Contrast‐enhanced ultrasound imaging in vivo with laser‐activated nanodroplets

PURPOSE: This study introduces a real-time contrast-enhanced ultrasound imaging method with recently developed laser-activated nanodroplets (LANDs), a new class of phase-change nanometer-scale contrast agents that provides perceptible, sustained high-contrast with ultrasound. METHODS: In response to pulsed laser irradiation, the LANDs-, which contain liquid perfluorohexane and optical fuses-blink (vaporize and recondense). That is, they change their state from liquid nanodroplets to gas microbubbles, and then back to liquid nanodroplets. In their gaseous microbubble state, the LANDs provide high-contrast ultrasound, but the microbubbles formed in situ typically recondense in tens of milliseconds. As a result, LAND visualization by standard, real-time ultrasound is limited. However, the periodic optical triggering of LANDs allows us to observe corresponding transient, periodic changes in ultrasound contrast. This study formulates a probability function that measures how ultrasound temporal signals vary in periodicity. Then, the estimated probability is mapped onto a B-scan image to construct a LAND-localized, contrast-enhanced image. We verified our method through phantom and in vivo experiments using an ultrasound system (Vevo 2100, FUJIFILM VisualSonics, Inc., Toronto, ON, Canada) operating with a 40-MHz linear array and interfaced with a 10 Hz Nd:YAG laser (Phocus, Opotek Inc., Carlsbad, CA, USA) operating at the fundamental 1064 nm wavelength. RESULTS: From the phantom study, the results showed improvements in the contrast-to-noise ratio of our approach over conventional ultrasound ranging from 129% to 267%, with corresponding execution times of 0.10 to 0.29 s, meaning that the developed method is computationally efficient while yielding high-contrast ultrasound. Furthermore, in vivo sentinel lymph node (SLN) imaging results demonstrated that our technique could accurately identify the SLN. CONCLUSIONS: The results indicate that our approach enables efficient and robust LAND localization in real time with substantially improved contrast, which is essential for the successful translation of this contrast agent platform to clinical settings.