Introduction:
Imaging technologies play a pivotal role in clinical examinations and the diagnosis and management of various ocular pathologies. Ocular imaging modalities for the anterior segment include Scheimpflug tomography, high-resolution ultrasound biomicroscopy, in vivo confocal microscopy, corneal topography, and optical coherence tomography (OCT). Initially, OCT was developed for imaging of the posterior segment, however, in 1994, Izatt et al first reported the OCT image of the corneal and anterior segment from then it has been gaining popularity for ocular surface and anterior segment imaging. Anterior-segment OCT (AS-OCT) has become an important tool in ophthalmology clinical practice.
What Is Anterior Segment Optical Coherence Tomography (AS-OCT)?
Anterior segment optical coherence tomography is a revolutionary technology that has changed the way healthcare professionals examine and treat anterior segment pathology. This technology provides ultra-high resolution, dynamic, cross-sectional images in a noninvasive and fast manner. They are user-friendly and can take images rapidly hence can be used by operators with any level of experience. These devices are well-tolerated by patients. OCT offers 3 to 5-micron-level resolution and real-time imaging capabilities that allow for the assessment of subtle nuances in cellular architecture and tissue morphology without the need for invasive procedures or contrast agents. These excellent-resolution images make it easy for novice as well as experienced clinicians to interpret.
How Does Anterior Segment Optical Coherence Tomography (AS-OCT) Work?
AS-OCT works on the principles of low-coherence tomography and near-infrared light. It is a non-invasive imaging technique that utilizes light to produce high-resolution cross-sectional images of the structures of the eyes. AS-OCT uses a near-infrared light source, generally with wavelengths ranging from 800 to 1300 nanometers. Near-infrared penetrates structures of the eyes with minimal scattering and absorption, making deeper imaging of the anterior segment structures possible. AS-OCT has also become an important diagnostic tool for preoperative planning and postoperative checking in ophthalmic surgeries.
During the procedure, a light beam is emitted from the AS-OCT device. The emitted light is directed towards the eye where some light is reflected from the different parts within the anterior segment. In AS-OCT, the emitted light is divided into two paths: the reference arm and the sample arm. The reference arm light is directed to a mirror, and the sample arm light is directed towards the eye. When the light from the sample arm interacts with the eye tissues, it creates interference patterns. This is because of the optical path length differences between the sample and reference arms. These interference patterns carry information about the reflectivity and depth of the structures within the anterior segment. The depth of the eye structures is calculated by measuring the reference and sample light beam time delay. This information about the depth allows the AS-OCT to produce cross-sectional images of the anterior segment of the eyes with micron-level resolution. Advanced signal processing techniques aid in enhancing image quality and reducing noise, resulting in high-quality pictures with more details.
The cross-sectional images are displayed in real-time on a monitor, allowing the eye care to view the structures of the anterior segment. AS-OCT systems come with different types of imaging modes and analysis tools, this is useful to provide image interpretation, comparison, and quantitative measurements. The images generated by AS-OCT are of high-resolution quality so they aid valuable diagnostic information for different kinds of anterior segment conditions, including glaucoma, corneal diseases, lens disorders, anterior segment tumors, and iris abnormalities.
What Are the Clinical Applications of Anterior Segment Optical Coherence Tomography?
1) AS-OCT For Anterior Segment Surgery Planning: AS-OCT has proven to be an efficient diagnostic tool to monitor the success and complications of several anterior segment surgical procedures, such as Descemet membrane endothelial keratoplasty (DMEK), Descemet stripping automated endothelial keratoplasty (DSAEK), laser-assisted in situ keratomileusis (LASIK), post-cataract surgery, and Boston Keratoprosthesis (KPro) implantation. It can be used to check for various abnormalities in individuals with persistent postoperative corneal edema. AS-OCT can also be used to check for flap dislocations after LASIK and also identify macro striae (larger and full-thickness folds in the cornea), flap edema, epithelial hyperplasia (increased number of cells) and epithelial ingrowth (ingrowth of epithelium into the corneal flap interface) after surgery.
It can be also used during lamellar keratoplasty surgery to assess the effectiveness of Descemet’s membrane stripping (peeled off the Descemet membrane), confirm graft adherence and centration, and check for the presence of interface fluid between the host cornea. With post-operative images overall graft thickness, long-term graft adherence, and even epithelial remodeling can be checked and verified.
2) Architectural Analysis: AS-OCT images can be used to analyze corneal microarchitecture and epithelial thickness in individuals with postoperative corneal ectasia and early keratoconus. AS-OCT can also be incorporated in evaluating the cross-linking effectiveness by monitoring corneal demarcation lines, which are areas with changes in stromal reflectivity and to calculate the depth of penetration of collagen crosslinking treatment. Additionally, it can also be used to check for any corneal changes and abnormalities in the depth and position of intracorneal ring segments.
3) Glaucoma Management: The anterior chamber angle which is formed by the cornea and iris, is important for the maintenance of intraocular pressure and outflow of aqueous humor. If the angle is narrow or closed it can lead to angle-closure glaucoma. AS-OCT provides a clear visualization of the angle structures. This helps the eye care specialist assess angle depth, width, and configuration. This information is vital in the diagnosis and treatment of glaucoma easing the treatment decisions such as trabeculectomy and laser peripheral iridotomy.
4) Infectious keratitis: AS-OCT is used to image microbial keratitis. In cases of Acanthamoeba keratitis, keratoneuritis can be noticed as highly reflective lines present from the anterior to the mid stroma. AS-OCT images can be used to monitor corneal thickness, especially where there is more thinning. Corneal scarring will appear as areas of stromal or subepithelial hyperreflectivity. By using AS-OCT images the depth of corneal opacities can be calculated which will be helpful for the surgeons in determining the optimal surgical procedure for rehabilitation, which can vary from deep anterior lamellar keratoplasty (DALK), phototherapeutic keratectomy (PTK), or penetrating keratoplasty (PK).
Conclusion:
Anterior segment optical coherence tomography (AS-OCT) provides ultra-high resolution, dynamic, cross-sectional images. It is a non-invasive technique that provides images rapidly. It works on the principle of low-coherence tomography and near-infrared light. It has revolutionized diagnostic and therapeutic approaches for various kinds of eye pathologies. With ongoing technological advancements, AS-OCT continues to evolve in the ophthalmic practice.
