B-scan ultrasonography, or B-scan, is a diagnostic test used in optometry and ophthalmology to produce a two-dimensional, cross-sectional view of the eye and the orbit. It is otherwise called brightness scan. It is commonly used to see inside the eye when media is hazy due to cataract or any corneal opacity. Your eye is numbed with medicine (anesthetic drops). The ultrasound wand (transducer) is placed against the front surface of the eye. The ultrasound uses high-frequency sound waves that travel through the eye. Reflections (echoes) of the sound waves form a picture of the structure of the eye.
B-scan ultrasonography is an important adjuvant for the clinical assessment of various ocular and orbital diseases. With understanding of the indications for ultrasonography and proper examination technique, one can gather a vast amount of information not possible with clinical examination alone. This article is designed to describe the principles, techniques, and indications for echographic examination, as well as to provide a general understanding of echographic characteristics of various ocular pathologies. A-scan ultrasound biometry, commonly referred to as an A-scan (short for Amplitude scan), is routine type of diagnostic test used in optometry or ophthalmology. The A-scan provides data on the length of the eye, which is a major determinant in common sight disorders.
Indications for Examination
B-scan ultrasound is most useful when direct visualization of intraocular structures is difficult or impossible. Situations that prevent normal examination include lid problems (eg, severe edema, partial or total tarsorrhaphy), keratoprosthesis, corneal opacities (eg, scars, severe edema), hyphema, hypopyon, miosis, pupillary membranes, dense cataracts, or vitreous opacities (eg, hemorrhage, inflammatory debris).
In such cases, diagnostic B-scan ultrasound can accurately image intraocular structures and give valuable information on the status of the lens, vitreous, retina, choroid, and sclera. However, in many instances, ultrasound is used for diagnostic purposes even though pathology is clinically visible. Such instances include differentiating iris or ciliary body lesions; ruling out ciliary body detachments; and differentiating intraocular tumors, serous versus hemorrhagic choroidal detachments, rhegmatogenous
versus exudative retinal detachments, and disc drusen versus papilledema.
Ultrasound Principles and Physics
Ophthalmic ultrasonography uses high-frequency sound waves, which are transmitted from a probe into the eye. As the sound waves strike intraocular structures, they are reflected back to the probe and converted into an electric signal. The signal is subsequently reconstructed as an image on a monitor, which can be used to make a dynamic evaluation of the eye or can be photographed to document pathology.
Sound is emitted in a parallel, longitudinal wave pattern, similar to that of light. The frequency of the sound wave is the number of cycles, or oscillations, per second, measured in hertz (Hz). For sound to be considered ultrasound, it must have a frequency of greater than 20,000 oscillations per second, or 20 KHz, rendering it inaudible to human ears. The higher the frequency of the ultrasound, the shorter the wavelength (distance from the peak of one wave to the peak of the next wave). A direct relationship exists between wavelength and depth of tissue penetration (the shorter the wavelength, the more shallow the penetration). However, as the wavelength shortens, the image resolution improves.
Given that ophthalmic examinations require little in the way of tissue penetration (an eye being 23.5 mm long on average) and much in the way of tissue resolution, ultrasound probes used for ophthalmic B-scan are manufactured with very high frequencies of about 10 million oscillations per second, or 10 MHz. In contrast, ultrasound probes used for purposes such as obstetrics use lower frequencies for deeper penetration into the body, and, because the structures being imaged are larger, they
do not require the same degree of resolution. Recently, high-resolution ophthalmic B-scan probes (ultrasound biomicroscopy or UBM) of 20-50 MHz have been manufactured that penetrate only about 5-10 mm into the eye for incredibly detailed resolution of the anterior segment.
The velocity of the sound wave is dependent on the density of the medium through which the sound travels. Sound travels faster through solids than liquids, an important principle to understand since the eye is composed of both. There are known velocities of different components of the eye, with sound traveling through both aqueous and vitreous at a speed of 1,532 meters/second (m/s) and through the cornea and lens at an average speed of 1,641 m/s.
When sound travels from one medium to another medium of different density, part of the sound is reflected from the interface between those media back into the probe. This is known as an echo; the greater the density difference at that interface, the stronger the echo, or the higher the reflectivity.
In A-scan ultrasonography, a thin, parallel sound beam is emitted, which passes through the eye and images one small axis of tissue; the echoes of which are represented as spikes arising from a baseline. The stronger the echo, the higher the spike. For example, the vitreous is less dense than the vitreous hyaloid, which, in turn, is much less dense than the retina. Therefore, the spike obtained as the sound strikes the interface of the vitreous and hyaloid is shorter than the spike obtained when the sound
strikes the hyaloid-retinal interface.
In B-scan ultrasonography, an oscillating sound beam is emitted, passing through the eye and imaging a slice of tissue; the echoes of which are represented as a multitude of dots that together form an image on the screen. The stronger the echo, the brighter the dot. Using the same example, the dots that form the posterior vitreous hyaloid membrane are not as bright as the dots that form the retinal membrane. This is very useful in differentiating a posterior vitreous detachment (a benign condition)
from a more highly reflective retinal detachment (a blinding condition).