18 Use of Optical Coherence Tomography in Femtosecond Laser Lens and Cornea Surgery
Since its development in 2001, optical coherence tomography (OCT) has demonstrated increasingly wide use across a range of ophthalmic applications. 1 , 2 Although the technology was first described for imaging of the peripapillary retina and coronary artery, it has subsequently been used for evaluation of the cornea, iridocorneal angle, crystalline lens, vitreous, vitreoretinal interface, optic nerve, retina, choroid, and vasculature of the eye. 3 High-resolution, noninvasive imaging of these structures has imparted significant value to practitioners in a variety of clinical scenarios: (1) OCT has become the gold standard for establishing some diagnoses (e.g. macular pucker), (2) it can be used to follow the effects of treatment (e.g., for exudative age-related macular degeneration), and (3) it can guide decision making during surgery (e.g., with intraoperative assessment of macular hole closure). 4 , 5 , 6 , 7 , 8 In coupling OCT with femtosecond lasers, however, we find the first instance of OCT (4) forming the imaging basis for direct guidance of a surgical device.
In early 2001, the same year in which OCT was first described in the literature, the IntraLase Corporation (Irvine, CA) received 510k clearance from the U.S. Food and Drug Administration (FDA) to market a femtosecond laser for corneal flap creation. The primary virtue of femtosecond laser technology was that it had the capacity to precisely photodissect ophthalmic tissue without collateral photodisruption. The marriage of optical coherence and femtosecond laser technologies into an integrated diagnostic-therapeutic device was consummated 8 years later. In August 2009, the U.S. FDA granted 510k approval for an OCT-guided femtosecond laser for the fashioning of an anterior capsulotomy at the time of cataract surgery.
18.1 Femtosecond Lasers Using OCT Guidance
Three OCT-guided laser platforms are presently available for use in femtosecond laser lens surgery and laser-assisted in-situ keratomileusis (LASIK) surgery. The Lensx (Alcon Laboratories, Ft. Worth, TX), Catalys (Abbott Medical Optics, Santa Ana, CA), and Victus (Bausch & Lomb, Rochester, NY) technologies use a three-dimensional (3-D) spectral-domain OCT along with a video microscope to enable image-guided femtosecond laser application. These lasers can perform anterior capsulotomy, crystalline lens fragmentation, corneal incisions, and astigmatic keratotomy. Additionally, the Victus and Lensx platforms can be used for lamellar flap creation at the time of LASIK surgery.
The Lensx was the first of these lasers introduced to the market. Its first commercial use was in December 2009 by Dr. Zoltan Nagy of Semmelweis University in Budapest, Hungary. 9 Lensx has received approval for anterior capsulotomy, phacofragmentation, partial- and full-thickness corneal incisions, and corneal flap creation in LASIK.
From an imaging standpoint, the Lensx laser system OCT uses spectral-domain OCT technology. It performs a high-resolution, cross-sectional tomographic scan of the internal microstructure of the eye by measuring backscattered light. It scans up to 8.5 mm deep to enable anterior-segment imaging from the anterior surface of the cornea to the posterior lens (Fig. 18.1). The Lensx laser system uses a combination of circle scan and line scan to identify anterior segment landmarks. On the video microscope OCT screen, the circle scan can be seen to the right of the video image during the docking process (Fig. 18.2). The OCT beam rotates about the laser system 360 degrees at a set diameter consistent with the capsulotomy size (Fig. 18.3). A column in the Z direction is collected at each point along the circumference of the circle scan; the summation of these points forms a cylinder. The cylinder is subsequently unwrapped, creating the displayed OCT image (Fig. 18.4 , Fig. 18.5). In the circle scan, several reference lines and control points are shown to indicate landmarks. The dotted crosshair identifies the highest and lowest points of the anterior capsule. The dotted vertical line identifies maximum lens tilt and dictates the position of the OCT line scan (Fig. 18.6). The line scan is a cross-sectional image at the highest angular deviation or maximum lens tilt identified on the circle scan (Fig. 18.7). Combination of the circle scan and line scan gives the 3-D corneal and lens alignment information. Image analysis algorithms assist users in identifying interfaces inside the anterior segment by prepositioning control points. The user must review and accept OCT images individually.
The Lensx OCT system is capable of imaging the posterior lens surface, even in patients with hypermature cataracts (Fig. 18.8) and brunescent cataracts (Fig. 18.9). In patients with subcapsular cataracts, the OCT gives high resolution and images that can assist the surgeon in planning lens removal in the operating room, even where cataract density prevents femtosecond laser phacofragmentation (Fig. 18.10).
Along with the other two platforms, the Catalys femtosecond laser system employs 3-D spectral-domain OCT to scan the anterior segment. Catalys computer guidance systems use these data to identify anterior-segment landmarks (Fig. 18.11) and assist in treatment planning.
The system’s 3-D OCT includes more than 10,000 A-scans of the anterior segment, which are completed in a spiral pattern, as illustrated in Fig. 18.12. This extent of OCT imaging coverage is similar to that of the Victus laser but distinct from the Lensx laser, which uses OCT imaging only along a z-axis corresponding to the anterior capsulotomy diameter. The OCT operates at a wavelength of 820 to 930nm with an axial resolution of 30 μm and a lateral resolution of 15 μm. Because of the unique laser wavelength and favorable signal-to-noise-ratio, the Catalys OCT can image through the posterior capsule. It has been reported as effective in the peer-reviewed literature and conference proceedings to achieve good visualization with intumescent 10 and brunescent 11 cataracts. It can effectively illustrate variations of normal anatomy, such as in the case of a thickened crystalline lens in a crowded anterior segment. It has also demonstrated good posterior capsule visualization for performing bag in-the-lens IOL implantation.
After OCT image acquisition, algorithms automatically map each surface (anterior cornea, posterior cornea, iris, limbus, anterior and posterior lens) and account for possible tilt. Catalys is the only platform to perform automatic mapping of these structures guided by OCT. The sagittal and axial cross-sections (Fig. 18.13 , Fig. 18.14 , Fig. 18.15) demonstrate the presence of lens tilt with surface maps overlaid (Fig. 18.16). Using ocular surface identification, the software can display central corneal thickness, aqueous depth, and anterior chamber depth, among other dimensions.
After the ocular surfaces are identified, safety zones and the incisions are calculated. In surface mapping guides, laser pulses are placed according to a specified pattern based on the full-volume scan without relying on manual surface selection. Catalys is unique among the laser platforms in that it is capable of compensating incisions for tilt, including capsulotomy, fragmentation, primary cataract incisions/side ports, and arcuate incisions. Each incision is confirmed independently on the cross-section and en face view (Fig. 18.17). Once the incisions have been customized, the OCT images begin streaming at a frequency of 0.5 Hz, continuously refreshing with the overlays displayed (Fig. 18.18). The surgeon has the capability to confirm the automated surface fits throughout the procedure, with the OCT images streaming underneath for confirmation or detection of eye movement. Once confirmation of surfaces is complete, the surgeon confirms the incisions and fires the laser (Fig. 18.19). The Catalys laser has received FDA approval for anterior capsulotomy, phacofragmentation, primary cataract incisions and side ports, and arcuate incisions.