ABSTRACT
The spectral parameters of echo signals from eyes are critical for designing the
receiving filter of the ophthalmological ultrasound device. It is inappropriate to
estimate the echo spectra of the eyes with the averaged acoustic attenuation coefficient
derived from soft tissues, because the ocular tissues are far from uniform. This paper
studies the echo spectrum from human eyes in vivo. A 10 MHz ultrasonic transducer is
used to scan the human eye and the result shows that the center frequencies of echo
signal from eyeball are almost unchanged from the lens till retina, but decline rapidly
with the depth about 3-5 mm behind the fundus with the average slope about -0.16
MHz/mm. When the contributions of the probe and electronic elements are eliminated,
the remaining spectrum of the posterior lens capsule is relatively flat which is similar
to the feature of a plane reflector with high reflectivity, while the remaining spectrum
of fundus present an ascending trend with the frequency, which is close to the
characteristic expected for Rayleigh scatters.
According to the frequencies of the eye tissue declining with depth, we design
different filters with bandwidths from 1MHz to 7MHz to deal with the echo signal
corresponding to the eye tissue. The result shows that dynamic filter can take better
care of high-frequency from shallow and low-frequency from deep signals than the
same fixed filter, and the former image is better too.
This paper discusses the ultrasound attenuation and scattering characteristics of
the eye tissues. Experimental results and analysis show that the spectrums have
obvious features with the changes of tissues in depth. According to the characteristics,
the dynamic filter compared with the fixed filter effect is excellent for the
ocular ultrasound diagnostic equipment. When the affects of the probe and others are
removed, the spectral characteristics of their own have some certain rules to follow,
which can be provided a path for the future to distinguish organizations.
Key words: ultrasound for ophthalmology, power spectrum
analysis, tissue characterization, dynamic filter