Elsevier

Ophthalmology

Volume 111, Issue 7, July 2004, Pages 1344-1351
Ophthalmology

Original article
High-resolution ultrasonic imaging of the posterior segment

https://doi.org/10.1016/j.ophtha.2003.10.029Get rights and content

Abstract

Purpose

Conventional ophthalmic ultrasonography is performed using 10-megahertz (MHz) transducers. Our aim was to explore the use of higher frequency ultrasound to provide improved resolution of the posterior pole.

Design

Prospective case series.

Participants

One normal subject and 5 subjects with pathologies affecting the posterior coats, including nevii, small melanomas, and macular hole.

Methods

We modeled the frequency-dependent attenuation of ultrasound across the eye to develop an understanding of the range of frequencies that might be practically applied for imaging of the posterior pole. We compared images of the posterior coats made at 10, 15, and 20 MHz, and 20-MHz ultrasound images of pathologies with 10-MHz ultrasound and optical coherence tomography (OCT).

Main outcome measures

Ability to resolve normal and pathologic structures affecting posterior coats of the eye.

Results

Modeling showed that frequencies of 20 to 25 MHz might be used for posterior pole imaging. Twenty-megahertz images allowed differentiation of the retina, choroid, and sclera. In addition, at 20 MHz the retina showed banding patterns suggesting an internal structure comparable in many respects to that seen in OCT and histology. Images of ocular pathology provided much improved detail relative to 10-MHz images and deeper penetration than OCT.

Conclusions

Twenty-megahertz ultrasound can be practically employed for imaging of the posterior pole of the eye, providing a 2-fold improvement in resolution relative to conventional 10-MHz instruments. Although not providing the resolution of OCT, ultrasound can be used in the presence of optical opacities and allows evaluation of deeper tissue structures.

Section snippets

Materials and methods

These studies were carried out under a protocol approved by the Institutional Review Board of the Weill Medical College of Cornell University.

We modeled the expected attenuation in scanning of the anterior and posterior segments of the eye. Attenuation increases exponentially both with frequency and with range. This is generally expressed using the formula α = α0νγ, where α0 represents attenuation in decibels (dB) per centimeter at 1 MHz, ν represents frequency in megahertz, and γ represents

Results

The results of the mathematical model are shown graphically in Figure 1. An immersion procedure (with lid speculum) or direct contact on the globe provides the least attenuation. The globe contact procedure shows attenuation values of 11, 15, and 21 dB at 20, 25, and 30 MHz, respectively. The immersion procedure, due to the longer acoustic path, resulted in slightly higher attenuation than the direct contact procedure: <2 dB of additional attenuation at 20 MHz, increasing to about a 4-dB

Discussion

Modeling of attenuation as a function of frequency shows that we may reasonably expect to obtain clinically useful images of the posterior segment at well above the 10 MHz used in current conventional ophthalmic B-mode instruments. The issue of how high in frequency one might go is complex, as it depends on the examination technique and the signal-to-noise ratio of specific instruments. It should be noted, however, that as the frequency bandwidth of electronic components is increased, noise

References (20)

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Manuscript no. 230315.

Supported by National Institutes of Health, Betheda, Maryland (grant nos.: EB00238 and P41-RR11795); the Dyson Foundation, Millbrook, New York; and Research to Prevent Blindness, New York, New York.

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