Ultrasonic imaging of the mechanical properties of tissues using localized, transient acoustic radiation force

Acoustic radiation force impulse (ARFI) imaging utilizes brief, high energy, focused acoustic pulses to generate radiation force in tissue, and ultrasonic correlation-based methods to detect the resulting tissue displacements in order to image the relative mechanical properties of tissue. The magnitude and spatial extent of the applied force is dependent upon the transmit beam parameters and the tissue attenuation. Forcing volumes are on the order of 5 mm³, pulse durations are less than 1 msec, and tissue displacements are typically several microns. Displacement is quantified using interpolation and cross-correlation methods. Noise reduction is accomplished by adaptively filtering the temporal response, and median filters are applied to the resulting images. Images of tissue displacement reflect local tissue stiffness, with softer tissues (e.g. fat) displacing farther than stiffer tissues (e.g. muscle). Parametric images of maximum displacement, time to peak displacement, and recovery time provide information about tissue material properties and structure. In both in vivo and ex vivo data, structures shown in matched B-mode images are in good agreement with those shown in ARFI images, with comparable resolution. Potential clinical applications under investigation include: soft tissue lesion characterization, assessment of focal atherosclerosis, and imaging of thermal lesion formation during tissue ablation procedures. Results from ongoing studies are presented.