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Anterior Segment Optical Coherence Tomography (AS-OCT)

Anterior Segment Optical Coherence Tomography (AS-OCT)AS-OCT is a non-contact imaging modality utilizing 1300nm of infrared light. Like Ultrasound Biomicroscopy (UBM), it is specifically designed for imaging various conditions involving the anterior segment of the eye. The AC Cornea OCT (Ophthalmic Technologies Inc. [OTI], Toronto, Ontario, Canada), in particular, is a prototype time-domain OCT system capable of scanning at a rate of 1,000 lines per second. It acquires both coronal (transversal, en face, C-scan) and cross-sectional (longitudinal, B-scan) OCT images from 1-12 frames per second. It has an axial resolution of 10-15µ and a transverse resolution of 15µ.

AS-OCT can image the conjunctiva, cornea, iris and sclera (Figs.1-2).

Figure 1 - OCT of normal anterior segment

Fig. 1 This is a horizontal cross-sectional OCT image of a normal anterior segment.

Figure 2A - Hazy cornea due to detachment of Descemet's membrane

Fig. 2A This is an eye with hazy cornea due to detachment of Descemet’s membrane as shown by the slit beam.

Figure 2B - Corneal bullae and separation of Descemet's membrane

Fig. 2B Horizontal OCT image just below the pupil shows the corneal bullae and demonstrates separation of Descemet’s membrane from the rest of the cornea.

The AC Cornea OCT, in particular, can uniquely display images in a coronal manner which helps define spatial relationships of structures observed on cross-sectional OCT views.

Figure 2C - Coronal OCT reveals the nature of detachment and location of the break

Fig. 2C Coronal OCT image reveals the nature of detachment and location of the break.

To a limited extent, the AS-OCT can image the ciliary body.

Figure 3 - Hypotonic eye depicting ciliochoroidal edema

Fig. 3 This is a diagonal OCT image of a hypotonic eye depicting ciliochoroidal edema.

The AS-OCT is a quick tool for screening the angles.

Figure 4 - Magnified OCT of the angle showing Schlemm's Canal

Fig. 4 This is a magnified OCT image of the angle showing Schlemm’s canal.

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AS-OCT is good for documenting implant devices (intraocular lenses, glaucoma implants) positioned under the conjunctiva or in the anterior chamber (Figs.5A,B). It is not effective for imaging implants in the posterior chamber unless the eye is fully dilated (Fig.6).

Figure 5A - Glaucoma tube under the conjunctiva

Fig. 5A This is a diagonal OCT image of a glaucoma tube under the conjunctiva.

Figure 5B - The tube disappears as it enters the posterior chamber

Fig. 5B The tube disappears as it enters the posterior chamber.

Figure 6A - Diagonal OCT image demonstrating a glaucoma tube implanted under the conjunctiva

Fig. 6A This is a diagonal OCT image demonstrating a glaucoma tube implanted under the conjunctiva and inserted into the anterior chamber; an intraocular lens (PCIOL) is also visible through a dilated pupil.

Figure 6B - Coronal OCT demonstrates the path of the tube

Fig. 6B Coronal OCT image demonstrates the path of the tube.

AS-OCT may be suitable for screening cases where UBM cannot be performed (infected eyes, ruptured globes, incompletely-healed post-surgical eyes, post-tarsorrhaphy cases). However, it is largely limited by light-scattering property of the sclera, iris and surface elements (dense opacities, pigment deposits, thick tumors, corneal grafts and/or abnormal blood vessels), diminishing its capability and the quality of AS-OCT images generated.

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