Nanodroplet-based ultrasound and photoacoustic super-resolution and functional imaging
Creators: name: Zhang, Ge
Publisher: name: Imperial College London
Year: 2019

description: Microbubble-enhanced ultrasound imaging, and the more recent advances in super-resolution imaging and molecular imaging, have shown great promise in a wide range of clinical applications. However, a number of challenges still exist during their clinical translations. For instance, in ultrasound superresolution imaging, the in vivo lifetime of microbubble is normally less than 5 minutes and the concentration of microbubbles in vivo is difficult to adjust after the injection. Nanodroplet, as a condensed version of microbubble, has a number of advantages in super-resolution imaging. Firstly, nanodroplet have a longer lifetime compared to microbubbles. This enables less contrast agents injected and a longer scanning time. Second, these nanodroplets can be controlled to be activated to provide the contrast signals both spatially and temporally on-demand. This potentially provides more flexibility during the ultrasound scanning. In the context of molecular imaging, the nano-size of droplets potentially allows extravasation into cancerous tissue due to its leaky vasculature and enhanced permeability and retention effects where microbubbles cannot. In the field of photoacoustic imaging, dye-coated nanodroplet was developed to provide a significant contrast enhancement in photoacoustic imaging upon the optical activation where microbubbles also cannot be used. Among a variety of research efforts on facilitating the pre-clinical translation, this work focuses on the applications of nanodroplets in ultrasound and photoacoustic super-resolution and molecular imaging. There are four main scientific contribution as follows. First, the feasibility of using low-boiling-point nanodroplets to perform ultrasound super-resolution imaging was investigated. ‘Acoustic wave sparsely activated localization microscopy (AWSALM)’, an acoustic counterpart of photo-activated localization microscopy (PALM), is developed to super-resolve structures which cannot be resolved by conventional B-mode imaging. AWSALM utilizes acoustic waves to sparsely and stochastically activate decafluorobutane nanodroplets by acoustic vaporization and to simultaneously deactivate the existing vaporized nanodroplets via acoustic destruction. This technique is less dependent on flow and does not require a low concentration of contrast agents, as is required by current ultrasound super resolution techniques. Acoustic activation and deactivation can be controlled by adjusting the acoustic pressure, which remains well within the FDA approved safety range. This study shows the promise of a flow and contrast agent concentration less dependent super-resolution ultrasound technique which has potential to be faster and go beyond vascular imaging. Second, to achieve super-resolved imaging frames with sub-second temporal resolution, fast acoustic wave sparsely activated localization microscopy (fast-AWSALM) was developed by using low-boiling-point octafluoropropane nanodroplets and high frame rate plane waves for activation, destruction, as well as imaging. The effects of the temperature and mechanical index on fast-AWSALM was investigated. The contrast signals were quantified as a function of acquisition time. The final results showed two orders of magnitude faster than the reported localization-based ultrasound super-resolution techniques, under a non-flow/very slow flow situations. Just as in AWSALM, fast-AWSALM is less dependent on flow, as is required by current microbubble based ultrasound super resolution techniques. This study shows the promise of fast-AWSALM, a superresolution ultrasound technique using nanodroplets, which can generate super-resolution images in milli-seconds. Third, to have a better design of targeted nanodroplets for imaging and therapeutic applications, the size and acoustic response of targeted nanodroplets under high-frame-rate ultrasound and optical imaging was investigated. A flow velocity mapping technique, Stokes’ theory and optical microscopy were used to estimate the size of both floating and attached vaporized nanodroplets immediately after activation. The floating vaporized nanodroplets were on average more than seven times larger than vaporized nanodroplets attached to the cells. The results also indicated that the acoustic signal of vaporized FR-targeted-nanodroplets persisted after activation, with 70% of the acoustic signals still present 1 s after activation, compared with the vaporized NT-nanodroplets, for which only 40% of the acoustic signal remained. The optical microscopic images revealed on average six times more vaporized FR-targeted-nanodroplets generated with a wider range of diameters (from 4 to 68mm) that were still attached to the cells, compared with vaporized NT-nanodroplets (from 1 to 7mm) with non-specific binding after activation. The mean size of attached vaporized FR-targetednanodroplets was on average about threefold larger than that of attached vaporized NT-nanodroplets. Taking advantage of high-frame-rate contrastenhanced ultrasound and optical microscopy, this study offers an improved understanding of the vaporization of the targeted nanodroplets in terms of their size and acoustic response in comparison with NT-nanodroplets. Such understanding would help in the design of optimized methodology for imaging and therapeutic applications. Fourth, the feasibility of using dye-coated nanodroplets to perform photoacoustic super-resolution imaging was investigated. A photoacoustic super-resolution imaging technique was developed through imaging the activation of Cyanine 7.5-coated phase-change nanodroplets using a preclinical photoacoustic imaging system and localizing the activated droplets. As a proof-of-concept experiment, photoacoustic images of microfluidic channels were obtained with a cylindrically focused curvedarray, while dye-coated nanodroplets flowed through the channels. Experimental results showed that super-resolution images can resolve structures which cannot be resolved by conventional beamformed images in vitro. The results also showed that the dye-coated phase-change nanodroplets can be optically activated in vivo and the activation signals can be separated from the image background by applying singular value decomposition filtering, and be used for further super-localization processing. Nanodroplets offer better biocompatibility, as well as more flexible and controllable droplet activation rates, with potential for superresolution imaging without flow and of extravascular targets, compared to contrast agents used in existing localization-based photoacoustic superresolution imaging techniques.