Ocular Imaging Physics

Light & Optics Basics - Ray's Eye View

• Light: Electromagnetic radiation; wave-particle duality. Visible spectrum: 400-700 nm.
• Reflection: Light bounces; angle of incidence = angle of reflection.
• Refraction: Light bends passing between media. Snell's Law: $n_1 \sin(\theta_1) = n_2 \sin(\theta_2)$.
  • Refractive index ($n$): $n = c/v$. Higher $n$ means slower light, more bending.
• Lenses:
  • Convex (converging, +): Corrects hyperopia.
  • Concave (diverging, -): Corrects myopia.
• Focal point: Where parallel rays converge/appear to diverge.
• Focal length ($f$): Lens to focal point distance.
• Diopter ($D$): Lens power; $D = 1/f$ ($f$ in meters).
• Vergence: Degree of light convergence/divergence (diopters).

⭐ The cornea is the eye's primary refractive surface, contributing ~+43D to the total ~+60D optical power.

Anterior Segment Imaging - Cornea's Close-Up

• Foundation: Slit-lamp biomicroscopy for initial detailed examination.
• Corneal Topography: Maps corneal surface power.
  • Placido disk: Anterior curvature via reflected mires.
  • Scheimpflug (e.g., Pentacam): Rotating camera; anterior/posterior curvature, pachymetry, anterior chamber depth (ACD).
• AS-OCT (Anterior Segment Optical Coherence Tomography): High-resolution optical cross-sections.
  • Measures: Central Corneal Thickness (CCT ~540 µm), epithelial maps, anterior chamber angle.
  • Uses: Glaucoma, refractive surgery planning.
• Specular Microscopy: Evaluates endothelial cell density (Normal: 2000-3000 cells/mm²) & morphology.
• Confocal Microscopy: In-vivo cellular detail (e.g., fungal/Acanthamoeba keratitis).
• UBM (Ultrasound Biomicroscopy): High frequency (35-50 MHz) ultrasound; images structures behind iris (ciliary body, sulcus).

⭐ Scheimpflug imaging uniquely provides both anterior and posterior corneal elevation and curvature data, vital for early keratoconus detection and refractive surgery screening.

Posterior Segment & Angio - Retina In Motion

• Dynamic Imaging: Visualizes blood flow & structural changes over time in the posterior segment.
• Fundus Fluorescein Angiography (FFA):
  • Dye: Sodium Fluorescein (IV).
  • Excitation: 465-490 nm (blue light), Emission: 520-530 nm (yellow-green light).
  • Phases: Choroidal (pre-arterial), Arterial, Arteriovenous, Venous (early, mid, late), Late staining/leakage.
  • Key findings: Hyperfluorescence (leakage, pooling, staining, window defect), Hypofluorescence (blockage, vascular filling defect).
• Indocyanine Green Angiography (ICGA):
  • Dye: Indocyanine Green (IV).
  • Near-infrared light: Deeper penetration, better visualization of choroidal circulation.
  • Uses: Occult Choroidal Neovascularization (CNV), Polypoidal Choroidal Vasculopathy (PCV), choroidal tumors.
• Optical Coherence Tomography Angiography (OCT-A):
  • Non-invasive, no dye required.
  • Detects motion of Red Blood Cells (RBCs) to create angiograms.
  • Provides depth-resolved 3D vascular maps (retinal & choroidal plexuses).
  • Applications: Diabetic Retinopathy (DR), Age-related Macular Degeneration (AMD), Retinal Vein Occlusion (RVO).

⭐ In FFA, a "hot disc" (diffuse optic disc hyperfluorescence) can indicate optic disc edema or papillitis, representing leakage from disc capillaries.

Cross-Sectional Imaging - Depth & Detail

• Ultrasound (USG): Uses sound waves.
  • A-Scan: Measures axial length (IOL power). Time vs. echo amplitude.
  • B-Scan: 2D cross-section. Posterior segment if opaque media.
  • UBM (Ultrasound Biomicroscopy): High frequency (35-50 MHz). Anterior segment (angle, ciliary body). Limited penetration.
• Optical Coherence Tomography (OCT): Light-based, high resolution (1-15 µm), near-histological.
  • Principle: Low-coherence interferometry. Measures optical backscattering.
  • Types: TD-OCT, SD-OCT (faster, ↑res), SS-OCT (fastest, ↑depth).
  • Applications: Macula (edema, AMD), glaucoma (RNFL), cornea.

  ⭐ SD-OCT & SS-OCT offer faster acquisition & higher axial resolution than TD-OCT.

• OCT Angiography (OCTA): Non-invasive, dye-less vascular imaging.
  • Detects RBC motion.
  • Applications: DR, RVO, AMD, neovascularization.

High‑Yield Points - ⚡ Biggest Takeaways

• A-scan USG measures axial length (IOL power); B-scan images posterior segment pathology.
• OCT uses low-coherence interferometry for high-res retinal/ONH imaging & RNFL analysis.
• FFA (fluorescein) shows retinal/choroidal circulation, leakage, and neovascularization.
• ICGA (indocyanine green) is superior for imaging choroidal vasculature and pathology.
• Corneal topography maps anterior curvature; tomography (Scheimpflug) provides 3D corneal analysis.
• Specular microscopy assesses corneal endothelial cell density and morphology.