Ultrasonography
Contact B-scan ultrasonography is the most common form of ultrasonography used in the clinic. In contact B-scan ultrasonography, a 10-MHz probe is placed on the patient’s eyelid. A piezoelectric crystal is used to send and receive sound waves for each A-scan. The ultrasound beam is about 1 mm wide at the level of the retina, which severely limits the lateral resolution. The axial resolution, which is the resolution along the axis of the ultrasound beam, is very different in the eye versus as measured with flat surfaces. This difference is due to the interaction of the eye’s curved surfaces with the lateral summation of the signal. Contact B-scan ultrasonography is used to evaluate tumor thickness, detect foreign bodies, assess choroidal or retinal detachments, and analyze the vitreous. Drusen of the optic nerve head can be detected by observing small areas of bright reflection in the nerve that cause shadows. Careful ultrasonography is able to detect retinal tears. Ultrasonography also has the ability to image the eye during saccades, because the imaging doesn’t have to occur through the pupil. Information gained during dynamic examinations is helpful when examining the vitreous and in differentiating retinoschisis from true detachment. Contact B-scan ultrasonography can produce images through opaque media (Figs 2-12 and 2-13; Videos 2-1 and 2-2).
Ultrasound biomicroscopy uses higher frequencies and thus offers higher resolution images, but at the cost of decreased tissue penetration depth. The probe, typically 50 MHz, is contained inside a soft, water-filled bag that is placed against the patient’s eye. Ultrasound biomicroscopy is used to evaluate the anterior chamber and ciliary body; it can also be used to visualize the vitreous insertion anteriorly. Typical uses in a retinal practice include the evaluation of tumors, iris cysts, foreign bodies, anterior suprachoroidal effusions, cyclodialysis clefts, intraocular lens placement, and hyphemas.
VIDEO 2-1 Contact B-scan ultrasonography showing massive choroidal effusions.
Courtesy of Yale Fisher, MD, and www.ophthalmicedge.org.
Access all Section 12 videos at www.aao.org/bcscvideo_section12.
VIDEO 2-2 Dislocated cataractous lens in the vitreous cavity.
Courtesy of Yale Fisher, MD, and www.ophthalmicedge.org.
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Coleman DJ, Silverman RH, Lizzi FL, et al. Ultrasonography of the Eye and Orbit. 2nd ed. Philadelphia: Lippincott Williams & Wilkins; 2006.
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DiBernardo CW, Greenberg EF. Ophthalmic Ultrasound: A Diagnostic Atlas. 2nd ed. New York: Thieme; 2006.
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Fisher Y. Ultrasound. Available at www.ophthalmicedge.org. Accessed March 15, 2018.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.