Review of Photoacoustic Imaging for Imaging-Guided Spinal Surgery

Advances in imaging modalities for spinal tumors

The spinal cord occupies a narrow region and is tightly surrounded by osseous and ligamentous structures; spinal tumors can damage this structure and deprive patients of their ability to independently perform activities of daily living. Hence, imaging is vital for the prompt detection and accurate diagnosis of spinal tumors, as well as determining the optimal treatment and follow-up plan. However, many clinicians may not be familiar with the imaging characteristics of spinal tumors due to their rarity. In addition, spinal surgeons might not fully utilize imaging for the surgical planning and management of spinal tumors because of the complex heterogeneity of these lesions. In the present review, we focus on conventional and advanced spinal tumor imaging techniques. These imaging modalities include computed tomography, positron emission tomography, digital subtraction angiography, conventional and microstructural magnetic resonance imaging, and high-resolution ultrasound. We discuss the advantages and disadvantages of conventional and emerging imaging modalities, followed by an examination of cutting-edge medical technology to complement current needs in the field of spinal tumors. Moreover, machine learning and artificial intelligence are anticipated to impact the application of spinal imaging techniques. Through this review, we discuss the importance of conventional and advanced spinal tumor imaging, and the opportunity to combine advanced technologies with conventional modalities to better manage patients with these lesions.

Monitoring angiogenesis in skin autografts using photoacoustic microscopy

BACKGROUND

Skin autografts have been broadly used to manage the skin and soft tissue defects. It is important for surgeons to assess the vitality of skin autografts via observing the angiogenesis. However, there is lack of reliable approach for giving the quantitative angiogenesis information on the skin autografts. Recently, photoacoustic microscopy imaging has attracted much attention based on its good performance in angiography.

METHODS

In this study, we aim to monitor angiogenesis in skin autografts via PAM, and further verify its clinical potential for the early prediction of skin autografts clinical outcome.

RESULTS AND CONCLUSIONS

The results indicate that PAM is a feasible, precise, high-resolution, noninvasive technique for the early prediction of necrosis of skin autografts via monitoring the angiogenesis, providing a promising tool for surgeons to use this surgical technology.

Sparse-view reconstruction for photoacoustic tomography combining diffusion model with model-based iteration

As a non-invasive hybrid biomedical imaging technology, photoacoustic tomography combines high contrast of optical imaging and high penetration of acoustic imaging. However, the conventional standard reconstruction under sparse view could result in low-quality image in photoacoustic tomography. Here, a novel model-based sparse reconstruction method for photoacoustic tomography via diffusion model was proposed. A score-based diffusion model is designed for learning the prior information of the data distribution. The learned prior information is utilized as a constraint for the data consistency term of an optimization problem based on the least-square method in the model-based iterative reconstruction, aiming to achieve the optimal solution. Blood vessels simulation data and the animal in vivo experimental data were used to evaluate the performance of the proposed method. The results demonstrate that the proposed method achieves higher-quality sparse reconstruction compared with conventional reconstruction methods and U-Net. In particular, under the extreme sparse projection (e.g., 32 projections), the proposed method achieves an improvement of ∼ 260 % in structural similarity and ∼ 30 % in peak signal-to-noise ratio for in vivo data, compared with the conventional delay-and-sum method. This method has the potential to reduce the acquisition time and cost of photoacoustic tomography, which will further expand the application range.

Tetrazine-Derived Near-Infrared Dye for Targeted Photoacoustic Imaging of Bone

A near-infrared photoacoustic probe was used to image bone in vivo through active and bioorthogonal pretargeting strategies that utilized coupling between a tetrazine-derived cyanine dye and a trans-cyclooctene-modified bisphosphonate. In vitro hydroxyapatite binding of the probe via active and pretargeting strategies showed comparable increases in percent binding vs a nontargeted control. Intrafemoral injection of the bisphosphonate-dye conjugate showed retention out to 24 h post-injection, with a 14-fold increase in signal over background, while the nontargeted dye exhibited negligible binding to bone and signal washout by 4 h post-injection. Intravenous injection, using both active and pretargeting strategies, demonstrated bone accumulation as earlier as 4 h post-injection, where the signal was found to be 3.6- and 1.5-fold higher, respectively, than the signal from the nontargeted dye. The described bone-targeted dye enabled in vivo photoacoustic imaging, while the synthetic strategy provides a convenient building block for developing new targeted photoacoustic probes.

Fast iterative reconstruction for photoacoustic tomography using learned physical model: Theoretical validation

Iterative reconstruction has demonstrated superior performance in medical imaging under compressed, sparse, and limited-view sensing scenarios. However, iterative reconstruction algorithms are slow to converge and rely heavily on hand-crafted parameters to achieve good performance. Many iterations are usually required to reconstruct a high-quality image, which is computationally expensive due to repeated evaluations of the physical model. While learned iterative reconstruction approaches such as model-based learning (MBLr) can reduce the number of iterations through convolutional neural networks, it still requires repeated evaluations of the physical models at each iteration. Therefore, the goal of this study is to develop a Fast Iterative Reconstruction (FIRe) algorithm that incorporates a learned physical model into the learned iterative reconstruction scheme to further reduce the reconstruction time while maintaining robust reconstruction performance. We also propose an efficient training scheme for FIRe, which releases the enormous memory footprint required by learned iterative reconstruction methods through the concept of recursive training. The results of our proposed method demonstrate comparable reconstruction performance to learned iterative reconstruction methods with a 9x reduction in computation time and a 620x reduction in computation time compared to variational reconstruction.

A Comprehensive Review on Photoacoustic-Based Devices for Biomedical Applications

The photoacoustic effect is an emerging technology that has sparked significant interest in the research field since an acoustic wave can be produced simply by the incidence of light on a material or tissue. This phenomenon has been extensively investigated, not only to perform photoacoustic imaging but also to develop highly miniaturized ultrasound probes that can provide biologically meaningful information. Therefore, this review aims to outline the materials and their fabrication process that can be employed as photoacoustic targets, both biological and non-biological, and report the main components' features to achieve a certain performance. When designing a device, it is of utmost importance to model it at an early stage for a deeper understanding and to ease the optimization process. As such, throughout this article, the different methods already implemented to model the photoacoustic effect are introduced, as well as the advantages and drawbacks inherent in each approach. However, some remaining challenges are still faced when developing such a system regarding its fabrication, modeling, and characterization, which are also discussed.

Nanotheranostic Strategies for Cancer Immunotherapy

Despite advancements in cancer immunotherapy, heterogeneity in tumor response impose barriers to successful treatments and accurate prognosis. Effective therapy and early outcome detection are critical as toxicity profiles following immunotherapies can severely affect patients' quality of life. Existing imaging techniques, including positron emission tomography, computed tomography, magnetic resonance imaging, or multiplexed imaging, are often used in clinics yet suffer from limitations in the early assessment of immune response. Conventional strategies to validate immune response mainly rely on the Response Evaluation Criteria in Solid Tumors (RECIST) and the modified iRECIST for immuno-oncology drug trials. However, accurate monitoring of immunotherapy efficacy is challenging since the response does not always follow conventional RECIST criteria due to delayed and variable kinetics in immunotherapy responses. Engineered nanomaterials for immunotherapy applications have significantly contributed to overcoming these challenges by improving drug delivery and dynamic imaging techniques. This review summarizes challenges in recent immune-modulation approaches and traditional imaging tools, followed by emerging developments in three-in-one nanoimmunotheranostic systems co-opting nanotechnology, immunotherapy, and imaging. In addition, a comprehensive overview of imaging modalities in recent cancer immunotherapy research and a brief outlook on how nanotheranostic platforms can potentially advance to clinical translations for the field of immuno-oncology is presented.

Dual-Modal Imaging and Synergistic Spinal Tumor Therapy Enabled by Hierarchical-Structured Nanofibers with Cascade Release and Postoperative Anti-adhesion

Most treatments for spinal cancer are accompanied by serious side effects including subsequent tumor recurrence, spinal cord compression, and tissue adhesion, thus a highly effective treatment is crucial for preserving spinal and neurological functionalities. Herein, trilayered electrospun doxorubicin@bovine serum albumin/poly(ε-caprolactone)/manganese dioxide (DOX@BSA/PCL/MnO₂) nanofibers with excellent antiadhesion ability, dual glutathione/hydrogen peroxide (GSH/H₂O₂) responsiveness, and cascade release of Mn²⁺/DOX was fabricated for realizing an efficient spinal tumor therapy. In detail, Fenton-like reactions between MnO₂ in the fibers outermost layer and intra-/extracellular glutathione within tumors promoted the first-order release of Mn²⁺. Then, sustained release of DOX from the fibers' core layer occurred along with the infiltration of degradation fluid. Such release behavior avoided toxic side effects of drugs, regulated inflammatory tumor microenvironment, amplified tumor elimination efficiency through synergistic chemo-/chemodynamic therapies, and inhibited recurrence of spinal tumors. More interestingly, magnetic resonance and photoacoustic dual-modal imaging enabled visualizations of tumor therapy and material degradation in vivo, achieving rapid pathological analysis and diagnosis. On the whole, such versatile hierarchical-structured nanofibers provided a reference for rapid and potent theranostic of spinal cancer in future clinical translations.

Multimodal Imaging and Phototherapy of Cancer and Bacterial Infection by Graphene and Related Nanocomposites

The advancements in nanotechnology and nanomedicine are projected to solve many glitches in medicine, especially in the fields of cancer and infectious diseases, which are ranked in the top five most dangerous deadly diseases worldwide by the WHO. There is great concern to eradicate these problems with accurate diagnosis and therapies. Among many developed therapeutic models, near infra-red mediated phototherapy is a non-invasive technique used to invade many persistent tumors and bacterial infections with less inflammation compared with traditional therapeutic models such as radiation therapy, chemotherapy, and surgeries. Herein, we firstly summarize the up-to-date research on graphene phototheranostics for a better understanding of this field of research. We discuss the preparation and functionalization of graphene nanomaterials with various biocompatible components, such as metals, metal oxides, polymers, photosensitizers, and drugs, through covalent and noncovalent approaches. The multifunctional nanographene is used to diagnose the disease with confocal laser scanning microscopy, magnetic resonance imaging computed tomography, positron emission tomography, photoacoustic imaging, Raman, and ToF-SMIS to visualize inside the biological system for imaging-guided therapy are discussed. Further, treatment of disease by photothermal and photodynamic therapies against different cancers and bacterial infections are carefully conferred herein along with challenges and future perspectives.

Photoacoustic-MR Image Registration Based on a Co-Sparse Analysis Model to Compensate for Brain Shift

Brain shift is an important obstacle to the application of image guidance during neurosurgical interventions. There has been a growing interest in intra-operative imaging to update the image-guided surgery systems. However, due to the innate limitations of the current imaging modalities, accurate brain shift compensation continues to be a challenging task. In this study, the application of intra-operative photoacoustic imaging and registration of the intra-operative photoacoustic with pre-operative MR images are proposed to compensate for brain deformation. Finding a satisfactory registration method is challenging due to the unpredictable nature of brain deformation. In this study, the co-sparse analysis model is proposed for photoacoustic-MR image registration, which can capture the interdependency of the two modalities. The proposed algorithm works based on the minimization of mapping transform via a pair of analysis operators that are learned by the alternating direction method of multipliers. The method was evaluated using an experimental phantom and ex vivo data obtained from a mouse brain. The results of the phantom data show about 63% improvement in target registration error in comparison with the commonly used normalized mutual information method. The results proved that intra-operative photoacoustic images could become a promising tool when the brain shift invalidates pre-operative MRI.

CuInS2 Quantum Dot and Polydimethylsiloxane Nanocomposites for All-Optical Ultrasound and Photoacoustic Imaging

Dual-modality imaging employing complementary modalities, such as all-optical ultrasound and photoacoustic imaging, is emerging as a well-suited technique for guiding minimally invasive surgical procedures. Quantum dots are a promising material for use in these dual-modality imaging devices as they can provide wavelength-selective optical absorption. The first quantum dot nanocomposite engineered for co-registered laser-generated ultrasound and photoacoustic imaging is presented. The nanocomposites developed, comprising CuInS2 quantum dots and medical-grade polydimethylsiloxane (CIS-PDMS), are applied onto the distal ends of miniature optical fibers. The films exhibit wavelength-selective optical properties, with high optical absorption (> 90%) at 532 nm for ultrasound generation, and low optical absorption (< 5%) at near-infrared wavelengths greater than 700 nm. Under pulsed laser irradiation, the CIS-PDMS films generate ultrasound with pressures exceeding 3.5 MPa, with a corresponding bandwidth of 18 MHz. An ultrasound transducer is fabricated by pairing the coated optical fiber with a Fabry-Pérot (FP) fiber optic sensor. The wavelength-selective nature of the film is exploited to enable co-registered all-optical ultrasound and photoacoustic imaging of an ink-filled tube phantom. This work demonstrates the potential for quantum dots as wavelength-selective absorbers for all-optical ultrasound generation.

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.

Prospects of Photo- and Thermoacoustic Imaging in Neurosurgery

The evolution of neurosurgery has been, and continues to be, closely associated with innovations in technology. Modern neurosurgery is wed to imaging technology and the future promises even more dependence on anatomic and, perhaps more importantly, functional imaging. The photoacoustic phenomenon was described nearly 140 yr ago; however, biomedical applications for this technology have only recently received significant attention. Light-based photoacoustic and microwave-based thermoacoustic technologies represent novel biomedical imaging modalities with broad application potential within and beyond neurosurgery. These technologies offer excellent imaging resolution while generally considered safer, more portable, versatile, and convenient than current imaging technologies. In this review, we summarize the current state of knowledge regarding photoacoustic and thermoacoustic imaging and their potential impact on the field of neurosurgery.

Deep-Learning Image Reconstruction for Real-Time Photoacoustic System

Recent advances in photoacoustic (PA) imaging have enabled detailed images of microvascular structure and quantitative measurement of blood oxygenation or perfusion. Standard reconstruction methods for PA imaging are based on solving an inverse problem using appropriate signal and system models. For handheld scanners, however, the ill-posed conditions of limited detection view and bandwidth yield low image contrast and severe structure loss in most instances. In this paper, we propose a practical reconstruction method based on a deep convolutional neural network (CNN) to overcome those problems. It is designed for real-time clinical applications and trained by large-scale synthetic data mimicking typical microvessel networks. Experimental results using synthetic and real datasets confirm that the deep-learning approach provides superior reconstructions compared to conventional methods.

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

Significance: Stem cell therapies are of interest for treating a variety of neurodegenerative diseases and injuries of the spinal cord. However, the lack of techniques for longitudinal monitoring of stem cell therapy progression is inhibiting clinical translation. Aim: The goal of this study is to demonstrate an intraoperative imaging approach to guide stem cell injection to the spinal cord in vivo. Results may ultimately support the development of an imaging tool that spans intra- or postoperative environments to guide therapy throughout treatment. Approach: Stem cells were labeled with Prussian blue nanocubes (PBNCs) to facilitate combined ultrasound and photoacoustic (US/PA) imaging to visualize stem cell injection and delivery to the spinal cord in vivo. US/PA results were confirmed by magnetic resonance imaging (MRI) and histology. Results: Real-time intraoperative US/PA image-guided injection of PBNC-labeled stem cells and three-dimensional volumetric images of injection provided feedback necessary for successful delivery of therapeutics into the spinal cord. Postoperative MRI confirmed delivery of PBNC-labeled stem cells. Conclusions: The nanoparticle-augmented US/PA approach successfully detected injection and delivery of stem cells into the spinal cord, confirmed by MRI. Our work demonstrated in vivo feasibility, which is a critical step toward the development of a US/PA/MRI platform to monitor regenerative spinal cord therapies.

CARS-imaging guidance for fs-laser ablation precision surgery

Due to ageing populations the number of tumors is increasing worldwide. Successful surgical treatment requires complete resection of tumors to reduce recurrence rates. To reach this goal, novel methods combining in vivo tumor and tumor margin detection with low invasive precision surgical tools are needed. Coherent anti-Stokes Raman scattering (CARS) imaging is a highly promising optical tool for visualizing tumors based on characteristic changes in tissue morphology and molecular composition, while fs-laser ablation is to date the most precise surgical tool established in ophthalmology. In this contribution, CARS imaging has been combined with fs-laser ablation as a new approach for image-guided precision surgery for the first time. CARS guided fs-ablation has been applied to ablate brain, liver, skin, muscular and vascular tissues with μm-precision using sub-100 fs pulses of μJ level. We demonstrate superior imaging performance and contrast as well as detection of tissue margins by coherent Raman microscopy in comparison to laser reflectance imaging. The combination of CARS-image-guided tissue ablation is a promising tool for minimally invasive surgeries particularly in the vicinity of functional structures in the future.

Photoacoustic image-guided interventions

Photoacoustic imaging has demonstrated its potential for diagnosis over the last few decades. In recent years, its unique imaging capabilities, such as detecting structural, functional and molecular information in deep regions with optical contrast and ultrasound resolution, have opened up many opportunities for photoacoustic imaging to be used during image-guided interventions. Numerous studies have investigated the capability of photoacoustic imaging to guide various interventions such as drug delivery, therapies, surgeries, and biopsies. These studies have demonstrated that photoacoustic imaging can guide these interventions effectively and non-invasively in real-time. In this minireview, we will elucidate the potential of photoacoustic imaging in guiding active and passive drug deliveries, photothermal therapy, and other surgeries and therapies using endogenous and exogenous contrast agents including organic, inorganic, and hybrid nanoparticles, as well as needle-based biopsy procedures. The advantages of photoacoustic imaging in guided interventions will be discussed. It will, therefore, show that photoacoustic imaging has great potential in real-time interventions due to its advantages over current imaging modalities like computed tomography, magnetic resonance imaging, and ultrasound imaging.