Ryan Alfonso, MPH
Managing Editor
Medcompare
It has been said that ocular ultrasound takes a weekend to learn and a lifetime to master. Significant training goes into learning what various ocular pathologies look like in a 2D cross-sectional view. Clearly, this is a bit more complicated than taking a simple IOP measurement. Nevertheless, B-scan ultrasonography is a particularly important part of an ophthalmologist's armamentarium. But in the same way that your proficiency with the technology has improved with time and experience, so have the diagnostic tools. Innovations in image resolution, acquisition, and subsequent digital image evaluation, provide for a higher level of diagnostic confidence than before.
When direct observation of intraocular anatomy is obscured, the B-scan is the tool of choice for the evaluation of the eye and its orbit. In the emergency department context, B-scan ultrasound can be used for diagnosing ocular pathology in patients presenting with acute conditions like central retinal artery occlusions or traumas resulting in retinal detachments, vitreous hemorrhages, or lens dislocations. So in this sense, it can help the ED staff differentiate between a pathology that legitimately requires urgent care and that which can be followed up by the appropriate specialist on an outpatient basis.
By far the most advantageous feature of the B-scan systems is that they enable you to “see” through and beyond structures that prevent normal visual examination. The best example of this is the dense cataract. Performing a cataract surgery may not make sense if there is another ocular pathology that would prevent good vision after the procedure. Similarly the B-scan can image through severely swollen lids, corneal opacities, vitreous opacities, uveitis, and persistent pupillary membrane. In such cases, diagnostic B-scan ultrasound can accurately image the lens, vitreous, retina, choroid, and sclera providing valuable clues as to their status.
In addition to being used for the detection of pathology hidden by other structures, B-scan ultrasound is frequently used for the characterization of clearly visible pathologies. A few examples are: differentiating rhegmatogenous from exudative retinal detachments, differentiating drusen from papilledema, and determining intraocular tumor type. In the ophthalmic oncology setting, B-scan ultrasonography is commonly used for the initial and follow-up evaluation of retinoblastoma – a highly malignant retinal cancer of childhood. Retinoblastoma has focal areas of calcification within the tumor that appear quite clearly by ultrasound.
The physics behind ophthalmic ultrasound technology is really interesting: sound wave absorption and reflection, angle of incidence and probe positioning. Probe frequency and depth are inversely proportional, but frequency and resolution are directly proportional. Ultrasound probes used for ophthalmic B-scans typically use very high frequencies (10 MHz) compared to typical ultrasound modalities. At this frequency a great deal of resolution can be gained, but at the expense of tissue penetration depth. In contrast, ultrasound probes used for GI exams or fetal gender determination use lower frequencies for deeper penetration into the body. Since the structures being imaged are much larger, the same degree of resolution isn’t necessary.
Recently, manufacturers such as Paradigm Medical and Ophthec BV have made devices with high-resolution ophthalmic B-scan probes of 20-50 MHz that penetrate only about 5-10 mm into the eye for extremely detailed resolution of the anterior segment. This new class of device has been called an ultrasonic bio-microscope, or UBM.
DGH, Accutome, and Micro Medical Devices, have all recently launched versions of a unique new kind of B-scan ultrasound device. It is a portable, self-contained, hand-held device that turns your PC into a B-Scan device capable of high-resolution imaging. The probe is integrated into the head of the unit and connects to a computer via a USB 2.0 port. This is particularly applicable to practices where the exam lanes and offices are already fitted with modern IT infrastructure creating the “paperless practice”. Each of these hand-held systems feature custom settings for multiple applications and a patient database limited only by the size of your hard-drive. Since the image capture is digital, you can store movies or JPEG images right to your PC and merge them with your EMR system.
The high-resolution imaging yields over 2000 sample points per line. There is also an adjustable pulse power functionality that improves its penetration capabilities. With a pulse frequency of 12MHz/15MHz it has a 3 to 10 cm penetration depth and with its dual calipers you can easily measure various aspects of ocular anatomy. This range of frequencies gives excellent posterior segment and fairly wide field anterior segment resolution. There’s also a zoom feature to allow better inspection of real-time or stored images. All together, it’s quite a package.
Like all modern technology-based medical devices, B-scan ultrasound systems continue to become smaller and more powerful.