Principles of ophthalmic ultrasound

ABSTRACTIntroduction Ultrasound imaging of the eye was introduced over 50 years ago. While the physical principles of ultrasound imaging have not changed, technology has undergone tremendous and ongoing development.Areas covered The fundamentals of ultrasound physics, biometry (A-scan), structural imaging (B-scan) and blood-flow imaging and measurement (Doppler) will be described. Emphasis will be placed on technological development and potential future advances.Expert opinion While A- and B-scan ultrasound of the eye has traditionally been performed with focused single-element transducers, the introduction of annular and linear arrays has enhanced clinical utility. Future advances, especially in multielement arrays, and point-of-care systems promise amazing new capabilities for diagnostic imaging of the eye and orbit.KEYWORDS: UltrasoundeyeDopplertransducerDisclaimerAs a service to authors and researchers we are providing this version of an accepted manuscript (AM). Copyediting, typesetting, and review of the resulting proofs will be undertaken on this manuscript before final publication of the Version of Record (VoR). During production and pre-press, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal relate to these versions also. Article highlightsMedical diagnostic ultrasound developed in the aftermath of the second world war as a spinoff of Sonar technology used for underwater range finding.While ophthalmic ultrasound has largely been based on mechanically scanned, focused single-element transducer technology, virtually all other clinical specialties use linear array-based systems.Array-based systems allow control of focal depth. Linear arrays offer high scan rates and can provide Doppler to visualize and measure blood-flow.The advantages and decreasing cost of linear array systems is leading towards greater utilization for ophthalmic imaging.While the principles of ultrasound imaging are unchanged, the technology, especially in array-based systems, continues to advance.Declaration of interestR H Silverman has a financial interest in Arcscan, Inc. The author has no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.Reviewer disclosuresPeer reviewers on this manuscript have no relevant financial or other relationships to disclose.Figure 1: Left: Plots representing ultrasound waveform of a highly damped (top) and a poorly damped (bottom) transducer of the same wavelength (λ). To the right of each plot is the power spectrum corresponding to each waveform. Bandwidth (usually measured at 50% of maximum spectral power) is inversely related to pulse-length.Display full sizeFigure 2: 1970’s setup for immersion ultrasonography. The probe was pivoted by hand to sweep out a B-scan or held steady for an A-scan while the shutter of the polaroid camera facing the display was held open. Analog images had limited dynamic range, but resolution was excellent. The immersion technique was particularly good for visualizing the anterior segment since the probe could be positioned far enough away to place its focus where needed.Display full sizeFigure 3: Biometric immersion A-scan and contact axial B-scan images of 73-year-old subject. Note faint echoes in mid vitreous in the A-scan due to floaters. Lens does not show internal echoes indicative of cataract. C=corneal, L=lens, R=retina.Display full sizeFigure 4: UBM images of anterior segment obtained by shallow arc scan (top) match to curvature of eye and by compounding of multiple arc and linear scans (bottom) on the Insight system. Due to specular reflectivity of lens capsule, the posterior capsule is only seen at the posterior pole in the arc scan (arrow) versus nearly full outline in compound image.Display full sizeFigure 5: 20 MHz B-scan images obtained with Quantel Absolu annular array in vitreous (left) and retina (right) mode settings. The images show a lesion supero-temporally at the site of laser-treated retinal tear that occurred following posterior vitreous detachment 12 years previously. Note greater detail in depiction of vitreous debris in vitreous-mode image and improved depiction of retina and orbital tissues in retina-mode.Display full sizeFigure 6: Axial B-scan and color-flow Doppler image obtained on GE Venue Go ultrasound system with L4-20t-RS linear array probe. The central retinal artery (red) and vein (blue) are visualized.Display full sizeFigure 7: Plane-wave Doppler color flow image and spectrograms of pre-term, low birthweight neonate evaluated at cribside in neonatal intensive care unit. Data were acquired from 6 compounded angled transmits with 3000 compound images acquired per second. Spectrograms depict flow velocity over 1.5 seconds in the central retinal artery (1), central retinal vein (2) and short posterior ciliary artery (3).Display full sizeAdditional informationFundingThis paper was funded by NIH grants R01 EY025215, R01 EB032082, P30 EY019007 and an unrestricted grant to the Columbia University Department of Ophthalmology from Research to Prevent Blindness.