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.

For the intern

At 3am, if a patient presents with acute vision loss and media opacity preventing fundoscopy, perform a B-scan ultrasound immediately to rule out retinal detachment, vitreous hemorrhage, or intraocular foreign body—these require urgent intervention. Document your findings with images and notify your senior if you see a bright, inserted membrane (possible RD) or an intraocular mass. Routine A-scan biometry for cataract surgery planning can wait until morning clinic hours when optical biometry equipment and experienced staff are available.

Common pitfalls

• Mistaking vitreous membrane for retinal detachment: RD is thicker, brighter, inserts at the optic disc and ora serrata, and has less aftermovement than a PVD membrane. Always scan all four quadrants—partial RDs are easily missed with incomplete examination.

• Corneal compression during contact A-scan biometry: Pressing too hard on the cornea artificially shortens axial length measurements, leading to incorrect intraocular lens (IOL) power calculations and postoperative refractive surprises. Use immersion technique when possible, or apply minimal pressure with contact method.

• Inadequate gain settings: Setting gain too low misses subtle vitreous hemorrhage or membranes; too high creates artifact that obscures real pathology. Adjust gain systematically during your scan and compare suspicious findings between eyes.

• Overlooking calcification in suspected tumors: High-gain B-scan may hide calcification that's obvious on low gain. In pediatric patients with leukocoria, failure to identify retinoblastoma calcifications can delay life-saving diagnosis.

Read this lecture as Markdown