Figure 1: Iris Cyst
Anterior Segment Optical Coherence Tomography (AS-OCT)
Sarah Moyer, CRA, OCT-C
Director, Ophthalmic Imaging
University of North Carolina
Chapel Hill, North Carolina
OCT is a well known and frequently used technology to image the posterior segment. Izatt et al. published the first report of OCT imaging of the cornea in 1994.1 In October 2005 Carl Zeiss Meditec, Inc, Dublin, CA released the Visante®, an anterior segment OCT. Using time domain OCT technology (TD-OCT), the Visante® creates cross-sectional images of anterior segment structures (Figure 2). It also provides measurement tools to document and follow changes in the cornea, angle, and anterior chamber.
Figure 2: Normal eye imaged on Visante®
In the fall of 2007, with patent restrictions out of the way for the new spectral domain OCT (SD-OCT) technology, several manufactures released new posterior segment SD-OCTs. Instead of developing stand alone AS-OCTs, several SD-OCT manufactures developed software and hardware solutions that allow their posterior segment SD-OCTs to also create AS-OCT images. While some of the manufacturer's hardware solutions are internal optics changes, others require external optics to be swapped out with posterior segment optics or to be mounted in addition to their posterior segment optics (Figure 3). Like the time domain AS-OCT, measurement tools are provided to document and follow changes specific to anterior segment anatomy.
Figure 3: Patient at RT-Vue® with CAM-L lens attached for AS-OCT.
Photo: Bruno Bertoni, CRA, OCT-C; Tamera Schoenholz, CRA, OCT-C
AS-OCT helps cornea and glaucoma specialists follow, diagnose and treat their patients. Cross-sectional images are most commonly used to review the cornea, angle and anterior chamber. Images of the cornea are performed on patients with keratoconus, corneal scars and corneal dystrophies. Both manual and automatic corneal pachymetry measurements quantify corneal disease. Images of the angle are commonly performed to quantify the angle for angle closure glaucoma and attempt to identify the scleral spur, Schlemm's canal, Schwalbe's line and trabecular meshwork.2 Anterior chamber biometry is helpful for refractive surgery. Imaging the iris can document iris cysts (Figure 1), iris nevus, iris melanoma and iridoschesis.3 Imaging the lens informs physicians of the location of intraocular lenses (IOLs). Because of the limitations of light, imaging the lens, posterior to the iris and ciliary body can be difficult. Images of anterior segment cysts and tumors are sometimes possible, but are generally imaged with ultrasound biomicrscopy (UBM) technologies.4 Images of the sclera are occasionally clinically useful.
AS-OCT is an excellent preoperative and postoperative tool to evaluate and manage patients with: blebs, intrastromal corneal rings, full-thickness penetrating keratoplasty (PK), descemet-stripping endothelial keratoplasty (DSEK), deep lamellar endothelial keratoplasty (DLEK), IOLs and laser-assisted in situ keratomileusis (LASIK).
Time Domain and Spectral Domain Advantages and Limitations
It is important to note the differences between the Visante® and SD-OCTs with AS-OCT capability. Both technologies have advantages and limitations based on the way they are designed. The advantage to the Visante® is its ability to image the ciliary body and pathology shadowed by the iris better than SD-OCTs. The Visante® uses a higher wavelength of light (1310nm instead of 830nm or 870nm used in SD-OCT) that is strongly absorbed by water. When using 1310nm, the vitreous protects the retina. With this added protection, the optical power of the Visante® is able to be 20 times more powerful than the SD-OCT technology that also images the retina.5 With a stronger optical power, the Visante® penetrates deeper to image the ciliary body better than OCTs with a lower wavelength of light (Figure 4). Even though the Visante® is able to penetrate deeper than the SD-OCT technology, UBM is able to penetrate deeper.
Figure 4: Normal eye of same patient. (Left) Imaged by Visante®. Photo: Rona Lyn Esquejo-Leon, CRA (Right) RT-Vue® Photo: Carl Denis, CRA, OCT-C. White Arrows: Visante's high wavelength of light and stronger optical power allows for better images of the ciliary body.
As the Visante® is a TD-OCT, it faces the same major restriction that posterior segment TD-OCT machines face compared to SD-OCT technology: TD-OCT uses a moving reference mirror that limits its scan speed and ultimately its resolution. The stable mirror in SD-OCT allows faster image capture and increased resolution. With higher resolution, details in anterior segment pathology are better appreciated (Figure 5).
Figure 5: Unsuccessful post-op DSEK time domain and spectral domain comparison. (Top) Visante® scan 16 x 6mm. (Bottom) RT-Vue® horizontal scan 6 x 2mm. The RT-Vue® image has increased resolution over the Visante® image. Note: Images are from different patients. Bottom photo: Bruno Bertoni, CRA, OCT-C; Tamera Schoenholz, CRA, OCT-C
Corneal Reflex Artifact
For all imaging modalities, it is important to understand the artifacts that can be produced. Recognizing artifacts and understanding how they were created is critical to differentiate artifacts from pathology. When the cross-section of an AS-OCT image is on a corneal meridian, a vertical white beam (central vertical flare) appears in the anterior chamber and a small hyper reflective area appears on the corneal surface on both the Visante® and SD-OCT images (Figure 6).5 When performing corneal pachymetry, it is important to create this corneal reflex artifact on the vertex in order to get an accurate pachymetry reading.
Figure 6: Corneal Reflex artifact. When the cross-section is centered on the vertex of the cornea, a vertical white beam appears in the anterior chamber and a small hyper reflective area appears on the corneal surface. (Left) on Visante®. Photo: Rona Lyn Esquejo-Leon, CRA (Right) on RT-Vue® Photo: Bruno Bertoni, CRA, OCT-C; Tamera Davis, CRA, OCT-C
- Izatt JA, Hee MR, Swanson EA et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112: 1584–9.
- See JLS. Imaging of the anterior segment in glaucoma. Clinical and Experimental Ophthalmology 2009; 37: 506-513
- Ramos BLJ, Li Y, Huang D. Clinical and research applications of anterior segment optical coherence tomography – a review. Clinical and Experimental Opthalmology 2009; 37: 81-89.
- Bianciotto C, Shields CL et al. Assessment of Anterior Segment Tumors with Ultrasound Biomicroscopy versus Anterior Segment Optical Coherence Tomography in 200 Cases. Ophthalmology 2011; 118: 1297-302.
- Steinert R, Huang D. Anterior Segment Optical Coherence Tomography,1st edn. Thorofare, NJ: SLACK Inc., 2008.