17.3 Enhanced Depth Imaging (EDI-OCT) & Anterior Segment OCT (AS-OCT)

Enhanced Depth Imaging (EDI-OCT) & Anterior Segment OCT (AS-OCT)

Clinical Core: Enhanced Depth Imaging (EDI-OCT) transforms standard spectral-domain platforms by shifting the zero-delay line closer to the chorio-scleral interface, enabling precise quantification of subfoveal choroidal thickness and deep lamina cribrosa pores. In parallel, Anterior Segment OCT (AS-OCT) leverages a longer ~1310 nm wavelength to penetrate opaque sclera and edematous corneas, providing objective biometry of anterior chamber angles, angle-closure mechanisms, and lamellar graft interfaces.

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Biophysical Principles of Enhanced Depth Imaging (EDI-OCT)

In conventional Spectral-Domain OCT imaging, sensitivity and optical signal-to-noise ratio (SNR) are not uniform across the imaging depth. The instrument's highest sensitivity lies directly at the zero delay line (the plane where the sample and reference arm optical paths are identical). In standard retinal OCT modes:

• The zero-delay line is positioned in the vitreous cavity above the retina.
• Because spectrometer resolution is limited by pixel cross-talk and finite detector elements, sensitivity exhibits a steep drop-off (sensitivity roll-off) as imaging depth increases away from the zero-delay line.
• Consequently, deep structures such as the outer choroid, choriocapillaris, and lamina cribrosa suffer from weak backscattering, poor SNR, and obscured anatomical margins.

The EDI Optical Technique

Enhanced Depth Imaging overcomes this limitation without modifying the underlying spectrometer hardware:

1. Instrument Displacement: The technologist moves the OCT objective lens slightly closer to the patient's cornea than in conventional imaging.
2. Zero-Delay Line Inversion: This physical shift repositions the zero-delay line from the vitreous down into the deep choroid, choriocapillaris, or sclerochoroidal junction.
3. Spectral Inversion: Software inverts the resulting Fourier spectrum, presenting the retina in its correct upright anatomical orientation while placing the region of peak sensitivity, highest SNR, and sharpest focus deep in the choroidal vascular beds and lamina cribrosa.

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Subfoveal Choroidal Thickness (SFCT) & The Pachychoroid Spectrum

Measurement Protocol

Subfoveal choroidal thickness (SFCT) is measured on a high-resolution, averaged EDI horizontal B-scan passing directly through the foveal center:

• Measurement Axis: A perpendicular caliper line drawn from the outer hyperreflective border of the RPE/Bruch's membrane complex down to the inner hyperreflective margin of the sclerochoroidal interface.
• Physiological Baselines: In healthy young adults, normal SFCT averages 250 to 350 µm.
• Physiological Variations:
  • Diurnal Variation: SFCT varies by 20 to 30 µm across 24 hours, being thickest in the early morning and thinnest in the late afternoon.
  • Age-Related Thinning: Normal choroidal thickness decreases predictably by approximately 15 to 20 µm per decade of life.
  • Axial Length / Myopia: SFCT decreases by approximately 15 to 25 µm for every millimeter increase in axial length.

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|                CHOROIDAL THICKNESS SPECTRUM IN CLINICAL DISEASE                   |
+-----------------------------------------------------------------------------------+
| THICK CHOROID: PACHYCHOROID SPECTRUM (SFCT >390 - 400 µm)                         |
| - Central Serous Chorioretinopathy (CSCR): Dilated Haller pachyvessels, subretinal|
|   fluid, choriocapillaris attenuation, RPE decompensation                         |
| - Pachychoroid Pigment Epitheliopathy (PPE): Asymptomatic RPE mottling over       |
|   pachyvessels; precursor to CSCR                                                 |
| - Pachychoroid Neovasculopathy (PNV): Type 1 shallow irregular choroidal          |
|   neovascularization arising over focal choroidal thickening                      |
| - Polypoidal Choroidal Vasculopathy (PCV): Aneurysmal dilatations / polyps of     |
|   branching vascular network at choriocapillaris interface                        |
+-----------------------------------------------------------------------------------+
| THIN CHOROID: CHOROIDAL ATROPHY (SFCT <100 - 120 µm)                              |
| - Pathologic / High Myopia: Posterior staphyloma, scleral stretching, extreme     |
|   choroidal thinning (<60 µm), lacquer cracks                                     |
| - Age-Related Choroidal Atrophy (ARCA): Primary choroidal attenuation in advanced |
|   elderly patients; distinct from dry AMD                                         |
| - Geographic Atrophy (Dry AMD): Severe localized choroidal vascular loss beneath  |
|   confluent RPE atrophy                                                           |
+-----------------------------------------------------------------------------------+

Hallmark Features of the Pachychoroid Spectrum

The hallmark of the pachychoroid spectrum on EDI-OCT is not merely global thickness, but the presence of pachyvessels—pathologically enlarged, engorged venous channels in Haller's layer that exert mechanical compression on the overlying Sattler's layer and choriocapillaris. This focal choriocapillaris ischemia and overlying RPE barrier breakdown lead to neurosensory detachments and subretinal fluid accumulation.

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Lamina Cribrosa Visualization & Glaucomatous Structural Alterations

In open-angle and angle-closure glaucomas, the mechanical and vascular site of axonal injury is the lamina cribrosa (LC) within the optic nerve head. Because overlying prelaminar neural tissue, dense glial columns, and central retinal vessels scatter 840 nm light, conventional SD-OCT cannot clearly image laminar pores.

EDI-OCT enables direct visualization of both the anterior laminar surface and posterior laminar surface:

• Laminar Thickness: Normal laminar thickness ranges from 250 to 300 µm; in eyes with advanced primary open-angle glaucoma, the lamina undergoes severe thinning to <150 µm.
• Posterior Laminar Bowing: Elevated intraocular pressure (IOP) drives backward mechanical bowing of the cribriform plates, quantified as increased lamina cribrosa depth (LCD) relative to the reference plane of Bruch's membrane opening (BMO).
• Focal Lamina Cribrosa Defects (FLCDs): EDI-OCT detects full-thickness holes, tears, or disinsertions in the laminar meshwork. These focal pores correspond directly to dense, localized visual field scotomas and recurring optic disc hemorrhages (Drance hemorrhages).

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EDI-OCT in Choroidal Tumors: Nevus vs. Melanoma Differentiation

Evaluating small choroidal melanocytic lesions is a vital clinical application for EDI-OCT. Technologists utilize the TFSOM-UHHD mnemonic (To Find Small Ocular Melanoma Using Helpful Hints Daily), correlating clinical findings with cross-sectional EDI-OCT biomarkers:

Clinical Significance of Photoreceptor Shagginess: Chronic accumulation of subretinal fluid over an active melanoma prevents RPE phagocytosis of shed photoreceptor outer segments. On EDI-OCT, these outer segments elongate into irregular, hyperreflective finger-like projections termed "shaggy photoreceptors."

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Anterior Segment OCT (AS-OCT): Instrumentation & Optical Modifications

While retinal OCT systems operate at ~840 nm to optimize resolution through the transparent ocular media, imaging the anterior segment presents different optical hurdles: dense scleral collagen fibers, pigmented iris stroma, and cloudy or edematous corneas induce extreme light scattering.

The 1310 nm Near-Infrared Wavelength Advantage

AS-OCT systems utilize a longer near-infrared center wavelength of approximately 1310 nm:

• Reduced Optical Scattering: Rayleigh scattering is inversely proportional to the fourth power of wavelength ($\sim 1/\lambda^4$). By increasing wavelength from 840 nm to 1310 nm, optical scattering in scleral and corneal collagen lamellae is reduced dramatically.
• Water Absorption and Retinal Safety: Water within ocular media absorbs 1310 nm light strongly. Because 1310 nm photons are absorbed by aqueous and vitreous humors before reaching the retina, higher laser incident power (up to 10–20 times higher than retinal systems) can be safely directed into the anterior eye without causing photochemical retinal damage.
• Imaging Depth: Permits complete cross-sectional imaging through the full thickness of the cornea, scleral spur, ciliary body, and crystalline lens surface.

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Quantitative Anterior Chamber Angle Assessment

The fundamental anatomical landmark for all AS-OCT angle biometry is the scleral spur—the inward circular projection of the scleral sulcus marking the insertion of the longitudinal ciliary muscle fibers.

+-----------------------------------------------------------------------------------+
|                    AS-OCT ANTERIOR CHAMBER ANGLE BIOMETRY                         |
+-----------------------------------------------------------------------------------+
|                               Corneal Endothelium                                 |
|                            --------------------------                             |
|                           /                          \                            |
|                          /  [AOD500]        [AOD750]  \                           |
|                         /      |               |       \                          |
|  [Scleral Spur (SS)] ->*       |               |        \                         |
|                         \      |   [TISA500]   |   [TISA750]                      |
|                          \     |               |          \                       |
|                           \____v_______________v___________\                      |
|                                    Anterior Iris Surface                          |
+-----------------------------------------------------------------------------------+

Standard Quantitative Angle Metrics

1. Trabecular-Iris Angle (TIA): The angle (measured in degrees) between the corneal endothelium/trabecular meshwork arm and the anterior iris surface, with the apex at the iridocorneal recess.
2. Angle Opening Distance at 500 µm and 750 µm (AOD500 / AOD750): The length of a perpendicular line originating from the trabecular meshwork at a point 500 µm (or 750 µm) anterior to the scleral spur, extending to meet the anterior iris surface.
3. Trabecular-Iris Space Area at 500 µm and 750 µm (TISA500 / TISA750): The trapezoidal area bounded anteriorly by the AOD500 (or AOD750) line, posteriorly by a line drawn perpendicular from the scleral spur to the iris, superiorly by the inner corneal wall, and inferiorly by the anterior iris surface.

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Angle-Closure Mechanisms on AS-OCT

AS-OCT allows dynamic, dark-room non-contact classification of angle-closure glaucoma subtypes:

• Pupillary Block: Manifests as pronounced anterior convex bowing of the iris (iris bombé) driven by relative pupillary resistance between the posterior and anterior chambers. Following laser peripheral iridotomy (LPI), the iris flattens immediately and the angle recess widens.
• Plateau Iris Configuration: Characterized by a flat central iris plane, a deep central anterior chamber, and an abrupt, steep anterior angulation of the peripheral iris root caused by anteriorly rotated ciliary processes that physically prop the iris against the trabecular meshwork. The angle remains narrow or closed on AS-OCT even after a widely patent LPI.
• Lens Vault (LV): Defined as the perpendicular distance from the anterior crystalline lens pole to the horizontal baseline connecting opposite scleral spurs. A high lens vault (LV >600–800 µm) indicates that the crystalline lens is the primary mechanical driver of angle crowding, directing treatment toward cataract extraction rather than iridotomy.

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AS-OCT Corneal Imaging & Surgical Evaluation

Epithelial Thickness Mapping in Keratoconus and Ectasia

Modern AS-OCT generates 9 mm wide epithelial pachymetric maps. Normal central corneal epithelium is uniformly 50 to 55 µm thick. In subclinical or manifest keratoconus, the epithelium thins focally directly over the apex of the cone and thickens concentrically around the base, creating a pathognomonic "donut pattern" (epithelial donut). Detecting this epithelial redistribution prevents patients with subclinical ectasia from undergoing laser in situ keratomileusis (LASIK).

LASIK Flap and Corneal Cross-Linking (CXL) Evaluation

• LASIK Flaps: AS-OCT measures flap thickness across the entire diameter, identifying flap thickness irregularities, microkeratome buttonholes, interface fluid syndrome, and epithelial ingrowth.
• CXL Demarcation Line: At 1 month following riboflavin-UVA corneal collagen cross-linking for progressive keratoconus, AS-OCT reveals a sharp, hyperreflective horizontal band across the corneal stroma known as the demarcation line. Located typically at a depth of 250 to 320 µm, this line represents the biological boundary of cross-linked stroma and confirms adequate treatment depth while verifying endothelial safety (must remain >100 µm anterior to endothelium).

Lamellar Endothelial Keratoplasty (DSAEK and DMEK)

In Descemet Stripping Automated Endothelial Keratoplasty (DSAEK) and Descemet Membrane Endothelial Keratoplasty (DMEK), AS-OCT provides immediate, non-contact evaluation of graft positioning in the early postoperative period:

• Confirms complete graft adherence or identifies partial graft detachment and interface fluid.
• Visualizes edge scroll separation, allowing targeted anterior chamber air or sulfur hexafluoride ($SF_6$) rebubbling even when the host cornea is too cloudy or edematous for slit-lamp visualization.