Magnetic resonance imaging-guided focused ultrasound to increase localized blood-spinal cord barrier permeability

Strategies for enhanced gene delivery to the central nervous system

The delivery of genes to the central nervous system (CNS) has been a persistent challenge due to various biological barriers. The blood-brain barrier (BBB), in particular, hampers the access of systemically injected drugs to parenchymal cells, allowing only a minimal percentage (<1%) to pass through. Recent scientific insights highlight the crucial role of the extracellular space (ECS) in governing drug diffusion. Taking into account advancements in vectors, techniques, and knowledge, the discussion will center on the most notable vectors utilized for gene delivery to the CNS. This review will explore the influence of the ECS - a dynamically regulated barrier-on drug diffusion. Furthermore, we will underscore the significance of employing remote-control technologies to facilitate BBB traversal and modulate the ECS. Given the rapid progress in gene editing, our discussion will also encompass the latest advances focused on delivering therapeutic editing in vivo to the CNS tissue. In the end, a brief summary on the impact of Artificial Intelligence (AI)/Machine Learning (ML), ultrasmall, soft endovascular robots, and high-resolution endovascular cameras on improving the gene delivery to the CNS will be provided.

Safety Review of Therapeutic Ultrasound for Spinal Cord Neuromodulation and Blood-Spinal Cord Barrier Opening

New focused ultrasound spinal cord applications have emerged, particularly those improving therapeutic agent delivery to the spinal cord via blood-spinal cord barrier opening and the neuromodulation of spinal cord tracts. One hurdle in the development of these applications is safety. It may be possible to use safety trends from seminal and subsequent works in focused ultrasound to guide the development of safety guidelines for spinal cord applications. We collated data from decades of pre-clinical studies and illustrate a clear relationship between damage, time-averaged spatial peak intensity and exposure duration. This relationship suggests a thermal mechanism underlies ultrasound-induced spinal cord damage. We developed minimum and mean thresholds for damage from these pre-clinical studies. When these thresholds were plotted against the parameters used in recent pre-clinical ultrasonic spinal cord neuromodulation studies, the majority of the neuromodulation studies were near or above the minimum threshold. This suggests that a thermal neuromodulatory effect may exist for ultrasonic spinal cord neuromodulation, and that the thermal dose must be carefully controlled to avoid damage to the spinal cord. By contrast, the intensity-exposure duration threshold had no predictive value when applied to blood-spinal cord barrier opening studies that employed injected contrast agents. Most blood-spinal cord barrier opening studies observed slight to severe damage, except for small animal studies that employed an active feedback control method to limit pressures based on measured bubble oscillation behavior. The development of new focused ultrasound spinal cord applications perhaps reflects the recent success in the development of focused ultrasound brain applications, and recent work has begun on the translation of these technologies from brain to spinal cord. However, a great deal of work remains to be done, particularly with respect to developing and accepting safety standards for these applications.

Simultaneous proton resonance frequency T1 - MR shear wave elastography for MR-guided focused ultrasound multiparametric treatment monitoring

PURPOSE

To develop an efficient MRI pulse sequence to simultaneously measure multiple parameters that have been shown to correlate with tissue nonviability following thermal therapies.

METHODS

A 3D segmented EPI pulse sequence was used to simultaneously measure proton resonance frequency shift (PRFS) MR thermometry (MRT), T1 relaxation time, and shear wave velocity induced by focused ultrasound (FUS) push pulses. Experiments were performed in tissue mimicking gelatin phantoms and ex vivo bovine liver. Using a carefully designed FUS triggering scheme, a heating duty cycle of approximately 65% was achieved by interleaving FUS ablation pulses with FUS push pulses to induce shear waves in the tissue.

RESULTS

In phantom studies, temperature increases measured with PRFS MRT and increases in T1 correlated with decreased shear wave velocity, consistent with material softening with increasing temperature. During ablation in ex vivo liver, temperature increase measured with PRFS MRT initially correlated with increasing T1 and decreasing shear wave velocity, and after tissue coagulation with decreasing T1 and increasing shear wave velocity. This is consistent with a previously described hysteresis in T1 versus PRFS curves and increased tissue stiffness with tissue coagulation.

CONCLUSION

An efficient approach for simultaneous and dynamic measurements of PRSF, T1 , and shear wave velocity during treatment is presented. This approach holds promise for providing co-registered dynamic measures of multiple parameters, which correlates to tissue nonviability during and following thermal therapies, such as FUS.

Disruption of the Blood-Spinal Cord Barrier using Low-Intensity Focused Ultrasound in a Rat Model

Low-intensity focused ultrasound (LIFU) uses ultrasonic pulsations at lower intensities than ultrasound and is being tested as a reversible and precise neuromodulatory technology. Although LIFU-mediated blood-brain barrier (BBB) opening has been explored in detail, no standardized technique for blood-spinal cord barrier (BSCB) opening has been established to date. Therefore, this protocol presents a method for successful BSCB disruption using LIFU sonication in a rat model, including descriptions of animal preparation, microbubble administration, target selection and localization, as well as BSCB disruption visualization and confirmation. The approach reported here is particularly useful for researchers who need a fast and cost-effective method to test and confirm target localization and precise BSCB disruption in a small animal model with a focused ultrasound transducer, evaluate the BSCB efficacy of sonication parameters, or explore applications for LIFU at the spinal cord, such as drug delivery, immunomodulation, and neuromodulation. Optimizing this protocol for individual use is recommended, especially for advancing future preclinical, clinical, and translational work.

Levetiracetam Attenuates the Spinal Cord Injury Induced by Acute Trauma via Suppressing the Expression of Perforin

The spinal cord injury (SCI) is one of the major reasons causing the motor dysfunctions of the patients. At present, few therapeutic strategies can effectively improve the symptom of SCI. Levetiracetam has been confirmed to alleviate the injury of nerve cells, while its functions in SCI remains unclear. In this study, C57BL/6J mice were used to establish SCI models to observe the effect of levetiracetam on SCI. The mice were fed with 180 mg/kg levetiracetam when suffering from SCI, and Basso mouse score (BMS) and CatWalk-assisted gait analysis were used to observe the motor functions of the mice. Nissl staining and TUNEL staining were used to observe the injury of nerve cells. The abundance of inflammatory factors was measured by ELISA. The permeability of blood-spinal cord barrier (BSCB) in mice was detected with macrophage infiltration analysis. Moreover, the abundance of perforin in the tissues was detected by western blot. The results showed that the SCI mice treated with levetiracetam exhibited lighter motor dysfunction compared with the mice treated with saline. Levetiracetam can effectively reduce the inflammatory reactions and alleviate apoptosis of the nerve cells. Moreover, levetiracetam remarkably decreased the BSCB permeability of SCI mice. Besides, it was also found that levetiracetam can significantly inhibit the expression of perforin. In conclusion, this study suggests that levetiracetam can attenuate the injury of BSCB to block the progression of SCI via suppressing the expression of perforin.

Establishing density-dependent longitudinal sound speed in the vertebral lamina

Focused ultrasound treatments of the spinal cord may be facilitated using a phased array transducer and beamforming to correct spine-induced focal aberrations. Simulations can non-invasively calculate aberration corrections using x-ray computed tomography (CT) data that are correlated to density (ρ) and longitudinal sound speed (c_L). We aimed to optimize vertebral lamina-specific c_L(ρ) functions at a physiological temperature (37 °C) to maximize time domain simulation accuracy. Odd-numbered ex vivo human thoracic vertebrae were imaged with a clinical CT-scanner (0.511 × 0.511 × 0.5 mm), then sonicated with a transducer (514 kHz) focused on the canal via the vertebral lamina. Vertebra-induced signal time shifts were extracted from pressure waveforms recorded within the canals. Measurements were repeated 5× per vertebra, with 2.5 mm vertical vertebra shifts between measurements. Linear functions relating c_L with CT-derived density were optimized. The optimized function was c_L(ρ)=0.35(ρ-ρ_w)+ c_L,w m/s, where w denotes water, giving the tested laminae a mean bulk density of 1600 ± 30 kg/m³ and a mean bulk c_L of 1670 ± 60 m/s. The optimized lamina c_L(ρ) function was accurate to λ/16 when implemented in a multi-layered ray acoustics model. This modelling accuracy will improve trans-spine ultrasound beamforming.

Trans-Spinal Focused Ultrasound Stimulation Selectively Modulates Descending Motor Pathway

Compared to current non-invasive methods utilizing magnetic and electrical means, focused ultrasound provides greater spatial resolution and penetration depth. Despite the broad application of ultrasound stimulation, there is a lack of studies dedicated to the investigation of acoustic neuromodulation on the spinal cord. This study aims to apply focused ultrasound on the spinal cord to modulate the descending pathways in a non-invasive fashion. The application of trans-spinal focused ultrasound (tsFUS) was examined on the motor deficit mouse model. tsFUS was achieved using a single-element focused ultrasound transducer operating at 3 MHz. The sonication was performed on anesthetized 6 week-old mice targeting T12 and L3 vertebrae. The effect was analyzed by comparing electromyography responses from the hindlimb induced by electrical stimulation of the motor cortex. Further, the mouse model with the Harmaline-induced essential tremor was selected to investigate the potential clinical application of tsFUS. The safety was verified by histological assessment. Sonication at the T12 area inhibited motor response, while sonication over the L3 region provided signal enhancement. Sonication of T12 of the ET mouse also showed the ability of ultrasound to suppress tremors. Meanwhile, the histological examination did not show any abnormalities with the highest applied acoustic pressure. In this work, a non-invasive motor signal modulation was achieved using tsFUS. Moreover, the results showed the ability of focused ultrasound to manage tremors in a safe manner. This study provides a stepping stone for the trans-spinal application of focused ultrasound to motor-related disorders.

Therapeutic oxygen delivery by perfluorocarbon-based colloids

After the protocol-related indecisive clinical trial of Oxygent, a perfluorooctylbromide/phospholipid nanoemulsion, in cardiac surgery, that often unduly assigned the observed untoward effects to the product, the development of perfluorocarbon (PFC)-based O₂ nanoemulsions ("blood substitutes") has come to a low. Yet, significant further demonstrations of PFC O₂-delivery efficacy have continuously been reported, such as relief of hypoxia after myocardial infarction or stroke; protection of vital organs during surgery; potentiation of O₂-dependent cancer therapies, including radio-, photodynamic-, chemo- and immunotherapies; regeneration of damaged nerve, bone or cartilage; preservation of organ grafts destined for transplantation; and control of gas supply in tissue engineering and biotechnological productions. PFC colloids capable of augmenting O₂ delivery include primarily injectable PFC nanoemulsions, microbubbles and phase-shift nanoemulsions. Careful selection of PFC and other colloid components is critical. The basics of O₂ delivery by PFC nanoemulsions will be briefly reminded. Improved knowledge of O₂ delivery mechanisms has been acquired. Advanced, size-adjustable O₂-delivering nanoemulsions have been designed that have extended room-temperature shelf-stability. Alternate O₂ delivery options are being investigated that rely on injectable PFC-stabilized microbubbles or phase-shift PFC nanoemulsions. The latter combine prolonged circulation in the vasculature, capacity for penetrating tumor tissues, and acute responsiveness to ultrasound and other external stimuli. Progress in microbubble and phase-shift emulsion engineering, control of phase-shift activation (vaporization), understanding and control of bubble/ultrasound/tissue interactions is discussed. Control of the phase-shift event and of microbubble size require utmost attention. Further PFC-based colloidal systems, including polymeric micelles, PFC-loaded organic or inorganic nanoparticles and scaffolds, have been devised that also carry substantial amounts of O₂. Local, on-demand O₂ delivery can be triggered by external stimuli, including focused ultrasound irradiation or tumor microenvironment. PFC colloid functionalization and targeting can help adjust their properties for specific indications, augment their efficacy, improve safety profiles, and expand the range of their indications. Many new medical and biotechnological applications involving fluorinated colloids are being assessed, including in the clinic. Further uses of PFC-based colloidal nanotherapeutics will be briefly mentioned that concern contrast diagnostic imaging, including molecular imaging and immune cell tracking; controlled delivery of therapeutic energy, as for noninvasive surgical ablation and sonothrombolysis; and delivery of drugs and genes, including across the blood-brain barrier. Even when the fluorinated colloids investigated are designed for other purposes than O₂ supply, they will inevitably also carry and deliver a certain amount of O₂, and may thus be considered for O₂ delivery or co-delivery applications. Conversely, O₂-carrying PFC nanoemulsions possess by nature a unique aptitude for ¹⁹F MR imaging, and hence, cell tracking, while PFC-stabilized microbubbles are ideal resonators for ultrasound contrast imaging and can undergo precise manipulation and on-demand destruction by ultrasound waves, thereby opening multiple theranostic opportunities.

Focused Ultrasound Combined with Microbubbles in Central Nervous System Applications

The blood–brain barrier (BBB) protects the central nervous system (CNS) from invasive pathogens and maintains the homeostasis of the brain. Penetrating the BBB has been a major challenge in the delivery of therapeutic agents for treating CNS diseases. Through a physical acoustic cavitation effect, focused ultrasound (FUS) combined with microbubbles achieves the local detachment of tight junctions of capillary endothelial cells without inducing neuronal damage. The bioavailability of therapeutic agents is increased only in the area targeted by FUS energy. FUS with circulating microbubbles is currently the only method for inducing precise, transient, reversible, and noninvasive BBB opening (BBBO). Over the past decade, FUS-induced BBBO (FUS-BBBO) has been preclinically confirmed to not only enhance the penetration of therapeutic agents in the CNS, but also modulate focal immunity and neuronal activity. Several recent clinical human trials have demonstrated both the feasibility and potential advantages of using FUS-BBBO in diseased patients. The promising results support adding FUS-BBBO as a multimodal therapeutic strategy in modern CNS disease management. This review article explores this technology by describing its physical mechanisms and the preclinical findings, including biological effects, therapeutic concepts, and translational design of human medical devices, and summarizes completed and ongoing clinical trials.

Technical Note: Quantification of blood-spinal cord barrier permeability after application of magnetic resonance-guided focused ultrasound in spinal cord injury

PURPOSE

To demonstrate that magnetic resonance-guided focused ultrasound (MRgFUS) facilitates blood-spinal cord barrier (BSCB) permeability and develop observer-independent MRI quantification of BSCB permeability after MRgFUS for spinal cord injury (SCI).

METHODS

Noninjured Sprague-Dawley rats (n = 3) underwent MRgFUS and were administered Evans blue post-MRgFUS to confirm BSCB opening. Absorbance was measured by spectrophotometry and correlated with its corresponding image intensity. Rats (n = 21) underwent T8-T10 laminectomy and extradural compression of the spinal cord (23g weighted aneurysm-type clip, 1 min). The intervention group (n = 11) was placed on a preclinical MRgFUS system, administered microbubbles (Optison, 0.2 mL/kg), and received 3 MRgFUS sonications (25 ms bursts, 1 Hz pulses for 3 min, 3 acoustic W, approximately 1.0-2.1 MPa peak pressure as measured via hydrophone). The sham group (n = 10) received equivalent procedures with no sonications. T1w MRI was obtained both pre- and post-MRgFUS BSCB opening. Spinal cords were segmented manually or semiautomatically and a Pearson correlation with P ≤ 0.001 was used to correlate the two segmentation methods. MRgFUS sonication and control regions intensity values were evaluated with a paired t-test with a P ≤ 0.01.

RESULTS

Semiautomatic segmentation reduced computational time by 95% and was correlated with manual segmentation (Pearson = 0.92, P < 0.001, n = 71 regions). In the noninjured rat group, Evans blue absorbance correlated with image intensity in the MRgFUS and control regions (Pearson = 0.82, P = 0.02, n = 6). In rats that underwent the SCI procedure, an increase in signal intensity in the MRgFUS targeted region relative to control was seen in all SCI rats (10.65 ± 12.4%, range: 0.96-43.9%, n = 11, P = 0.002). SCI sham MRgFUS revealed no change (0.63 ± 0.52%, 95% CI 0.320.95, n = 10). This result was significant between both groups (P = 0.003).

CONCLUSION

The implemented semiautomatic segmentation procedure improved data analysis efficiency. Quantitative methods using contrast-enhanced MRI with histological validation are sensitive for detection of blood-spinal cord barrier opening induced by magnetic resonance-guided focused ultrasound.

Focused Ultrasound-Induced Blood-Spinal Cord Barrier Opening Using Short-Burst Phase-Keying Exposures in Rats: A Parameter Study

Transient opening of the blood-spinal cord barrier has the potential to improve drug delivery options to the spinal cord. We previously developed short-burst phase-keying exposures to reduce focal depth of field and mitigate standing waves in the spinal canal. However, optimal short-burst phase-keying parameters for drug delivery have not been identified. Here, the effects of pressure, treatment duration, pulse length, burst repetition frequency and burst length on resulting tissue effects were investigated. Increased in situ pressures (0.23-0.33 MPa) led to increased post-treatment T₁-weighted contrast enhancement in magnetic resonance imaging (p = 0.015). Increased treatment duration (120 vs. 300 s) led to increased enhancement, but without statistical significance (p = 0.056). Increased burst repetition frequency (20 vs. 40 kHz) yielded a non-significant increase in enhancement (p = 0.064) but corresponded with increased damage observed on histology. No difference was observed in enhancement between pulse lengths of 2 and 10 ms (p = 0.912), corresponding with a sharp drop in the recorded second harmonic signal during the first 2 ms of the pulse. Increasing the burst length from two to five cycles (514 kHz) led to increased enhancement (p = 0.014). Results indicate that increasing the burst length may be the most effective method to enhance drug delivery. Additionally, shorter pulse lengths may allow more interleaved targets, and therefore a larger treatment volume, within one sonication.

Ultrasound in Traumatic Spinal Cord Injury: A Wide-Open Field

Traumatic spinal cord injury (SCI) is a common and devastating condition. In the absence of effective validated therapies, there is an urgent need for novel methods to achieve injury stabilization, regeneration, and functional restoration in SCI patients. Ultrasound is a versatile platform technology that can provide a foundation for viable diagnostic and therapeutic interventions in SCI. In particular, real-time perfusion and inflammatory biomarker monitoring, focal pharmaceutical delivery, and neuromodulation are capabilities that can be harnessed to advance our knowledge of SCI pathophysiology and to develop novel management and treatment options. Our review suggests that studies that evaluate the benefits and risks of ultrasound in SCI are severely lacking and our understanding of the technology's potential impact remains poorly understood. Although the complex anatomy and physiology of the spine and the spinal cord remain significant challenges, continued technological advances will help the field overcome the current barriers and bring ultrasound to the forefront of SCI research and development.

Characterization of ultrasound-mediated delivery of trastuzumab to normal and pathologic spinal cord tissue

Extensive studies on focused ultrasound (FUS)-mediated drug delivery through the blood-brain barrier have been published, yet little work has been published on FUS-mediated drug delivery through the blood-spinal cord barrier (BSCB). This work aims to quantify the delivery of the monoclonal antibody trastuzumab to rat spinal cord tissue and characterize its distribution within a model of leptomeningeal metastases. 10 healthy Sprague-Dawley rats were treated with FUS + trastuzumab and sacrificed at 2-h or 24-h post-FUS. A human IgG ELISA (Abcam) was used to measure trastuzumab concentration and a 12 ± fivefold increase was seen in treated tissue over control tissue at 2 h versus no increase at 24 h. Three athymic nude rats were inoculated with MDA-MB-231-H2N HER2 + breast cancer cells between the meninges in the thoracic region of the spinal cord and treated with FUS + trastuzumab. Immunohistochemistry was performed to visualize trastuzumab delivery, and semi-quantitative analysis revealed similar or more intense staining in tumor tissue compared to healthy tissue suggesting a comparable or greater concentration of trastuzumab was achieved. FUS can increase the permeability of the BSCB, improving drug delivery to specifically targeted regions of healthy and pathologic tissue in the spinal cord. The achieved concentrations within the healthy tissue are comparable to those reported in the brain.

In silico feasibility assessment of extracorporeal delivery of low-intensity pulsed ultrasound to intervertebral discs within the lumbar spine

Low intensity pulsed ultrasound (LIPUS) may have utility for non-invasive treatment of discogenic lower back pain through stimulating, remodeling and accelerating healing of injured or degenerated intervertebral disc (IVD) tissues. This study investigates the feasibility of delivering LIPUS to lumbar IVDs between L2 and S1 spine vertebra using a planar extracorporeal phased array (8 × 8 cm, 1024 elements, 500 kHz). Three 3D anatomical models with heterogenous tissues were generated from patient CT image sets and used in the simulation-based analysis. Time-reversal acoustic modeling techniques were applied to optimize posterior-lateral placement of the array with respect to the body to facilitate energy deposition in discrete target regions spanning the annulus fibrosus and central nucleus of each IVD. Forward acoustic and biothermal simulations were performed with time-reversal optimized array placements and driving amplitude/phase settings to predict LIPUS intensity distributions at target sites and to investigate off-target energy deposition and heating potential. Simulation results demonstrate focal intensity gain of 5-168 across all IVD targets and anatomical models, with greater average intensity gain (>50) and energy localization in posterior, posterolateral, and lateral target sites of IVDs. Localized LIPUS delivery was enhanced in thinner patient anatomies and in the high lumbar levels (L2-L3 and L3-L4). Multiple amplitude/phasing illumination patterns could be sequenced at a fixed array position for larger regional energy coverage in the IVD. Biothermal simulations demonstrated that LIPUS-appropriate exposures of 100 mW cm^-2 I_SPTA to the target disc region would result in <1 °C global peak temperature elevation for all cases. Hence, simulations suggest that spatially-precise extracorporeal delivery of therapeutically relevant LIPUS doses to discrete regions of lumbar IVDs is feasible and may be useful in clinical management of discogenic back pain.

A Porcine Model of Transvertebral Ultrasound and Microbubble-Mediated Blood-Spinal Cord Barrier Opening

Blood-spinal cord barrier opening, using focused ultrasound and microbubbles, has the potential to improve drug delivery for the treatment of spinal cord pathologies. Delivering and detecting ultrasound through the spine is a challenge for clinical translation. We have previously developed short burst, phase keying exposures, which can be used in a dual-aperture configuration to address clinical scale targeting challenges. Here we demonstrate the use of these pulses for blood-spinal cord barrier opening, in vivo in pigs.

Methods: The spinal cords of Yorkshire pigs (n=8) were targeted through the vertebral laminae, in the lower thoracic to upper lumbar region using focused ultrasound (486 kHz) and microbubbles. Four animals were treated with a combination of pulsed sinusoidal exposures (1.0-4.0 MPa, non-derated) and pulsed short burst, phase keying exposures (1.0-2.0 MPa, non-derated). Four animals were treated using ramped short burst, phase keying exposures (1.8-2.1 MPa, non-derated). A 250 kHz narrowband receiver was used to detect acoustic emissions from microbubbles. Blood-spinal cord barrier opening was assessed by the extravasation of Evans blue dye. Histological analysis of the spinal cords was used to assess tissue damage and excised vertebral samples were used in benchtop experiments.

Results: Ramped short burst, phase keying exposures successfully modified the blood-spinal cord barrier at 16/24 targeted locations, as assessed by the extravasation of Evans blue dye. At 4 of these locations, opening was confirmed with minimal adverse effects observed through histology. Transmission measurements through excised vertebrae indicated a mean transmission of (47.0 ± 7.0 %) to the target.

Conclusions: This study presents the first evidence of focused ultrasound-induced blood-spinal cord barrier opening in a large animal model, through the intact spine. This represents an important step towards clinical translation.

Ultrasound-Responsive Cavitation Nuclei for Therapy and Drug Delivery

Therapeutic ultrasound strategies that harness the mechanical activity of cavitation nuclei for beneficial tissue bio-effects are actively under development. The mechanical oscillations of circulating microbubbles, the most widely investigated cavitation nuclei, which may also encapsulate or shield a therapeutic agent in the bloodstream, trigger and promote localized uptake. Oscillating microbubbles can create stresses either on nearby tissue or in surrounding fluid to enhance drug penetration and efficacy in the brain, spinal cord, vasculature, immune system, biofilm or tumors. This review summarizes recent investigations that have elucidated interactions of ultrasound and cavitation nuclei with cells, the treatment of tumors, immunotherapy, the blood-brain and blood-spinal cord barriers, sonothrombolysis, cardiovascular drug delivery and sonobactericide. In particular, an overview of salient ultrasound features, drug delivery vehicles, therapeutic transport routes and pre-clinical and clinical studies is provided. Successful implementation of ultrasound and cavitation nuclei-mediated drug delivery has the potential to change the way drugs are administered systemically, resulting in more effective therapeutics and less-invasive treatments.

Noninvasive, Targeted Creation of Neuromyelitis Optica Pathology in AQP4-IgG Seropositive Rats by Pulsed Focused Ultrasound

Neuromyelitis optica spectrum disorders (herein called NMO) is an autoimmune disease of the CNS characterized by astrocyte injury, inflammation, and demyelination. In seropositive NMO, immunoglobulin G autoantibodies against aquaporin-4 (AQP4-IgG) cause primary astrocyte injury. A passive transfer model of NMO was developed in which spatially targeted access of AQP4-IgG into the CNS of seropositive rats was accomplished by pulsed focused ultrasound through intact skin. Following intravenous administration of microbubbles, pulsed ultrasound at 0.5 MPa peak acoustic pressure was applied using a 1 MHz transducer with 6-cm focal length. In brain, the transient opening of the blood-brain barrier (BBB) in an approximately prolate ellipsoidal volume of diameter ∼3.5 mm and length ∼44 mm allowed entry of IgG-size molecules for up to 3-6 hours. The ultrasound treatment did not cause erythrocyte extravasation or inflammation. Ultrasound treatment in AQP4-IgG seropositive rats produced localized NMO pathology in brain, with characteristic astrocyte injury, inflammation, and demyelination after 5 days. Pathology was not seen when complement was inhibited, when non-NMO human IgG was administered instead of AQP4-IgG, or in AQP4-IgG seropositive AQP4 knockout rats. NMO pathology was similarly created in cervical spinal cord in seropositive rats. These results establish a noninvasive, spatially targeted model of NMO in rats, and demonstrate that BBB permeabilization, without underlying injury or inflammation, is sufficient to create NMO pathology in AQP4-IgG seropositive rats.

First-in-human trial of blood-brain barrier opening in amyotrophic lateral sclerosis using MR-guided focused ultrasound

MR-guided focused ultrasound (MRgFUS) is an emerging technology that can accurately and transiently permeabilize the blood-brain barrier (BBB) for targeted drug delivery to the central nervous system. We conducted a single-arm, first-in-human trial to investigate the safety and feasibility of MRgFUS-induced BBB opening in eloquent primary motor cortex in four volunteers with amyotrophic lateral sclerosis (ALS). Here, we show successful BBB opening using MRgFUS as demonstrated by gadolinium leakage at the target site immediately after sonication in all subjects, which normalized 24 hours later. The procedure was well-tolerated with no serious clinical, radiologic or electroencephalographic adverse events. This study demonstrates that non-invasive BBB permeabilization over the motor cortex using MRgFUS is safe, feasible, and reversible in ALS subjects. In future, MRgFUS can be coupled with promising therapeutics providing a targeted delivery platform in ALS.

Ultrasound-Induced Blood-Spinal Cord Barrier Opening in Rabbits

The blood-spinal cord barrier (BSCB) considerably limits the delivery and efficacy of treatments for spinal cord diseases. The blood-brain barrier can be safely opened with low-intensity pulsed ultrasound when microbubbles are simultaneously administered intravenously. This technique was tested on the BSCB in a rabbit model in this work. Twenty-three segments of spinal cord were sonicated with a 1-MHz unfocused pulsed ultrasound device and compared with non-sonicated segments. BSCB disruption was assessed using Evan's blue dye (EBD) extravasation. Tolerance was assessed by histologic analysis. An increased EBD concentration indicating BSCB disruption was clearly observed in sonicated segments compared with controls (p = 0.004). On one animal, which received 10 sonications, repetitive BSCB disruptions revealed no evidence of cumulative toxicity. BSCB can be disrupted using an unfocused pulsed ultrasound device in combination with microbubbles without neurotoxicity even in case of repeated sonications.

Enhanced Detection of Bubble Emissions Through the Intact Spine for Monitoring Ultrasound-Mediated Blood-Spinal Cord Barrier Opening

OBJECTIVE

We previously developed short burst, phase keying (SBPK) focused ultrasound (FUS) to mitigate standing waves in the human vertebral canal. Here, we show microbubble emissions from these pulses can be detected through the human vertebral arch and that these pulses are effective for blood-spinal cord barrier (BSCB) opening.

METHODS

At f₀ = 514 kHz, circulating microbubbles were sonicated through ex vivo human vertebrae (60 kPa-1 MPa) using a dual-aperture approach and SBPK exposures engineered to incorporate pulse inversion (PI). Signals from a 250 kHz receiver were analyzed using PI, short-time Fourier analysis and the maximum projection over the pulse train. In rats (n = 14), SBPK FUS+microbubbles was applied to 3 locations/spinal cord at fixed pressures (∼0.20-0.47 MPa). MRI and histology were used to assess opening and tissue damage.

RESULTS

In human vertebrae between 0.2-0.4 MPa, PI amplified the microbubble/baseline ratio at f₀/2 and 2f₀ by 202 ± 40% (132-291%). This was maximal at 0.4 MPa, coinciding with the onset of broadband emissions. In vivo, opening was achieved at 40/42 locations, with mean MRI enhancement of 46 ± 32%(16%-178%). Using PI, f₀/2 was detected at 14/40 opening locations. At the highest pressures (f₀/2 present) histology showed widespread bleeding throughout the focal region. At the lowest pressures, opening was achieved without bleeding.

CONCLUSION

This study confirmed that PI can increase sensitivity to transvertebral detection of microbubble signals. Preliminary in vivo investigations show that SBPK FUS can increase BSCB permeability without tissue damage.

SIGNIFICANCE

SBPK is a clinically relevant pulse scheme and, in combination with PI, provides a means of mediating and monitoring BSCB opening noninvasively.

Focused Ultrasound for Neuromodulation

For more than 70 years, the promise of noninvasive neuromodulation using focused ultrasound has been growing while diagnostic ultrasound established itself as a foundation of clinical imaging. Significant technical challenges have been overcome to allow transcranial focused ultrasound to deliver spatially restricted energy into the nervous system at a wide range of intensities. High-intensity focused ultrasound produces reliable permanent lesions within the brain, and low-intensity focused ultrasound has been reported to both excite and inhibit neural activity reversibly. Despite intense interest in this promising new platform for noninvasive, highly focused neuromodulation, the underlying mechanism remains elusive, though recent studies provide further insight. Despite the barriers, the potential of focused ultrasound to deliver a range of permanent and reversible neuromodulation with seamless translation from bench to the bedside warrants unparalleled attention and scientific investment. Focused ultrasound boasts a number of key features such as multimodal compatibility, submillimeter steerable focusing, multifocal, high temporal resolution, coregistration, and the ability to monitor delivered therapy and temperatures in real time. Despite the technical complexity, the future of noninvasive focused ultrasound for neuromodulation as a neuroscience and clinical platform remains bright.

Analysis of Multifrequency and Phase Keying Strategies for Focusing Ultrasound to the Human Vertebral Canal

Focused ultrasound has been shown to increase the permeability of the blood-brain barrier and its feasibility for opening the blood-spinal cord barrier has also been demonstrated in small animal models, with great potential to impact the treatment of spinal cord (SC) disorders. For clinical translation, challenges to transvertebral focusing of ultrasound energy on the human spinal canal, such as a focal depth of field and standing-wave formation, must be addressed. A dual-aperture approach using multifrequency and phase-shift keying (PSK) strategies for achieving a controlled focus in human thoracic vertebrae was investigated through numerical simulations and benchtop experiments in ex vivo human vertebrae. An ~85% reduction in the focal depth of field was achieved compared to a single-aperture approach at 564 kHz. Short-burst (two-cycle) excitations in combination with PSK were found to suppress the formation of standing waves in ex vivo human thoracic vertebrae when focusing through the vertebral laminae. The results make an important contribution toward the development of a clinical-scale approach for targeting ultrasound therapy to the SC.

Laser-activated perfluorocarbon nanodroplets: a new tool for blood brain barrier opening

A major obstacle in the monitoring and treatment of neurological diseases is the blood brain barrier (BBB), a semipermeable barrier that prevents the delivery of many therapeutics and imaging contrast agents to the brain. In this work, we explored the possibility of laser-activated perfluorocarbon nanodroplets (PFCnDs) to open the BBB and deliver agents to the brain tissue. Specifically, near infrared (NIR) dye-loaded PFCnDs comprised of a perfluorocarbon (PFC) core with a boiling point above physiological temperature were repeatedly vaporized and recondensed from liquid droplet to gas bubble under pulsed laser excitation. As a result, this pulse-to-pulse repeated behavior enabled the recurring interaction of PFCnDs with the endothelial lining of the BBB, allowing for a BBB opening and extravasation of dye into the brain tissue. The blood brain barrier opening and delivery of agents to tissue was confirmed on the macro and the molecular level by evaluating Evans Blue staining, ultrasound-guided photoacoustic (USPA) imaging, and histological tissue analysis. The demonstrated PFCnD-assisted pulsed laser method for BBB opening, therefore, represents a tool that has the potential to enable non-invasive, cost-effective, and efficient image-guided delivery of contrast and therapeutic agents to the brain.

Simulating transvertebral ultrasound propagation with a multi-layered ray acoustics model

The simulation accuracy of transvertebral ultrasound propagation using a multi-layered ray acoustics model based on CT-derived vertebral geometry was investigated through comparison with experimental measurements of pressure fields in ex vivo human vertebral foramen. A spherically focused transducer (5 cm diameter, f-number 1.2, 514 kHz) was geometrically focused to the centre of individual thoracic vertebral foramen, through the posterior bony elements. Transducer propagation paths through the laminae and the spinous processes were tested. Simulation transducer-vertebra configurations were registered to experiment transducer-vertebra configurations, and simulation accuracy of the simulation model was evaluated for predicting maximum transmitted pressure to the canal, voxel pressure in the canal, and focal distortion. Accuracy in predicting maximum transmitted pressure was calculated by vertebra, and it is shown that simulation predicts maximum pressure with a greater degree of accuracy than a vertebra-specific insertion loss. Simulation error in voxel pressure was evaluated using root-mean-square error and cross-correlation, and found to be similar to the water-only case. Simulation accuracy in predicting focal distortion was evaluated by comparing experiment and simulation maximum pressure location and weighted  >50% focal volume location. Average simulation error across all measurements and simulations in maximum pressure location and weighted  >50% focal volume location were 2.3 mm and 1.5 mm, respectively. These errors are small relative to the dimensions of the transducer focus (4.9 mm full width half maximum), the spinal cord (10 mm diameter), and vertebral canal diameter (15-20 mm diameter). These results suggest that ray acoustics can be applied to simulating transvertebral ultrasound propagation.

Preliminary Investigation of Focused Ultrasound-Facilitated Drug Delivery for the Treatment of Leptomeningeal Metastases

Leptomeningeal metastases (LM) are a serious complication of cancer in the central nervous system (CNS) and are diagnosed in approximately 5% of patients with solid tumors. Effective treatment using systemically administered therapeutics is hindered by the barriers of the CNS. Ultrasound can mediate delivery of drugs through these barriers. The goal of this study was to test the feasibility of using ultrasound-mediated drug delivery to improve the treatment of LM. LM was induced in the spinal cord of athymic rats by injecting HER2-expressing breast cancer cells into the subarachnoid space of the thoracic spine. Animals were divided into three groups: no treatment (n = 5), trastuzumab only (n = 6) or trastuzumab + focused ultrasound + microbubbles (FUS + MBs) (n = 7). Animals in groups 2 and 3 were treated weekly with intravenous trastuzumab +/− FUS + MBs for three weeks. Suppression in tumor growth was qualitatively observed by MRI in the group receiving ultrasound, and was confirmed by a significant difference in the tumor volume measured from the histology data (25 ± 17 mm³ vs 8 ± 5 mm³, p = 0.04 in the trastuzumab-only vs trastuzumab + FUS + MBs). This pilot study demonstrates the potential of ultrasound-mediated drug delivery as a novel treatment for LM. Future studies will extend this work to larger cohorts and the investigation of LM arising from other cancers.