Visualization of ultrasonically induced shear wave propagation using phase sensitive optical coherence tomography

Shear wave elastography measures the stiffness of soft tissues from the speed of propagating shear waves induced in tissue. Optical coherence tomography (OCT) is a promising detection modality given its high sensitivity and spatial resolution, making it suitable for elastic characterization of skin, peripheral vasculature or ocular tissues. For clinical applications, it would be valuable to use a non-contact shear source. Thus, we propose acoustic radiation force as a remote shear source combined with OCT for visualization. A single-element focused transducer (central frequency 7.5 MHz) was used to apply a maximal pressure of ~3 MPa for 100 μs in agar phantoms. It induced shear waves with an amplitude of several hundreds of nanometers and a broadband spectrum in the kilohertz range. Phasesensitive OCT was used to track shear waves at an equivalent frame rate of 47 kHz. We reconstructed shear modulus maps in a heterogeneous phantom. In addition, we use 3-ms long coded excitation to increase the displacement signal-to-noise ratio. We applied digital pulse compression to the resulting displacement field to obtain a gain of ~15 dB compared to standard pulse excitation while maintaining the US pressure level and the shear wave spatial and temporal resolution. This is a promising result for shear wave generation at low US pressures (~ 1 MPa).