Acoustic micro-tapping for non-contact 4D imaging of tissue elasticity

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.

Optical coherence elastography – OCT at work in tissue biomechanics [Invited]

“…Most recently, technological innovations in piezoceramic materials from the same group have enabled the realization of high-power ultrasound transducers capable of air-coupled ARF excitation [95]. Such transducers, recently demonstrated for OCE in ex vivo porcine cornea [96], present intriguing possibilities for noncontact OCE.…”

High-speed OCT light sources and systems [Invited]

“…Moreover, repetitive imaging of flow can yield not only velocity, but also the total flow [175,176]. Another functional extension allows for determination of tissue elasticity [177]. In developmental biology, high speed imaging with FDML lasers has demonstrated time-resolved imaging of heart motion [178,179].…”

Applanation optical coherence elastography: noncontact measurement of intraocular pressure, corneal biomechanical properties, and corneal geometry with a single instrument

“…28 There are some limitations in our work as well. The surface wave equation does not take into account the corneal viscoelastic nonlinearity, 25,28 corneal curvature and thickness, 29 aqueous humor, 30 dispersion of the elastic wave, 31,32 or corneal mechanical anisotropy. 24,33 Additionally, the limited spatial and temporal data points resulted in the large variance in measured corneal biomechanical properties at 20 mmHg (stiff conditions), and the limited bandwidth of the air-pulse induced waves (below 1 kHz) combined with limited number of data points prevented dispersion analysis of the elastic wave.…”

“…There are still several steps for improvement, which are the focus of our future work. For example, utilizing the both scans of the resonant scanner, 21 dispersion analysis 31,32,34 with a more rigorous mechanical wave model 31 will enable more accurate elasticity measurements. Acoustic techniques can also provide noncontact measurement of corneal biomechanical parameters, 33 but the presence of the coupling medium can be uncomfortable for in vivo corneal applications.…”

“…In this case, air-coupled acoustic radiation force excitation with ultrafast OCE shows great promise for providing truly noncontact quantification of corneal viscoelasticity. 32 Nevertheless, our OCE technique has the ability to measure the biomechanical properties of the cornea in its natural resting state in the eye-globe with small displacements that minimize the effects of the corneal nonlinear biomechanical properties. Noncontact OCE has also shown the ability to measure the depthwise microscale biomechanical properties of the cornea 34 as well as changes in these properties from different cross-linking techniques.…”