Optical Coherence Tomography in Cornea and Refractive Surgery

Volkan Hurmeric; Sonia H Yoo; Fatih Mehmet Mutlu

Disclosures

Expert Rev Ophthalmol. 2012;7(3):241-250. 

In This Article

Comparison of AS-OCT With Alternative Anterior Segment Imaging Techniques

All anterior segment imaging devices promise quantitative information and qualitative imaging of the ocular structures. There are relative strengths and weaknesses of these techniques (Table 1). It must not be forgotten that the results of different anterior segment imaging modalities are correlated, but they are not interchangeable. Every system depends on its own technology, which is not equivalent and the results could be significantly different.[4–8] Because of this, anterior segment surgeons must be aware of the limitations of every imaging system in various clinical scenarios (Table 1).[9]

Lateral slit illumination, slit scanning tomography and Scheimpflug imaging are the most commonly used noncontact optical methods in anterior segment evaluation.[10] Optical systems prepare repeatable and reliable topographic and elevation maps and analyze the morphology of the anterior segment.[11,12] The most important drawback of the optical systems is that they are affected by cornea clarity, pupil diameter and iris reflections.[13,14] The resolutions of optical imaging systems are also limited and cannot be used in the evaluation of corneal layers in detail.

Ultrasound biomicroscopy (UBM) is the first available anterior segment imaging system that is especially useful in the evaluation of structures behind the iris. However, UBM is a time-consuming and relatively invasive technique that requires water immersion.[5] UBM also requires a highly skilled operator and most of the systems require supine positioning of the patients. The image quality of UBM is also limited in the evaluation of the cornea, which makes it rarely used in the preoperative and postoperative evaluation of patients undergoing corneal surgeries.[9]

There are several advantages of AS-OCT over the above-mentioned imaging modalities. The image resolution of AS-OCT is higher than that of the optical analyzers and UBM (Table 1). AS-OCT can be used to prepare high-quality cross-sectional and 3D images of the anterior segment.[15–17] AS-OCT systems can also prepare reliable pachymetric maps of the cornea (Figure 1).[18] Recent advancements have enabled several AS-OCT systems to prepare not only topographic analysis but also anterior and posterior elevation maps of the cornea.[19] Besides the noninvasive nature, the quick acquisition time (it is completed in seconds) makes AS-OCT an ideal imaging technique for use in handicapped and elderly patients.

Figure 1.

Pachymetric map of a patient with keratoconus prepared with Visante® optical coherence tomography.

AS-OCT systems use light sources with longer infrared wavelengths (1310 nm) to improve the penetration through tissues that scatter light, such as sclera and limbus, and allow the visualization of the iridocorneal angle. Unlike posterior segment OCT, AS-OCT systems require greater depth of field, which decreases transverse resolution. AS-OCT systems also require higher energy levels than the retinal OCT systems. Furthermore, dense opacities in the ocular surface and iris pigments deteriorate the quality of images in AS-OCT.[7] Because of this, visualization of retro-iris structures is very limited in AS-OCT. UBM has a better image quality for imaging structures posterior to the iris plane (Table 1).[20]

processing....