Effect of inter-element apodization on passive cavitation images

Acoustic cavitation has been correlated with a variety of ultrasound-mediated bioeffects. Recently developed passive cavitation imaging methods provide spatially resolved maps of cavitation activity with good azimuthal resolution but poor axial resolution. Here, inter-element apodization is investigated as a means of improving image quality. Cavitation was induced from echogenic liposomes in a flow phantom exposed to 6 MHz Doppler ultrasound (Philips HDI-5000). The resulting acoustic emissions were passively recorded on 64 elements of a linear array (L8-3 transducer, Zonare z.one ultra scanner). Amplitude scaling of each waveform by its root-mean-square value improved axial resolution at the expense of creating an ‘X-shaped’ artifact. Cosine amplitude apodization of the received waveforms across the array and centered about the azimuthal location of the beamformed image pixel was found to reduce grating lobe artifacts. Numerical time reversal of the received waveforms, using the Fresnel approximation for the acoustic field of each array element, resulted in an effective apodization due to element directivity and also reducing grating lobe artifacts. Applying apodization may be an effective means of increasing passive image quality for certain cavitation distributions, which will be discussed. [Supported in part by NIH grants F32HL104916, R01HL074002, R21EB008483, R01HL059586, and R01NS047603.]