Acoustic radiation force impulse imaging: ex vivo and in vivo demonstration of transient shear wave propagation

Acoustic radiation force impulse (ARFI) imaging utilizes brief, high energy, focused acoustic pulses to generate radiation force in remote locations in tissue, and conventional diagnostic ultrasound methods to detect the resulting tissue displacements in order to provide information about the mechanical properties of tissue. Tissue displacement magnitude is inversely related to local tissue stiffness, and the temporal response of the tissue is related to its viscosity. In addition, ARM imaging allows visualization of the transient shear waves generated by the impulsive radiation force, whose propagation velocity and attenuation reflect the local tissue properties. ARFI imaging is implemented on a modified Siemens Elegra scanner with a 7.5 MHz linear array transducer using radiation force application times ranging from .03 to 1 msec. Good correlation is observed between the matched pathology, B-mode, and ARM images of ex vivo breast and cervical tissues. ARFI image representation of tissue stiffness is consistent with manual palpation. In addition, localized radiation force induced shear wave propagation is visualized by ARFI imaging in phantoms, ex vivo breast tissue, and in vivo abdominal tissue. Shear wave speeds ranging from 1 to 5 m/s are observed in abdominal tissue in vivo. These results suggest that ARM imaging holds clinical promise.