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.