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G. Sun, R. Silverman, J. Mamou, H. Lloyd, R. Chan, D. Coleman; Ultrasound Sensitivity Enhancement by Pulse Encoding. Invest. Ophthalmol. Vis. Sci. 2009;50(13):3301.
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The normal vitreous is optically and acoustically transparent, but fluid spaces, membranes, blood and cellular debris may occur with aging, disease or trauma. These structures may be difficult to visualize ultrasonically due to their low relative acoustic impedance. Pulse encoding using chirp excitation offers an increase in the signal-to-noise ratio (SNR) proportional to the time-bandwidth product of the pulse. In this study, we compare different pulse modes in imaging the vitreous and other ocular structures.
Six human subjects with faint to significant vitreous inhomogeneities were scanned with a focused 13-MHz transducer (focal length=35 mm, aperture=12 mm) using an immersion technique. The transducer was excited with either a 13-MHz monocycle, a 3-cycle 13-MHz toneburst, or a 4.4-µsec chirp ranging from 8 to 20 MHz. Calibrated hydrophone measurements showed acoustic power levels to comply with standards for ophthalmic exposure. Echo data were acquired at a 100-MHz sample rate with 12-bit resolution. B-mode images were formed from digitized echo data. Chirp data were convolved with a tapered time-reversed chirp waveform by multiplication in the frequency domain to reconstruct the compressed image.
The -6 dB bandwidth of the transducer was 8-MHz (62%) for both the monocycle and chirp excitations and was 4-MHz (31%) for the 3-cycle toneburst. B-mode images showed improved sensitivity but reduced axial resolution with the narrowband toneburst in comparison to the monocycle. Compressed chirp images provided a 15-dB improvement in SNR, enhancing sensitivity to vitreous inhomogeneities and depiction of deep orbital tissues.
While use of a narrowband pulse improved sensitivity compared to a monocycle, it does so at the cost of reduced axial resolution. Coded excitation provides enhanced sensitivity while maintaining resolution. Chirp excitation allowed clearer depiction of vitreous inhomogeneities and deeper penetration into the orbit. These findings suggest that this mode may be of useful for evaluation of pathologies involving the vitreous, such as vitreoretinal traction, and for improved depiction of the optic nerve and other orbital structures.
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