Ultrasound Biomicroscopy (UBM) is a specialized, non-invasive imaging technique used in ophthalmology to generate detailed, high-resolution cross-sectional images of the eye’s anterior segment. This method allows eye care professionals to obtain pictures of structures at the front of the eye that are often too small or hidden for standard examination tools. UBM provides nearly microscopic detail, enabling precise measurements and assessments of the frontal ocular anatomy. The information gathered assists doctors in understanding complex anatomical relationships and diagnosing various conditions affecting these structures.
How UBM Works
UBM operates on the same principle as standard ultrasound but utilizes extremely high-frequency sound waves to achieve superior resolution. While conventional ophthalmic ultrasound (B-scan) typically operates at 10 to 20 MHz, UBM employs frequencies ranging from 50 to 100 MHz. This significant increase in frequency shortens the wavelength of the sound waves, allowing the instrument to detect finer details within the tissue structure. The resulting images offer an axial resolution as fine as 20 to 50 micrometers, providing clarity that approaches light microscopy.
The trade-off for superior detail is a reduction in penetration depth. At these high frequencies, the UBM signal effectively images only the anterior segment, usually reaching a depth of 3 to 5 millimeters. This limited penetration means UBM is exclusively used for the front of the eye, while lower-frequency ultrasound images posterior structures like the retina and vitreous. The UBM probe emits high-frequency pulses, and a transducer converts the returning echoes, which reflect off different tissue densities, into a detailed, two-dimensional grayscale image.
Unique Visualization of the Anterior Eye
The utility of UBM lies in its ability to visualize structures that are optically opaque or hidden behind other tissues, distinguishing it from light-based imaging tools like the slit lamp or Anterior Segment Optical Coherence Tomography (AS-OCT). Since sound waves are not obstructed by pigment or cloudy media, UBM can effectively see through the iris to image the structures immediately behind it. This allows for clear visualization of the ciliary body, the ring-shaped structure responsible for producing the fluid that fills the eye.
UBM provides unparalleled access to the anterior chamber angle, the junction where the cornea and iris meet, and the location of the eye’s primary drainage system. Doctors can precisely map the scleral spur, the attachment point for the trabecular meshwork, which regulates fluid outflow. UBM also images the posterior surface of the iris, the lens zonules (fibers holding the lens in place), and the peripheral sclera. This detailed anatomical mapping is necessary when conditions like a cloudy cornea or a hyphema (blood in the anterior chamber) block the view of other imaging modalities.
Primary Diagnostic Uses
The high-resolution visualization offered by UBM makes it an important tool in diagnosing and managing several complex anterior segment disorders. A frequent application is the assessment of glaucoma, particularly narrow-angle or angle-closure types. UBM allows the physician to objectively measure the angle width and determine the specific mechanism causing closure, such as a forward bowing of the iris (pupillary block) or an abnormal, anteriorly positioned ciliary body (plateau iris configuration).
For patients who have experienced ocular trauma, UBM is useful for assessing hidden internal damage. It is the preferred method for locating tiny intraocular foreign bodies lodged in the anterior structures, especially those that are non-metallic and difficult to see on X-ray. The scan can also reveal a cyclodialysis cleft—a separation of the ciliary body from the scleral spur—which is an injury that can lead to low eye pressure.
UBM also functions in the staging and evaluation of tumors and cysts originating in the iris or ciliary body. The technique helps differentiate between solid masses, such as melanomas, and benign fluid-filled cysts, guiding treatment planning. By accurately determining the size, depth of penetration, and location of a lesion relative to surrounding structures, UBM assists the doctor in planning surgical interventions or monitoring mass growth.
What to Expect During the Procedure
The UBM examination is an outpatient procedure that is generally comfortable and requires no specific preparation. The patient is typically positioned lying down (supine) to allow for stable and accurate imaging. Before the scan begins, the doctor administers topical anesthetic drops to numb the surface of the eye.
A small, sterile plastic eye cup, sometimes called a scleral shell, is then placed on the anesthetized eye and filled with a clear, water-based solution (such as distilled water or methylcellulose). This fluid acts as a coupling medium, necessary for the high-frequency sound waves to travel from the probe to the eye without interference. The UBM probe is placed directly into this water bath, gently touching the eye’s surface to capture images. The entire process is quick, often taking 10 to 20 minutes, and the patient can usually resume normal activities immediately afterward.