Supershear Rayleigh wave imaging for quantitative assessment of biomechanical properties of brain using air-coupled optical coherence elastography

Abstract

Recently, supershear Rayleigh waves (SRWs) have been proposed to characterize the biomechanical properties of soft tissues. The SRWs propagate along the surface of the medium, unlike surface Rayleigh waves, SRWs propagate faster than bulk shear waves. However, their behavior and application in biological tissues is still elusive. In brain tissue elastography, shear waves combined with magnetic resonance elastography or ultrasound elastography are generally used to quantify the shear modulus, but high spatial resolution elasticity assessment in 10 μm scale is still improving. Here, we develop an air-coupled ultrasonic transducer for noncontact excitation of SRWs and Rayleigh waves in brain tissue, use optical coherent elastography (OCE) to detect, and reconstruct the SRW propagation process; in combing with a derived theoretical model of SRWs on a free boundary surface, we quantify the shear modulus of brain tissue with high spatial resolution. We first complete validation experiments using a homogeneous isotropic agar phantom, and the experimental results clearly show the SRW is 1.9649 times faster than the bulk shear waves. Furthermore, the propagation velocity of SRWs in both the frontal and parietal lobe regions of the brain is all 1.87 times faster than the bulk shear wave velocity. Finally, we evaluated the anisotropy in different brain regions, and the medulla oblongata region had the highest anisotropy index. Our study shows that the OCE system using the SRW model is a new potential approach for high-resolution assessment of the biomechanical properties of brain tissue.

FIG. 1.

(a) Schematic diagram of the propagation of different mechanical wave modes in soft tissue. (b) Schematic diagram of SRW propagation in brain tissue after acoustic radiation force excitation.

FIG. 2.

The AcUT-OCE results of the SAW and SRW propagation processes in the phantom. (a) 2D vibration displacement propagation images of the SAW and SRW in the phantom at different times. The yellow arrow in the top image indicates the excitation point. (b) 3D surface reconstruction (x, z, t) of the SAW and SRW; the white arrow indicates the SAW, and the black arrow indicates the SRW. (c) The 2D structure of phantom. (d) The displacement profiles of the SRW and SAW at the depth marked by the blue line in the bottom slice of Fig. 2(a) at 2.7 ms. (e) The spatiotemporal displacement maps of the SRW and SAW at the depth marked by the blue line in the bottom slice of Fig. 2(a).

FIG. 3.

The 2D OCT structure results of different regions of brain tissue. (a) The frontal lobe (FL) region. (b) The parietal lobe (PL) region. (c) The medulla oblongata (MO) region.

FIG. 4.

The AcUT-OCE results of the SRW and SAW propagation processes in the FL. (a) The different detection regions in ex vivo brain tissue. (b) The SRW and SAW propagation processes at different times, the yellow arrow indicates the location of acoustic radiation force excitation. (c) 3D (x, z, t) surface reconstruction results, with the white and black arrows indicating the SAW and SRW, respectively. (d) Spatiotemporal image of SRW and SAW propagation, and the white arrow indicates the time contours of the SRW and SAW.

FIG. 5.

The AcUT-OCE results of the SRW and SAW propagation processes in the PL and MO. (a) Vibration displacement image of SRW and SAW propagation in the PL at 2.7 ms, the yellow arrow indicates the location of acoustic radiation force excitation. (b) 3D surface reconstruction results of the SRW and SAW at 2.7 ms; the white and black arrows indicate the SAW and SRW, respectively. (c) Spatiotemporal image of SRW and SAW propagation in the PL. (d) Vibration displacement image of SAW propagation in the MO at 2.7 ms, the yellow arrow indicates the location of acoustic radiation force excitation. (e) 3D surface reconstruction result of SAW at 2.7 ms; the white arrow indicates the SAW. (f) Spatiotemporal image of SAW propagation in the MO.

FIG. 6.

The results in the FL, PL, and MO. (a) Experimental protocol in the FL, PL, and MO regions; elastic wave data were acquired in four directions: 0°, 60°, 120°, and 180°. (b) The shear modulus results in different directions in the FL region. (c) The shear modulus results in different directions in the PL region. (d) The shear modulus results in different directions in the MO region.

FIG. 7.

Detailed calculations of the average shear modulus in different brain regions, and the error bars indicate the standard deviation.

FIG. 8.

Schematic diagram of the AcUT-OCE system. A laser with a center wavelength of 1310 nm was divided into a sample arm (99%) and a reference arm (1%). The M-B scan protocol was completed by a two-dimensional (2D) galvanometer, which was driven by a computer. The λ trigger and k clock of the laser were used to perform synchronized control between the optical detection and ultrasound excitation systems.