What are the recommendations for generating a good image of anterior segment optical coherence tomography (AS-OCT)?

Generating High-Quality Anterior Segment OCT Images

To generate optimal AS-OCT images, ensure proper patient positioning with minimal head tilt, use the crosshair alignment system to reduce eye misalignment, and obtain images in a dark room to maximize angle visualization—the quality of AS-OCT imaging depends critically on proper technique and patient cooperation. 1

Key Technical Considerations

Patient Positioning and Alignment

• Minimize head tilt and eye misalignment using crosshair alignment systems built into AS-OCT devices, as misalignment significantly degrades image quality and measurement accuracy 2
• Position the patient comfortably with their chin and forehead firmly against the rests to reduce motion artifact 3
• Instruct patients to maintain steady fixation on the internal target to prevent eye movement during scanning 4

Optimal Imaging Environment

• Perform imaging in a dark room to avoid pupillary constriction, which can artificially widen the anterior chamber angle and provide misleading anatomical information 1
• Use a bright, short beam (approximately 1 mm in length) that does not pass through the pupil when evaluating angle structures 1

Handling Corneal Opacity

• When corneal edema or opacity impairs visualization, topical glycerin may be applied to temporarily clear the cornea and improve image quality 1
• AS-OCT has advantages over other imaging modalities in that it can penetrate moderate corneal edema better than specular microscopy, though severe edema may still limit image quality 1

Device-Specific Capabilities

Resolution and Depth Considerations

• Spectral domain AS-OCT provides higher resolution but less depth of field compared to time-domain systems 1
• Time-domain instruments using longer wavelength light (1310 nm) can image deeper structures including the ciliary body, though with less clarity than ultrasound biomicroscopy 1
• Modern systems with extended imaging depth (up to 12 mm) enable visualization of the entire anterior segment in a single scan 5

Image Acquisition Parameters

• Utilize high A-scan rates (120 kHz or higher) to minimize motion artifacts and improve image quality 5
• Ensure adequate axial resolution (approximately 10 μm) for detailed structural visualization 5
• Apply full-range complex techniques when available to maximize imaging depth 5

Common Pitfalls to Avoid

Technical Errors

• Avoid scanning through the pupil when evaluating angle structures, as this induces pupillary constriction and falsely widens the angle appearance 1
• Be aware that AS-OCT has limitations in imaging the posterior iris and ciliary body—ultrasound biomicroscopy is superior for these structures 1
• Recognize that isolated peripheral anterior synechiae or small neovascular tufts may be missed if not in the imaging plane 1

Image Distortion Correction

• Correct for image distortion induced by nontelecentric scanning systems, particularly when making quantitative measurements of ocular surface shape 2
• Document the specific AS-OCT device type used, as measurements can vary between different systems and are not directly interchangeable 6

Clinical Applications Requiring High-Quality Images

Corneal Assessment

• High-definition cross-sectional imaging enables visualization of corneal thickness, epithelial thickness mapping (showing characteristic "donut-shaped" thinning in keratoconus), and detection of retrocorneal structures 1
• AS-OCT can identify Descemet's membrane breaks, central stromal clefts in keratoconic hydrops, and assess LASIK flap thickness 1

Angle Evaluation

• Quantitative angle measurements including angle opening depth, trabecular-iris space area, and iridotrabecular contact index require high-quality images for accuracy 1
• AS-OCT provides good agreement with gonioscopy for angle assessment, though it should be considered complementary rather than replacement 1

Post-Surgical Monitoring

• High-resolution imaging is essential for evaluating endothelial keratoplasty graft adherence, with detachments >30% of total graft area predictive of persistent detachment 1
• AS-OCT enables non-contact assessment of glaucoma surgery outcomes including trabeculectomy blebs and drainage device positioning 6

Quality Assurance

Reproducibility

• AS-OCT demonstrates high inter- and intraobserver reproducibility when proper technique is employed 6
• Standard deviations for height measurements should be less than 35 μm within a 14-mm range for acceptable repeatability 2
• Maximum height error should not exceed approximately 17 μm when measuring curved surfaces 2