ANTERIOR SEGMENT OCT
Analyzing the Anterior Segment
Optical coherence tomography (OCT) has proven to be a useful tool in diagnosing and managing retinal and optic nerve disease. Recent technology has progressed to include examining the anterior segment. Anterior segment OCT (AS-OCT) generates in vivo, cross-sectional scans of the tissue to assist in analyzing the cornea, anterior chamber angle, iris and lens. Exceptional quality images, captured at a high speed rate, allow practitioners to embrace the new technology once reserved for the posterior segment.
What is an OCT?
OCT utilizes near-infrared light waves to measure distances of anatomical structures. A beam of light is directed onto the structure and the echo time delay of light is then recorded. Employing low-coherence interferometry, the reflected light from the eye is compared to a reference value of a known length. A series of axial scans (A-scans) are combined to form two-dimensional images of the ocular structures in a process similar to ultrasound biomicroscopy; however, light (as opposed to sound waves) is used in OCT. Cross-sectional images are then generated by scanning the incident optical beam. The resultant scans are displayed in a color scale. Using these principles, two OCT platforms have been developed: time domain and spectral (or Fourier) domain. Time domain OCT (TD-OCT) instrumentation utilizes a moveable reference mirror. The mirror moves for each A-scan to determine the ocular structure’s depth, thereby limiting the speed at which the image is acquired. Spectral domain OCT (SD-OCT) has a fixed reference mirror to measure the depth information and uses a Fourier transformation algorithm of the spectral interferogram to produce the A-scan, which results in faster acquisition and better image quality. AS-OCT resolution is based on wavelength and bandwidth. A 1,310nm wavelength captures data at a rate 20 times faster than the original 820nm wavelength utilized in posterior segment examination by TD-OCT. The longer wavelength also allows for better penetration into the sclera, iris and anterior chamber angle.
AS-OCT is a non-contact procedure and is more user-friendly when compared to ultrasound biomicroscopy (UBM). Limitations still exist with the advancing technology. AS-OCT cannot completely image beyond the pigmented epithelium of the iris due to light absorption by this layer. Comparatively, UBM can clearly image the ciliary body, lens and zonules. However, AS-OCT can capture the crystalline lens, posterior chamber intraocular implants or phakic implantable lens.
AS-OCT and the Cornea:
AS-OCT can assist in diagnosis and documentation of corneal conditions such as dystrophies and degenerations, as well as assorted inflammatory pathologies. This technology can be used to diagnose and manage corneal infiltrates, ulcers, dellen or scars. The depth and extent as obtained by AS-OCT are superior to that of anterior segment photography. Monitoring of corneal infection is possible especially when deep, aggressive infection occurs. Hypopyon or hyphema involvement can also be monitored as a result of severe infection or trauma. AS-OCT has the capability to determine the depth of a foreign body or the presence of residue once removed.
Corneal dystrophies affect one or more layers of the cornea. High definition images obtained by AS-OCT allow the practitioner to visualize the epithelium, stroma and endothelium with clear differentiation. Monitoring Fuchs’ endothelial dystrophy is another capability of this technology. Descemet’s detachment, corneal guatta and edema can be imaged and observed. In some severe cases, Fuchs’ results in bullous keratopathy. AS-OCT can document corneal topography preand postDescemet’s stripping automated endothelial keratoplasty (DSAEK) in the treatment of Fuchs’ or pseudophakic bullous keratopathy (PBK). Surgeons can visualize the interface and document its depth in DSAEK patients. AS-OCT can also document thickness changes over time. Corneal inlay position with possible resultant refractive effect can also be monitored by OCT imaging. AS-OCT has the capability to image keratoconus and other ectastias to determine and document the severity of thinning and scarring. While moderate to severe keratoconus can be identified via slit lamp examination and topography, subclinical or form fruste keratoconus and other thinning conditions can be recognized with pachymetry maps. These cases are especially important to identify prior to refractive laser surgeries.
AS-OCT and Refractive Surgery:
Refractive surgery has benefitted greatly from AS-OCT technology. Preoperatively, patients can be scanned for any abnormalities. Postoperatively, LASIK epithelial ingrowth can be monitored closely (figure 3). Not all epithelial ingrowths need to be treated, therefore measuring thickness of the epithelium and the LASIK flap help differentiate between the cases that must be treated by lifting the flap and those that can be monitored. A thinning flap due to epithelial ingrowth, especially centrally, warrants significant concern. AS-OCT also images the architecture of the flap and the thickness of the residual corneal bed. When considering post-LASIK enhancement, proper assessment of the flap and residual bed thickness can easily be made with accurate measurement using the caliper tools. Unexpected corneal ectasia postoperatively can also be assessed. AS-OCT has been utilized in monitoring the epithelial healing progression under therapeutic contact lenses (TCL) following lamellar keratoplasty and epiLASIK.10 Visualizing the epithelium postoperatively allows the practitioner to remove the TCL at the appropriate time without traumatizing the new epithelium or risking a secondary infection by leaving the TCL in longer than necessary. OCT imaging of the anterior segment can also be beneficial in difficult RGP fits. AS-OCT can measure the curvature of the peripheral cornea and sclera, giving practitioners better insight to a successful fit.
AS-OCT and Glaucoma:
AS-OCT provides direct visualization of the anterior chamber angle including the scleral spur, ciliary body and ciliary sulcus. Schlemm’s canal can also be observed on some scans. Studies have shown AS-OCT analysis of the angle is accurate, repeatable, and correlated to findings of traditional gonioscopy. Traditional gonioscopy suffers from artifact due to light and indentation errors. AS-OCT allows the practitioner to view the anatomical angle features in both light and dark conditions, thereby allowing a better evaluation for possible angle closure. AS-OCT can identify the different mechanisms responsible for angle closure glaucoma including pupillary block, plateau iris syndrome and lens-related mechanisms. The scleral spur insertion landmark is located where the less reflective ciliary muscle contacts the more reflective sclera. By first identifying the scleral spur, the trabecular meshwork distance can be measured in addition to viewing the ciliary body’s curvature and the depth of the anterior chamber. Properly identifying patients in need of an iridotomy is an important responsibility of eye care practitioners. Iridotomy procedures have shown dramatic improvement of the angle structure when pupillary block mechanisms are present. However, when certain patients’ angles do not improve as expected following iridotomy, nonpupillary block mechanisms (i.e., plateau iris, lens-related anterior rotation of the iris) can be easily identified. The characteristic iris configuration and thick profile in plateau iris syndrome is visualized by the AS-OCT. Practitioners can also observe blebs and implants by scanning the affected tissue. A limitation of the ASOCT during angle imaging is the inability to visualize pathology causing primary or secondary glaucoma including trabecular pigment or narrow bands of peripheral synechiae.
AS-OCT is a valuable tool in surgical planning. Preoperative assessment of phakic IOL patients—as well as those who have undergone corneal transplant, LASIK, DSAEK and other corneal procedures—can assist in obtaining accurate measurements. These measurements include angle recess-to-angle recess distance estimates, measurements of posterior phakic lens vault and predicting potential for iris pigmentary dispersion. The risk of pigment dispersion is calculated using crystalline lens rise (CLR). CLR is an indirect measurement of iris convexity found by using the distance between the anterior pole of the crystalline lens and the horizontal line joining the iridocorneal recesses.17 This measurement can also be utilized in determining whether laser peripheral iridotomy (LPI) or crystalline lens extraction is appropriate in treatment of narrow angle patients.17 Lensectomy is indicated in patients with a CLR of 0.8mm, a chamber depth less than 2mm, and angles less than 15º. Eye care practitioners can also use AS-OCT to examine the density of nuclear sclerosis or subcapsular cataracts and the centration of phakic or pseudophakic implants. AS-OCT can also be useful in helping surgeons determine the proper ablation for phototherapeutic keratectomy (PTK). By establishing the opacity depth and the epithelial thickness, the practitioner can properly remove the opacity and estimate the amount of induced refractive change. It also allows the practitioner to determine if the scar is too deep for PTK to be as effective as with deep posterior stromal scars. OCT imaging of the anterior segment can quantitatively monitor pigmented lesions of the iris, sclera, and angle. Changes of these lesions, including seeding into the angle or enlargement, can be easily identified.
INTEGRAL GUIDANCE System using Catalys Procedure
During the CATALYS® System procedure, the ocular surfaces are visualized by a proprietary, integrated Optical Coherence Tomography (OCT) system. The OCT is enhanced by sophisticated algorithms designed to ensure that the femtosecond laser pulses are delivered precisely to the intended location.