Acoustic wave sparsely activated localization microscopy (AWSALM): Super-resolution ultrasound imaging using acoustic activation and deactivation of nanodroplets

Abstract

Photo-activated localization microscopy (PALM) has revolutionized the field of fluorescence microscopy by breaking the diffraction limit in spatial resolution. In this study, “acoustic wave sparsely activated localization microscopy (AWSALM),” an acoustic counterpart of 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. In this method, activation, imaging, and deactivation are all performed using acoustic waves. Experimental results show that sub-wavelength micro-structures not resolvable by standard B-mode ultrasound images can be separated by AWSALM. This technique is flow independent 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. In conclusion, this study shows the promise of a flow and contrast agent concentration independent super-resolution ultrasound technique which has potential to be faster and go beyond vascular imaging.

Super-resolution Ultrasound Imaging

“…The switching-off mechanism can be either the recondensation of the resulting microbubble back into a nano-agent or its disappearance through disruption or dissolution. This has recently been observed by Zhang et al (2018) through acoustic wave sparsely activated localization microscopy (AWSALM) in two proof-of-concept studies (Zhang et al , 2019a. One of the fundamental distinctions between bubble-and droplet-based super-resolution imaging techniques is that, with bubble-based super-resolution approaches, accumulated signals are limited by low bubble concentration and depend on flow decorrelation, which is slow in the smaller vessels.…”

“…The transmitted ultrasound amplitude has to match the vaporization threshold of the droplets to activate them, and the threshold depends on the boiling point of the perflurocarbon gas, their size distribution and whether the droplets are encapsulated by a shell. In its initial realization, Zhang et al (2018) used shelled decafluorobutane (boiling point: À1.7˚C) nanodroplets and focused waves with relatively high ultrasound amplitude (mechanical index [MI] = 1.3) were swept through the imaging area to switch on new droplets, while switching off previously activated droplets at the same time, followed by loweramplitude (MI = 0.25) imaging plane waves. Such an approach of separate activating and imaging pulses is necessary in this case as the activation of the decafluorobutane droplets requires acoustic pressure that is difficult for plane-wave to achieve.…”

3-D Super-Resolution Ultrasound Imaging Using a 2-D Sparse Array with High Volumetric Imaging Rate

“…If motion is present and subsequently corrected post-acquisition, then the motion correction accuracy can also limit the achievable spatial resolution [3], [4]. Researchers demonstrated the use of 2-D SR-US imaging in many studies using microbubbles [5]- [13] and nanodroplets [14]- [16]. All of these studies were based on imaging with 1-D probes that can only display 2-D slices of a 3-D structure, thus making the volumetric observations more challenging.…”

Short Acquisition Time Super-Resolution Ultrasound Microvessel Imaging via Microbubble Separation

“…Contrast MBs can be considered as acoustic point scatterers, since they are substantially smaller than the typical ultrasound wavelength, but also provide strong acoustic backscatter, permitting the application of signal processing methods that can overcome the inherent compromise between ultrasound frequency and attenuation. ULM has been shown by numerous research groups [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] to break the diffraction limit of conventional ultrasound, resulting in an approximately tenfold improvement in imaging resolution while maintaining imaging penetration depth. Furthermore, as an ultrasound-based imaging modality, ULM is safe, non-invasive, low-cost, and lacks ionizing radiation, thereby promising great clinical significance by providing capillary-level imaging resolution at acoustic-level imaging penetrations.…”

“…The dilution method also requires a constant infusion of MBs or multiple bolus injections, both of which are challenging in a clinical setting. Phase-change droplets, which can change phase from liquid to gas when activated by ultrasound, have been utilized as a contrast agent for ULM and have recently demonstrated the potential for real-time super-resolution imaging in vitro 15,18 . Distinct from clinically used MBs, the droplet-based super-resolution method can sparsely activate droplets for localization, allowing for high droplet concentrations, and does not necessarily require frame-to-frame droplet flow for contrast-agent detection.…”