Frequency-sum passive cavitation imaging

Abstract

Passive cavitation imaging (PCI) is a method for spatially mapping acoustic emissions caused by microbubble activity, including subharmonic and ultraharmonic emissions that denote stable cavitation. The point spread function (PSF) of passive cavitation images is diffraction limited. When typical clinical diagnostic linear arrays are used for PCI, the diffraction limit results in high azimuthal resolution but low axial resolution. Abadi et al. (2013)recently demonstrated a method called frequency-sum beamforming, which employs second-order or higher products of the acoustic emissions to manufacture higher frequencies, thereby reducing the size of the PSF. We applied this approach to cavitation emissions recorded from albumin-shelled bubbles insonified by 2 MHz ultrasound. Cavitation emissions were recorded on a 5 MHz, 128 element linear array using a Vantage scanner (Verasonics Inc.). Quadratic and fourth-order frequency-sum beamforming was applied to both harmonic and ultraharmonic cavitation emissions. Corresponding simulations were also performed to illustrate frequency-sum passive cavitation imaging of multiple bubbles. In comparison to delay-and-sum PCI, apparent areas of cavitation activity decreased when products of the emissions were used to perform frequency-sum beamforming. However, frequency-sum beamforming also produced artifacts, including the appearance of spurious emission sources.