Vibro-acoustography: The most promising approaches and inferred needs for transducers and arrays

Vibro-acoustography is an imaging modality that uses the dynamic radiation force of ultrasound to induce local oscillatory motion. Two ultrasound beams at slightly different frequencies, f₀ and f₀ + Df, are caused to intersect at the focus of the ultrasound beams and interfere to create a stress field that causes the tissue in the focal region to locally vibrate harmonically at Df. The object responds to the oscillatory force by creating a sound field called acoustic emission that is measured with a nearby hydrophone. Production of the ultrasound beams may be accomplished in a variety of ways. The simplest method is with single element transducers that can be used by summing two ultrasound frequencies on the element causing a modulated beam. The disadvantage of this method is that all of the tissue in the beam experiences modulation and causes acoustic emission of varying degrees. Another method is to divide the aperture into two or more regions, each region driven by a separate frequency. If the aperture is appropriately focused, this method produces a modulated radiation force and thus an acoustic emission only at the intersection of the beams. Multiple harmonic motions of the focal region can be produced in this manner. Methods for dividing the aperture depend on the geometry of the aperture and the required beam shape, including side lobes and grating lobes. Because of the large number of parameters that can be varied, the number of dimensions of the parameter space is large and optimization of the selected aperture and associated drive frequencies is a complex process.