Do bubble characteristics affect recanalization in stroke patients treated with microbubble-enhanced sonothrombolysis?

Effect of microbubbles on transcranial doppler ultrasound-assisted intracranial recombinant tissue-type plasminogen activator thrombolysis

OBJECTIVES

The aim of this study was to evaluate the effect of microbubbles on the efficacy of transcranial doppler (TCD) ultrasound-assisted thrombolytic therapy of recombinant tissue-type plasminogen activator (rt-PA).

METHODS

Male New Zealand white rabbits (n = 36) were randomly divided into an rt-PA group (n = 18) and an rt-PA plus microbubble group (n = 18). After the cerebral infarction model was constructed with autologous blood clots, rt-PA and rt-PA plus microbubble intervention were performed, respectively. The hemodynamic changes and infarct size of the two groups were recorded. In addition, the ELISA method was used to detect the level of nitric oxide (NO), superoxide dismutase (SOD), and malondialdehyde (MDA) in the brain tissue of the two-group graph model and high-sensitivity C-reactive protein (hs-CRP) in the serum.

RESULTS

In the rt-PA group, the recanalization rate was 38.9% and the average infarct size was 11.8%. In the rt-PA plus microbubble group, the recanalization rate was 66.7% and the average infarct size was 8.2%. In addition, the average values for NO, SOD, MDA, and hs-CRP were 16.48 ± 5.39 μmol/L, 730.2 ± 9.86 U/mg, 0.92 ± 0.43 nmol/mg, and 8.56 ± 1.64 mg/L in the rt-PA group, respectively, and the average values were 9.18 ± 3.37 μmol/L, 426.2 ± 6.39 U/mg, 0.73 ± 0.44 nmol/mg, and 5.23 ± 0.94 mg/L in the rt-PA plus microbubble group, respectively.

CONCLUSIONS

The addition of microbubbles enhanced the effects of TCD-assisted rrt-PA thrombolysis.

Focused Ultrasound, an Emerging Tool for Atherosclerosis Treatment: A Comprehensive Review

Focused ultrasound (FUS) has emerged as a promising noninvasive therapeutic modality for treating atherosclerotic arterial disease. High-intensity focused ultrasound (HIFU), a noninvasive and precise modality that generates high temperatures at specific target sites within tissues, has shown promising results in reducing plaque burden and improving vascular function. While low-intensity focused ultrasound (LIFU) operates at lower energy levels, promoting mild hyperthermia and stimulating tissue repair processes. This review article provides an overview of the current state of HIFU and LIFU in treating atherosclerosis. It focuses primarily on the therapeutic potential of HIFU due to its higher penetration and ability to achieve atheroma disruption. The review summarizes findings from animal models and human trials, covering the effects of FUS on arterial plaque and arterial wall thrombolysis in carotid, coronary and peripheral arteries. This review also highlights the potential benefits of focused ultrasound, including its noninvasiveness, precise targeting, and real-time monitoring capabilities, making it an attractive approach for the treatment of atherosclerosis and emphasizes the need for further investigations to optimize FUS parameters and advance its clinical application in managing atherosclerotic arterial disease.

A narrative review on the application of high-intensity focused ultrasound for the treatment of occlusive and thrombotic arterial disease

Objectives

High-intensity focused ultrasound (HIFU) is a noninvasive therapeutic modality with a variety of applications. It is approved for the treatment of essential tremors, ablation of prostate, hepatic, breast, and uterine tumors. Although not approved for use in the treatment of atherosclerotic arterial disease, there is a growing body of evidence investigating applications of HIFU. Currently, percutaneous endovascular techniques are predominant for the treatment of arterial pathology; however, there are no endovascular techniques of HIFU available. This study aims to review the state of current evidence for the application of HIFU in atherosclerotic arterial disease.

Methods

All English-language articles evaluating the effect of HIFU on arterial occlusive and thrombotic disease until 2021 were reviewed. Both preclinical and human clinical studies were included. Study parameters such as animal or clinical model and outcomes were reviewed. In addition, details pertaining to settings on the HIFU device used were also reviewed.

Results

In preclinical models, atherosclerotic plaque progression was inhibited by HIFU, through decreases in oxidized low-density lipoprotein cholesterol and increases in macrophage apoptosis. Additionally, HIFU promotes angiogenesis in hindlimb ischemic models by the upregulation of angiogenic and antiapoptotic factors, with increased angiogenesis at higher line densities of HIFU. HIFU also promotes thrombolysis and conversely induces platelet activation at low frequencies and higher intensities. Various clinical studies have attempted to translate some of these properties and demonstrated positive clinical outcomes for arterial recanalization after thrombotic stroke, decreased atherosclerotic plaque burden in carotid arteries, increase in tissue perfusion and a decrease in diameter stenosis in patients with atherosclerotic arterial disease.

Conclusions

In current preclinical and clinical data, the safety and efficacy of HIFU shows great promise in the treatment of atherosclerotic arterial disease. Future focused studies are warranted to guide the refinement of HIFU settings for more widespread adoption of this technology.

The Updated Role of Transcranial Ultrasound Neuromodulation in Ischemic Stroke: From Clinical and Basic Research

Ischemic stroke is a common cause of death and disability worldwide, which leads to serious neurological and physical dysfunction and results in heavy economic and social burdens. For now, timely and effective dissolution of thrombus, and ultimately improvement in the recovery of neurological functions, is the treatment strategy focus. Recently, many studies have reported that transcranial ultrasound stimulation (TUS), as a non-invasive method, can dissolve thrombus, improve cerebral blood circulation, and exert a neuroprotective effect post-stroke. TUS can promote functional recovery and improve rehabilitation efficacy among patients with ischemic stroke. This mini-review summarizes the potential mechanism and limitation of TUS in stroke aims to provide a new strategy for the future treatment of patients with ischemic stroke.

Nanomedicines, an emerging therapeutic regimen for treatment of ischemic cerebral stroke: A review

Owing to its intricate pathophysiology, cerebral stroke is a serious medical condition caused by interruption or obstruction of blood supply (blockage of vasculature) to the brain tissues which results in diminished supply of essential nutrients and oxygen (hypoxia) and ultimate necrosis of neuronal tissues. A prompt risks assessment and immediate rational therapeutic plan with proficient neuroprotection play critically important role in the effective management of this neuronal emergency. Various conventional medications are being used for treatment of acute ischemic cerebral stroke but fibrinolytic agents, alone or in combination with other agents are considered the mainstay. These clot-busting agents effectively restore blood supply (reperfusion) to ischemic regions of the brain; however, their clinical significance is hampered due to various factors such as short plasma half-life, limited distribution to brain tissues due to the presence of highly efficient physiological barrier, blood brain barrier (BBB), and lacking of target-specific delivery to the ischemic brain regions. To alleviate these issues, various types of nanomedicines such as polymeric nanoparticles (NPs), liposomes, nanoemulsion, micelles and dendrimers have been designed and evaluated. The implication of these newer therapies (nanomedicines) have revolutionized the therapeutic outcomes by improving the plasma half-life, permeation across BBB, efficient distribution to ischemic cerebral tissues and neuroprotection. Furthermore, the adaptation of some diverse techniques including PEGylation, tethering of targeting ligands on the surfaces of nanomedicines, and pH responsive features have also been pondered. The implication of these emerging adaptations have shown remarkable potential in maximizing the targeting efficiency of drugs to ischemic brain tissues, simultaneous delivery of drugs and imaging agents (for early prognosis as well as monitoring of therapy), and therapeutic outcomes such as long-term neuroprotection.

Therapeutic Use of Microbubbles and Ultrasound in Acute Peripheral Arterial Thrombosis: A Systematic Review

Catheter-directed thrombolysis (CDT) for acute peripheral arterial occlusion is time consuming and carries a risk of major hemorrhage. Contrast-enhanced sonothrombolysis (CEST) might enhance outcomes compared with standard CDT. In the study described here, we systematically reviewed all in vivo studies on contrast-enhanced sonothrombolysis in a setting of arterial thrombosis. A systematic search of the PubMed, Embase, Cochrane Library and Web of Science databases was conducted. Two reviewers independently performed the study selection, quality assessment and data extraction. Primary outcomes were recanalization rate and thrombus weight. Secondary outcome was any possible adverse event. The 35 studies included in this review were conducted in four different (pre)clinical settings: ischemic stroke, myocardial infarction, (peripheral) arterial thrombosis and arteriovenous graft occlusion. Because of the high heterogeneity among the studies, it was not possible to conduct a meta-analysis. In almost all studies, recanalization rates were higher in the group that underwent a form of CEST. One study was terminated early because of a higher incidence of intracranial hemorrhage. Studies on CEST suggest that adding microbubbles and ultrasound to standard intra-arterial CDT is safe and might improve outcomes in acute peripheral arterial thrombosis. Further research is needed before CEST can be implemented in daily practice.

Emerging paradigms in treating cerebral infarction with nanotheranostics: opportunities and clinical challenges

Interest is increasing in the use of nanotheranostics as diagnosis, imaging and therapeutic tools for stroke management, but movement to the clinic remains challenging.

In-vitro sonothrombolysis using thick-shelled polymer microbubbles - a comparison with thin-shelled microbubbles

BACKGROUND

Vascular thrombosis can be treated pharmacologically, however, serious shortcomings such as bleeding may occur. Several studies suggest that sonothrombolysis can induce lysis of the clots using ultrasound. Moreover, intravenously injected thin-shelled microbubbles (MBs) combined with ultrasound can further improve clot lysis. Thick-shelled MBs have been used for drug delivery, targeting and multimodal imaging. However, their capability to enhance sonothrombolysis is unknown. In this study, using an in-vitro set-up, the enhancement of clot lysis using ultrasound and thick-shelled MBs was investigated. Thin-shelled MBs was used for comparison.

METHOD

The main components in the in-vitro set-up was a vessel mimicking phantom, a pressure mearing system and programmable ultrasound machine. Blood clots were injected and entrapped on a pore mesh in the vessel phantom. Four different protocols for ultrasound transmission and MB exposure (7 blood clots/protocol) were considered together with a control test were no MBs and ultrasound were used. For each protocol, ultrasound exposure of 20 min was used. The upstream pressure of the partially occluded mesh was continuously measured to assess clot burden. At the end of each protocol blood clots were removed from the phantom and the clot mass loss was computed.

RESULTS

For the thick-shelled MBs no difference in clot mass loss compared with the control tests was found. A 10% increase in the clot mass loss compared with the control tests was found when using thin-shelled MBs and low pressure/long pulses ultrasound exposure. Similarly, in terms of upstream pressure over exposure time, no differences were found when using the thick-shelled MBs, whereas thin-shelled MBs showed a 15% decrease achieved within the first 4 min of ultrasound exposure.

CONCLUSION

No increase in clot lysis was achieved using thick-shelled MBs as demonstrated by no significant change in clot mass or upstream pressure. Although thick-shelled MBs are promising for targeting and drug delivery, they do not enhance clot lysis when considering the ultrasound sequences used in this study. On the other hand, ultrasound in combination with thin-shelled MBs can facilitate thrombolysis when applying long ultrasound pulses with low pressure.

Engineered microparticles and nanoparticles for fibrinolysis

Fibrinolytic agents including plasmin and plasminogen activators improve outcomes in acute ischemic stroke and thrombosis by recanalizing occluded vessels. In the decades since their introduction into clinical practice, several limitations of have been identified in terms of both efficacy and bleeding risk associated with these agents. Engineered nanoparticles and microparticles address some of these limitations by improving circulation time, reducing inhibition and degradation in circulation, accelerating recanalization, improving targeting to thrombotic occlusions, and reducing off-target effects; however, many particle-based approaches have only been used in preclinical studies to date. This review covers four advances in coupling fibrinolytic agents with engineered particles: (a) modifications of plasminogen activators with macromolecules, (b) encapsulation of plasminogen activators and plasmin in polymer and liposomal particles, (c) triggered release of encapsulated fibrinolytic agents and mechanical disruption of clots with ultrasound, and (d) enhancing targeting with magnetic particles and magnetic fields. Technical challenges for the translation of these approaches to the clinic are discussed.

Recent strategies on targeted delivery of thrombolytics

Thrombus formed in blood vessel is a progressive process, which would lead to life-threatening thrombotic diseases such as ischemic stroke. Unlike other diseases, the recognition of thrombus is usually in the late stage where blood vessels are largely blocked. So acute thrombotic diseases have a narrow therapeutic window, and remain leading causes of morbidity and mortality, whereas current thrombolysis therapy has limited therapeutic effects and bleeding complications. Thrombolytic agents in unwanted sites would cause hemorrhage due to the activation of plasminogen. Moreover, untargeted thrombolysis therapy require large amounts of thrombolytic agents, which in return would enhance hemorrhage risk. To improve the efficiency while minimizing the adverse effects of traditional thrombolysis therapy, novel drug delivery systems have been investigated. Various targeting strategies including ultrasound and magnetic field directed targeting, and specific binding, have been designed to deliver thrombolytic drugs to the thrombotic sites. These strategies demonstrate promising results in reducing bleeding risk as well as allowing less dosage of thrombolytic drugs with lowered clot lysis time. In this review, we discuss recent progress on targeted delivery of thrombolytics, and summarize treatment advantages and shortcomings, potentially helping to further promote the development of targeted thrombolysis.

Efficacy and safety of sonothombolysis versus non-sonothombolysis in patients with acute ischemic stroke: A meta-analysis of randomized controlled trials

Recent studies have shown that inconsistent results of safety and efficacy between sonothombolysis vs. non-sonothombolysis in acute ischemic stroke (AIS). We implemented a meta-analysis to explore the value of sonothrombolysis in AIS treatment. The MEDLINE, EMBASE, and Cochrane Library databases were searched for randomized controlled trials (RCTs) which had evaluated sonothrombolysis or ultrasound thrombolysis in AIS. One hundred five studies were retrieved and analyzed, among them, 7 RCTs were included in the current meta-analysis. In comparison with the non-sonothombolysis, sonothrombolysis significantly improved complete recanalization (RR 2.16, 95% CI 1.51 to 3.08, P < 0.001), complete or partial recanalization (RR 1.90, 95% CI 1.26 to 2.88, P = 0.002), there is also a tendency to improvement of ≥ 4 points in NIHSS score (RR 1.43, 95% CI 0.99 to 2.07, P = 0.057). However, sonothrombolysis and non-sonothrombolysis had insignificant differences in neurological recovery and adverse events. In subgroup analysis, we found that “With t-PA”, “NIHSS > 15”, “Treatment time ≤ 150min”, and “Age ≤ 65 years” are potential favorable factors for efficacy outcomes of sonothombolysis. Sonothrombolysis can significantly increase the rate of recanalization in patients with AIS compared with non-sonothrombolysis, but there is no significant effect on improving neurological functional recovery and avoiding complications.

Fibrinolysis: strategies to enhance the treatment of acute ischemic stroke

Stroke is a major cause of disability worldwide, and is the second leading cause of death after ischemic heart disease. Until recently, tissue-type plasminogen activator (t-PA) was the only treatment for acute ischemic stroke. If administered within 4.5 h of symptom onset, t-PA improves the outcome in stroke patients. Mechanical thrombectomy is now the preferred treatment for patients with acute ischemic stroke resulting from a large-artery occlusion in the anterior circulation. However, the widespread use of mechanical thrombectomy is limited by two factors. First, only ⁓ 10% of patients with acute ischemic stroke have a proximal large-artery occlusion in the anterior circulation and present early enough to undergo mechanical thrombectomy within 6 h; an additional 9-10% of patients presenting within the 6-24-h time window may also qualify for the procedure. Second, not all stroke centers have the resources or expertise to perform mechanical thrombectomy. Nonetheless, patients who present to hospitals where thrombectomy is not an option can receive intravenous t-PA, and those with qualifying anterior circulation strokes can then be transferred to tertiary stroke centers where thrombectomy is available. Therefore, despite the advances afforded by mechanical thrombectomy, there remains a need for treatments that improve the efficacy and safety of thrombolytic therapy. In this review, we discuss: (i) current treatment options for acute ischemic stroke; (ii) the mechanism of action of fibrinolytic agents; and (iii) potential strategies to manipulate the fibrinolytic system to promote endogenous fibrinolysis or to enhance the efficacy of fibrinolytic therapy.

NOR-SASS (Norwegian Sonothrombolysis in Acute Stroke Study): Randomized Controlled Contrast-Enhanced Sonothrombolysis in an Unselected Acute Ischemic Stroke Population

Background and Purpose—

The NOR-SASS (Norwegian Sonothrombolysis in Acute Stroke Study) aimed to assess effect and safety of contrast-enhanced ultrasound treatment in an unselected acute ischemic stroke population.

Methods—

Patients treated with intravenous thrombolysis within 4.5 hours after symptom onset were randomized 1:1 to either contrast-enhanced sonothrombolysis (CEST) or sham CEST. A visible arterial occlusion on baseline computed tomography angiography was not a prerequisite for inclusion. Pulse-wave 2 MHz ultrasound was given for 1 hour and contrast (SonoVue) as an infusion for ≈30 minutes. Magnetic resonance imaging and angiography were performed after 24 to 36 hours. Primary study end points were neurological improvement at 24 hours defined as National Institutes of Health Stroke Scale score 0 or reduction of ≥4 National Institutes of Health Stroke Scale points compared with baseline National Institutes of Health Stroke Scale and favorable functional outcome at 90 days defined as modified Rankin scale score 0 to 1.

Results—

A total of 183 patients were randomly assigned to either CEST (93 patient) or sham CEST (90 patients). The rates of symptomatic intracerebral hemorrhage, asymptomatic intracerebral hemorrhage, or mortality were not increased in the CEST group. Neurological improvement at 24 hours and functional outcome at 90 days was similar in the 2 groups both in the intention-to-treat analysis and in the per-protocol analysis.

Conclusions—

CEST is safe among unselected ischemic stroke patients with or without a visible occlusion on computed tomography angiography and with varying grades of clinical severity. There was, however, statistically no significant clinical effect of sonothrombolysis in this prematurely stopped trial.

Clinical Trial Registration—

URL: http://www.clinicaltrials.gov. Unique identifier: NCT01949961.

Expanding the concept of neuroprotection for acute ischemic stroke: The pivotal roles of reperfusion and the collateral circulation

This review surveys the efforts taken to achieve clinically efficacious protection of the ischemic brain and underscores the necessity of expanding our purview to include the essential role of cerebral perfusion and the collateral circulation. We consider the development of quantitative strategies to measure cerebral perfusion at the regional and local levels and the application of these methods to elucidate flow-related thresholds of ischemic viability and to characterize the ischemic penumbra. We stress that the modern concept of neuroprotection must consider perfusion, the necessary substrate upon which ischemic brain survival depends. We survey the major mechanistic approaches to neuroprotection and review clinical neuroprotection trials, focusing on those phase 3 multicenter clinical trials for acute ischemic stroke that have been completed or terminated. We review the evolution of thrombolytic therapies; consider the lessons learned from the initial, negative multicenter trials of endovascular therapy; and emphasize the highly successful positive trials that have finally established a clinical role for endovascular clot removal. As these studies point to the brain's collateral circulation as key to successful reperfusion, we next review the anatomy and pathophysiology of collateral perfusion as it relates to ischemic infarction, as well as the molecular and genetic influences on collateral development. We discuss the current MR and CT-based diagnostic methods for assessing the collateral circulation and the prognostic significance of collaterals in ischemic stroke, and we consider past and possible future therapeutic directions.

Sonothrombolysis with BR38 Microbubbles Improves Microvascular Patency in a Rat Model of Stroke

Background

Early recanalization of large cerebral vessels in ischemic stroke is associated with improved clinical outcome, however persisting hypoperfusion leads to poor clinical recovery despite large vessel recanalization. Limited experimental sonothrombolysis studies have shown that addition of microbubbles during treatment can improve microvascular patency. We aimed to determine the effect of two different microbubble formulations on microvascular patency in a rat stroke model.

Methods

We tested BR38 and SonoVue® microbubble-enhanced sonothrombolysis in Wistar rats submitted to 90-minute filament occlusion of the middle cerebral artery. Rats were randomized to treatment (n = 6/group): control, rt-PA, or rt-PA+3-MHz ultrasound insonation with BR38 or SonoVue® at full or 1/3 dose. Treatment duration was 60 minutes, beginning after withdrawal of the filament, and sacrifice was immediately after treatment. Vascular volumes were evaluated with microcomputed tomography.

Results

Total vascular volume of the ipsilateral hemisphere was reduced in control and rt-PA groups (p<0.05), but was not significantly different from the contralateral hemisphere in all microbubble-treated groups (p>0.1).

Conclusions

Microbubble-enhanced sonothrombolysis improves microvascular patency. This effect is not dose- or microbubble formulation-dependent suggesting a class effect of microbubbles promoting microvascular reopening. This study demonstrates that microbubble-enhanced sonothrombolysis may be a therapeutic strategy for patients with persistent hypoperfusion of the ischemic territory.

Nanomedicine as a strategy to fight thrombotic diseases

This review highlights the preclinical and clinical research based on the use of nano- and micro-carriers in thrombolytic drug delivery. Ischemic heart and stroke caused by thrombosis are the main causes of death in the world. Because of their inactivation in the blood, high doses of thrombolytics are administered to patients, increasing the risk of intracranial hemorrhage. Preclinical research conducted with lipid, polymer or magnetic nanoparticles loaded with thrombolytic drugs showed an enhancement of thrombolysis and a reduction of undesirable side effects. Targeted nanocarriers exhibited an increased accumulation into clot. Clinical trials were already conducted with lipid-based microbubbles combined with ultrasound and thrombolytic drug and showed thrombolysis improvement. Future validation of nanosystems is awaited in clinic. This research opens new strategies for the management of thrombotic diseases.

To dissolve a thrombus, thrombolytic drugs are administered, but they are rapidly inactivated in the blood. High amounts are thus injected to patients with the risk to develop intracranial hemorrhages. Nanocarriers and microbubbles have been tested in preclinical models to deliver thrombolytic drugs. These systems have the advantage to protect the drug from the degradation. In clinical trials, galactose and lipid-based microbubbles associated to ultrasound and thrombolytic drugs showed an enhancement of thrombolysis. Other systems are also expected with new drugs combined or not with endovascular intervention to treat ischemic heart or stroke.

Efficacy and spatial distribution of ultrasound-mediated clot lysis in the absence of thrombolytics

Ultrasound and microbubble (MB) contrast agents accelerate clot lysis, yet clinical trials have been performed without defining optimal acoustic conditions. Our aim was to assess the effect of acoustic pressure and frequency on the extent and spatial location of clot lysis. Clots from porcine blood were created with a 2-mm central lumen for infusion of lipid-shelled perfluorocarbon MBs (1×10(7) ml(-1)) or saline. Therapeutic ultrasound at 0.04, 0.25, 1.05, or 2.00 MHz was delivered at a wide range of peak rarefactional acoustic pressure amplitudes (PRAPAs). Ultrasound was administered over 20 minutes grouped on-off cycles to allow replenishment of MBs. The region of lysis was quantified using contrast-enhanced ultrasound imaging. In the absence of MBs, sonothrombolysis did not occur at any frequency. Sonothrombolysis was also absent in the presence of MBs despite their destruction at 0.04 and 2.00 MHz. It occurred at 0.25 and 1.05 MHz in the presence of MBs for PRAPAs > 1.2 MPa and increased with PRAPA. At 0.25 MHz the clot lysis was located in the far wall. At 1.05 MHz, however, there was a transition from far to near wall as PRAPA was increased. The area of clot lysis measured by ultrasound imaging correlated with that by micro-CT and quantification of debris in the effluent. In conclusion, sonothrombolysis with MBs was most efficient at 0.25 MHz. The spatial location of sonothrombolysis varies with pressure and frequency indicating that the geometric relation between therapeutic probe and vascular thrombosis is an important variable for successful lysis clinically.

Reperfusion therapies of acute ischemic stroke: potentials and failures

Over the past 20 years, clinical research has focused on the development of reperfusion therapies for acute ischemic stroke (AIS), which include the use of systemic intravenous thrombolytics (alteplase, desmoteplase, or tenecteplase), the augmentation of systemic intravenous recanalization with ultrasound, the bridging of intravenous with intra-arterial thrombolysis, the use of multi-modal approaches to reperfusion including thrombectomy and thromboaspiration with different available retrievers. Clinical trials testing these acute reperfusion therapies provided novel insight regarding the comparative safety and efficacy, but also raised new questions and further uncertainty on the field. Intravenous alteplase (tPA) remains the fastest and easiest way to initiate acute stroke reperfusion treatment, and should continue to be the first-line treatment for patients with AIS within 4.5 h from onset. The use of tenecteplase instead of tPA and the augmentation of systemic thrombolysis with ultrasound are both novel therapeutical modalities that may emerge as significant options in AIS treatment. Endovascular treatments for AIS are rapidly evolving due to technological advances in catheter-based interventions and are currently emphasizing speed in order to result in timely restoration of perfusion of still-salvageable, infarcted brain tissue, since delayed recanalization of proximal intracranial occlusions has not been associated with improved clinical outcomes. Comprehensive imaging protocols in AIS may enable better patient selection for endovascular interventions and for testing multi-modal combinatory strategies.

Interactions between individual ultrasound-stimulated microbubbles and fibrin clots

The use of ultrasound-stimulated microbubbles (USMBs) to promote thrombolysis is well established, but there remains considerable uncertainty about the mechanisms of this process. Here we examine the microscale interactions between individual USMBs and fibrin clots as a function of bubble size, exposure conditions and clot type. Microbubbles (n = 185) were placed adjacent to clot boundaries ("coarse" or "fine") using optical tweezers and exposed to 1-MHz ultrasound as a function of pressure (0.1-0.39 MPa). High-speed (10 kfps) imaging was employed, and clots were subsequently assessed with 2-photon microscopy. For fine clots, 46% of bubbles "embedded" within 10 μm of the clot boundary at pressures of 0.1 and 0.2 MPa, whereas at 0.39 MPa, 53% of bubbles penetrated and transited into the clots with an incidence inversely related to their diameter. A substantial fraction of penetrating bubbles induced fibrin network damage and promoted the uptake of nanobeads. In coarse clots, penetration occurred more readily and at lower pressures than in fine clots. The results therefore provide direct evidence of therapeutically relevant effects of USMBs and indicate their dependence on size, exposure conditions and clot properties.

Intravenous thrombolysis in acute ischemic stroke: standard and potential future applications

Acute ischemic stroke is a medical emergency requiring urgent treatment. Randomized clinical trial and Phase IV data have provided unequivocal evidence that intravenous thrombolysis with recombinant tissue plasminogen activator (rt-PA) improves early functional outcomes by restoring brain perfusion. Moreover, these studies have shed substantial light on the factors which are associated with more favorable outcome with tPA and are related to the highest benefit-to-risk ratio. Stroke physicians should consider vascular imaging techniques to aid decision making with thrombolytic therapy. The presence of intracranial occlusion is the target of treatment with early recanalization being the goal. Successful use of intravenous thrombolysis depends on a sound understanding of the decision-making process and organization of the treating team who strives for early treatment initiation and strict adherence to the protocol. Intravenous rt-PA within 4.5 h of onset should now be a standard treatment of acute disabling ischemic stroke throughout the world. This review also summarizes intravenous thrombolysis contraindications as well as the safety of novel reperfusion therapies including tenecteplase, sonothrombolysis and the combination of alteplase with direct thrombin inhibitors or glycoprotein IIb/IIIa receptor antagonists.

The role of sonolysis and sonothrombolysis in acute ischemic stroke: a systematic review and meta-analysis of randomized controlled trials and case-control studies

OBJECTIVES

To assess the evidence on the safety and efficacy of sonothrombolysis in acute stroke.

SEARCH METHODS

Electronic databases and grey literature were searched under different MeSH terms from 1970 to present.

SELECTION CRITERIA

Randomized control trials (RCTs) and case control studies (CCSs) on sonolysis and sonothrombolysis alone or with microsphere in acute stroke patients (>18 old). Outcome measures included complete recanalization (CR) at 1-2 and 24 hours, 3 months modified Rankin Scale (mRS), and symptomatic intracerebral hemorrhage (sICH). Data was extracted to Review Manager software.

RESULTS

Fifty-seven studies were retrieved and analyzed. Ten studies (7 RCTs and 3 CCSs) were included in our meta-analysis, which revealed that sonolysis and sonothrombolysis are safe (OR of sICH: 1.14; 95% confidence interval (CI): 0.56- 2.34;P=0.71) and effective (OR of CR at 1-2 hours: 2.95;95% CI: 1.81-4.81;P<0.00001) and have more than two-fold higher likelihood of favourable long-term outcome (3-month mRS 0-2; OR: 2.20; CI:1.52-3.19;P<0.0001). Further subgroup analysis based on the presence of microsphere revealed that it is safe (OR of sICH: 1.18; CI:0.433.24;P=0.75) and effective (OR of CR: 2.61; CI: 1.36-4.99;P=0.004). Subgroup analysis based on sonolysis revealed to be safe and effective.

CONCLUSIONS

This novel treatment appears safe and effective. The evidence of microsphere as an enhancement of sonothrombolysis is evolving.

Pulsed focused ultrasound-induced displacements in confined in vitro blood clots

Ultrasound has been shown to potentiate the effects of tissue plasminogen activator to improve clot lysis in a range of in vitro and in vivo studies as well as in clinical trials. One possible mechanism of action is acoustic radiation force-induced clot displacements. In this study, we investigate the temporal and spatial dynamics of clot displacements and strain initiated by focused ultrasound pulses. Displacements were produced by a 1.51 MHz f-number 1 transducer over a range of acoustic powers (1-85 W) in clots constrained within an agar vessel phantom channel. Displacements were tracked during and after a 5.45 ms therapy pulse using a 20 MHz high-frequency ultrasound imaging probe. Peak thrombus displacements were found to be linear as a function of acoustic power up to 60 W before leveling off near 128 μm for the highest transmit powers. The time to peak displacement and recovery time of blood clots was largely independent of acoustic powers with measured values near 2 ms. A linear relationship between peak axial strain and transmit power was observed, reaching a peak value of 11% at 35 W. The peak strain occurred ~0.75 mm from the focal zone for all powers investigated in both lateral and axial directions. These results indicate that substantial displacements can be induced by focused ultrasound in confined blood clots, and that the spatial and temporal displacement patterns are complex and highly dependent on exposure conditions, which has implications for future work investigating their link to clot lysis and for developing approaches to exploit these effects.

Sonothrombolysis for the treatment of acute stroke: current concepts and future directions

Achieving rapid reperfusion transcranial color-coded duplex is the critical issue in acute stroke treatment. Ultrasound (US) generates negative pressure waves that are associated with an increase in either intrinsic or intravenous tissue plasminogen activator (tPA)-induced fibrinolytic activity. Higher rates of tPA-induced arterial recanalization, associated with a trend towards better functional outcome, have been safely achieved by using high-frequency US. By contrast, the use of low-frequency US and transcranial color-coded duplex has been linked to significant hemorrhagic complications. US-accelerated thrombolysis has been safely enhanced by lowering the amount of energy needed for acoustic cavitation with the administration of microbubbles. Other applications of US are being studied, including its intra-arterial use. Operator-independent devices, which will spread the use of these US techniques further, are also being developed. This article reviews the present status of sonothrombolysis in acute stroke treatment, highlighting both experimental and clinical studies addressing this issue, and discusses its future regarding both efficacy and safety.

Potentiating intra-arterial sonothrombolysis for acute ischemic stroke by the addition of the ultrasound contrast agents (Optison™ & SonoVue(®))

Transcranial ultrasound in combination with intravenously administered ultrasound contrast agents (UCA) in the presence or absence of recombinant tissue plasminogen activator (rt-PA) has been widely evaluated as a new modality for treatment of ischemic stroke. Despite the successful demonstration of accelerated clot lysis there are inherent limitations associated with this modality such as inconsistency in temporal window thickness and/or potential serious cardiopulmonary reactions to intravenous administration of UCA that prevent broad application to ischemic stroke populations. As a complementary modality, we evaluated potential lysis enhancement by intra-arterial ultrasound with concurrent intra-clot delivery of UCA and rt-PA. To this end, clots were formed with average pore diameter similar to clinically retracted clots by adjusting the thrombin concentration. Physical characteristic and retention of UCA after delivery through the catheter as a function of clinically relevant flow rates of 6, 12, 18 ml/h were determined using a microscopic method. The ability of the UCA employed in this study, Optison and SonoVue, to penetrate into the clot was verified using ultrasound B-mode imaging. Clot lysis as a function of rt-PA concentration, 0.009 through 0.5 mg/ml, in the presence and absence of UCA diluted to 1:10, 1:100, and 1:200 v/v at two Peak rarefaction acoustic pressures of 1.3 and 2.1 MPa were evaluated using a weighing method. The study results suggest the addition of only 0.02 ml of 1:100 diluted UCA to rt-PA of 0.009, 0.05, 0.3, and 0.5 mg/ml can enhance the lysis rate by 3.9, 2.6, 1.9 and 1.8 fold in the presence of peak rarefaction acoustic pressure of 1.3 MPa and by 5.1, 3.4, 2.6, 3.1 in the presence of peak rarefaction acoustic pressure of 2.1 MPa, respectively. In addition, Optison and SonoVue demonstrated comparable effectiveness in enhancing the clot lysis rate. Addition of UCA to intra-arterial sonothrombolysis could be considered as a viable treatment option for ischemic stroke patients.

Sonothrombolysis: an emerging modality for the treatment of acute ischemic and hemorrhagic stroke

To date, it is believed that rapid removal of impedances hindering normal blood circulation in the brain would salvage ischemic tissue. Hence, most treatment modalities undergoing clinical evaluation for treatment of stroke are focused on faster recanalization in acute ischemic stroke or faster hematoma mass reduction in hemorrhagic stroke. Therapeutic ultrasound is among the promising emerging modalities being clinically evaluated to meet this purpose. This review provides an overview of existing clinical data in evaluating sonothrombolysis applications in treatment of acute ischemic and hemorrhagic stroke. Furthermore, the present status of clinical evaluation of microbubbles as a potential adjuvant to this modality is reviewed.

Changes in hemocoagulation in acute stroke patients after one-hour sono-thrombolysis using a diagnostic probe

The aim was to monitor the changes in hemocoagulation parameters in acute ischemic stroke (AIS) patients after sono-thrombolysis of the occluded middle cerebral artery using a duplex transcranial probe with 2.0-MHz frequency in Doppler mode. Sixteen AIS patients indicated for intravenous thrombolysis (IVT) (8 males; mean age 68.3 +/- 7.1 y) and 16 AIS patients contraindicated for IVT (11 males; mean age 67.9 +/- 7.9 y) were randomized for sono-thrombolysis (8 + 8 patients) or standard treatment (control group) (8 + 8 patients). The significant decrease of plasminogen activator inhibitor-1, plasminogen and alpha-2-antiplasmin activity by a mean of 60, 32 and 24%, respectively, and the increase of tissue plasminogen activator by a mean of 56% was found after sono-thrombolysis when compared with control group (p < 0.0125); these changes were more evident in patients treated with a combination of sono-thrombolysis and IVT (79, 38, 50 and 82%, respectively) than in patients treated by sono-thrombolysis alone (34, 13, 17 and 30%, respectively).

Sonothrombolysis in the management of acute ischemic stroke

Multiple in vitro and animal models have demonstrated the efficacy of ultrasound to enhance fibrinolysis. Mechanical pressure waves produced by ultrasound energy improve the delivery and penetration of alteplase (recombinant tissue plasminogen activator [tPA]) inside the clot. In human stroke, the CLOTBUST phase II trial showed that the combination of alteplase plus 2 hours of continuous transcranial Doppler (TCD) increased recanalization rates, producing a trend toward better functional outcomes compared with alteplase alone. Other small clinical trials also showed an improvement in clot lysis when transcranial color-coded sonography was combined with alteplase. In contrast, low-frequency ultrasound increased the symptomatic intracranial hemorrhage rate in a clinical trial. Administration of microbubbles (MBs) may further enhance the effect of ultrasound on thrombolysis by lowering the ultrasound-energy threshold needed to induce acoustic cavitation. Initial clinical trials have been encouraging, and a multicenter international study, TUCSON, determined a dose of newly developed MBs that can be safely administered with alteplase and TCD. Even in the absence of alteplase, the ultrasound energy, with or without MBs, could increase intrinsic fibrinolysis. The intra-arterial administration of ultrasound with the EKOS NeuroWave catheter is another ultrasound application for acute stroke that is currently being studied in the IMS III trial. Operator-independent devices, different MB-related techniques, and other ultrasound parameters for improving and spreading sonothrombolysis are being tested.

Current and future recanalization strategies for acute ischemic stroke

In a quest for stroke treatment, reperfusion proved to be the first key to the puzzle. Systemic tissue plasminogen activator (tPA), the first and currently the only approved treatment, is also the fastest way to initiate thrombolyis for acute ischemic stroke. tPA works by induction of mostly partial recanalization since stroke patients often have large thrombus burden. Thus, early augmentation of fibrinolysis and multi-modal approach to improve recanalization are desirable. This review focuses on the following strategies available to clinicians now or being tested in clinical trials: (a) faster initiation of tPA infusion; (b) sonothrombolysis; (c) intra-arterial revascularization, bridging intravenous and intra-arterial thrombolysis, mechanical thrombectomy and aspiration; and (d) novel experimental approaches. Despite these technological advances, no single strategy was yet proven to be a 'silver bullet' solution to reverse acute ischemic stroke. Better outcomes are expected with faster treatment leading to early, at times just partial flow improvement rather than achieving complete recanalization with lengthy procedures. Arterial re-occlusion can occur with any of these approaches, and it remains a challenge since it leads to poor outcomes and no clinical trial data are available yet to determine safe strategies to prevent or reverse re-occlusion.

Fibrinolytic effects of transparietal ultrasound associated with intravenous infusion of an ultrasound contrast agent: study of a rat model of acute cerebral stroke

The aim of this study was to evaluate the thrombolytic effect of focused transparietal ultrasound in combination with a specific contrast agent (microbubbles) in acute cerebral ischemia. Acute cerebral ischemia was induced in 10 rats by intra-arterial clots injection. Five rats (group 1) were treated with a combination of transparietal ultrasound (probe 2 MHz, acoustic power 500 mW/cm(2)) and intravenous injection of 0.6 mL of the ultrasound contrast agent (UCA) sulfur hexafluoride. Five rats (group 2) were treated by fibrinolytic intravenous infusion (recombinant tissue plasminogen activator). Cerebral cellular energy production was determined by measuring the cellular phosphorylation using phosphorus magnetic spectroscopy before and during ischemia induction and after treatment. Measures were performed on a dedicated 2.35T magnet. The ratio phosphocreatine (P(Cr)) on inorganic phosphate (P(i)), P(Cr)/P(i), estimation of the oxidative phosphorylation metabolism and the intracellular pH (pHi) were measured in the two groups. Compared with the ischemia induction period, both treatments were associated with an increase of P(Cr)/P(i) and pHi values, respectively, +80% and +100% in group 1 (p=0.07) and +100% and +80% in group 2 (p=0.04). There was no significant difference between the two groups for the response treatment. To conclude, treatment with intravenous fibrinolytic infusion and treatment with focused ultrasound in combination with UCA seems to be equally effective in treating acute cerebral ischemia in rats. (E-mail: j.p.tasu@chu-poitiers.fr).

Changes in lipid-encapsulated microbubble population during continuous infusion and methods to maintain consistency

Stabilized microbubbles are used as ultrasound contrast agents. These micron-sized gas capsules are injected into the bloodstream to provide contrast enhancement during ultrasound imaging. Some contrast imaging strategies, such as destruction-reperfusion, require a continuous injection of microbubbles over several minutes. Most quantitative imaging strategies rely on the ability to administer a consistent dose of contrast agent. Because of the buoyancy of these gas-filled agents, their spatial distribution within a syringe changes over time. The population of microbubbles that is pumped from a horizontal syringe outlet differs from initial population as the microbubbles float to the syringe top. In this manuscript, we study the changes in the population of a contrast agent that is pumped from a syringe caused by microbubble flotation. Results are presented in terms of change in concentration and change in mean diameter, as a function of time, suspension medium and syringe diameter. Data illustrate that the distribution of contrast agents injected from a syringe changes in both concentration and mean diameter over several minutes without mixing. We discuss the application of a mixing system and viscosity agents to keep the contrast solution more evenly distributed in a syringe. These results are significant for researchers using microbubble contrast agents in continuous-infusion applications where it is important to maintain consistent contrast agent delivery rate, or in situations where the injection syringe cannot be mixed immediately before administration.

Transcranial ultrasound in clinical sonothrombolysis (TUCSON) trial

OBJECTIVE

Microspheres (microS) reach intracranial occlusions and transmit energy momentum from an ultrasound wave to residual flow to promote recanalization. We report a randomized multicenter phase II trial of microS dose escalation with systemic thrombolysis.

METHODS

Stroke patients receiving 0.9mg/kg tissue plasminogen activator (tPA) with pretreatment proximal intracranial occlusions on transcranial Doppler (TCD) were randomized (2:1 ratio) to microS (MRX-801) infusion over 90 minutes (Cohort 1, 1.4ml; Cohort 2, 2.8ml) with continuous TCD insonation, whereas controls received tPA and brief TCD assessments. The primary endpoint was symptomatic intracerebral hemorrhage (sICH) within 36 hours after tPA.

RESULTS

Among 35 patients (Cohort 1 = 12, Cohort 2 = 11, controls = 12) no sICH occurred in Cohort 1 and controls, whereas 3 (27%, 2 fatal) sICHs occurred in Cohort 2 (p = 0.028). Sustained complete recanalization/clinical recovery rates (end of TCD monitoring/3 month) were 67%/75% for Cohort 1, 46%/50% for Cohort 2, and 33%/36% for controls (p = 0.255/0.167). The median time to any recanalization tended to be shorter in Cohort 1 (30 min; interquartile range [IQR], 6) and Cohort 2 (30 min; IQR, 69) compared to controls (60 min; IQR, 5; p = 0.054). Although patients with sICH had similar screening and pretreatment systolic blood pressure (SBP) levels in comparison to the rest, higher SBP levels were documented in sICH+ patients at 30 minutes, 60 minutes, 90 minutes, and 24-36 hours following tPA bolus.

INTERPRETATION

Perflutren lipid microS can be safely combined with systemic tPA and ultrasound at a dose of 1.4ml. Safety concerns in the second dose tier may necessitate extended enrollment and further experiments to determine the mechanisms by which microspheres interact with tissues. In both dose tiers, sonothrombolysis with microS and tPA shows a trend toward higher early recanalization and clinical recovery rates compared to standard intravenous tPA therapy. Ann Neurol 2009;66:28-38.

Ultrasound Enhancement of Fibrinolysis

Systemic administration of tissue plasminogen activator (tPA) remains the fastest way to initiate treatment for acute ischemic stroke. The presence of a proximal arterial occlusion should not be viewed as an insurmountable predictor of tPA failure. Because tPA works by induction of partial recanalization of large thrombi, early augmentation of fibrinolysis to improve recanalization is desirable. This augmentation is feasible and can be safely achieved at the bedside with diagnostic Doppler ultrasound. In the CLOTBUST trial, 83% of patients achieved any recanalization (46% complete, 27% partial) with tPA+transcranial Doppler vs 50% (17% complete, 33% partial) with tPA alone within 2 hours of treatment ( P <0.001). Sustained, complete recanalization at 2 hours was 38% vs 13%, respectively ( P =0.03). A recent meta-analysis of 6 randomized and 3 nonrandomized clinical studies of sonothrombolysis showed that any diagnostic ultrasound monitoring can at least double the chance of early complete arterial recanalization at no increase in the risk of symptomatic intracerebral hemorrhage. Because application in humans of frequencies below the diagnostic range resulted in increased symptomatic bleeding rates, mechanisms by which megahertz and kilohertz frequencies interact with the clot–residual flow interface and endothelium are currently under renewed investigations. Catheter-based ultrasound delivery to arterial thrombi and intraventricular clots is the subject of ongoing clinical trials. Addition of gaseous perflutren-lipid microspheres to tPA and transcranial Doppler can further facilitate early flow improvement, with a 50% rate of early, complete recanalization in a recent feasibility study. Transcranial ultrasound delivery in an operator-independent and dose-controlled manner is being tested in a clinical trial.