The overall time for in vivo imaging is constrained by blinking rate and tear-film stability, whereas involuntary eye motion during fixation (drift and microsaccades) constrain exposure time. Microsaccades as found in
Figure 1are reported
41 to have amplitudes ranging from approximately 0.25° to 2.3°. Depending on their time scale, they appear every 0.5 to 3 seconds with accelerations of up to 14,500°/s
2 and speeds of 30° to 360°/s.
42 Axial and rotational motions, caused by head movements and activity of the oblique eye muscles, result in further distortions in the scanned volumetric image.
43 If these motions are faster than the raster scan, scanning positions become ambiguous, leading to image distortions that cannot be compensated. Hence, the time spent and, therefore, the number of depth scans in the fast-scanning direction of a typical rectangular raster scanning retinal imaging device are limited because of restriction on the advancement speed of the slow scanning axis. For typical isotropic wide scans of angle α approximately 20°, corresponding to
A∼5.7 mm (using a conversion factor of 288 μm/degree), corresponds to a depth scan separation of Δ
x slow =
A ·
N −1. For healthy subjects with maximum eye motions of ω
mot = 30 to 100°s
−1 (
v mot approximately 8–28 mm/s),
44 the number of depth scans (
N) in the fast direction at
f s = 47 kHz is limited by
\[N{<}\sqrt{\frac{A\ {\cdot}\ f_{\mathrm{s}}}{v_{\mathrm{mot}}}}{=}\sqrt{\frac{{\alpha}\ {\cdot}\ f_{\mathrm{s}}}{{\omega}_{\mathrm{mot}}}}.\]
to approximately 100 samples, enabling compensation only of microsaccades. Another solution to avoid image distortions by microsaccades is to restrict the complete acquisition time to less than 0.5 to 1 second. On the other hand, the 10-fold slower drift can be completely compensated by interframe registration rather than mechanical tracking. This will allow acquisition of volume scans representing the 3D structure at high precision. As shown in a recent study in which an experimental 1060-nm tunable laser system was used,
31 higher speeds can be used to further improve sampling density or to evade microsaccades. In contrast, the spectrometer-based system permits the user to freely choose the light source and to emphasize longer wavelengths within the 1060-nm water window for higher penetration through melanin and blood. Furthermore, more sophisticated signal processing is possible because of the higher stability of the static spectrometer. This enables visualization of different portions of the circle of Zinn-Haller and feeder vessels that perforate the sclera and proves that penetration at 1060 nm is sufficient to monitor the full retina supporting vasculature of the choroid. Wide-field, high-speed scanning, especially in future combinations with optional Doppler flow measurements, opens the possibility to partially replace invasive and risky fluorescein angiography by a completely noninvasive technique. The ability to distinguish and visualize individual layers of blood vessels above and beneath the RPE at high resolution has great potential to improve diagnostic abilities for diseases such as age-related macular degeneration, diabetes, and the effects of retinal occlusions. Better visualization of the ONH, including the lamina cribrosa and the depth-dependent density and shape of its pores, holds promise to improve glaucoma assessment. At the ONH, thick vessels or thicker NFL limit access to deeper portions at the center of the optic disc in young, healthy subjects. Older patients, especially those with glaucoma, are known to have a shallower nerve fiber layer and a more visible lamina cribrosa. Further studies on variability will probably aid our understanding of the impact of morphologic and functional differences on the visualization and structural parameters of different tissues, including layer thickness, vessel density, and melanin content. Optimizations of the patient interface, as found in the technologically similar commercial systems, are likely to improve overall sensitivity. The spectrometer allows the integration of sources with bandwidths greater than 110 nm that also operate in the better-penetrating, long-wavelength side of the 1060-nm water transmission window to improve axial OCT resolution to approximately 4 μm and to reduce speckle size, whereas camera development will improve imaging speeds. It is expected that clinical studies of abnormalities with 1060 nm OCT will lead to new insights into disease progression and will help early diagnosis and optimized treatment for a variety of patients, especially when optical access to the retina is limited because of scatterers in the anterior eye segment (e.g., cataract or corneal haze).