Original articleHigh-resolution ultrasonic imaging of the posterior segment☆
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
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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.