Cell size relations for sonolysis

Prediction of vascular injury by cavitation microbubbles in a focused ultrasound field

Many studies have shown that microbubble cavitation is one mechanism for vascular injury under ultrasonic excitation. Previous work has attributed vascular damage to vessel expansions and invaginations due to the expansion and contraction of microbubbles. However, the mechanisms of vascular damage are not fully understood. In this paper, we investigate, theoretically and experimentally, the vessel injury due to stress induced by ultrasound-induced cavitation (UIC). A bubble-fluid-vessel coupling model is constructed to investigate the interactions of the coupling system. The dynamics process of vessel damage due to UIC is theoretically simulated with a finite element method, and a focused ultrasound (FU) setup is carried out and used to assess the vessel damage. The results show that shear stress contributes to vessel injury by cell detachment while normal stress mainly causes distention injury. Similar changes in cell detachment in a vessel over time can be observed with the experimental setup. The severity of vascular injury is correlated to acoustic parameters, bubble-wall distance, and microbubble sizes, and the duration of insonation..

A 3D finite element model to study the cavitation induced stresses on blood-vessel wall during the ultrasound-only phase of photo-mediated ultrasound therapy

Photo-mediated ultrasound therapy (PUT) is a novel technique utilizing synchronized ultrasound and laser to generate enhanced cavitation inside blood vessels. The enhanced cavitation inside blood vessels induces bio-effects, which can result in the removal of micro-vessels and the reduction in local blood perfusion. These bio-effects have the potential to treat neovascularization diseases in the eye, such as age-related macular degeneration and diabetic retinopathy. Currently, PUT is in the preclinical stage, and various PUT studies on in vivo rabbit eye models have shown successful removal of micro-vessels. PUT is completely non-invasive and particle-free as opposed to current clinical treatments such as anti-vascular endothelial growth factor therapy and photodynamic therapy, and it precisely removes micro-vessels without damaging the surrounding tissue, unlike laser photocoagulation therapy. The stresses produced by oscillating bubbles during PUT are responsible for the induced bio-effects in blood vessels. In our previous work, stresses induced during the first phase of PUT due to combined ultrasound and laser irradiation were studied using a 2D model. In this work, stresses induced during the third or last phase of PUT due to ultrasound alone were studied using a 3D finite element method-based numerical model. The results showed that the circumferential and shear stress increased as the bubble moves from the center of the vessel toward the vessel wall with more than a 16 times increase in shear stress from 1.848 to 31.060 kPa as compared to only a 4 times increase in circumferential stress from 211 to 906 kPa for a 2 µm bubble placed inside a 10 µm vessel on the application of 1 MHz ultrasound frequency and 130 kPa amplitude. In addition, the stresses decreased as the bubble was placed in smaller sized vessels with a larger decrease in circumferential stress. The changes in shear stress were found to be more dependent on the bubble-vessel wall distance, and the changes in circumferential stress were more dependent on the bubble oscillation amplitude. Moreover, the bubble shape changed to an ellipsoidal with a higher oscillation amplitude in the vessel's axial direction as it was moved closer to the vessel wall, and the bubble oscillation amplitude decreased drastically as it was placed in vessels of a smaller size.

Pulsating Microbubble in a Micro-vessel and Mechanical Effect on Vessel Wall: A Simulation Study

Background:

Microbubbles are widely used in diagnostic ultrasound applications as contrast agents. Recently, many studies have shown that microbubbles have good potential for the use in therapeutic applications such as drug and gene delivery and opening of blood- brain barrier locally and transiently. When microbubbles are located inside an elastic microvessel and activated by ultrasound, they oscillate and induce mechanical stresses on the vessel wall. However, the mechanical stresses have beneficial therapeutic effects, they may induce vessel damage if they are too high. Microstreaming-induced shear stress is one of the most important wall stresses.

Objective:

The overall aim of this study is to simulate the interaction between confined bubble inside an elastic microvessel and ultrasound field and investigate the effective parameters on microstreaming-induced shear stress.

Material and Methods:

In this Simulation study, we conducted a 2D finite element simulation to study confined microbubble dynamics, also we investigated both acoustical and bubble material parameters on microbubble oscillation and wall stress.

Results:

Based on our results, for acoustic parameters in the range of therapeutic applications, the maximum shear stress was lower than 4 kPa. Shear stress was approximately independent from shell viscosity whereas it decreased by increasing the shell stiffness. Moreover, shear stress showed an increasing trend with acoustic pressure.

Conclusion:

Beside the acoustical parameters, bubble properties have important effects on bubble behavior so that the softer and larger bubbles are more appropriate for therapeutic application as they can decrease the required frequency and acoustic pressure while inducing the same biological effects.

Hemocoagulase Combined with Microbubble-Enhanced Ultrasound Cavitation for Augmented Ablation of Microvasculature in Rabbit VX2 Liver Tumors

We investigated a new method for combining microbubble-enhanced ultrasound cavitation (MEUC) with hemocoagulase (HC) atrox. Our goal was to induce embolic effects in the vasculature and combine these with an anti-angiogenic treatment strategy. Fourteen days after being implanted with a single slice of the liver VX2 tumor, rabbits were randomly divided into five groups: (i) a control group injected intra-venously with saline using a micropump; (ii) a group given only an injection of HC; (iii) a group treated only with ultrasound cavitation; (iv) a group treated with MEUC; (v) a group treated with MEUC + HC. Contrast-enhanced ultrasound was performed before treatment and 1 h and 7 d post-treatment to measure tumor size, enhancement and necrosis range. QontraXt software was used to determine the time-intensity curve of tumor blood perfusion and microvascular changes. At 1 h and 7 d after treatment with MEUC + HC, the parameters of the time-intensity curve, which included peak value, regional blood volume, regional blood flow and area under the curve value and which were measured using contrast-enhanced ultrasound, were significantly lower than those of the other treatment groups. The MEUC + HC treatment group exhibited significant growth inhibition relative to the ultrasound cavitation only, HC and MEUC treatment groups. No damage was observed in the surrounding normal tissues. These results support the feasibility of reducing the blood perfusion of rabbit VX2 liver tumors using a new method that combines MEUC and HC.

Microbubble oscillating in a microvessel filled with viscous fluid: A finite element modeling study

Understanding the dynamics of coated-microbubble oscillating in an elastic microvessel is important for effective and safe applications of ultrasound contrast agents (UCAs) in imaging and therapy. Numerical simulations are performed based on a two-dimensional (2D) asymmetric finite element model to investigate the influences of both acoustic driving parameters (e.g., pressure and frequency) and material properties (vessel size, microbubble shell visco-elastic parameters and fluid viscosity) on the dynamic interactions in the bubble-blood-vessel system. The results show that, the constrained effect of the blood vessel along the radial direction will induce the asymmetric bubble oscillation and vessel deformation, as well as shifting the bubble resonance frequency toward the higher frequency range. For a bubble (1.5-μm radius) activated by 1-MHz ultrasound pulses in a microvessel with a radius varying between 2 and 6.5 μm, up to 26.95 kPa shear stress could be generated on the vessel wall at a driving pressure of 0.2 MPa, which should be high enough to damage the vascular endothelial cells. The asymmetrical oscillation ratio of the bubble can be aggravated from 0.12% to 79.94% with the increasing acoustic driving pressure and blood viscosity, or the decreasing vessel size and microbubble shell visco-elastic properties. The maximum compression velocity on the bubble shell will be enhanced from 0.19 to 22.79 m/s by the increasing vessel size and acoustic pressure, or the decreasing microbubble shell visco-elasticity and blood viscosity. As the results, the peak values of microstreaming-induced shear stress on the vessel wall increases from 0.003 to 26.95 kPa and the deformation degree of vessel is raised from 1.01 to 1.49, due to the enhanced acoustic amplitude, or the decreasing vessel size, blood viscosity and microbubble shell visco-elasticity. Moreover, it also suggests that, among above impact parameters, microbubble resonance frequency and UCA shell elasticity might play more dominant roles in dynamic interactions of the bubble-blood-vessel system.

Mechanisms of prostate permeability triggered by microbubble-mediated acoustic cavitation

The objective of this research was to study the mechanisms of opening of the blood-prostate barrier and increased permeability of prostate tissue induced by microbubble cavitation. Thirty-five rabbits were randomly divided into four study groups: (1) control group and groups exposed to (2) microbubble alone, (3) ultrasound alone, or (4) combined intervention (ultrasound + microbubble group). Evans blue (EB) tracer was used to gauge the changes of permeability of prostate tissue. Furthermore, light and electron microscopy analyses were conducted, as well as the western blot analysis of expression of gap junction (Cx43) protein. We observed that EB concentration in prostate tissue was significantly greater in the ultrasound + microbubble group compared with either intervention alone (p < 0.05, both comparisons). Furthermore, light microscopy of tissue samples from animals exposed to ultrasound + microbubble showed epithelial cell disarrangement, loss of interstitial structure, and thickness of fibrous stroma. In line with these findings, electron microscopy analysis demonstrated widening of cell gaps and broken cell connections, as well as more dense lysosomes and secretary granules, and mitochondrial swelling. These changes were absent in the animals exposed to microbubble or ultrasound alone. Finally, only combined treatment with microbubble or ultrasound significantly elevated expression of Cx43 (p < 0.05 vs. control group). In conclusion, increases of permeability of prostate tissue by acoustic cavitation appear to involve opening of tight junctions, widening of intracellular spaces, changes in the structure of acinar cell membrane, enhancement of vesicular transport, and loosening of fibrous stroma. Increased expression of cell gap junction protein will help to restore normal connections between cells and the blood-prostate barrier after the treatment.

Vascular effects of microbubble-enhanced, pulsed, focused ultrasound on liver blood perfusion

The purpose of this study was to investigate the vascular effects of microbubble-enhanced pulsed high-pressure ultrasound on liver blood perfusion. In the presence of circulating lipid-shell microbubbles, a focused ultrasound transducer was used to transcutaneously treat eight livers of healthy rabbits for perfusion analysis and to treat three livers with the abdomen open for histologic analysis. Twenty-two livers treated with the ultrasound only (n = 11) or microbubbles only (n = 11) served as the controls. The focused ultrasound was operated at a frequency of 1.22 MHz with a peak negative pressure of 4.6 MPa. The liver blood perfusion was estimated by performing contrast-enhanced ultrasound and gray-scale quantification on the livers before and after treatment. A temporary, nonenhanced region occurred in all of the experimental livers. The regional contrast gray-scale values of the experimental group dropped significantly from 88.4 before treatment to 2.7 after treatment. The liver perfusion also demonstrated a gradual recovery over a 60-min period. The liver perfusion of the control groups remained the same after treatment. We found microvascular rupture, hemorrhage and swelling hepatocytes upon histologic examination of the experimental group. Regional liver blood perfusion can be temporarily blocked by microbubble-enhanced focused ultrasound with high-pressure amplitude. These vascular effects can be explained as acute microvascular injury of the liver and may have clinical implications.

Determination of potassium in red blood cells using unmeasured volumes of whole blood and combined sodium/potassium-selective membrane electrode measurements

Recent studies suggest that the measurement of intracellular potassium concentrations in red blood cells (RBC-K) can be a marker for assessing the risk, development, and treatment of hypertension. In this work, the combined use of miniature potassium- and sodium-selective membrane electrodes is evaluated as a simple means to determine RBC-K. The proposed method requires two separate sets of electrode measurements: (i) potassium and sodium concentrations in the plasma phase of an unmeasured volume of a whole blood sample, and (ii) determination of potassium and sodium concentrations in the same sample of blood after complete hemolysis by ultrasonic disruption of the RBC membranes. The dilution of sodium concentration after hemolysis can be used to determine hematocrit (Hct) (volume of red cells per unit volume of blood) of the blood. The concentration of potassium within the red blood cells (RBCs) is then calculated using the measured change in potassium levels before and after RBCs lysis and the hematocrit level determined from the sodium electrode measurements and/or a conventional centrifugation method. Good correlation for RBC-K between the proposed method and traditional flame photometry is observed for animal blood samples that possess the range of potassium levels found within human RBCs (80-120 mM). However, when potassium is much lower than that found in human RBCs (known to occur for certain animal species), the Hct measured by the sodium electrode method is falsely low, compared to traditional spun hematocrit values, because of an increased level of sodium within the RBCs, necessitating use of spun Hct levels to assess RBC-K accurately. It is envisioned that this new approach could be further miniaturized into a single-use disposable cartridge type electrode system that would enable rapid point-of-care screening of RBC-K levels in human subjects.

Dynamic adsorption properties of n-alkyl glucopyranosides determine their ability to inhibit cytolysis mediated by acoustic cavitation

Suspensions of human leukemia (HL-60) cells readily undergo cytolysis when exposed to ultrasound above the acoustic cavitation threshold. However, n-alkyl glucopyranosides (hexyl, heptyl, and octyl) completely inhibit ultrasound-induced (1057 kHz) cytolysis (Sostaric, et al. Free Radical Biol. Med. 2005, 39, 1539-1548). The efficacy of protection from ultrasound-induced cytolysis was determined by the n-alkyl chain length of the glucopyranosides, indicating that protection efficacy depended on adsorption of n-alkyl glucopyranosides to the gas/solution interface of cavitation bubbles and/or the lipid membrane of cells. The current study tests the hypothesis that "sonoprotection" (i.e., protection of cells from ultrasound-induced cytolysis) in vitro depends on the adsorption of glucopyranosides at the gas/solution interface of cavitation bubbles. To test this hypothesis, the effect of ultrasound frequency (from 42 kHz to 1 MHz) on the ability of a homologous series of n-alkyl glucopyranosides to protect cells from ultrasound-induced cytolysis was investigated. It is expected that ultrasound frequency will affect sonoprotection ability since the nature of the cavitation bubble field will change. This will affect the relative importance of the possible mechanisms for ultrasound-induced cytolysis. Additionally, ultrasound frequency will affect the lifetime and rate of change of the surface area of cavitation bubbles, hence the dynamically controlled adsorption of glucopyranosides to their surface. The data support the hypothesis that sonoprotection efficiency depends on the ability of glucopyranosides to adsorb at the gas/solution interface of cavitation bubbles.

Biophysical response to pulsed laser microbeam-induced cell lysis and molecular delivery

Cell lysis and molecular delivery in confluent monolayers of PtK(2) cells are achieved by the delivery of 6 ns, lambda = 532 nm laser pulses via a 40x, 0.8 NA microscope objective. With increasing distance from the point of laser focus we find regions of (a) immediate cell lysis; (b) necrotic cells that detach during the fluorescence assays; (c) permeabilized cells sufficient to facilitate the uptake of small (3 kDa) FITC-conjugated Dextran molecules in viable cells; and (d) unaffected, viable cells. The spatial extent of cell lysis, cell detachment, and molecular delivery increased with laser pulse energy. Hydrodynamic analysis from time-resolved imaging studies reveal that the maximum wall shear stress associated with the pulsed laser microbeam-induced cavitation bubble expansion governs the location and spatial extent of each of these regions independent of laser pulse energy. Specifically, cells exposed to maximum wall shear stresses tau(w, max) > 190 +/- 20 kPa are immediately lysed while cells exposed to tau(w, max) > 18 +/- 2 kPa are necrotic and subsequently detach. Cells exposed to tau(w, max) in the range 8-18 kPa are viable and successfully optoporated with 3 kDa Dextran molecules. Cells exposed to tau(w, max) < 8 +/- 1 kPa remain viable without molecular delivery. These findings provide the first direct correlation between pulsed laser microbeam-induced shear stresses and subsequent cellular outcome.

Effects of shear stress on endothelial cells: possible relevance for ultrasound applications

This review forms part of a series of papers resulting from a workshop on safety of ultrasound applications. The physical effects of ultrasound include generation of steady streaming in large fluid volumes, and micro-streaming around contrast bubbles. Such streaming induces shear stress acting on the vascular endothelium. This review provides a discussion on the levels of endothelial shear stress associated with diagnostic ultrasound applications, and on the biological effects of shear stress acting on the endothelial cells. Depending on vessel size and ultrasound characteristics, shear stresses associated with streaming and micro-streaming may exceed the physiological levels associated with the flow of blood by many orders of magnitude. The resulting biological effects could range anywhere from activation of normal shear stress sensors such as ion channels, damage of the endothelial surface layer, reversible perforation of the membrane, to cell detachment and lysis. The possible presence of such biological effects does not necessarily mean that the effects are harmful for the individual. However, considering the ever-increasing use of ultrasound, a further investigation into these shear stress-related effects, using both experiments and modelling, is desired. Apart from safety concerns, such effects may provide a base for strategies aimed at targeted delivery of drugs.