In vivo photoacoustic guidance of stem cell injection and delivery for regenerative spinal cord therapies

Dual photoacoustic/ultrasound technologies for preclinical research: current status and future trends

Photoacoustic (PA) imaging, by integrating optical and ultrasound (US) modalities, combines high spatial resolution with deep tissue penetration, making it a transformative tool in biomedical research. This review presents a comprehensive analysis of the current status of dual PA/US imaging technologies, emphasising their applications in preclinical research. It details advancements in light excitation strategies, including tomographic and microscopic modalities, innovations in pulsed laser and alternative light sources, and US instrumentation. The review further explores preclinical methodologies, encompassing dedicated instrumentation, signal processing, and data analysis techniques essential for PA/US systems. Key applications discussed include the visualisation of blood vessels, micro-circulation, and tissue perfusion; diagnosis and monitoring of inflammation; evaluation of infections, atherosclerosis, burn injuries, healing, and scar formation; assessment of liver and renal diseases; monitoring of epilepsy and neurodegenerative conditions; studies on brain disorders and preeclampsia; cell therapy monitoring; and tumour detection, staging, and recurrence monitoring. Challenges related to imaging depth, resolution, cost, and the translation of contrast agents to clinical practice are analysed, alongside advancements in high-speed acquisition, artificial intelligence-driven reconstruction, and innovative light-delivery methods. While clinical translation remains complex, this review underscores the crucial role of preclinical studies in unravelling fundamental biomedical questions and assessing novel imaging strategies. Ultimately, this review delves into the future trends of dual PA/US imaging, highlighting its potential to bridge preclinical discoveries with clinical applications and drive advances in diagnostics, therapeutic monitoring, and personalised medicine.

In Vivo Ultrasound and Photoacoustic Imaging of Nanoparticle-Engineered T Cells and Post-Treatment Assessment to Guide Adoptive Cell Immunotherapy

Despite great promise, adoptive cell therapy (ACT) continues to fail at treating a majority of cancers, especially solid tumors. To inform development and expedite the translation of more potent cellular immunotherapies, advanced immunoimaging tools are needed to better understand the in vivo requirements for generating a robust immune response. Even methods to evaluate the delivery, location, and status of transferred T cells at the tumor target are lacking. Therefore, a real-time, safe, noninvasive, longitudinal imaging method is critically needed to 1) monitor adoptive T cell location and status and 2) assess treatment progression and response through imaging biomarkers. Here, we developed a combined ultrasound (US) and photoacoustic (PA) imaging approach to enable T cell tracking following adoptive transfer for cancer immunotherapy. Our approach leverages highly photostable gold nanorods and cell surface engineering to tag the T cells without impacting effector functions, as well as generate PA contrast for imaging post-transfer. Our in vivo US/PA imaging approach detected nanoparticle-labeled T cell accumulation at the tumor, visualized changes in tumor volume, and conveyed accompanying changes in blood biomarkers. US/PA data also showed different trends according to a positive or negative antitumor response to T cell therapy over 7 days. Results highlight the potential of the approach and motivate future development to expand the platform for advanced, theranostic immunoimaging.

A comprehensive review on nanoparticle-based photo acoustic: current application and future prospective

In vivo, molecular imaging is prevalent for biology research and therapeutic practice. Among advanced imaging technologies, photoacoustic (PA) imaging and sensing is gaining interest around the globe due its exciting features like high resolution and good (~ few cm) penetration depth. PA imaging is a recent development in ultrasonic technology that generates acoustic waves by absorbing optical energy. However, poor light penetration through tissue continues to be the key obstacle in the field. The NPs as contrast agents can assist in overcoming tissue penetration depth as NPs can produce high signal to noise (SNR) PA signal which aids reconstruction of high resolution of the PA images in deep tissue sights. Subsequently, NPs are very effective in PA based targeted and precise theranostic applications. This article detail about various NPs (organic, inorganic and hybrid) used in PA imaging and spectroscopy applications including various disease diagnosis, therapy and theranostic. It also features optical property, advantages and limitations of various NPs utilised in PA techniques which would comprehend readers about the potential of NPs in evolving PA technique from laboratory to clinical modality in future.

Prussian blue nanotechnology in the treatment of spinal cord injury: application and challenges

Spinal cord injury (SCI) is a serious neurological condition that currently lacks effective treatments, placing a heavy burden on both patients and society. Prussian blue nanoparticles exhibit great potential for treating spinal cord injuries due to their excellent physicochemical properties and biocompatibility. These nanoparticles have strong anti-inflammatory and antioxidant capabilities, effectively scavenge free radicals, and reduce oxidative stress damage to cells. Prussian blue nanotechnology shows broad application potential in drug delivery, bioimaging, cancer therapy, anti-inflammatory and oxidative stress treatment, and biosensors. This article reviewed the potential applications of Prussian blue nanotechnology in treating spinal cord injuries, explored the challenges and solutions associated with its application, and discussed the future prospects of this technology in SCI treatment.

Exploring the future of regenerative medicine: Unveiling the potential of optical microscopy for structural and functional imaging of stem cells

Regenerative medicine, which utilizes stem cells for tissue and organ repair, holds immense promise in healthcare. A comprehensive understanding of stem cell characteristics is crucial to unlock their potential. This study explores the pivotal role of optical microscopy in advancing regenerative medicine as a potent tool for stem cell research. Advanced optical microscopy techniques enable an in-depth examination of stem cell behavior, morphology, and functionality. The review encompasses current optical microscopy, elucidating its capabilities and constraints in stem cell imaging, while also shedding light on emerging technologies for improved stem cell visualization. Optical microscopy, complemented by techniques like fluorescence and multiphoton imaging, enhances our comprehension of stem cell dynamics. The introduction of label-free imaging facilitates noninvasive, real-time stem cell monitoring without external dyes or markers. By pushing the boundaries of optical microscopy, researchers reveal the intricate cellular mechanisms underpinning regenerative processes, thereby advancing more effective therapeutic strategies. The current study not only outlines the future of regenerative medicine but also underscores the pivotal role of optical microscopy in both structural and functional stem cell imaging.

Imaging increased metabolism in the spinal cord in mice after middle cerebral artery occlusion

Emerging evidence indicates crosstalk between the brain and hematopoietic system following cerebral ischemia. Here, we investigated metabolism and oxygenation in the spleen and spinal cord in a transient middle cerebral artery occlusion (tMCAO) model. Sham-operated and tMCAO mice underwent [18F]fluorodeoxyglucose (FDG)-positron emission tomography (PET) to assess glucose metabolism. Naïve, sham-operated and tMCAO mice underwent multispectral optoacoustic tomography (MSOT) assisted by quantitative model-based reconstruction and unmixing algorithms for accurate mapping of oxygenation patterns in peripheral tissues at 24 h after reperfusion. We found increased [18F]FDG uptake and reduced MSOT oxygen saturation, indicating hypoxia in the thoracic spinal cord of tMCAO mice compared with sham-operated mice but not in the spleen. Reduced spleen size was observed in tMCAO mice compared with sham-operated mice ex vivo. tMCAO led to an increase in the numbers of mature T cells in femoral bone marrow tissues, concomitant with a stark reduction in these cell subsets in the spleen and peripheral blood. The combination of quantitative PET and MSOT thus enabled observation of hypoxia and increased metabolic activity in the spinal cord of tMCAO mice at 24 h after occlusion compared to sham-operated mice.

Photoacoustic Imaging and Characterization of Bone in Medicine: Overview, Applications, and Outlook

Photoacoustic techniques have shown promise in identifying molecular changes in bone tissue and visualizing tissue microstructure. This capability represents significant advantages over gold standards (i.e., dual-energy X-ray absorptiometry) for bone evaluation without requiring ionizing radiation. Instead, photoacoustic imaging uses light to penetrate through bone, followed by acoustic pressure generation, resulting in highly sensitive optical absorption contrast in deep biological tissues. This review covers multiple bone-related photoacoustic imaging contributions to clinical applications, spanning bone cancer, joint pathologies, spinal disorders, osteoporosis, bone-related surgical guidance, consolidation monitoring, and transsphenoidal and transcranial imaging. We also present a summary of photoacoustic-based techniques for characterizing biomechanical properties of bone, including temperature, guided waves, spectral parameters, and spectroscopy. We conclude with a future outlook based on the current state of technological developments, recent achievements, and possible new directions.

Ultrasonic photoacoustic emitter of graphene-nanocomposites film on a flexible substrate

Photoacoustic devices generating high-amplitude and high-frequency ultrasounds are attractive candidates for medical therapies and on-chip bio-applications. Here, we report the photoacoustic response of graphene nanoflakes - Polydimethylsiloxane composite. A protocol was developed to obtain well-dispersed graphene into the polymer, without the need for surface functionalization, at different weight percentages successively spin-coated onto a Polydimethylsiloxane substrate. We found that the photoacoustic amplitude scales up with optical absorption reaching 11 MPa at ∼ 228 mJ/cm² laser fluence. We observed a deviation of the pressure amplitude from the linearity increasing the laser fluence, which indicates a decrease of the Grüneisen parameter. Spatial confinement of high amplitude (> 40 MPa, laser fluence > 55 mJ/cm²) and high frequency (Bw-6db ∼ 21.5 MHz) ultrasound was achieved by embedding the freestanding film in an optical lens. The acoustic gain promotes the formation of cavitation microbubbles for moderate fluence in water and in tissue-mimicking material. Our results pave the way for novel photoacoustic medical devices and integrated components.

Video-Rate Dual-Modal Wide-Beam Harmonic Ultrasound and Photoacoustic Computed Tomography

Dual-modal ultrasound (US) and photoacoustic (PA) imaging has tremendous advantages in biomedical applications, such as pharmacokinetics, cancer screening, and imaging-guided therapy. Compared with ring-shaped arrays, a linear piezoelectric transducer array applies to more anatomical sites and has been widely used in US/PA imaging. However, the linear array may limit the imaging quality due to narrow bandwidth, partial detection view, or sparse spatial sampling. To meet clinic demand of high-quality US/PA imaging with the linear transducer, we develop dual-modal wide-beam harmonic ultrasound (WBHUS) and photoacoustic computed tomography at video rate. The harmonic US imaging employs pulse phase inversion to reduce clutters and improve spatial resolution. Wide-beam US transmission can shorten the scanning times by 267% and enables a 20-Hz imaging rate, which can minimize motion artifacts in in vivo imaging. The harmonic US imaging does not only provide accurate anatomical references for locating PA features but also reduces artifacts in PA images. The improved image quality allows us to acquire high-resolution anatomical structures in deep tissue without labeling. The fast-imaging speed enables visualizing interventional procedures and monitoring the pulsations of the thoracic aorta and radial artery in real-time. The video-rate dual-modal harmonic US and single-shot PA computed tomography use a clinical-grade linear-array transducer and thus can be readily implemented in clinical US imaging.

Magnetic particles in motion: magneto-motive imaging and sensing

Superparamagnetic nanoparticles have become an important tool in biomedicine. Their biocompatibility, controllable small size, and magnetic properties allow manipulation with an external magnetic field for a variety of diagnostic and therapeutic applications. Recently, the magnetically-induced motion of superparamagnetic nanoparticles has been investigated as a new source of imaging contrast. In magneto-motive imaging, an external, time-varying magnetic field is applied to move a magnetically labeled subject, such as labeled cells or tissue. Several major imaging modalities such as ultrasound, photoacoustic imaging, optical coherence tomography, and laser speckle tracking can utilize magneto-motive contrast to monitor biological events at smaller scales with enhanced contrast and sensitivity. In this review article, an overview of magneto-motive imaging techniques is presented, including synthesis of superparamagnetic nanoparticles, fundamental principles of magneto-motive force and its utility to excite labeled tissue within a viscoelastic medium, current capabilities of magneto-motive imaging modalities, and a discussion of the challenges and future outlook in the magneto-motive imaging domain.

Visualizing stem cells in vivo using magnetic resonance imaging

Stem cell (SC) therapies displayed encouraging efficacy and clinical outcome in various disorders. Despite this huge hype, clinical translation of SC therapy has been disheartening due to contradictory results from clinical trials. The ability to monitor migration and engraftment of cells in vivo represents an ideal strategy in cell therapy. Therefore, suitable imaging approach to track MSCs would allow understanding of migratory and homing efficiency, optimal route of delivery and engraftment of cells at targeted location. Hence, longitudinal tracking of SCs is crucial for the optimization of treatment parameters, leading to improved clinical outcome and translation. Magnetic resonance imaging (MRI) represents a suitable imaging modality to observe cells non-invasively and repeatedly. Tracking is achieved when cells are incubated prior to implantation with appropriate contrast agents (CA) or tracers which can then be detected in an MRI scan. This review explores and emphasizes the importance of monitoring the distribution and fate of SCs post-implantation using current contrast agents, such as positive CAs including paramagnetic metals (gadolinium), negative contrast agents such as superparamagnetic iron oxides and ¹⁹ F containing tracers, specifically for the in vivo tracking of MSCs using MRI. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Therapeutic Approaches and Drug Discovery > Emerging Technologies.

Contrast agents for photoacoustic imaging: a review of stem cell tracking

With the advent of stem cell therapy for spinal cord injuries, stroke, burns, macular degeneration, heart diseases, diabetes, rheumatoid arthritis and osteoarthritis; the need to track the survival, migration pathways, spatial destination and differentiation of transplanted stem cells in a clinical setting has gained increased relevance. Indeed, getting regulatory approval to use these therapies in the clinic depends on biodistribution studies. Although optoacoustic imaging (OAI) or photoacoustic imaging can detect functional information of cell activities in real-time, the selection and application of suitable contrast agents is essential to achieve optimal sensitivity and contrast for sensing at clinically relevant depths and can even provide information about molecular activity. This review explores OAI methodologies in conjunction with the specific application of exogenous contrast agents in comparison to other imaging modalities and describes the properties of exogenous contrast agents for quantitative and qualitative monitoring of stem cells. Specific characteristics such as biocompatibility, the absorption coefficient, and surface functionalization are compared and how the labelling efficiency translates to both short and long-term visualization of mesenchymal stem cells is explored. An overview of novel properties of recently developed optoacoustic contrast agents and their capability to detect disease and recovery progression in clinical settings is provided which includes newly developed exogenous contrast agents to monitor stem cells in real-time for multimodal sensing.

Iron Oxide Nanoparticles in Regenerative Medicine and Tissue Engineering

In recent years, many promising nanotechnological approaches to biomedical research have been developed in order to increase implementation of regenerative medicine and tissue engineering in clinical practice. In the meantime, the use of nanomaterials for the regeneration of diseased or injured tissues is considered advantageous in most areas of medicine. In particular, for the treatment of cardiovascular, osteochondral and neurological defects, but also for the recovery of functions of other organs such as kidney, liver, pancreas, bladder, urethra and for wound healing, nanomaterials are increasingly being developed that serve as scaffolds, mimic the extracellular matrix and promote adhesion or differentiation of cells. This review focuses on the latest developments in regenerative medicine, in which iron oxide nanoparticles (IONPs) play a crucial role for tissue engineering and cell therapy. IONPs are not only enabling the use of non-invasive observation methods to monitor the therapy, but can also accelerate and enhance regeneration, either thanks to their inherent magnetic properties or by functionalization with bioactive or therapeutic compounds, such as drugs, enzymes and growth factors. In addition, the presence of magnetic fields can direct IONP-labeled cells specifically to the site of action or induce cell differentiation into a specific cell type through mechanotransduction.

Combined Ultrasound and Photoacoustic Image Guidance of Spinal Pedicle Cannulation Demonstrated With Intact ex vivo Specimens

OBJECTIVE

Spinal fusion surgeries require accurate placement of pedicle screws in anatomic corridors without breaching bone boundaries. We are developing a combined ultrasound and photoacoustic image guidance system to avoid pedicle screw misplacement and accidental bone breaches, which can lead to nerve damage.

METHODS

Pedicle cannulation was performed on a human cadaver, with co-registered photoacoustic and ultrasound images acquired at various time points during the procedure. Bony landmarks obtained from coherence-based ultrasound images of lumbar vertebrae were registered to post-operative CT images. Registration methods were additionally tested on an ex vivo caprine vertebra.

RESULTS

Locally weighted short-lag spatial coherence (LW-SLSC) ultrasound imaging enhanced the visualization of bony structures with generalized contrast-to-noise ratios (gCNRs) of 0.99 and 0.98-1.00 in the caprine and human vertebrae, respectively. Short-lag spatial coherence (SLSC) and amplitude-based delay-and-sum (DAS) ultrasound imaging generally produced lower gCNRs of 0.98 and 0.84, respectively, in the caprine vertebra and 0.84-0.93 and 0.34-0.99, respectively, in the human vertebrae. The mean ± standard deviation of the area of -6 dB contours created from DAS photoacoustic images acquired with an optical fiber inserted in prepared pedicle holes (i.e., fiber surrounded by cancellous bone) and holes created after intentional breaches (i.e., fiber exposed to cortical bone) was 10.06 ±5.22 mm ² and 2.47 ±0.96 mm ², respectively (p 0.01).

CONCLUSIONS

Coherence-based LW-SLSC and SLSC beamforming improved visualization of bony anatomical landmarks for ultrasound-to-CT registration, while amplitude-based DAS beamforming successfully distinguished photoacoustic signals within the pedicle from less desirable signals characteristic of impending bone breaches.

SIGNIFICANCE

These results are promising to improve visual registration of ultrasound and photoacoustic images with CT images, as well as to assist surgeons with identifying and avoiding impending bone breaches during pedicle cannulation in spinal fusion surgeries.

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.

Photoacoustic-guided surgery from head to toe [Invited]

Photoacoustic imaging-the combination of optics and acoustics to visualize differences in optical absorption - has recently demonstrated strong viability as a promising method to provide critical guidance of multiple surgeries and procedures. Benefits include its potential to assist with tumor resection, identify hemorrhaged and ablated tissue, visualize metal implants (e.g., needle tips, tool tips, brachytherapy seeds), track catheter tips, and avoid accidental injury to critical subsurface anatomy (e.g., major vessels and nerves hidden by tissue during surgery). These benefits are significant because they reduce surgical error, associated surgery-related complications (e.g., cancer recurrence, paralysis, excessive bleeding), and accidental patient death in the operating room. This invited review covers multiple aspects of the use of photoacoustic imaging to guide both surgical and related non-surgical interventions. Applicable organ systems span structures within the head to contents of the toes, with an eye toward surgical and interventional translation for the benefit of patients and for use in operating rooms and interventional suites worldwide. We additionally include a critical discussion of complete systems and tools needed to maximize the success of surgical and interventional applications of photoacoustic-based technology, spanning light delivery, acoustic detection, and robotic methods. Multiple enabling hardware and software integration components are also discussed, concluding with a summary and future outlook based on the current state of technological developments, recent achievements, and possible new directions.

A Trimodal Ultrasound, Photoacoustic and Magnetic Resonance Imaging Approach for Longitudinal Post-operative Monitoring of Stem Cells in the Spinal Cord

Longitudinal monitoring of stem cells in the spinal cord could unveil critical information needed to understand regenerative processes, thereby expediting therapy development and translation. We introduce a post-operative trimodal imaging approach to monitor stem cells in the spinal cord over time. A key aspect of the approach is to label the stem cells with Prussian blue nanocubes (PBNCs), which simultaneously possess optical and magnetic properties for ultrasound-guided photoacoustic (US/PA) and magnetic resonance imaging (MRI) contrast. PBNC-Labeled stem cells were injected into the spinal cord of immunodeficient rats and tracked with US/PA imaging and MRI up to 14 d post-injection. Good agreement was observed between imaging modalities in vivo. Our results suggest that further development of the US/PA/MR imaging approach may create a powerful tool to aid development of regenerative therapies of the spinal cord, and the non-invasive imaging approach can ultimately be deployed in intra- and post-operative environments.