As opposed to approximately 800 nm, the typical wavelength range for ophthalmic OCT imaging used in previous studies (Kagemann L, et al.
IOVS 2009;50:ARVO E-Abstract 813),
4 a central wavelength of 1310 nm was selected for this study. Indeed, axial resolution is lower for longer central wavelength assuming the same wavelength scan range. The theoretical axial resolution of our system is 7.5 μm, whereas that of the previous studies (Kagemann L, et al.
IOVS 2009;50:ARVO E-Abstract 813)
4 was 1.3 μm. However, as shown in
Figure 5, one can easily identify and locate CCs from the images acquired by the probe. The 7.5-μm axial resolution was sufficient because the dimensions of the structures were usually on the order of 100 microns. The fact that the previous systems (Kagemann L, et al.
IOVS 2009;50:ARVO E-Abstract 813)
4 with higher axial resolution could not provide images of the same quality could have been due primarily to light scattering and absorption by intervening tissues. Although no blood was involved in this ex vivo study, one can expect an improved performance over shorter wavelengths for in vivo studies because shorter wavelengths are more greatly affected by optical scattering.
6 Higher water absorption at 1310 nm should not impair image contrast as much as expected in external OCT systems because the distance from the probe tip to the tissue of interest is usually in the 0.5- to 1-mm range during the intended endoscopic imaging procedures.
This wavelength selection of 1310 nm has been verified in the initial penetration verification experiments. Although we did not happen to capture a CC during those experiments (which were not designed to search CCs after all), based on the images we obtained with TM intact, the clarity should have been comparable if a CC had been captured. In addition to the advantages of 1310 nm light, this occurred primarily because the beam has bypassed most of the intervening tissues; the only remaining tissue is a thin, flimsy layer of TM with a thickness usually around 10–20 microns that can be easily penetrated, as shown in
Figure 5a.
Although the rotation speed of the motor was well maintained as constant, the angular speed of the deflection was not constant because the deflection angle is not linearly related to the rotation angle.
8 An accurate relationship between the two angles is critical for accurate image reconstruction. In this study, which was based on a simplified model,
8 we developed a more accurate theoretical model to estimate the deflection angle. A numerical simulation (ZEMAX Development Corporation, Bellevue, WA) was also used to verify this calculation. Finally, we experimentally measured the relationship. These results are shown in
Figure 6. We applied this result to the final image reconstruction to obtain the correct geometry of the structures.
In this endoscopic probe design, the actuation system is located far away from the probe tip, which enables easy miniaturization. The diameter of the probe was limited primarily by that of the GRIN lens. We had already achieved a narrower probe
9 encased in two needles of 23 and 21 gauge, respectively, for the inner and outer needles. Now GRIN lenses with diameters smaller than 400 μm are commercially available. This can further reduce the probe size, making it small enough to be introduced through a clear corneal incision for OCT visualization of CC location before or while implanting a bypass shunt.
As mentioned in our previous publication,
8 inherent in this technology is the capability to perform volumetric scanning by using different rotation modes. By driving the lenses at different angular speeds and switching their rotation directions, we can engineer many volumetric scanning patterns in addition to the planar fan-shape scan pattern. Those patterns might permit visualization of larger areas, leading to more rapid identification of tissue structures.
The frame rate of the OCT system (0.5 fps) was limited by the scan rate of the swept laser. To accumulate enough A scans (666 depth scans in our case) for each OCT image frame, the rotation speed of our probe had to be kept at a constant value of 15 rpm, which is much lower than what it can support. OCT engines with A-scan rates greater than 100 kHz have been demonstrated,
10 and OCT systems greater than 20 kHz are now commercially available as well. These could significantly increase the frame rate of our endoscopic system.