Research

Hand-Held Forward-Imaging Needle Endoscope for Ophthalmic OCT Inspection

One potential application of our PARS probe is aiding vitrectomy, which is a surgical procedure to remove the vitreous humor (sketched in Fig. 1). Vitrectomy is a required precursor to other surgeries for the treatment of severe eye diseases such as retinal detachment. The success of the procedure and the long term prognosis for patients depend critically on the complete removal of vitreous humor around retinal tears and holes. To guarantee this, surgeons will typically insert a light pipe through an incision on the patient’s eye and examine the remnant vitreous by direct illumination during eye surgery. As the vitreous is transparent, this examination is often very difficult. OCT can aid in this procedure by providing depth resolved images without the introduction of a contrast agent. A forward-imaging OCT endoscopic probe can be brought in close proximity to the retinal surface, avoiding the necessity of imaging through the cornea and crystalline lens.

Based on our PARS technology, the narrowest to-date (21 gauge, 820 um in diameter) hand-held forward-imaging optical coherence tomography (OCT) needle endoscope has been demonstrated recently. The feasibility of retinal imaging has been tested on enucleated ex vivo porcine eyes, where structural features including remnant vitreous humor, retina, and choroid can be clearly distinguished. The narrow probe can easily fit through the standard cannula or scleral incisions employed in ophthalmic surgery. The probe can potentially serve as a better alternative to traditional visual inspection by white-light illumination during vitreoretinal surgery (e.g. vitrectomy).

The PARS-OCT probe design utilizes two angle polished gradient-index (GRIN) lenses (d = 500 mm, N.A. = 0.22) which are rotated about the optical axis to steer the optical beam position. As sketched in Fig. 1, the GRIN lenses are encased in two counter-rotating concentric needles (23 gauge/21 gauge for the inner/outer needle). The inner faces of both GRIN lenses are polished at a 15o angle, and separated by a designed air gap. Refraction at the first angle polished surface directs the beam off axis to the second GRIN lens, providing a maximum tilt angle of 15.3o. The first GRIN lens is ¼ pitch (l = 3.11 mm). The second GRIN lens is less than ¼ pitch (l = 2.6 mm). The working distance of the probe was measured to be 0.78 ± 0.02 mm. The scan confinement can be seen from Fig. 2, which was recorded by suspending the scanning PARS-OCT probe tip above a planar CCD camera at a distance of 2 mm.

We designed the prototype for operation at 1310 nm, as opposed to ~800 nm which is the typical wavelength for OCT retinal imaging. A central wavelength of 1310 nm is desirable as we can expect to achieve improved visibility of the choroid and choriocapillaries over the more common shorter wavelengths. An additional benefit of using 1310 nm illumination with the PARS-OCT probe is that its higher absorption in vitreous can be expected to improve visualization of remnant vitreous adhering to the retina. Capitalizing on the inherent sensitivity advantage of Fourier domain systems, a swept laser was selected as the source illumination (Micron Optics, lo = 1310 nm, Dl = 70 nm, 250 Hz A-scan rate). The k-domain sampling clock signal was generated by a fiber Fabry-Perot interferometer.

We measured the focal spot size of our prototype at different tilt angles by projecting the focal spot onto a CCD camera through a 20X objective combined with an achromat doublet. We found that the minimum spot size of 7.6 mm (FWHM) was obtained when the beam exited the probe with no tilt. The spot size increased moderately to 10.4 mm (26.4% increase) at the maximum tilt angle.

For a preliminary test on its feasibility in clinical application, the PARS-OCT probe was used to image the retina of an enucleated porcine eye with cornea, lens, and vitreous removed. The power delivered at the sample was about 0.6 mW. The experimental system sensitivity was measured to be 92 dB. In the resulting images, shown in Fig. 3, structures as deep as ~2.5 mm from the probe tip were clearly observed, including top remnant vitreous, retina, and choroid underneath the retina layer. In several places, we were also able to observe the detachments of retina from choroid, which likely occurred during removal of the vitreous humor. An image of the enucleated retina after lensectomy and vitrectomy captured by a commercial 1310 nm OCT microscope (Thorlabs OCM1300SS) is also shown for comparison. The images acquired with the PARS-OCT probe are seen to have comparable quality to the commercial system.

Figure 1. (a) Swept source OCT setup with PARS-OCT probe. (b) Schematic of the application of PARS-OCT probe during vitrectomy, a surgical procedure for removal of vitreous humor. (c) Photograph of PARS-OCT probe and its beam steering principle.

Fig. 2. (a) Planar sweep pattern achieved when the two GRIN lenses are rotated with equal and opposite velocities. (b) Scan pattern as recorded by a CCD camera placed 2 mm from probe tip. (c) Measured spot size (FWHM) vs. tilt angle. Spot profile is also shown for selected tilt angles.

Figure 3. (a, b, and c) Porcine retinal images acquired by PARS-OCT probe. The retina layer was partially detached during the removal of vitreous humor. Some remnant vitreouses on the retina were still visible. (d) Retinal image from a commercial OCT microscope.