Phase-Coded Pulse Sequence for Passive Detection and Mapping of Ultrasound Cavitation

Focused ultrasound (FUS) therapy requires reliable and effective monitoring tools to understand the underlying physical mechanism, controlling the treatment process, and assessing the treatment effects. Passive acoustic mapping (PAM), extended from passive cavitation detection (PCD), can provide noninvasive real-time quantitative imaging associated with treatment effects, and has been widely used in more and more applications of ultrasound therapy. Most PAM applications are based on common one-dimensional arrays that are used for ultrasound diagnosis, assisting with the simplest time exposure acoustics algorithm. However, due to the limited frequency-dependent diffraction mode of these arrays, this algorithm results in a large point spread function (PSF). To address this challenge, in the present study, we proposed a phase-coded pulse sequence for FUS exposure to improve the PAM performance. In the experiments, K sinusoidal signals with equidistant phases were used to driven the FUS transducer, and a PCD transducer and a linear array were utilized to receive the acoustic emissions. The PCD signals of different phases were summed in time domain and then the spectrum analyses were performed, and the passive array signals were processed by a modified passive beamforming algorithm. Ex vivo experimental results illustrated that in PCD spectrum, the K-order and its integer harmonics were enhanced while the other harmonics were suppressed. The PAM results suggested that compared with single phase and pulse inversion, the PSF size was reduced by using 4-phase coding. This work may be beneficial to the accurate control of ultrasound treatment, particularly cavitation-mediated applications.