Wave-based optical coherence elastography (OCE) is a rapidly emerging technique for localized elasticity assessment of tissues due to its high displacement sensitivity and simple implementation. This method does not require prior knowledge of mechanical load characteristics, such as the applied preload and applied stress on the sample. Currently, noncontact wave excitation has been accomplished with various methods, such as focused micro air-pulse and acoustic techniques. However, they are limited by the inability to target specific tissues and usually only image the transversely propagating elastic wave, which generally requires scanning the probe beam across the sample. In addition, the upper frequency components of the elastic waves are limited to a few kilohertz, which are sensitive to boundary conditions due to their long wavelengths. In this study, we demonstrated that rapid vaporization of perfluorocarbon inside dye nanoparticles that was excited by a pulsed laser excitation, termed “nanobombs”, can produce high frequency longitudinal elastic waves in tissue mimicking phantoms. The nanoparticles were excited by a 1064 nm pulsed laser, which was co-focused with the OCT probe beam. The longitudinal elastic waves, which propagated axially (i.e., following the optical path), were directly imaged by a phase-sensitive Fourier domain mode-locked based OCT system. The detected elasticity was validated with well-established air-pulse OCE and the “gold standard” uniaxial mechanical testing. The results demonstrate the feasibility of performing nanobomb elastography in tissue with the potential for targeting specific tissues and producing longitudinal elastic waves with high frequency content.
Wave-based optical elastography is a rapidly emerging technique for viscoelastic assessment of tissues due to its high displacement sensitivity and simple implementation. This method does not require prior knowledge of mechanical load characteristics, such as the applied preload and applied stress on the sample. However, current truly noncontact excitation methods are limited by their inability to produce broadband waves with high frequency content. Lower frequency wave content is constrained by boundary conditions, and thus, requires specifically tailored mechanical models that consider the sample geometry. In this work, we demonstrate that rapid vaporization of perfluorocarbon inside dye nanoparticles (NP) with a pulsed laser can produce high frequency and broadband elastic waves in tissue mimicking agar phantoms. As a comparison, a focused air-pulse was used as an alternative excitation method. The elastic waves were imaged by an ultra-fast low-coherence line-field holography system. Our results show that the NPs produced elastic waves with frequencies up to ~9 kHz, while the air-pulse was only able to produce waves with frequency content up to ~2 kHz. The elastic wave dispersion curves were fitted to the analytical solution of a Rayleigh wave model to quantify viscoelasticity. Analysis of the broadband high-frequency waves produced by the NPs yielded more accurate quantification of the sample viscoelasticity, demonstrating the benefits of optically excited elastic waves.