phase Microscopy and Nanoscopy - page 259

Figure 13.5
Possible digital processing for phase OCM.

13.4.1 Imaging of the Outer Retina

The cone photoreceptor outer segment (OS) changes over time in response to visible
stimuli and due to the daily course of renewal and shedding. This results in submicron
changes in the OPD, which are too small to be studied using standard OCM. Ref. [15]
shows that it is possible to measure phase information from the outer retina in vivo. It does
so by quantifying of phase differences within the retina, which improves the sensitivity to
OS length change by more than an order of magnitude, down to 45 nm, slightly thicker
than a single OS disk. This technique was applied for measuring changes in the OS
for hours in hundreds of cones over time.

In this method, the phase difference between different reflective layers in the retina is
quantified, which, unlike absolute phase, is immune to decorrelation due to axial motion.
Critical to the technique is the ability to measure phase differences between reflective
layers located at different retinal depths. A number of preliminary analytical steps are
required, the most important of which is the selection of one dimensional scans or A-lines
for analysis and spatial unwrapping of phase within the regions of interest.

The setup consists of an ultrahigh-resolution SD-OCM system, combined with a
woofer tweeter adaptive optics system, which provides cellular-level lateral resolution.