Session: P2G

Background: Vibro-acoustography is an elasticity imaging method that images the acoustic response of a material to a localized harmonic excitation generated by ultrasound radiation force [Fatemi and Greenleaf, Science 280:82-85, 1998]. Proper beam forming for the stress field (radiation force) of the probing ultrasound is very important because it determines the resolution of the imaging system. Ideally, the stress field should be confined to a small 3D region. Approach: Three beam forming geometries are studied: AM, confocal, and xfocal. Low frequency oscillatory radiation force (e.g., 10 kHz) can be obtained by amplitude modulating a single ultrasound beam sinusoidally (AM), or by interfering two beams of slightly different frequencies. For instance, the confocal transducer is a two-element spherically focused annular array and the x-focal has two transducers whose beam axes cross at their foci at an angle. The amplitude of radiation force on a unit point target is calculated from the ultrasound energy density averaged over a short period of time. This force has an oscillatory component whose magnitude is proportional to the product of the pressure field amplitudes of the two beams. The profiles of radiation stress amplitude for a point target on the focal plane and on the beam axis are derived for all three geometries. These profiles represent the point-spread-function of the system. Methods: The theory is validated by experiments using a small sphere target (radius = 200 μm) glued on a thin membrane. A laser vibrometer is used to measure the velocity profiles of the sphere, which are proportional to the radiation stress exerted on the target, as the transducer is scanned over the focal plane and the beam axis. Results: The experimental velocity profiles for all three beam forming geometries agree well with the corresponding theoretical results. Conclusion: The theoretical models for the three beam forming methods are validated by direct measurements. The results can serve as guidance for transducer design for vibro-acoustography. The theory and experimental technique may be useful in designing linear array transducers for vibro-acoustography. This work is supported by grants EB2640 and IMG0100744.