Cyclic generation of strain inside object using dual acoustic radiation force

Abstract

There are many studies on measurement of tissue mechanical properties by applying an acoustic radiation force induced by ultrasound to an object. However, when the elastic modulus of the object is much higher than that of the surrounding tissue (such like a tumor in the breast tissue), an acoustic radiation force might generate only the change in position of the object and the strain of the object is hardly generated. In such cases, mechanical properties of the object cannot be evaluated. In this study, two cyclic acoustic radiation forces are simultaneously applied to an object to effectively generate the strain inside the object even when the object is much harder than the surrounding tissue. I. INTRODUCTION In recent years, some remote actuation methods based on acoustic radiation force have been reported. Fatemi and coworkers proposed an imaging method called ultrasound- stimulated acoustic emission (USAE) (1), (2). Their system consists of two confocal ultrasonic transducers, and two ul- trasound beams with two slightly different frequencies of f and (f +∆ f) are transmitted. Acoustic radiation pressure, PR(t), exerted on the interface between two different media is a function of the energy density, e(t), and the specific acoustic impedances, Z1 and Z2, of the media (3). The energy density, e(t), is proportional to the square of the sum of the sound pressures, p1(t) and p2(t), generated by the two transducers. In the intersectional area of the two beams, therefore, an oscillatory radiation pressure PR(t) with the frequency difference, ∆f, is applied to the interface. The radiation force produces acoustic emission which is closely related to the mechanical frequency response of the medium. By measuring the acoustic emission with a hydrophone, hard inclusions in a soft material were experimentally detected. The spatial resolution in the depth direction corresponds to the size of the intersectional area. Nightingale et al. proposed an alternative imaging method in which the pulsed ultrasound is employed for applying the radiation force to a soft tissue during short durations (less than 1 ms). The viscoelastic properties of the tissue are measured from the magnitude and the transient response of the displacement, d(t), of the tissue (4). In order to generate measurable displacement, d(t), by several ultrasonic pulses, high-intensity pulsed ultrasound of 1,000 W/cm 2 is employed. According to the safety guideline shown by the Japan society of ultrasonics in medicine (JSUM), however, the intensity of ultrasound is recommended to be less than 240 mW/cm 2 and 1 E2 E1