Pulse-Encoded Ultrasound Imaging of the Vitreous With an Annular Array

Abstract

The vitreous body is nearly transparent both optically and ultrasonically. Conventional 10- to 12-MHz diagnostic ultrasound can detect vitreous inhomogeneities at high gain settings, but has limited resolution and sensitivity, especially outside the fixed focal zone near the retina. To improve visualization of faint intravitreal fluid/gel interfaces, the authors fabricated a spherically curved 20-MHz five-element annular array ultrasound transducer, implemented a synthetic-focusing algorithm to extend the depth-of-field, and used a pulse-encoding strategy to increase sensitivity. The authors evaluated a human subject with a recent posterior vitreous detachment and compared the annular array with conventional 10-MHz ultrasound and spectral-domain optical coherence tomography. With synthetic focusing and chirp pulse-encoding, the array allowed visualization of the formed and fluid components of the vitreous with improved sensitivity and resolution compared with the conventional B-scan. Although optical coherence tomography allowed assessment of the posterior vitreoretinal interface, the ultrasound array allowed evaluation of the entire vitreous body.

Synthetic-focusing strategies for real-time annular-array imaging

“…This approach to beamforming sacrifices overall signal strength because only a subset of the full transmit aperture is utilized, but we have demonstrated that ultimate image quality is not compromised [5], [6], [14]. The advantage to using this approach with an annular array is that the element count is low and it takes minimal time to acquire all TR data pairs at a single location.…”

“…For low-megahertz systems, this was a major drawback because of relatively long lateral displacements and the difficulty in obtaining real-time frame rates [4]. However, high-frequency ultrasound (HFU, >15-MHz) applications for which penetration depths are on the order of 1 to 3 cm and image widths are 1 to 2 cm, such as small-animal [5] and ophthalmic [6] imaging, are well suited for annular arrays.…”

“…We demonstrated that SF with five-element annular arrays that use all 25 data TR combinations [12] acquired over five passes were very effective at forming high-quality images when operating around 20 and 40 MHz [5], [6], [14]. However, with this approach we were only able to achieve frame rates on the order of 1 frame per second (fps) which was not sufficient for real-time applications such as ophthalmic imaging.…”

Imaging in Myopia

“…However, the anterior vitreous and anteriorsegment structures (cornea, iris, ciliary body, and lens) are poorly visualized because they fall within the unfocused near field of the transducer which makes current ophthalmic systems unsuitable for imaging the entire vitreous. Recent advancements in annular-array imaging technology overcome these technical limits [54].…”

Design, modeling and characterization of a 35MHz 1-D CMUT phased array

“…In small animal studies it provides the resolution required for evauation of anatomical features under physiological conditions such as operation of heart valves in mice [1]. In ophthalmology it is used to image and evaluate therapy methods [2]. High frequency ultrasound is also used to image thin layers of skin [3].…”