# 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:

| Internal Reflectivity | Tissue Type | Examples |
|----------------------|-------------|----------|
| Low | Vascular, melanocytic, fluid | Melanoma, hemorrhage, mucus |
| Medium | Lymphoid, vascular | Lymphoma, meningioma |
| Medium-high | Solid cellular | Metastatic carcinoma, inflammation |
| High | Calcified, fibrous | Calcification, foreign body, fibrous tissue |

**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)

![A-scan display showing measurement of axial length with labeled spikes for cornea, anterior lens, posterior lens, and retina](images/a-scan-axial-length.jpg)

## 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.

![B-scan ultrasound showing a funnel-shaped total retinal detachment](images/bscan-total-rd.jpg)

## 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.

![UBM image of plateau iris configuration showing anteriorly rotated ciliary processes](images/ubm-plateau-iris.jpg)

## 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.