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Sunny Kwok, Thomas Sandwisch, Keyton Clayson, Yanhui Ma, Jun Liu; Scleral Strain Artefacts due to Spatiotemporal Distortion in High-Resolution, High-Frame Rate Ultrasound Imaging. Invest. Ophthalmol. Vis. Sci. 2019;60(11):PB0168.
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Ultrasound imaging of objects in motion may suffer from image distortion due to sequential A-line scanning. This study aims to characterize and reduce distortion-induced strain artefacts in ultrasound elastography of the sclera.
A 50 MHz ultrasound probe (Vevo 2100, VisualSonics) was used to acquire radiofrequency B-mode frames at 128 frames per second of a porcine sclera during whole eye motion. The eye was displaced either parallel or perpendicular to the imaging axis of the ultrasound probe to simulate axial and lateral displacements, respectively (Fig 1A). The globe was cyclically displaced at a frequency of 1.5 Hz for five cycles of 10, 50, and 100 µm in both directions, simulating a potential range of whole eye motion during fixation. A previously validated ultrasound speckle tracking algorithm (Tang & Liu, JBME 2012) was used to estimate the strains for the image frames corresponding to peak and trough displacements of each cycle (Fig 1B). Frames immediately next to the peak and trough but within the same half-cycle (Fig 1C) were used to evaluate whether the strain artefacts can be reduced by selecting the frames obtained during eye motion of the same speed and direction.
Parallel and perpendicular motion both produced axial and lateral strain artefacts of small amplitudes (0.010% and 0.032%, respectively, Fig 2). Lateral strain artefacts are more prominent at higher displacements, and the magnitude of both axial and lateral strain artefacts appear to increase with increased speed of motion. Frames within the same half-cycle had significantly smaller strain artefacts, reducing to 0.01%, 0.02%, and 0.03% for 10, 50, and 100 µm, respectively (Fig 2).
Ultrasound imaging modalities relying on sequential A-line scanning may create image distortion when the imaged object is in motion, and this distortion may generate measureable strain artefacts in high-resolution ultrasound elastography. Using frames within the same half cycle, i.e. frames obtained at the same velocity, could minimize strain artefacts over a range of eye motion.
This abstract was presented at the 2019 ARVO Imaging in the Eye Conference, held in Vancouver, Canada, April 26-27, 2019.
Fig 1: (A) Scleral motion relative to ultrasound imaging axis; (B) initial frame selection process to calculate strain; (C) improved frame selection within same half-cycle
Fig 2: Strain artefacts before (black) and after (red) reduction of (A) axial and (B) lateral strain in both parallel and perpendicular probe orientations
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