2D and 3D real-time passive cavitation imaging of pulsed cavitation ultrasound therapy in moving tissues

Advancements in strategies for overcoming the blood-brain barrier to deliver brain-targeted drugs

The blood-brain barrier is known to consist of a variety of cells and complex inter-cellular junctions that protect the vulnerable brain from neurotoxic compounds; however, it also complicates the pharmacological treatment of central nervous system disorders as most drugs are unable to penetrate the blood-brain barrier on the basis of their own structural properties. This dramatically diminished the therapeutic effect of the drug and compromised its biosafety. In response, a number of drugs are often delivered to brain lesions in invasive ways that bypass the obstruction of the blood-brain barrier, such as subdural administration, intrathecal administration, and convection-enhanced delivery. Nevertheless, these intrusive strategies introduce the risk of brain injury, limiting their clinical application. In recent years, the intensive development of nanomaterials science and the interdisciplinary convergence of medical engineering have brought light to the penetration of the blood-brain barrier for brain-targeted drugs. In this paper, we extensively discuss the limitations of the blood-brain barrier on drug delivery and non-invasive brain-targeted strategies such as nanomedicine and blood-brain barrier disruption. In the meantime, we analyze their strengths and limitations and provide outlooks on the further development of brain-targeted drug delivery systems.

Quantifying the Effect of Acoustic Parameters on Temporal and Spatial Cavitation Activity: Gauging Cavitation Dose

OBJECTIVE

Cavitation-enhanced delivery of therapeutic agents is under development for the treatment of cancer and neurodegenerative and cardiovascular diseases, including sonothrombolysis for deep vein thrombosis. The objective of this study was to quantify the spatial and temporal distribution of cavitation activity nucleated by Definity infused through the EKOS catheter over a range of acoustic parameters controlled by the EKOS endovascular system.

METHODS

Three insonation protocols were compared in an in vitro phantom mimicking venous flow to measure the effect of peak rarefactional pressure, pulse duration and pulse repetition frequency on cavitation activity energy, location and duration. Inertial and stable cavitation activity was quantified using passive cavitation imaging, and a metric of cavitation dose based on energy density was defined.

RESULTS

For all three insonation protocols, cavitation was sustained for the entire 30 min Definity infusion. The evolution of cavitation energy during each pulse duration was similar for all three protocols. For insonation protocols with higher peak rarefactional acoustic pressures, inertial and stable cavitation doses also increased. A complex relationship between the temporal behavior of cavitation energy within each pulse and the pulse repetition frequency affected the cavitation dose for the three insonation protocols. The relative predominance of stable or inertial cavitation dose varied according to insonation schemes. Passive cavitation images revealed the spatial distribution of cavitation activity.

CONCLUSION

Our cavitation dose metric based on energy density enabled the impact of different acoustic parameters on cavitation activity to be measured. Depending on the type of cavitation to be promoted or suppressed, particular pulsing schemes could be employed in future studies, for example, to correlate cavitation dose with sonothrombolytic efficacy.

Barrier-breaking effects of ultrasonic cavitation for drug delivery and biomarker release

Recently, emerging evidence has demonstrated that cavitation actually creates important bidirectional channels on biological barriers for both intratumoral drug delivery and extratumoral biomarker release. To promote the barrier-breaking effects of cavitation for both therapy and diagnosis, we first reviewed recent technical advances of ultrasound and its contrast agents (microbubbles, nanodroplets, and gas-stabilizing nanoparticles) and then reported the newly-revealed cavitation physical details. In particular, we summarized five types of cellular responses of cavitation in breaking the plasma membrane (membrane retraction, sonoporation, endocytosis/exocytosis, blebbing and apoptosis) and compared the vascular cavitation effects of three different types of ultrasound contrast agents in breaking the blood-tumor barrier and tumor microenvironment. Moreover, we highlighted the current achievements of the barrier-breaking effects of cavitation in mediating drug delivery and biomarker release. We emphasized that the precise induction of a specific cavitation effect for barrier-breaking was still challenged by the complex combination of multiple acoustic and non-acoustic cavitation parameters. Therefore, we provided the cutting-edge in-situ cavitation imaging and feedback control methods and suggested the development of an international cavitation quantification standard for the clinical guidance of cavitation-mediated barrier-breaking effects.

Ultrasound-mediated blood-brain barrier opening: An effective drug delivery system for theranostics of brain diseases

Blood-brain barrier (BBB) remains a significant obstacle to drug therapy for brain diseases. Focused ultrasound (FUS) combined with microbubbles (MBs) can locally and transiently open the BBB, providing a potential strategy for drug delivery across the BBB into the brain. Nowadays, taking advantage of this technology, many therapeutic agents, such as antibodies, growth factors, and nanomedicine formulations, are intensively investigated across the BBB into specific brain regions for the treatment of various brain diseases. Several preliminary clinical trials also have demonstrated its safety and good tolerance in patients. This review gives an overview of the basic mechanisms, ultrasound contrast agents, evaluation or monitoring methods, and medical applications of FUS-mediated BBB opening in glioblastoma, Alzheimer's disease, and Parkinson's disease.

High-spatial-resolution, instantaneous passive cavitation imaging with temporal resolution in histotripsy: a simulation study

Purpose

In histotripsy, a shock wave is transmitted, and the resulting inertial bubble cavitation that disrupts tissue is used for treatment. Therefore, it is necessary to detect when cavitation occurs and track the position of cavitation occurrence using a new passive cavitation (PC) imaging method.

Methods

An integrated PC image, which is constructed by collecting the focused signals at all times, does not provide information on when cavitation occurs and has poor spatial resolution. To solve this problem, we constructed instantaneous PC images by applying delay and sum beamforming at instantaneous time instants. By calculating instantaneous PC images at all data acquisition times, the proposed method can detect cavitation when it occurs by using the property that when signals from the cavitation are focused, their amplitude becomes large, and it can obtain a high-resolution PC image by masking out side lobes in the vicinity of cavitation.

Results

Ultrasound image simulation confirmed that the proposed method has higher resolution than conventional integrated PC imaging and showed that it can determine the position and time of cavitation occurrence as well as the signal strength.

Conclusion

Since the proposed novel PC imaging method can detect each cavitation separately when the incidence of cavitations is low, it can be used to monitor the treatment process of shock wave therapy and histotripsy, in which cavitation is an important mechanism of treatment.

Contrast Ultrasound, Sonothrombolysis and Sonoperfusion in Cardiovascular Disease: Shifting to Theragnostic Clinical Trials

Contrast ultrasound has a variety of applications in cardiovascular medicine, both in diagnosing cardiovascular disease as well as providing prognostic information. Visualization of intravascular contrast microbubbles is based on acoustic cavitation, the characteristic oscillation that results in changes in the reflected ultrasound waves. At high power, this acoustic response generates sufficient shear that is capable of enhancing endothelium-dependent perfusion in atherothrombotic cardiovascular disease (sonoperfusion). The oscillation and collapse of microbubbles in response to ultrasound also induces microstreaming and jetting that can fragment thrombus (sonothrombolysis). Several preclinical studies have focused on identifying optimal diagnostic ultrasound settings and treatment regimens. Clinical trials have been performed in acute myocardial infarction, stroke, and peripheral arterial disease often with improved outcome. In the coming years, results of ongoing clinical trials along with innovation and improvements in sonothrombolysis and sonoperfusion will determine whether this theragnostic technique will become a valuable addition to reperfusion therapy.

Real-Time Visualization of a Focused Ultrasound Beam Using Ultrasonic Backscatter

Focused ultrasound (FUS) therapies induce therapeutic effects in localized tissues using either temperature elevations or mechanical stresses caused by an ultrasound wave. During an FUS therapy, it is crucial to continuously monitor the position of the FUS beam in order to correct for tissue motion and keep the focus within the target region. Toward the goal of achieving real-time monitoring for FUS therapies, we have developed a method for the real-time visualization of an FUS beam using ultrasonic backscatter. The intensity field of an FUS beam was reconstructed using backscatter from an FUS pulse received by an imaging array and then overlaid onto a B-mode image captured using the same imaging array. The FUS beam visualization allows one to monitor the position and extent of the FUS beam in the context of the surrounding medium. Variations in the scattering properties of the medium were corrected in the FUS beam reconstruction by normalizing based on the echogenicity of the coaligned B-mode image. On average, normalizing by echogenicity reduced the mean square error between FUS beam reconstructions in nonhomogeneous regions of a phantom and baseline homogeneous regions by 21.61. FUS beam visualizations were achieved, using a single diagnostic imaging array as both an FUS source and an imaging probe, in a tissue-mimicking phantom and a rat tumor in vivo with a frame rate of 25-30 frames/s.