Ultrasound biomicroscopy, or UBM, is a non-invasive imaging test that gives ophthalmologists a highly detailed, cross-sectional view of the front portion of the eye. It is used to create a more detailed image than a regular ultrasound. The primary purpose of a UBM exam is to provide high-resolution images of the eye’s anterior segment, helping doctors diagnose and manage various ocular conditions, including trauma and foreign bodies within the eye.
How Ultrasound Biomicroscopy Works
The technology behind UBM relies on very high-frequency sound waves, from 35 to 100 megahertz (MHz), which are significantly higher than those in standard B-scan ultrasounds. This allows UBM to achieve microscopic resolution, capturing details as small as 20 micrometers. The trade-off for this resolution is a limited penetration depth of about four to five millimeters into the tissue.
During the procedure, a specialized probe sends these sound waves toward the eye. The waves travel through the eye’s structures and bounce back, creating echoes that are detected by the same probe. A computer then processes these echo patterns to generate a detailed, two-dimensional sonogram of the eye’s anterior segment, providing a clear visualization of the cornea, iris, ciliary body, and the front portion of the lens.
Clinical Uses for UBM
Ultrasound biomicroscopy is employed to diagnose, monitor, and plan treatments for conditions affecting the front of the eye. Its ability to visualize structures behind the iris makes it suited for evaluating different forms of glaucoma. For angle-closure glaucoma, UBM can identify the mechanism causing the closure, such as a pupillary block or plateau iris, and measure the anterior chamber angle for diagnosis and management.
The high resolution of UBM is also valuable for identifying and measuring growths such as cysts and tumors on the iris or ciliary body. Because light-based imaging cannot penetrate these opaque structures, UBM is the preferred method for viewing what lies beneath. This visualization helps determine the size and extent of such lesions, guiding decisions about treatment.
In cases of eye trauma, UBM assesses injury to the anterior segment, detecting subtle damage to the lens or ciliary body. It is also used to check the positioning of intraocular lens (IOL) implants after cataract surgery, especially if complications are suspected. UBM can confirm if an IOL is correctly placed or interacting with surrounding tissues.
The UBM Procedure for Patients
For patients, the UBM exam is straightforward. Anesthetic eye drops numb the surface of the eye to ensure the procedure is painless. Patients lie down and are asked to remain still, following instructions to look in different directions for a comprehensive scan.
There are two common methods for the scan. One involves placing a small immersion shell on the eye, which is filled with a sterile water solution. An alternative method uses a handheld probe with a gel-covered tip placed directly onto the anesthetized eye.
The examination lasts between 15 and 30 minutes for both eyes. During this time, the operator moves the probe to capture images from various angles. After the test, vision may be blurry for a short period from the anesthetic and coupling fluid.
UBM Compared to Other Eye Imaging
Compared to a standard ocular B-scan ultrasound, the main difference is the frequency of the sound waves. B-scans use lower-frequency waves to penetrate deeper into the eye to visualize posterior structures like the retina. However, this deeper view comes at the cost of lower resolution.
Another common tool is Anterior Segment Optical Coherence Tomography (AS-OCT), which uses light waves instead of sound. AS-OCT can produce images with resolution comparable to UBM and is excellent for visualizing the cornea and the angle.
The defining limitation of AS-OCT is that light cannot pass through opaque tissues like the iris. UBM’s use of sound waves allows it to bypass these tissues, providing a clear view of hidden structures. This capability is why a doctor chooses UBM to evaluate conditions involving the ciliary body, diagnose the cause of angle-closure glaucoma, or assess tumors behind the iris.