Characterization of ultrasound-contrast agents by short-burst excitation

Abstract

A two-frequency method has been adapted to the use of short ultrasonic bursts for the determination of bubble size. The advantage of a burst method is threefold. The overall energy transfer into the volume of interest is smaller as compared to cw. Second, it allows simple methods for improving spatial resolution. Finally, only one transducer will eventually be required in a pulse-echo mode. The experimental setup is designed to fully compensate for transducer characteristics and sample artifacts such as attenuation through window material and contrast agent solution. This leads to a constant sound pressure exposure for contrast agent in the focal volume. The data acquisition employs the frequency tracking procedure where spectra for narrow-band excition are analyzed for fundamental, second harmonic, and sidebands. A technique of nonconstant imaging frequency is used to avoid ambiguities due to coincidences between higher harmonics of the fundamental and multiples of the sideband frequencies. Measurements with shell-encapsulated microbubbles showed the behavior demonstrated by previous cw methods. Higher harmonics as well as sidebands were present due to the nonlinear dynamics of the contrast agents. [This work was supported in part by USPHS Grant Nos. RO1 DK42290 and 1RO1 HL54201.]