Evaluation of capacitive micromachined ultrasonic transducers for passive monitoring of microbubble-assisted ultrasound therapies

Low-boiling-point perfluorocarbon nanodroplets for adaptable ultrasound-induced blood-brain barrier opening

Low-boiling point perfluorocarbon nanodroplets (NDs) are valued as effective sonosensitive agents, encapsulating a liquid perfluorocarbon that would instantaneously vaporize at body temperature without the NDs shell. Those NDs have been explored for both therapeutic and diagnostic purposes. Here, phospholipid-shelled nanodroplets containing octafluoropropane (C3F8) or decafluorobutane (C4F10) formed by condensation of microbubbles were thoroughly characterized before blood-brain (BBB) permeabilization. Transmission electron microscopy (TEM) and cryo-TEM were employed to confirm droplet formation while providing high-resolution insights into the droplet surface and lipid arrangement assessed from electron density observation after condensation. The vaporization threshold of NDs was determined with a high-speed camera, and the frequency signal emitted by the freshly vaporized bubbles was analyzed using cavitation detection. C3F8 NDs exhibited vaporization at 0.3 MPa (f0 = 1.5 MHz, 50 cycles), and emitted signals at 2 f0 and 1.5 f0 from 0.45 MPa onwards (f0 = 1.5 MHz, 50 cycles), while broadband noise was measured starting from 0.55 MPa. NDs with the higher boiling point C4F10 vaporized at 1.15 MPa and emitted signals at 2 f0 from 0.65 MPa and 1.5 f0 from 0.9 MPa, while broadband noise was detected starting from 0.95 MPa. Both ND formulations were used to permeabilize the BBB in healthy mice using tailored ultrasound sequences, allowing for the identification of optimal applications for each NDs type. C3F8 NDs proved suitable and safe for permeabilizing a large area, potentially the entire brain, at low acoustic pressure. Meanwhile, C4F10 droplets facilitated very localized (400 μm isotropic) permeabilization at higher pressure. This study prompts a closer examination of the structural rearrangements occurring during the condensation of microbubbles into NDs and highlights the potential to tailor solutions for different brain pathologies by choosing the composition of the NDs and adjusting the ultrasound sequence.

A High Sensitivity CMUT-Based Passive Cavitation Detector for Monitoring Microbubble Dynamics During Focused Ultrasound Interventions

Tracking and controlling microbubble (MB) dynamics in the human brain through acoustic emission (AE) monitoring during transcranial focused ultrasound (tFUS) therapy are critical for attaining safe and effective treatments. The low-amplitude MB emissions have harmonic and ultra-harmonic components, necessitating a broad bandwidth and low-noise system for monitoring transcranial MB activity. Capacitive micromachined ultrasonic transducers (CMUTs) offer high sensitivity and low noise over a broad bandwidth, especially when they are tightly integrated with electronics, making them a good candidate technology for monitoring the MB activity through human skull. In this study, we designed a 16-channel analog front-end (AFE) electronics with a low-noise transimpedance amplifier (TIA), a band-gap reference circuit, and an output buffer stage. To assess AFE performance and ability to detect MB AE, we combined it with a commercial CMUT array. The integrated system has 12.3 - [Formula: see text] receive sensitivity with 0.085 - [Formula: see text] minimum detectable pressure (MDP) up to 3 MHz for a single element CMUT with 3.78 [Formula: see text] area. Experiments with free MBs in a microfluidic channel demonstrate that our system is able to capture key spectral components of MBs' harmonics when sonicated at clinically relevant frequencies (0.5 MHz) and pressures (250 kPa). Together our results demonstrate that the proposed CMUT system can support the development of novel passive cavitation detectors (PCD) to track MB activity for attaining safe and effective focused ultrasound (FUS) treatments.

State of the art on microbubble cavitation monitoring and feedback control for blood-brain-barrier opening using focused ultrasound

Focused ultrasound (FUS) is a non-invasive and highly promising method for targeted and reversible blood-brain barrier permeabilization. Numerous preclinical studies aim to optimize the localized delivery of drugs using this method in rodents and non-human primates. Several clinical trials have been initiated to treat various brain diseases in humans using simultaneous BBB permeabilization and drug injection. This review presents the state of the art ofin vitroandin vivocavitation control algorithms for BBB permeabilization using microbubbles (MB) and FUS. Firstly, we describe the different cavitation states, their physical significance in terms of MB behavior and their translation into the spectral composition of the backscattered signal. Next, we report the different indexes calculated and used during the ultrasonic monitoring of cavitation. Finally, the differentin vitroandin vivocavitation control strategies described in the literature are presented and compared.

Towards controlled drug delivery in brain tumors with microbubble-enhanced focused ultrasound

Brain tumors are particularly challenging malignancies, due to their location in a structurally and functionally distinct part of the human body - the central nervous system (CNS). The CNS is separated and protected by a unique system of brain and blood vessel cells which together prevent most bloodborne therapeutics from entering the brain tumor microenvironment (TME). Recently, great strides have been made through microbubble (MB) ultrasound contrast agents in conjunction with ultrasound energy to locally increase the permeability of brain vessels and modulate the brain TME. As we elaborate in this review, this physical method can effectively deliver a wide range of anticancer agents, including chemotherapeutics, antibodies, and nanoparticle drug conjugates across a range of preclinical brain tumors, including high grade glioma (glioblastoma), diffuse intrinsic pontine gliomas, and brain metastasis. Moreover, recent evidence suggests that this technology can promote the effective delivery of novel immunotherapeutic agents, including immune check-point inhibitors and chimeric antigen receptor T cells, among others. With early clinical studies demonstrating safety, and several Phase I/II trials testing the preclinical findings underway, this technology is making firm steps towards shaping the future treatments of primary and metastatic brain cancer. By elaborating on its key components, including ultrasound systems and MB technology, along with methods for closed-loop spatial and temporal control of MB activity, we highlight how this technology can be tuned to enable new, personalized treatment strategies for primary brain malignancies and brain metastases.