Abstract
Elastography plays a key role in characterizing soft media such as biological tissue. Although this technology has found widespread use in both clinical diagnostics and basic science research, nearly all methods require direct physical contact with the object of interest and can even be invasive. For a number of applications, such as diagnostic measurements on the anterior segment of the eye, physical contact is not desired and may even be prohibited. Here we present a fundamentally new approach to dynamic elastography using non-contact mechanical stimulation of soft media with precise spatial and temporal shaping. We call it acoustic micro-tapping (AμT) because it employs focused, air-coupled ultrasound to induce significant mechanical displacement at the boundary of a soft material using reflection-based radiation force. Combining it with high-speed, four-dimensional (three space dimensions plus time) phase-sensitive optical coherence tomography creates a non-contact tool for high-resolution and quantitative dynamic elastography of soft tissue at near real-time imaging rates. The overall approach is demonstrated in ex-vivo porcine cornea.
Highlights
For nearly thirty years, elastography has been used to spatially map the shear/Young’s modulus of soft materials such as biological tissue[1,2,3,4]
The US wave reflected from this interface provides acoustic radiation force (ARF) to the medium surface, determined by the spatial shape of the pump beam and the duration of the ultrasound pulse[16]
We have shown that reflection-based ARF from air can excite mechanical transverse waves in porcine eye cornea with sufficient displacement amplitude to be tracked with an imaging system even at very low acoustic pressures
Summary
Elastography has been used to spatially map the shear/Young’s modulus (or elasticity) of soft materials such as biological tissue[1,2,3,4]. Non-contact mechanical wave generation in soft media has been demonstrated in a limited number of studies. It is very difficult to spatially shape the air-puff source, a very important feature for quantitative measurements Given these limitations, it may not be possible to produce high resolution maps of tissue elasticity with an air-puff source. There is still great need for a non-contact method to generate shear mechanical waves in soft media with the spatial and temporal compactness required for high spatial resolution elasticity imaging
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