Ultrasonic concentration imaging of cavitation bubbles using Nakagami statistical model

Cavitation is regarded as the primary mechanism in pulsed high-intensity focused ultrasound (pHIFU) therapy. Residual cavitation bubbles can be served as nuclei for the subsequent pHIFU cavitation. Therefore, information on spatial distribution especially concentration distribution of residual cavitation bubbles is important to better control pHIFU therapy. Acoustic spatial imaging of cavitation bubbles has been widely studied, while no research on quantitative bubble concentration imaging has been found. In this work, ultrasonic concentration imaging of cavitation bubbles was proposed based on Nakagami statistical model. The feasibility of Nakagami parameter m on concentration imaging of microbubbles was investigated firstly by simulation. Afterward, concentration imaging of cavitation bubbles by parameter m was performed in pHIFU experiments. Parameter m of cavitation bubbles increased with increasing concentrations and got saturated when concentrations above 16 bubbles/mm2, which indicated that parameter m could reflect bubble concentration at certain levels. Meanwhile, for the same concentration, parameter m kept almost constant under different imaging parameters, while the corresponding B-mode echoes varied greatly. Parameter m of cavitation bubbles in pHIFU decreased as bubbles dissipated gradually. All results showed that Nakagami parameter m can reflect the concentration distribution of cavitation bubbles, which provides a new method for better monitoring and optimization of pHIFU therapy.