Instruments
The first models of anterior segment OCT used the time-domain principle with an approximate axial resolution of 18 µm and a 20,000 A-scans/sec scanning speed, allowing a wide image of the anterior segment (16 × 6 mm) with a possible scanning protocol with a higher-resolution scan of the cornea (5 × 5 mm).
Spectral-domain OCT was then developed; the first instruments increased axial resolution to 5 µm and the scanning speed to 26,000 A-scans/sec at the expense of scan width (6 mm). Wider scans, higher resolution, and faster acquisition are requirements.
Swept-source spectral domain OCT was incorporated into instruments, allowing a faster scanning speed (30,000–100,000 scans/sec), higher resolution (2.5–5 µm), and a larger scanning width. OCT instruments used for posterior segment evaluation can be adapted with lenses in order to evaluate the anterior segment.
Morphologic evaluation and quantitative evaluation of the cornea and the anterior chamber are available in all instruments capable of imaging the anterior segment. Different OCT manufacturers have different approaches to the imaging technique of the anterior segment. Some instruments are able to image anterior and posterior segments and some instruments are dedicated only to the anterior segment, with 3-dimensional reconstruction and corneal topography based on OCT.
Considering OCT angiography, the anterior segment vasculature can be evaluated modifying parameters of the equipment (as segmentation) to focus the structure to be studied: iris, corneal, or conjunctival vasculature. This technique is susceptible to the microsaccadic eye movements generating movement artifacts that could be compensated by a built-in eye-tracking system.
Scanning Protocols and Measurements
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Pachymetry map: Central, paracentral, and peripheral corneal thickness with information on average, minimum, and maximum thickness ( Figs. 6.1A and 6.1B ). Differential maps can be calculated when preoperative and postoperative scans are available. Pachymetry maps are displayed in sections and diameters from the central part of the cornea to the periphery.
Fig. 6.1
Quantitative analysis of the cornea in refractive surgery candidates. (A–D) SS-OCT, DRI OCT Triton OCT, Topcon. (A) Upper right shows the horizontal corneal scan, inferior left shows the direction of the scan from nasal to temporal, upper left shows the overlay of the area scanned in the anterior segment image. Inferior right: Display of the 12 corneal scans taken to generate the pachymetry map (scale 300 to 800 μm, pachymetry varying from 500 to 580 μm). (B) Correspondent pachymetry map generated. (C) Each of the 12 corneal scans was analyzed to determine epithelium thickness (inferior) and one scan showing how epithelium was outlined (upper) by the software. (D) Correspondent epithelial map thickness map (30 to 80 μm). (E) SD-OCT, Avanti TR-Vue XR, Optovue. Anterior segment image with the representation of the scan orientation, vertical from inferior to superior (upper left) , measurement details (inferior left) for pachymetry and epithelium thicknesses (minimum pachymetry: 516 μm; minimum epithelium thickness: 52 μm); correspondent image of the 6.0 mm vertical central scan of the cornea (upper right) , and representation of the pachymetry map on the left (average central pachymetry = 525 μm, mean thickness displayed for 2.0, 5.0, and 6.0 mm zones in 8 sectors); and epithelium thickness map to the right (mean central epithelium thickness: 56 μm, display for 2.0, 5.0, and 6.0 mm zones in 8 sectors).
(Courtesy Tecn. Claudio Zett Lobos.)
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Epithelial map: Epithelium thickness is evaluated in different quadrants and distances to the center ( Figs. 6.1C to 6.1E ). Differential maps can be calculated when preoperative and postoperative scans are available.
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Flap and stromal bed thickness can be measured with the flap caliper from central to periphery and displayed ( Fig. 6.2A ). Differential maps can be calculated when preoperative and postoperative scans are available.
Fig. 6.2
(A) Postoperative images after myopic laser in situ keratomileusis procedure. Flap and correspondent stromal bed thicknesses are displayed from central to periphery. Measurements adjacent to the central part are considered more reliable: flap varied from 163 to 180 μm and stromal bed thickness varied from 329 to 293 μm. (B) Anterior chamber depth was measured from endothelium to crystalline lens: 3.20 mm; central corneal thickness: 480 μm, and internal anterior chamber diameter: 12.70 mm. (C) Safety distance measured from endothelium to central and anterior border of the phakic intraocular lens (IOL). In the example, an angle-supported phakic IOL. Central safety distance: 1.77 mm (left side) and 1.64 mm (right side) . Lens vault or the distance of the phakic IOL to the crystalline lens is also measured: 0.86 mm. (D) Angle-supported phakic IOL. Ideal safety distance of the phakic IOL to the endothelium is higher than 1.5 mm, displayed in green. A safety distance smaller than 0.5 mm, displayed in red, would be indication for IOL explantation. (E) Posterior chamber phakic IOL. Lens vault measured: 0.71 mm.
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Anterior chamber depth: Measurement of the distance between the endothelial face of the cornea and the anterior face of the crystalline lens ( Fig. 6.2B ).
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Internal anterior chamber diameter ( Fig. 6.2B ): Distance measured from the endothelium to the anterior lens capsule.
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Lens rise: Measurement of the perpendicular distance between the anterior pole of the crystalline lens and a horizontal line joining the 2 scleral spurs on a horizontal scan. In a normal eye, “lens rise” can increase by 20 µm per year and can be related to complications seen in anterior segment phakic intraocular lenses (PIOLs).
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Safety distance: Distance measured from the endothelium to the anterior surface of the anterior chamber intraocular lens (IOLs; phakic or aphakic lens; Figs. 6.2C and 6.2D ).
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Lens vault: Distance measured from the posterior face of the PIOL to the crystalline lens ( Figs. 6.2C and 6.2E ).
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Distance measured from scleral spur to scleral spur or posterior chamber internal diameter: Significant in the preoperative planning of posterior chamber PIOLs.
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Calipers: Measurements of any structure can be provided using manually placed calipers.
Preoperative Evaluation
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Corneal pachymetry map: The pachymetry map is not usually integrated with the keratometric map. There are systems that display the pachymetry map acquired by anterior segment optical coherence tomography (ASCOT) integrated with corneal topography acquired by the topographer when acquisition is performed in both machines in order to generate a combined map.
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Epithelial thickness map: Preoperative acquisition is needed in order to follow epithelial thickness during the follow-up of corneal procedures (surface and lamellar procedures (photorefractive keratectomy [PRK] and laser in situ keratomileusis [LASIK]).
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Anterior chamber depth: Important in the preoperative evaluation of PIOL implantation, decisive in angle-supported PIOLs and evaluated with care in iris-fixated and posterior chamber PIOLs.
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Angle opening: Used as screening for PIOLs that cannot be implanted in a shallow angle.
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Anterior chamber diameter: Can be used to determine the total diameter of angle-supported PIOLs.
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Posterior chamber diameter or sulcus-to-sulcus: Used to determine adequate total diameter of posterior chamber PIOLs.
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Crystalline lens rise: A positive anterior lens vault is related to a potential angle closure and can be assessed preoperatively in order to detect this condition or to have this information in order to follow this parameter in the postoperative period.
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In the preoperative evaluation of PIOLs, angle opening, iris configuration, and patency of laser peripheral iridotomies can be evaluated.
Postoperative Evaluation
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Radial and astigmatic keratotomy incisions: Scar depth, posterior stroma thickness, and epithelial plugs can be identified using AS-OCT. Complications at local microperforation sites can be identified when there is no posterior stroma left. Epithelial plugs, epithelial hyperplasia, and hypertrophic scars can be detected.
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PRK and different surface procedures: Differential maps can compare preoperative and postoperative corneal thickness; the difference is related to the treatment obtained or to the depth of ablation ( Figs. 6.3A and 6.3B) .
