Acoustic radiation force impulse imaging: remote palpation of the mechanical properties of tissue

Acoustic Radiation Force Impulse (ARFI) imaging is a new modality that utilizes brief, high energy, focused acoustic pulses to generate radiation force in tissue, and conventional diagnostic ultrasound methods to detect the resulting tissue displacements in order to image 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. A high resolution and wide dynamic range ARFI image can be formed in 30 to 40 msec using modified diagnostic ultrasound scanners with maximum tissue displacements less than 15 microns and tissue heating less than 1 degree Celsius. Tissue recovery to its original position typically occurs in 1 to 2 msec. In vivo images of human breast, abdomen, and arteries demonstrate good correlation between structures on matched B-mode and ARFI images. ARFI images are speckle free and demonstrate resolution similar to that of matched B-mode images. Differences in both peak displacement and recovery response are observed for different tissues. In general, fatty tissues move farther and take longer to reach their maximum displacement than fibrous or muscular tissues, and fibrous tissues recover more slowly than fat or muscle. In some cases, malignant breast lesions appear larger in the ARFI images than they do on matched B-mode images. Results from ongoing human in vivo and ex vivo studies evaluating the correlation between ARFI images and tissue pathology are presented. The spatial and temporal patterns of radiation force induced target motion in phantoms and in vivo for varying transmit aperture characteristics, transducer frequencies, and target characteristics are discussed. Images of radiation force induced shear waves both in vivo and ex vivo are also presented. Tissue structural boundaries are apparent in the propagating shear wave images. ARFI imaging is currently implemented on both a modified Siemens Elegra scanner with a 75L40 linear array transducer and a modified Siemens Antares scanner using a VF13-5 linear array using radiation force application times ranging from .03 to 1 msec, with real-time data acquisition and off line post processing. Differences in tissue response to radiation force suggest that ARFI imaging has the potential to provide clinicians with valuable insight into local differences in tissue mechanical properties that are not currently possible.