Buckling of lipidic ultrasound contrast agents under quasi-static load

Controlling the stability of monodisperse phospholipid-coated microbubbles by tuning their buckling pressure

HYPOTHESIS

Monodisperse phospholipid-coated microbubbles, with a size and resonance frequency tuned to the ultrasound driving frequency, have strong potential to enhance sensitivity, efficiency, and control in emerging diagnostic and therapeutic applications involving bubbles and ultrasound. A key requirement is that they retain their gas volume and shell material during physiologic pressure changes and withstand the overpressure during intravenous injection. The shell typically comprises a mixture of a phospholipid (e.g., DSPC) mixed with a PEGylated phospholipid (e.g., DPPE-PEG5000). We hypothesize that (i) lipid-coated microbubbles destabilize when shell buckling occurs under pressurization, (ii) the overpressure at which buckling occurs (buckling pressure) is linked to the molar fraction of PEGylated lipid in the shell, and (iii) PEGylated lipid can be selectively expelled from the shell by fluidizing it at elevated temperatures.

EXPERIMENTS

The buckling pressure was measured using ultrasound attenuation spectroscopy while the ambient pressure was varied. When the ambient pressure increased, the microbubble resonance frequency dropped sharply due to shell buckling and the associated loss of elasticity. The buckling pressure Pb was obtained for monodisperse microbubbles formed by microfluidic flow-focusing, with DPPE-PEG5000 mixed with DSPC at molar fractions from 1.5% to 10%. Additionally, Pb was quantified for microbubbles containing 10 mol% PEG after heating at temperatures ranging from 40∘C to 70∘C. The molar PEG content of the microbubbles was analyzed using high-performance liquid chromatography.

FINDINGS

Quasi-static compression of a microbubble above its buckling pressure leads to its destabilization. Lowering the PEG molar fraction from 10 to 1.5% increased the buckling pressure from 3 kPa to 27 kPa. Similarly, heating the 10 mol% bubble suspension at 60∘C for one hour raised the buckling pressure by 20 kPa, due to the selective loss of PEGylated lipid from the shell, without affecting the monodispersity of the bubbles. The higher buckling pressure significantly improved microbubble stability, allowing them to withstand pressurization cycles of up to 45 kPa, nearly three times the systolic blood pressure in vivo.

Whole Organ Volumetric Sensing Ultrasound Localization Microscopy for Characterization of Kidney Structure

Glomeruli are the filtration units of the kidney and their function relies heavily on their microcirculation. Despite its obvious diagnostic importance, an accurate estimation of blood flow in the capillary bundle within glomeruli defies the resolution of conventional imaging modalities. Ultrasound Localization Microscopy (ULM) has demonstrated its ability to image in-vivo deep organs in the body. Recently, the concept of sensing ULM or sULM was introduced to classify individual microbubble behavior based on the expected physiological conditions at the micrometric scale. In the kidney of both rats and humans, it revealed glomerular structures in 2D but was severely limited by planar projection. In this work, we aim to extend sULM in 3D to image the whole organ and in order to perform an accurate characterization of the entire kidney structure. The extension of sULM into the 3D domain allows better localization and more robust tracking. The 3D metrics of velocity and pathway angular shift made glomerular mask possible. This approach facilitated the quantification of glomerular physiological parameter such as an interior traveled distance of approximately 7.5 ±0.6 microns within the glomerulus. This study introduces a technique that characterize the kidney physiology which can serve as a method to facilite pathology assessment. Furthermore, its potential for clinical relevance could serve as a bridge between research and practical application, leading to innovative diagnostics and improved patient care.

RF-ULM: Ultrasound Localization Microscopy Learned From Radio-Frequency Wavefronts

In Ultrasound Localization Microscopy (ULM), achieving high-resolution images relies on the precise localization of contrast agent particles across a series of beamformed frames. However, our study uncovers an enormous potential: The process of delay-and-sum beamforming leads to an irreversible reduction of Radio-Frequency (RF) channel data, while its implications for localization remain largely unexplored. The rich contextual information embedded within RF wavefronts, including their hyperbolic shape and phase, offers great promise for guiding Deep Neural Networks (DNNs) in challenging localization scenarios. To fully exploit this data, we propose to directly localize scatterers in RF channel data. Our approach involves a custom super-resolution DNN using learned feature channel shuffling, non-maximum suppression, and a semi-global convolutional block for reliable and accurate wavefront localization. Additionally, we introduce a geometric point transformation that facilitates seamless mapping to the B-mode coordinate space. To understand the impact of beamforming on ULM, we validate the effectiveness of our method by conducting an extensive comparison with State-Of-The-Art (SOTA) techniques. We present the inaugural in vivo results from a wavefront-localizing DNN, highlighting its real-world practicality. Our findings show that RF-ULM bridges the domain shift between synthetic and real datasets, offering a considerable advantage in terms of precision and complexity. To enable the broader research community to benefit from our findings, our code and the associated SOTA methods are made available at https://github.com/hahnec/rf-ulm.

Influence of the liquid ionic strength on the resonance frequency and shell parameters of lipid-coated microbubbles

The correct measurement of the resonance frequency and shell properties of coated microbubbles (MBs) is essential in understanding and optimizing their response to ultrasound (US) exposure parameters. In diagnostic and therapeutic ultrasound, MBs are typically surrounded by blood; however, the influence of the medium charges on the MB resonance frequency has not been systematically studied using controlled measurements. This study aims to measure the medium charge interactions on MB behavior by measuring the frequency-dependent attenuation of the same size MBs in mediums with different charge densities. In-house lipid-coated MBs with C3F8 gas core were formulated. The MBs were isolated to a mean size of 2.35 μm using differential centrifugation. MBs were diluted to ≈8×105 MBs/mL in distilled water (DW), Phosphate-Buffered Saline solution (PBS1x) and PBS10x. The frequency-dependent attenuation of the MBs solutions was measured using an aligned pair of PVDF transducers with a center frequency of 10MHz and 100% bandwidth in the linear oscillation regime (7 kPa pressure amplitude). The MB shell properties were estimated by fitting the linear equation to experiments. Using a pendant drop tension meter, the surface tension at the equilibrium of ≈6 mm diameter size drops of the same MB shell was measured inside DW, PBS1x and PBS10x. The surface tension at the C3F8/solution interface was estimated by fitting the Young-Laplace equation from the recorded images. The frequency of the peak attenuation at different salinity levels was 13, 7.5 and 6.25 MHz in DW, PBS1x and PBS-10x, respectively. The attenuation peak increased by ≈140% with increasing ion density. MBs' estimated shell elasticity decreased by 64% between DW and PBS-1x and 36% between PBS-1x and PBS-10x. The drop surface tension reduced by 10.5% between DW and PBS-1x and by 5% between PBS-1x and PBS-10x, respectively. Reduction in the shell stiffness is consistent with the drop surface tension measurements. The shell viscosity was reduced by ≈40% between DW and PBS-1x and 42% between PBS-1x and PBS-10x. The reduction in the fitted stiffness and viscosity is possibly due to the formation of a densely charged layer around the shell, further reducing the effective surface tension on the MBs. The changes in the resonance frequency and estimated shell parameters were significant and may potentially help to better understand and explain bubble behavior in applications.

Effect of temperature on the acoustic response and stability of size-isolated protein-shelled ultrasound contrast agents and SonoVue

Limited work has been reported on the acoustic and physical characterization of protein-shelled UCAs. This study characterized bovine serum albumin (BSA)-shelled microbubbles filled with perfluorobutane gas, along with SonoVue, a clinically approved contrast agent. Broadband attenuation spectroscopy was performed at room (23 ± 0.5 °C) and physiological (37 ± 0.5 °C) temperatures over the period of 20 min for these agents. Three size distributions of BSA-shelled microbubbles, with mean sizes of 1.86 μm (BSA1), 3.54 μm (BSA2), and 4.24 μm (BSA3) used. Viscous and elastic coefficients for the microbubble shell were assessed by fitting de Jong model to the measured attenuation spectra. Stable cavitation thresholds (SCT) and inertial cavitation thresholds (ICT) were assessed at room and physiological temperatures. At 37 °C, a shift in resonance frequency was observed, and the attenuation coefficient was increased relative to the measurement at room temperature. At physiological temperature, SCT and ICT were lower than the room temperature measurement. The ICT was observed to be higher than SCT at both temperatures. These results enhance our understanding of temperature-dependent properties of protein-shelled UCAs. These findings study may guide the rational design of protein-shelled microbubbles and help choose suitable acoustic parameters for applications in imaging and therapy.