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Shear wave elasticity imaging (SWEI) was employed to track acoustic radiation force impulse (ARFI)-induced shear waves in the mid-myocardium of the left ventricular free wall (LVFW) of a beating canine heart. Shear waves were generated and tracked with a linear ultrasound transducer that was placed directly on the exposed epicardium. Acquisition was ECG-gated and coincided with the mid-diastolic portion of the cardiac cycle. Axial displacement profiles consistent with shear wave propagation were clearly evident in all SWEI acquisitions (i.e., those including an ARFI excitation); displacement data from control cases (i.e., sequences lacking an ARFI excitation) offered no evidence of shear wave propagation and yielded a peak absolute mean displacement below 0.31 μm after motion filtering. Shear wave velocity estimates ranged from 0.82 to 2.65 m/s and were stable across multiple heartbeats for the same interrogation region, with coefficients of variation less than 19% for all matched acquisitions. Variations in velocity estimates suggest a spatial dependence of shear wave velocity through the mid-myocardium of the LVFW, with velocity estimates changing, in limited cases, through depth and lateral position.

Acoustic radiation force (ARF) has become a common excitation mechanism in elasticity imaging. The high acoustic intensities and subsequent generation of harmonics, however, hamper the effectiveness of using conventional radiofrequency (rf) tracking to investigate the dynamics of tissues and catheter-based transducers, especially during the excitation. Optical tracking, on the other hand, does not utilize acoustic echo and thus is not affected by ARF-generated interference. Additionally, it is able to track equally well in two dimensions, something that rf tracking is unable to do. Despite the inherent near-field scattering that will likely preclude optical (i.e., visible spectrum) tracking from supplanting current ultrasound-based methods in a clinical setting, it could offer valuable new tools in the pursuit of a better understanding of ARF-induced dynamic responses. We utilized an optically-based method to track the dynamic response resulting from an ARF-induced excitation on the surface of a tissue-mimicking phantom and on an unbounded catheter. These tracking data were then compared to tracking data obtained from the conventional rf tracking method. Both impulsive and harmonic (i.e., amplitude-modulated) excitations were investigated. In general, there was good agreement between the conventional (i.e., ultrasound-based) and optically-based tracking methods. Disparities between displacement estimates from the two tracking methods is thought to be a result of the finite length of the tracking marker, which was assumed to move as an infinitesimal point, and aberration of reflected light due to surface waves. Given the reasonable agreement seen for the harmonic and impulsive excitation cases, an optical tracking method could be insightful in future investigations of tissue/transducer response to ARF-induced excitations in controlled experimental settings.

Acoustic radiation force (ARF) has become a common excitation mechanism in elasticity imaging. The high acoustic intensities and subsequent generation of harmonics, however, hamper the effectiveness of using conventional radiofrequency (rf) tracking to investigate the dynamics of tissues and catheter-based transducers, especially during the excitation. Optical tracking, on the other hand, does not utilize acoustic echo and thus is not affected by ARF-generated interference. Additionally, it is able to track equally well in two dimensions, something that rf tracking is unable to do. Despite the inherent near-field scattering that will likely preclude optical (i.e., visible spectrum) tracking from supplanting current ultrasound-based methods in a clinical setting, it could offer valuable new tools in the pursuit of a better understanding of ARF-induced dynamic responses. We utilized an optically-based method to track the dynamic response resulting from an ARF-induced excitation on the surface of a tissue-mimicking phantom and on an unbounded catheter. These tracking data were then compared to tracking data obtained from the conventional rf tracking method. Both impulsive and harmonic (i.e., amplitude-modulated) excitations were investigated. In general, there was good agreement between the conventional (i.e., ultrasound-based) and optically-based tracking methods. Disparities between displacement estimates from the two tracking methods is thought to be a result of the finite length of the tracking marker, which was assumed to move as an infinitesimal point, and aberration of reflected light due to surface waves. Given the reasonable agreement seen for the harmonic and impulsive excitation cases, an optical tracking method could be insightful in future investigations of tissue/transducer response to ARF-induced excitations in controlled experimental settings.

The goal of this work is to develop and characterize an integrated indenter-ARFI (acoustic radiation force impulse) imaging system. This system is capable of acquiring matched datasets of ARFI images and stiffness profiles from ex vivo tissue samples, which will facilitate correlation of ARFI images of tissue samples with independently-characterized material properties. For large and homogeneous samples, the indenter can be used to measure the Young's moduli by using Boussinesq's solution for a load on the surface of a semi-infinite isotropic elastic medium. Experiments and finite element method (FEM) models were designed to determine the maximum indentation depth and minimum sample size for accurate modulus reconstruction using this solution. Applying these findings, indentation measurements were performed on three calibrated commercial tissue-mimicking phantoms and the results were in good agreement with the calibrated stiffness. For heterogeneous tissue samples, indentation can be used independently to characterize relative stiffness variation across the sample surface, which can then be used to validate the stiffness variation in registered ARFI images. Tests were performed on heterogeneous phantoms and freshly-excised colon cancer specimens to detect the relative stiffness and lesion sizes using the combined system. Normalized displacement curves across the lesion surface were calculated and compared. Good agreement of the lesion profiles was observed between indentation and ARFI imaging.