Shear Wave Elasticity Imaging Based on Acoustic Radiation Force and Optical Detection

Abstract

Abstract:Tissue elasticity is closely related to the velocity of shear waves within biological tissue.Shear waves can be generated by an acoustic radiation force and tracked by, e.g., ultrasound or MRI measurements. This has been shown to be able to non-invasively map tissue elasticity in depth and has great potential in a wide range of clinical applications including cancer and cardiovascular diseases. In this study a highly sensitive optical measurement technique is proposed as an alternative way to track shear waves generated by the acoustic radiation force.A CCD camera was used to capture diffuse photons from tissue mimicking phantoms illuminated by a laser source at 532nm. CCD images were recorded at different delays after the transmission of an ultrasound burst, and were processed to obtain the time of flight for the shear wave. A differential measurement scheme involving generation of shear waves at two different positions was used to improve the accuracy and spatial resolution of the system. The results from measurements on both homogeneous and heterogeneous phantoms were compared to measurements from other instruments, and demonstrate the feasibility and accuracy of the technique for imaging and quantifying elasticity. The relative error in estimation of shear wave velocity can be as low as 5.5% with a spatial resolution of 2 mm.The system is shown to be highly sensitive and is able to track shear waves propagating over several centimetres given the ultrasound excitation amplitude and the phantom material used in this study. It was also found that the reflection of shear waves from boundaries between regions with different elastic properties can cause significant bias in the estimation of elasticity, which also applies to other shear wave tracking techniques. This bias can be reduced at the expense of reduced spatial resolution.

Tracking shear waves in turbid medium by light: theory, simulation, and experiment

“…In this Letter, we present a method to track the shear wave propagation in optically turbid media by laser speckle contrast analysis. Part of our experimental method was discussed in [6], where the shear wave speed was tracked in phantoms of varying elasticity. A coherent green laser was used, and the speckle patterns were received on a CCD camera.…”

“…Part of our experimental method was discussed in [6], where the shear wave speed was tracked in phantoms of varying elasticity. A coherent green laser was used, and the speckle patterns were received on a CCD camera.…”

“…In the experiment, a tissue-mimicking phantom was made with 0.8% agar in weight and 4% intralipid in volume to achieve similar mechanical and optical parameters to the simulation [6,13,15]. Shear waves were generated by the transient acoustic radiation force launched by a 1 ms 5 MHz ultrasound burst, and a 1 ms CCD exposure length was used to record the laser speckle patterns.…”

“…However, experimental results show the peak value at approximately 15 ms. Further simulation demonstrated that this mismatch was an effect of the shear wave reflection at the phantom boundaries in the experiment (the reflection coefficients of obliquely incident shear waves were calculated based on [16]), which broadened the shear wave and prolonged the measured time-of-flight. Though some reflection bias could exist in real situations, an accurate measurement of shear wave speed can be still obtained by a differential method [6].…”

“…Compared with ultrasound tracking methods (sensitive to μm displacement), the optical speckle-contrast analysis can be more sensitive because optical wavelengths are a few orders of magnitude smaller than ultrasound wavelengths. The approach could be used in medicine for quantitative elastography by tracking shear waves via time-of-flight [6]. In practice, blood diffusion, Brownian motion, and blood flow will cause speckle decorrelation, and, while this should not affect the time-of-flight estimation, it can reduce the signal-to-noise ratio of the measurements.…”