One major challenge in current microbubble (MB) and tissue plasminogen activator (tPA)-mediated sonothrombolysis techniques is effectively treating retracted blood clots, owing to the high density and low porosity of retracted clots. Nanodroplets (NDs) have the potential to enhance retracted clot lysis owing to their small size and ability to penetrate into retracted clots to enhance drug delivery. For the first time, we demonstrate that a sub-megahertz, forward-viewing intravascular (FVI) transducer can be used for ND-mediated sonothrombolysis, in vitro. In this study, we determined the minimum peak negative pressure to induce cavitation with low-boiling point phase change nanodroplets and clot lysis. We then compared nanodroplet mediated sonothrombolysis to MB and tPA mediate techniques. The clot lysis as a percent mass decrease in retracted clots was 9 ± 8%, 9 ± 5%, 16 ± 5%, 14 ± 9%, 17 ± 9%, 30 ± 8%, and 40 ± 9% for the control group, tPA alone, tPA + US, MB + US, MB + tPA + US, ND + US, and ND + tPA + US groups, respectively. In retracted blood clots, combined ND- and tPA-mediated sonothrombolysis was able to significantly enhance retracted clot lysis compared with traditional MB and tPA-mediated sonothrombolysis techniques. Combined nanodroplet with tPA-mediated sonothrombolysis may provide a feasible strategy for safely treating retracted clots.

One of the great advancements in the applications of piezoelectric materials is the application for therapeutic medical ultrasound for sonothrombolysis. Sonothrombolysis is a promising ultrasound based technique to treat blood clots compared to conventional thrombolytic treatments or mechanical thrombectomy. Recent clinical trials using transcranial Doppler ultrasound, microbubble mediated sonothrombolysis, and catheter directed sonothrombolysis have shown promise. However, these conventional sonothrombolysis techniques still pose clinical safety limitations, preventing their application for standard of care. Recent advances in sonothrombolysis techniques including targeted and drug loaded microbubbles, phase change nanodroplets, high intensity focused ultrasound, histotripsy, and improved intravascular transducers, address some of the limitations of conventional sonothrombolysis treatments. Here, we review the strengths and limitations of these latest pre-clincial advancements for sonothrombolysis and their potential to improve clinical blood clot treatments.

Thrombo-occlusive disease is one of the leading causes of death worldwide. There has been active research on safe and effective thrombolysis in preclinical and clinical studies. Recently, the dual-frequency transcutaneous sonothrombolysis with contrast agents [microbubbles (MBs)] has been reported to be more efficient in trigging the acoustic cavitation, which leads to a higher lysis rate. Therefore, there is increasing interest in applying dual-frequency technique for more significant efficacy improvement in intravascular sonothrombolysis since a miniaturized intravascular ultrasound transducer typically has a limited power output to fully harness cavitation effects. In this work, we demonstrated this efficacy enhancement by developing a new broadband intravascular transducer and testing dual-frequency sonothromblysis in vitro . A broadband intravascular transducer with a center frequency of 750 kHz and a footprint size of 1.4 mm was designed, fabricated, and characterized. The measured −6-dB fractional bandwidth is 68.1%, and the peak negative pressure is 1.5 MPa under the driving voltage of 80 V _pp . By keeping one frequency component at 750 kHz, the second frequency component was selected from 450 to 650 kHz with an interval of 50 kHz. The in vitro sonothrombolysis tests were conducted with a flow model and the results indicated that the MB-mediated, dual-frequency (750+500 kHz) sonothrombolysis yields an 85% higher lysis rate compared with the single-frequency treatment, and the lysis rate of dual-frequency sonothrombolysis increases with the difference between the two frequency components. These findings suggest a dual-frequency excitation technique for more efficient intravascular sonothrombolysis than conventional single-frequency excitation

While skeletal muscle is often assumed to be structurally transversely isotropic (TI), its mechanical anisotropy is not well characterized in vivo. VisR ultrasound, a technique that provides information about the viscoelastic mechanical response of tissue, can be used for such in vivo characterization. Specifically, VisR-derived Relative Elasticity (RE) provides directionally dependent information about the shear modulus of materials. We hypothesize that VisR RE can be used in healthy older adults and in stroke patients to 1) assess the degree of anisotropy (DoA) of the biceps brachii (BB) and 2) characterize the effect of passive loading on the BB. VisR imaging was performed in the BB long head of 2 healthy controls and 1 stroke patient. The subjects' arms were imaged in full extension and approximately 90° flexion at rest. Data were acquired in the transverse and longitudinal imaging planes at the medial location of the BB. The ratio of RE obtained in the transverse to the longitudinal muscle views was found to reflect the DoA in shear modulus, where DoA = 1 indicates perfect isotropy. In control subjects, RE values for flexion and extension in the transverse imaging plane (previously shown to reflect the longitudinal shear modulus, μL) were higher than the RE values in the longitudinal imaging plane (which, primarily reflects the transverse shear modulus, μT). Additionally, both the longitudinal and transverse shear moduli increased in extension vs. flexion. Both the magnitude and direction of anisotropy appeared to change in the stroke patient compared to the healthy controls. This work indicates VisR can be used to assess DoA in skeletal muscle of older adults and examine effects of passive loading on mechanical properties, in vivo, which may be clinically relevant to studying impairment post stroke.

In transversely isotropic (TI) materials, mechanical properties differ along versus across the axis of symmetry (AoS). The degree of these directional differences has been interrogated by ultrasound-based measures of shear wave velocity (SWV) and ARFI peak displacements (PD) acquired along versus across the AoS. When performed using a conventional linear array transducer, such measurements require a 90° transducer rotation, which can be cumbersome and prone to misalignment errors. Rather than 90°, we hypothesize that smaller rotation angles can be used to assess mechanical anisotropy. This hypothesis was tested in silico using finite-element method (FEM) models and Field II. ARFI and Shear Wave Elasticity Imaging (SWEI) were simulated in five TI elastic materials with longitudinal versus transverse shear elastic modulus ratios (SRs) of 1.22 to 5.44. The simulated rotation angles were 15°, 30°, and 90°. The ratio of PD was measured with the long-axis of the ARF PSF aligned along the AoS versus long-axis rotated by 90°, 30°, or 15° from the AoS. The ratio of SWV was measured along the AoS versus 90°, 30°, or 15° rotation from the AoS. For a 90° rotation angle, both SWV and PD ratios were statistically different (2-sample t-test, p<;0.05) in all 10 pairings of the simulated materials. For the 30° rotation angle, PD ratios were statistically different for all 10 material pairings, but SWV ratios were not statistically different for two material pairings. For the 15° rotation angle, PD ratio statistically differentiated all but one material pairing, while SWV ratio did not differentiate six material pairings. These results suggest that rotation angles smaller than 90° support assessment of elastic anisotropy in TI materials, and that ARFI PD ratios perform better than SWV ratios for distinguishing TI materials with different SRs given smaller angles of rotation.