An approach to ultrasonically evaluating the mechanical properties of tissue is acoustic radiation force impulse (ARFI) imaging . ARFI is distinguished from alternative elastographic imaging methods in that the mechanical excitation used to induce tissue motion is delivered with temporal and spatial precision via impulsive, focused acoustic radiation force (ARF). Relative to compression elastography, an advantage to ARFI imaging is that the force is applied directly to the tissue of interest, which obviates challenges associated with indirect force coupling and minimizes the stresses needed to achieve sufficient contrast of mechanical features. In this chapter, the fundamental physics of impulsive ARF are presented first, followed by detailed descriptions of both qualitative and quantitative ARFI imaging methods.
22.2 Impulsive Acoustic Radiation Force
A critical component of ARFI imaging is the impulsive ARF excitation force, which is delivered by a focused ultrasound pulse with longer length and/or higher acoustic power than conventional B‐mode imaging pulses [2, 3]. The force magnitude, in a tissue medium in which absorption is the primary cause of attenuation, is directly proportional ...