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R. H. Silverman, H. O. Lloyd, J. A. Ketterling, J. Mamou, D. Coleman; Ultrasonic Imaging of the Vitreous With Chirped Annual Array. Invest. Ophthalmol. Vis. Sci. 2010;51(13):5775.
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© ARVO (1962-2015); The Authors (2016-present)
The function of the vitreous requires optical transparency, making imaging of vitreous structure quite challenging. The normal vitreous is acoustically clear, but at high sensitivity, vitreous organization can be detected, especially following posterior vitreous detachment (PVD). Our aim was to demonstrate improved depiction of faint vitreous structure by use of advanced ultrasound technology.
The ultrasound probe consisted of five 20 MHz annular rings. The face of the probe was spherically curved to produce a geometric focus at 31 mm. During scanning, each element would take its turn as the transmitter while all received, for a total of 25 sequences of echo data along each line-of-sight. A ‘chirp’ of 4 µsec duration was employed rather than a conventional excitation pulse. The chirp swept the usable bandwidth of the transducer from 6.5 MHz to 30 MHz. After compression of the chirp data by convolution with a mismatched filter, a synthetic focusing algorithm was used to combine all 25 channels to dynamically extend the depth-of-field. After testing on phantoms and ex vivo eyes and measurement of output to assure compliance with safety standards, we imaged a human subject five days following PVD, also acquiring 2-D and 3-D spectral domain optical coherence tomography (OCT) and conventional 10 MHz ultrasound images.
OCT images demonstrated the posterior vitreous face and a funnel-like conformation of the retracted vitreous. Conventional ultrasound demonstrated the PVD as well, including its anterior conformation and visualization of dynamic motion in response to eye movement. The chirped annular array had higher sensitivity (an 18 dB improvement in signal-to-noise ratio [SNR]) and better resolution than 10 MHz ultrasound, especially anteriorly due to the 6-fold improvement in depth-of-field arising from the use of dynamic focusing.
We demonstrated the first clinical application of combined annular array with chirp excitation. The extended depth-of-field provided by the annular array enhanced sensitivity and lateral resolution. Chirp excitation further improved sensitivity because a gain in SNR equal to its time-bandwidth product is expected theoretically. The prototype is being modified to a more compact configuration to facilitate clinical application. The high-resolution and sensitivity of this technology make it ideal for imaging of faint vitreous structure and pathology.
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