Residency · Residency · Ophthalmology

Ophthalmic Ultrasound: A-Scan and B-Scan Techniques

Introduction

Ophthalmic ultrasound is an indispensable diagnostic tool in ophthalmology, providing real-time imaging of intraocular and orbital structures when direct visualization is limited. A-scan (amplitude modulation) provides one-dimensional measurements critical for biometry and tissue characterization, while B-scan (brightness modulation) generates two-dimensional cross-sectional images. Together, they form the foundation of diagnostic ophthalmic ultrasonography.

Physics of Ophthalmic Ultrasound

Basic Principles

Sound waves are generated and received by a piezoelectric transducer (crystal) Frequency: ophthalmic ultrasound uses 8-20 MHz (higher than most medical ultrasound) Higher frequency provides better axial resolution but lower tissue penetration. Sound waves reflect at tissue interfaces with different acoustic impedance (density x velocity) Time delay between emission and echo return determines the distance to the reflecting surface.

Key Terms

Axial resolution: ability to distinguish two closely spaced structures along the beam axis. Lateral resolution: ability to distinguish structures perpendicular to the beam. Gain: amplification of the received signal; adjustable to optimize image quality. Attenuation: progressive weakening of the sound beam as it passes through tissue.

A-Scan Ultrasonography

Principles

Displays returning echoes as vertical spikes on a horizontal time baseline. Spike height (amplitude) reflects the strength of the echo (acoustic reflectivity) Spike position on the baseline reflects the depth of the interface. Two primary modes: biometry A-scan and standardized diagnostic A-scan.

Biometry A-Scan (IOL Calculation)

Measures axial length of the eye (average 23.5 mm) Critical for IOL power calculation prior to cataract surgery. Components measured: anterior chamber depth, lens thickness, vitreous chamber length. Techniques: Contact (applanation): probe contacts the cornea; risk of corneal compression (falsely short axial length) Immersion: probe suspended in a fluid-filled scleral shell; avoids corneal compression; more accurate.

Optical biometry (IOLMaster, Lenstar): non-contact, laser-based; preferred for most cases; uses partial coherence interferometry. Ultrasound biometry remains essential when optical biometry fails (dense media, posterior staphyloma, silicone oil)

Standardized Diagnostic A-Scan (Ossoinig Technique)

Uses a focused, standardized 8 MHz probe with calibrated gain settings. Tissue identification based on internal reflectivity patterns:

Kappa angle: angle of incidence affects spike height; perpendicular incidence maximizes reflectivity. Kinetic evaluation: assess spike movement with eye movement (useful for differentiating mobile from fixed lesions)

B-Scan Ultrasonography

Principles

Displays returning echoes as dots of varying brightness on a two-dimensional image. Creates a cross-sectional image of the eye and orbit. Frequency: typically 10 MHz for globe imaging; lower for orbit. Real-time imaging allows dynamic assessment (eye movements, blood flow) Gain adjustment: higher gain detects weaker echoes (vitreous opacities); lower gain highlights strong echoes (calcification)

Technique

Patient seated or supine; coupling gel applied to closed eyelid or directly to the eye (with topical anesthesia) Contact technique: probe placed on the eyelid (through gel) or on the globe. Systematic examination: axial, transverse, longitudinal, and oblique scan orientations. Document all four quadrants; compare with fellow eye.

Probe Orientations

Axial: probe centered over the cornea; beam passes through the visual axis. Transverse: probe perpendicular to limbus; sweep around the clock hours. Longitudinal: probe parallel to limbus; provides meridional views. Each quadrant examined in at least two orientations.

Normal B-Scan Anatomy

Cornea: thin, bright anterior echo. Anterior chamber: anechoic (echo-free) Lens: anterior and posterior capsule echoes; interior normally anechoic. Vitreous: normally anechoic (liquefied vitreous may show faint mobile echoes) Retina-choroid-sclera complex: bright, continuous posterior echo. Optic nerve: hypoechoic shadow posterior to the disc.

Clinical Applications

Vitreous Pathology

Vitreous hemorrhage: diffuse, mobile, low-to-medium amplitude echoes; gravity-dependent; aftermovement with eye movement. Posterior vitreous detachment (PVD): membrane floating in the vitreous; attached at the disc; mobile. Asteroid hyalosis: bright, highly reflective, mobile echoes throughout the vitreous; no aftermovement. Vitreous membranes: inflammatory or fibrotic; may be thick, folded, and less mobile.

Retinal Pathology

Retinal detachment: bright, continuous membrane inserted at the optic disc and ora serrata. Total RD: funnel configuration (open or closed) Partial RD: convex toward the probe; undulates with eye movement. Distinguish from vitreous membrane (RD is thicker, brighter, and moves less) Retinoschisis: thin, smooth, immobile splitting of the retina; less mobile than RD. Choroidal detachment: smooth, dome-shaped, thick membrane anterior to the sclera; does not extend to the disc; may be kissing choroidals (large, touching in the center)

Intraocular Tumors

Choroidal melanoma: dome or mushroom shape, low internal reflectivity (A-scan), acoustic hollowing on B-scan, choroidal excavation. Choroidal metastasis: flat or minimally elevated, medium-high internal reflectivity, irregular internal structure. Choroidal hemangioma: dome-shaped, high internal reflectivity on A-scan (differs from melanoma) Retinoblastoma: calcification (highly echogenic foci with shadowing); characteristic in pediatric patients.

Orbital Pathology

Orbital tumors, extraocular muscle enlargement (thyroid eye disease) Optic nerve diameter measurement (normal < 5 mm including sheath) Optic nerve sheath distension in elevated intracranial pressure. Orbital foreign body localization.

Ultrasound Biomicroscopy (UBM)

Principles

Very high-frequency ultrasound: 35-50 MHz. Provides exquisite resolution of anterior segment structures (50-micron axial resolution) Limited penetration depth (4-5 mm); only images the anterior segment.

Applications

Angle anatomy: plateau iris, angle closure mechanisms, iris and ciliary body tumors. Ciliary body assessment: detachment, tumors, inflammation. IOL position: sulcus vs. bag placement; haptic location. Anterior segment trauma: cyclodialysis cleft, iridodialysis. Anterior segment tumors: iris melanoma, ciliary body melanoma depth and extent.

Emerging Technologies

High-Resolution B-Scan

20 MHz transducers providing improved resolution for posterior segment. Better differentiation of retinal layers.

Three-Dimensional Ultrasound

Volumetric reconstruction from serial B-scan images. Tumor volume measurement and monitoring.

Doppler Ultrasound

Color Doppler imaging of orbital and ocular blood flow. Ophthalmic artery, central retinal artery, posterior ciliary arteries. Applications: carotid occlusive disease screening, orbital tumors, ocular ischemic syndrome.

Key Clinical Pearls

B-scan ultrasound is essential when fundus visualization is precluded by media opacity; always rule out retinal detachment and intraocular mass in any eye with vitreous hemorrhage. Choroidal melanoma shows low internal reflectivity on A-scan with acoustic hollowing on B-scan, while choroidal hemangioma shows high internal reflectivity -- this distinction is diagnostically critical. Immersion A-scan biometry is more accurate than contact biometry because it avoids corneal compression; ultrasound biometry remains necessary when optical biometry fails. UBM at 35-50 MHz provides unparalleled anterior segment imaging for angle assessment, ciliary body pathology, and IOL positioning.

References

1. Byrne SF, Green RL. Ultrasound of the Eye and Orbit. 2nd ed. Mosby; 2002.
2. Ossoinig KC. Standardized echography: basic principles, clinical applications, and results. Int Ophthalmol Clin. 1979;19(4):127-210.
3. Silverman RH. High-resolution ultrasound imaging of the eye: a review. Clin Exp Ophthalmol. 2009;37(1):54-67.
4. Finger PT, Tran HV, Turbin RE, et al. High-frequency ultrasonographic evaluation of conjunctival intraepithelial neoplasia and squamous cell carcinoma. Arch Ophthalmol. 2003;121(2):168-172.

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