High frame rate 3-D ultrasound imaging technology combined with super-resolution processing method can visualize 3-D microvascular structures by overcoming the diffraction limited resolution in every spatial direction. However, 3-D superresolution ultrasound imaging using a full 2-D array requires a system with large number of independent channels, the design of which might be impractical due to the high cost, complexity, and volume of data produced.In this study, a 2-D sparse array was designed and fabricated with 512 elements chosen from a density-tapered 2-D spiral layout. High frame rate volumetric imaging was performed using two synchronized ULA-OP 256 research scanners. Volumetric images were constructed by coherently compounding 9-angle plane waves acquired in 3 milliseconds at a pulse repetition frequency of 3000 Hz. To allow microbubbles sufficient time to move between consequent compounded volumetric frames, a 7millisecond delay was introduced after each volume acquisition. This reduced the effective volume acquisition speed to 100 Hz and the total acquired data size by 3.3-fold. Localization-based 3-D super-resolution images of two touching sub-wavelength tubes were generated from 6000 volumes acquired in 60 seconds. In conclusion, this work demonstrates the feasibility of 3D superresolution imaging and super-resolved velocity mapping using a customized 2D sparse array transducer.

Localization-based ultrasound super-resolution imaging using microbubble contrast agents and phase-change nanodroplets has been developed to visualize microvascular structures beyond the diffraction limit. However, the long data acquisition time makes the clinical translation more challenging. In this study, fast acoustic wave sparsely activated localization microscopy (fast-AWSALM) was developed to achieve superresolved frames with subsecond temporal resolution, by using low-boiling-point octafluoropropane nanodroplets and high frame rate plane waves for activation, destruction, as well as imaging. Fast-AWSALM was demonstrated on an in vitro microvascular phantom to super-resolve structures that could not be resolved by conventional B-mode imaging. The effects of the temperature and mechanical index on fast-AWSALM were investigated. The experimental results show that subwavelength microstructures as small as 190 µm were resolvable in 200 ms with plane-wave transmission at a center frequency of 3.5 MHz and a pulse repetition frequency of 5000 Hz. This is about a 3.5-fold reduction in point spread function full-width-half-maximum compared to that measured in the conventional B-mode, and two orders of magnitude faster than the recently reported AWSALM under a nonflow/very slow flow situations and other localization-based methods. Just as in AWSALM, fast-AWSALM does not require flow, as is required by current microbubblebased ultrasound super-resolution techniques. In conclusion, this study shows the promise of fast-AWSALM, a super-resolution ultrasound technique using nanodroplets, which can generate super-resolution images in milliseconds and does not require flow.

Molecular targeted nanodroplets, promising to extravasate beyond the vascular space, have great potential to improve tumor detection and characterization. High frame rate ultrasound, on the other hand, is an emerging tool for imaging at a frame rate 1-2 orders of magnitude higher than common existing ultrasound operating systems. In this study, we used high frame rate ultrasound combined with optics to study the acoustic response and size distribution of Folate Receptor (FR) -targeted versus Non-Targeted (NT)-nanodroplets in vitro with MDA-MB-231 breast cancer cells immediately after ultrasound activation. 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. It was found that the size of floating vaporized nanodroplets was on average more than 7 times larger than the size of vaporized nanodroplets attached to the cells. The results also showed that the acoustic signal of vaporized FR-nanodroplets was persistent after activation, with 70% of the acoustic signals still present 1 second after activation, compared to the vaporized NT-nanodroplets where only 40% of the acoustic signal remains. The optical microscopic images showed on average 6 times more vaporized FR-nanodroplets generated with a wider range of diameters (from 4 to 68 µm) that still attach to the cells compared to vaporized NTnanodroplets (from 1 to 7 µm) with non-specific binding after activation. It was also found that the mean size of attached vaporized FR-nanodroplets was on average about 3-fold larger than that of attached vaporized NT-nanodroplets. The study offers an improved understanding of the vaporization of the targeted nanodroplets in terms of their sizes and acoustic response in comparison with non-targeted ones, taking advantage of high-frame-rate contrast-enhanced ultrasound and optical microscopy. Such understanding would help design optimized methodology for imaging and therapeutic applications.