PHOTOACOUSTIC IMAGING FOR MEDICAL DIAGNOSTICS

Vascular-centric mapping of in vivo blood oxygen saturation in preclinical models

Assessing intravascular blood oxygen saturation (SO₂) is crucial for characterizing in vivo microenvironmental changes in preclinical models of injury and disease. However, most conventional optical imaging techniques for mapping in vivo SO₂ assume or compute a single value of the optical path-length in tissue. This is especially detrimental when mapping in vivo SO₂ in experimental disease or wound healing models that are characterized by vascular and tissue remodeling. Therefore, to circumvent this limitation we developed an in vivo SO₂ mapping technique that utilizes hemoglobin-based intrinsic optical signal (IOS) imaging combined with a vascular-centric estimation of optical path-lengths. In vivo arterial and venous SO₂ distributions derived with this approach closely matched those reported in the literature, while those derived using the single path-length (i.e. conventional) approach did not. Moreover, in vivo cerebrovascular SO₂ strongly correlated (R² > 0.7) with changes in systemic SO₂ measured with a pulse oximeter during hypoxia and hyperoxia paradigms. Finally, in a calvarial bone healing model, in vivo SO₂ assessed over four weeks was spatiotemporally correlated with angiogenesis and osteogenesis (R² > 0.6). During the early stages of bone healing (i.e. day 10), angiogenic vessels surrounding the calvarial defect exhibited mean SO₂ that was elevated by10 % (p < 0.05) relative to that observed at a later stage (i.e., day 26), indicative of their role in osteogenesis. These correlations were not evident with the conventional SO₂ mapping approach. The feasibility of our wide field-of-view in vivo SO₂ mapping approach illustrates its potential for characterizing the microvascular environment in applications ranging from tissue engineering to cancer.

The role of imaging in targeted delivery of nanomedicine for cancer therapy

Nanomedicines overcome the pharmacokinetic limitations of traditional drug formulations and have promising prospect in cancer treatment. However, nanomedicine delivery in vivo is still facing challenges from the complex physiological environment. For the purpose of effective tumor therapy, they should be designed to guarantee the five features principle, including long blood circulation, efficient tumor accumulation, deep matrix penetration, enhanced cell internalization and accurate drug release. To ensure the excellent performance of the designed nanomedicine, it would be better to monitor the drug delivery process as well as the therapeutic effects by real-time imaging. In this review, we summarize strategies in developing nanomedicines for efficiently meeting the five features of drug delivery, and the role of several imaging modalities (fluorescent imaging (FL), magnetic resonance imaging (MRI), computed tomography (CT), photoacoustic imaging (PAI), positron emission tomography (PET), and electron microscopy) in tracing drug delivery and therapeutic effect in vivo based on five features principle.

In vivo photoacoustic image-guided tumor photothermal therapy and real-time temperature monitoring using a core-shell polypyrrole@CuS nanohybrid

Near-infrared (NIR) laser triggered theranostic platforms are increasingly used in clinical nanomedicine applications. In this work, a core-shell composite consisting of polypyrrole (PPy) coated copper sulfide (CuS) nanospheres with high photothermal efficiency and good photostability has been fabricated via a facile interfacial polymerization. The PPy@CuS nanohybrid had a hydrodynamic diameter of 58.5 nm with a CuS core and PPy shell and exhibited strong optical absorption and photon-to-heat conversion in the NIR region, leading to a sufficient photohyperthermic effect under irradiation with a 808 nm continuous wave laser. In vivo studies showed that the Ppy@CuS nanohybrids produced significant photoacoustic signals and exhibited remarkable photothermal therapeutic efficacy. Furthermore, the core-shell composites exhibited improved temperature elevation and photostability. The temperature-induced changes can be detected and monitored using photoacoustic imaging, thus allowing the control of the thermal dose while minimizing photothermal damage to surrounding healthy tissues. In summary, this study demonstrates that this novel platform could potentially be used for photoacoustic image-guided photothermal therapy and real-time temperature monitoring in cancer theranostics.

Targeting Cancer Stem Cells: Therapeutic and diagnostic strategies by the virtue of nanoparticles

Cancer stem cells (CSCs) are the subpopulation of cells present within a tumor with the properties of self-renewing, differentiating, and proliferating. Owing to the presence of ATP-binding cassette drug pumps and increased expression of anti-apoptotic proteins, the conventional chemotherapeutic agents have failed to eliminate CSCs resulting in relapse and resistance of cancer. Therefore, to obtain long-lasting clinical responses and avoid the recurrence of cancer, it is crucial to develop an efficient strategy targeting CSCs by either employing a differentiation therapy or specifically delivering drugs to CSCs. Several intracellular and extracellular cancer specific biomarkers are overexpressed by CSCs and are utilized as targets for the development of new approaches in the diagnosis and treatment of CSCs. Moreover, several nanostructured particles, alone or in combination with current treatment approaches, have been used to improve the detection, imaging, and targeting of CSCs, thus addressing the limitations of cancer therapies. Targeting CSC surface markers, stemness-related signaling pathways, and tumor microenvironmental signals has improved the detection and eradication of CSCs and, therefore, tumor diagnosis and treatment. This review summarizes a variety of promising nanoparticles targeting the surface biomarkers of CSCs for the detection and eradication of tumor-initiating stem cells, used in combination with other treatment regimens.

Gold nanoparticles conjugated with DNA aptamer for photoacoustic detection of human matrix metalloproteinase-9

Matrix metalloproteinase-9 (MMP-9) plays major roles in extracellular matrix (ECM) remodeling and membrane protein cleavage, suggesting a high correlation with cancer cell invasion and tumor metastasis. Here, we present a contrast agent based on a DNA aptamer that can selectively target human MMP-9 in the tumor microenvironment (TME) with high affinity and sensitivity. Surface modification of plasmonic gold nanospheres with the MMP-9 aptamer and its complementary sequences allows the nanospheres to aggregate in the presence of human MMP-9 through DNA displacement and hybridization. Aggregation of gold nanospheres enhances the optical absorption in the first near-infrared window (NIR-I) due to the plasmon coupling effect, thereby allowing us to detect the aggregated gold nanospheres within the TME via ultrasound-guided photoacoustic (US/PA) imaging. Selective and sensitive detection of human MMP-9 via US/PA imaging is demonstrated in solution of nanosensors with the pre-treatment of human MMP-9, in vitro in cell culture, and in vivo in a xenograft murine model of human breast cancer.

Recent Advances in Photosensitizers as Multifunctional Theranostic Agents for Imaging-Guided Photodynamic Therapy of Cancer

In recent years tremendous effort has been invested in the field of cancer diagnosis and treatment with an overall goal of improving cancer management, therapeutic outcome, patient survival, and quality of life. Photodynamic Therapy (PDT), which works on the principle of light-induced activation of photosensitizers (PS) leading to Reactive Oxygen Species (ROS) mediated cancer cell killing has received increased attention as a promising alternative to overcome several limitations of conventional cancer therapies. Compared to conventional therapies, PDT offers the advantages of selectivity, minimal invasiveness, localized treatment, and spatio-temporal control which minimizes the overall therapeutic side effects and can be repeated as needed without interfering with other treatments and inducing treatment resistance. Overall PDT efficacy requires proper planning of various parameters like localization and concentration of PS at the tumor site, light dose, oxygen concentration and heterogeneity of the tumor microenvironment, which can be achieved with advanced imaging techniques. Consequently, there has been tremendous interest in the rationale design of PS formulations to exploit their theranostic potential to unleash the imperative contribution of medical imaging in the context of successful PDT outcomes. Further, recent advances in PS formulations as activatable phototheranostic agents have shown promising potential for finely controlled imaging-guided PDT due to their propensity to specifically turning on diagnostic signals simultaneously with photodynamic effects in response to the tumor-specific stimuli. In this review, we have summarized the recent progress in the development of PS-based multifunctional theranostic agents for biomedical applications in multimodal imaging combined with PDT. We also present the role of different imaging modalities; magnetic resonance, optical, nuclear, acoustic, and photoacoustic in improving the pre-and post-PDT effects. We anticipate that the information presented in this review will encourage future development and design of PSs for improved image-guided PDT for cancer treatment.

Automatic threshold selection algorithm to distinguish a tissue chromophore from the background in photoacoustic imaging

The adaptive matched filter (AMF) is a method widely used in spectral unmixing to classify different tissue chromophores in photoacoustic images. However, a threshold needs to be applied to the AMF detection image to distinguish the desired tissue chromophores from the background. In this study, we propose an automatic threshold selection (ATS) algorithm capable of differentiating a target from the background, based on the features of the AMF detection image. The mean difference between the estimated thickness, using the ATS algorithm, and the known values was 0.17 SD (0.24) mm for the phantom inclusions and -0.05 SD (0.21) mm for the tissue samples of malignant melanoma. The evaluation shows that the thickness and the width of the phantom inclusions and the tumors can be estimated using AMF in an automatic way after applying the ATS algorithm.

Micro-Bio-Chemo-Mechanical-Systems: Micromotors, Microfluidics, and Nanozymes for Biomedical Applications

Wireless nano-/micromotors powered by chemical reactions and/or external fields generate motive forces, perform tasks, and significantly extend short-range dynamic responses of passive biomedical microcarriers. However, before micromotors can be translated into clinical use, several major problems, including the biocompatibility of materials, the toxicity of chemical fuels, and deep tissue imaging methods, must be solved. Nanomaterials with enzyme-like characteristics (e.g., catalase, oxidase, peroxidase, superoxide dismutase), that is, nanozymes, can significantly expand the scope of micromotors' chemical fuels. A convergence of nanozymes, micromotors, and microfluidics can lead to a paradigm shift in the fabrication of multifunctional micromotors in reasonable quantities, encapsulation of desired subsystems, and engineering of FDA-approved core-shell structures with tuneable biological, physical, chemical, and mechanical properties. Microfluidic methods are used to prepare stable bubbles/microbubbles and capsules integrating ultrasound, optoacoustic, fluorescent, and magnetic resonance imaging modalities. The aim here is to discuss an interdisciplinary approach of three independent emerging topics: micromotors, nanozymes, and microfluidics to creatively: 1) embrace new ideas, 2) think across boundaries, and 3) solve problems whose solutions are beyond the scope of a single discipline toward the development of micro-bio-chemo-mechanical-systems for diverse bioapplications.

Towards non-contact photoacoustic imaging [review]

Photoacoustic imaging (PAI) takes advantage of both optical and ultrasound imaging properties to visualize optical absorption with high resolution and contrast. Photoacoustic microscopy (PAM) is usually categorized with all-optical microscopy techniques such as optical coherence tomography or confocal microscopes. Despite offering high sensitivity, novel imaging contrast, and high resolution, PAM is not generally an all-optical imaging method unlike the other microscopy techniques. One of the significant limitations of photoacoustic microscopes arises from their need to be in physical contact with the sample through a coupling media. This physical contact, coupling, or immersion of the sample is undesirable or impractical for many clinical and pre-clinical applications. This also limits the flexibility of photoacoustic techniques to be integrated with other all-optical imaging microscopes for providing complementary imaging contrast. To overcome these limitations, several non-contact photoacoustic signal detection approaches have been proposed. This paper presents a brief overview of current non-contact photoacoustic detection techniques with an emphasis on all-optical detection methods and their associated physical mechanisms.

Photoacoustic Imaging Quantifies Drug Release from Nanocarriers via Redox Chemistry of Dye-Labeled Cargo

We report a new approach to monitor drug release from nanocarriers via a paclitaxel-methylene blue conjugate (PTX-MB) with redox activity. This construct is in a photoacoustically silent reduced state inside poly(lactic-co-glycolic acid) (PLGA) nanoparticles (PTX-MB@PLGA NPs). During release, PTX-MB is spontaneously oxidized to produce a concentration-dependent photoacoustic signal. An in vitro drug-release study showed an initial burst release (25 %) between 0-24 h and a sustained release between 24-120 h with a cumulative release of 40.6 % and a 670-fold increase in photoacoustic signal. An in vivo murine drug release showed a photoacoustic signal enhancement of up to 649 % after 10 hours. PTX-MB@PLGA NPs showed an IC50 of 78 μg mL-1 and 44.7±4.8 % decrease of tumor burden in an orthotopic model of colon cancer via luciferase-positive CT26 cells.

Carbon nanotubes: An effective platform for biomedical electronics

Cylindrical fullerenes (or carbon nanotubes (CNTs)) have been extensively investigated as potential sensor platforms due to effective and practical manipulation of their physical and chemical properties by functionalization/doping with chemical groups suitable for novel nanocarrier systems. CNTs play a significant role in biomedical applications due to rapid development of synthetic methods, structural integration, surface area-controlled heteroatom doping, and electrical conductivity. This review article comprehensively summarized recent trends in biomedical science and technologies utilizing a promising nanomaterial of CNTs in disease diagnosis and therapeutics, based on their biocompatibility and significance in drug delivery, implants, and bio imaging. Biocompatibility of CNTs is essential for designing effective and practical electronic applications in the biomedical field particularly due to their growing potential in the delivery of anticancer agents. Furthermore, functionalized CNTs have been shown to exhibit advanced electrochemical properties, responsible for functioning of numerous oxidase and dehydrogenase based amperometric biosensors. Finally, faster signal transduction by CNTs allows charge transfer between underlying electrode and redox centres of biomolecules (enzymes).

Biosensors for Epilepsy Management: State-of-Art and Future Aspects

Epilepsy is a serious neurological disorder which affects every aspect of patients’ life, including added socio-economic burden. Unfortunately, only a few suppressive medicines are available, and a complete cure for the disease has not been found yet. Excluding the effectiveness of available therapies, the timely detection and monitoring of epilepsy are of utmost priority for early remediation and prevention. Inability to detect underlying epileptic signatures at early stage causes serious damage to the central nervous system (CNS) and irreversible detrimental variations in the organ system. Therefore, development of a multi-task solving novel smart biosensing systems is urgently required. The present review highlights advancements in state-of-art biosensing technology investigated for epilepsy diseases diagnostics and progression monitoring or both together. State of art epilepsy biosensors are composed of nano-enabled smart sensing platform integrated with micro/electronics and display. These diagnostics systems provide bio-information needed to understand disease progression and therapy optimization timely. The associated challenges related to the development of an efficient epilepsy biosensor and vision considering future prospects are also discussed in this report. This review will serve as a guide platform to scholars for understanding and planning of future research aiming to develop a smart bio-sensing system to detect and monitor epilepsy for point-of-care (PoC) applications.

Photoacoustic imaging to assess pixel-based sO2 distributions in experimental prostate tumors

A protocol for photoacoustic imaging (PAI) has been developed to assess pixel-based oxygen saturation (sO2) distributions of experimental tumor models. The protocol was applied to evaluate the dependence of PAI results on measurement settings, reproducibility of PAI, and for the characterization of the oxygenation status of experimental prostate tumor sublines (Dunning R3327-H, -HI, -AT1) implanted subcutaneously in male Copenhagen rats. The three-dimensional (3-D) PA data employing two wavelengths were used to estimate sO2 distributions. If the PA signal was sufficiently strong, the distributions were independent from signal gain, threshold, and positioning of animals. Reproducibility of sO2 distributions with respect to shape and median values was demonstrated over several days. The three tumor sublines were characterized by the shapes of their sO2 distributions and their temporal response after external changes of the oxygen supply (100% O2 or air breathing and clamping of tumor-supplying artery). The established protocol showed to be suitable for detecting temporal changes in tumor oxygenation as well as differences in oxygenation between tumor sublines. PA results were in accordance with histology for hypoxia, perfusion, and vasculature. The presented protocol for the assessment of pixel-based sO2 distributions provides more detailed information as compared to conventional region-of-interest-based analysis of PAI, especially with respect to the detection of temporal changes and tumor heterogeneity.

cis-Platinum pro-drug-attached CuFeS2 nanoplates for in vivo photothermal/photoacoustic imaging and chemotherapy/photothermal therapy of cancer

Photothermal therapy (PTT) has attracted considerable attention in cancer treatment. Herein, the facile synthesis of copper iron sulfide (chalcopyrite, CuFeS₂) nanoplates (NPs) with well-defined shape was achieved by a template-mediated method. Chitosan (CS), a linear cationic polysaccharide, was used to improve the physiological stability and biocompatibility. CuFeS₂ NPs with strong near-infrared (NIR) absorbance enabled contrasts in photothermal and photoacoustic (PA) imaging. In vitro and in vivo tumor ablation studies further demonstrated that CS-functionalized CuFeS₂ (CuFeS₂-CS) NPs could convert 808 nm NIR light into heat for PTT with a photothermal conversion efficiency up to 30.5%, which was clearly higher than that of CuS NPs (only 21.4%). Furthermore, CuFeS₂-CS NPs could also load cis-platinum pro-drug (CuFeS₂-CS-Pt), and CuFeS₂-CS-Pt showed a better synergistic therapeutic effect with respect to either chemotherapy or PTT.

Ultrasound-guided spectral photoacoustic imaging of hemoglobin oxygenation during development

Few technologies are capable of imaging in vivo function during development. In this study, we have implemented spectral photoacoustic imaging to estimate tissue oxygenation longitudinally in pregnant mice. We used the spectral photoacoustic signal to estimate hemoglobin oxygen saturation within intact, in vivo mouse concepti from developmental day (E) 8.5 to E16.5-a first step towards functional imaging of the maternal-fetal environment. Future work will apply these methods to compare longitudinal functional changes during normal vs abnormal development of embryos, fetuses, and placentas.

Development of a mathematical model to estimate intra-tumor oxygen concentrations through multi-parametric imaging

Background

Tumor hypoxia is involved in every stage of solid tumor development: formation, progression, metastasis, and apoptosis. Two types of hypoxia exist in tumors—chronic hypoxia and acute hypoxia. Recent studies indicate that the regional hypoxia kinetics is closely linked to metastasis and therapeutic responses, but regional hypoxia kinetics is hard to measure. We propose a novel approach to determine the local pO₂ by fusing the parameters obtained from in vivo functional imaging through the use of a modified multivariate Krogh model.

Methods

To test our idea and its potential to translate into an in vivo setting through the use of existing imaging techniques, simulation studies were performed comparing the local partial oxygen pressure (pO₂) from the proposed multivariate image fusion model to the referenced pO₂ derived by Green’s function, which considers the contribution from every vessel segment of an entire three-dimensional tumor vasculature to profile tumor oxygen with high spatial resolution.

Results

pO₂ derived from our fusion approach were close to the referenced pO₂ with regression slope near 1.0 and an r² higher than 0.8 if the voxel size (or the spatial resolution set by functional imaging modality) was less than 200 μm. The simulation also showed that the metabolic rate, blood perfusion, and hemoglobin concentration were dominant factors in tissue oxygenation. The impact of the measurement error of functional imaging to the pO₂ precision and accuracy was simulated. A Gaussian error function with FWHM equal to 20 % of blood perfusion or fractional vascular volume measurement contributed to average 7 % statistical error in pO₂.

Conclusion

The simulation results indicate that the fusion of multiple parametric maps through the biophysically derived mathematical models can monitor the intra-tumor spatial variations of hypoxia in tumors with existing imaging methods, and the potential to further investigate different forms of hypoxia, such as chronic and acute hypoxia, in response to cancer therapies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12938-016-0235-5) contains supplementary material, which is available to authorized users.

Advances in Imaging Techniques and Genetically Encoded Probes for Photoacoustic Imaging

Photoacoustic (PA) imaging is a rapidly emerging biomedical imaging modality that is capable of visualizing cellular and molecular functions with high detection sensitivity and spatial resolution in deep tissue. Great efforts and progress have been made on the development of various PA imaging technologies with improved resolution and sensitivity over the past two decades. Various PA probes with high contrast have also been extensively developed, with many important biomedical applications. In comparison with chemical dyes and nanoparticles, genetically encoded probes offer easier labeling of defined cells within tissues or proteins of interest within a cell, have higher stability in vivo, and eliminate the need for delivery of exogenous substances. Genetically encoded probes have thus attracted increasing attention from researchers in engineering and biomedicine. In this review, we aim to provide an overview of the existing PA imaging technologies and genetically encoded PA probes, and describe further improvements in PA imaging techniques and the near-infrared photochromic protein BphP1, the most sensitive genetically encoded probe thus far, as well as the potential biomedical applications of BphP1-based PA imaging in vivo.

Visualization of molecular composition and functionality of cancer cells using nanoparticle-augmented ultrasound-guided photoacoustics

Assessment of molecular signatures of tumors in addition to their anatomy and morphology is desired for effective diagnostic and therapeutic procedures. Development of in vivo imaging techniques that can identify and monitor molecular composition of tumors remains an important challenge in pre-clinical research and medical practice. Here we present a molecular photoacoustic imaging technique that can visualize the presence and activity of an important cancer biomarker - epidermal growth factor receptor (EGFR), utilizing the effect of plasmon resonance coupling between molecular targeted gold nanoparticles. Specifically, spectral analysis of photoacoustic images revealed profound changes in the optical absorption of systemically delivered EGFR-targeted gold nanospheres due to their molecular interactions with tumor cells overexpressing EGFR. In contrast, no changes in optical properties and, therefore, photoacoustic signal, were observed after systemic delivery of non-targeted gold nanoparticles to the tumors. The results indicate that multi-wavelength photoacoustic imaging augmented with molecularly targeted gold nanoparticles has the ability to monitor molecular specific interactions between nanoparticles and cell-surface receptors, allowing visualization of the presence and functional activity of tumor cells. Furthermore, the approach can be used for other cancer cell-surface receptors such as human epidermal growth factor receptor 2 (HER2). Therefore, ultrasound-guided molecular photoacoustic imaging can potentially aid in tumor diagnosis, selection of customized patient-specific treatment, and monitor the therapeutic progression and outcome in vivo.