Photoacoustic Contrast Agents Research Articles

Functional photoacoustic calcium imaging using chlorophosphonazo III in a 3D tumor cell culture.

This study demonstrates that chlorophosphonazo III (CPZ III) can be used as a contrast agent for photoacoustic calcium imaging. CPZ III can pass across the plasma membrane for labeling intracellular Ca2+ without cytotoxicity. In optical-resolution photoacoustic microscopy (OR-PAM), the photoacoustic (PA) signal intensity was strongly correlated with the presence of CPZ III and Ca2+ at various concentrations. The sensitivity of PA signal reception was enhanced by using an 8 MHz single-element focused ultrasound detector due to their matched frequency characteristics. Differences in the PA signal intensity were successfully found between the core and margin areas of tumorspheres in three-dimensional cell cultures. These findings indicate that CPZ III can serve as a novel PA contrast agent for functional Ca2+ imaging using OR-PAM.

Integration of Multitargeted Polymer-Based Contrast Agents with Photoacoustic Computed Tomography: An Imaging Technique to Visualize Breast Cancer Intratumor Heterogeneity.

One of the primary challenges in breast cancer diagnosis and treatment is intratumor heterogeneity (ITH), i.e., the coexistence of different genetically and epigenetically distinct malignant cells within the same tumor. Thus, the identification of ITH is critical for designing better treatments and hence to increase patient survival rates. Herein, we report a noninvasive hybrid imaging technology that integrates multitargeted and multiplexed patchy polymeric photoacoustic contrast agents (MTMPPPCAs) with single-impulse panoramic photoacoustic computed tomography (SIP-PACT). The target specificity ability of MTMPPPCAs to distinguish estrogen and progesterone receptor-positive breast tumors was demonstrated through both fluorescence and photoacoustic measurements and validated by tissue pathology analysis. This work provides the proof-of-concept of the MTMPPPCAs/SIP-PACT system to identify ITH in nonmetastatic tumors, with both high molecular specificity and real-time detection capability.

Phthalocyanine-based photoacoustic contrast agents for imaging and theranostics.

Phthalocyanine, as an organic dye, has attracted much attention due to its high molar absorption coefficient in the near-infrared region (NIR). It is precisely because of this advantage that phthalocyanine is very beneficial to photoacoustic imaging (PAI). At present, many different strategies have been adopted to design phthalocyanine-based contrast agents with photoacoustic (PA) effect, including increasing water solubility, changing spectral properties, prolonging the circulation time, constructing activatable supramolecular nanoparticles, increasing targeting, etc. Based on this, this minireview highlighted the above ways to enhance the PA effect of phthalocyanine. What's more, the application of phthalocyanine-based PA contrast agents in biomedical imaging and image-guided phototherapy has been discussed. Finally, this minireview also provides the prospects and challenges of phthalocyanine-based PA contrast agents in order to provide some reference for the application of phthalocyanine-based PA contrast agents in biomedical imaging and guiding tumor treatment.

A high-contrast photoacoustic agent with near-infrared emission.

Benzobisthiadiazole as a typical electron acceptor, has been widely used to design fluorescent dyes and photoacoustic (PA) agents. With the strategy of constructing donor-acceptor-donor (D-A-D) type of electron characteristics, benzobisthiadiazole derivatives tend to behave stable in near-infrared absorption and emission, which is beneficial to PA imaging. In this chapter, two molecular design strategies are combined to improve the photoacoustic imaging effects of new PA contrast agent IR-1302 NPs, by installing strengthened conjugated bridges and electron donors. The nanoparticles exhibit high-contrast noninvasive photoacoustic imaging in tumor models with longer wavelength absorption and emission and show potential as a clinic contrast agent.

Organic nanoparticle-doped microdroplets as dual-modality contrast agents for ultrasound microvascular flow and photoacoustic imaging

Tumor blood vessels are chaotic and abundantly distributed, owing to their heterogeneity. Therefore, imaging techniques which reveal abnormalities of tumor vasculature play significant roles in both mechanistic and clinical diagnostic tumor studies. Photoacoustic (PA) imaging uses the intrinsic characteristics of hemoglobin, to acquire tumor hemodynamic information, while ultrasound (US) imaging provides information about tumoral vessel structures and blood flow. To improve the imaging contrast performance, hydrogel-based microdroplets were designed for both US blood flow and PA imaging in this study. The microdroplets served as carriers for PA contrast agent solution in the innermost part while oil and hydrogel formed the inner and outer layers of the droplets. In vitro experiments firstly demonstrated the dual modality contrast effects of the microdroplets on US flow determination and PA imaging. In vivo experiments were then carried out in both healthy nude mice and nude mice with subcutaneous tumor to validate the contrast effects and to monitor the duration of contrast effects in animals. Using the dual-modality microdroplets, we were able to obtain distinct edges of tumor and blood flow mapping of the tumor microvascular with improved sensitivity up to 11.09 dB for PA and 6.69 dB for US flow. Besides, the in vivo evaluation with microdroplets showed US flow enhancement for more than 60 min. Therefore, the microdroplets are able to provide the contrast effects for both US flow and PA in a relative long duration and have potential to be applied in the tumor related diagnoses and studies.

A Review of Endogenous and Exogenous Contrast Agents Used in Photoacoustic Tomography with Different Sensing Configurations.

Optical-based sensing approaches have long been an indispensable way to detect molecules in biological tissues for various biomedical research and applications. The advancement in optical microscopy is one of the main drivers for discoveries and innovations in both life science and biomedical imaging. However, the shallow imaging depth due to the use of ballistic photons fundamentally limits optical imaging approaches’ translational potential to a clinical setting. Photoacoustic (PA) tomography (PAT) is a rapidly growing hybrid imaging modality that is capable of acoustically detecting optical contrast. PAT uniquely enjoys high-resolution deep-tissue imaging owing to the utilization of diffused photons. The exploration of endogenous contrast agents and the development of exogenous contrast agents further improve the molecular specificity for PAT. PAT’s versatile design and non-invasive nature have proven its great potential as a biomedical imaging tool for a multitude of biomedical applications. In this review, representative endogenous and exogenous PA contrast agents will be introduced alongside common PAT system configurations, including the latest advances of all-optical acoustic sensing techniques.

Manganese(II) Texaphyrin: A Paramagnetic Photoacoustic Contrast Agent Activated by Near-IR Light.

The NIR absorptivity of the metallotexaphyrin derivatives MMn, MGd, and MLu for photoacoustic (PA)-based imaging is explored in this study. All three complexes demonstrated excellent photostabilities; however, MMn provided the greatest PA signal intensities in both doubly distilled water and RAW 264.7 cells. In vivo experiments using a prostate tumor mouse model were performed. MMn displayed no adverse toxicity to major organs as inferred from hematoxylin and eosin (H&E) staining and cell blood count testing. MMn also allowed for PA-based imaging of tumors with excellent in vivo stability to provide 3D tumor diagnostic information. Based on the present findings and previous magnetic resonance imaging (MRI) studies, we believe MMn may have a role to play either as a stand-alone PA contrast agent or as a single molecule dual modal (PA and MR) imaging agent for tumor diagnosis.

Tetrazine-Derived Near-Infrared Dye as a Facile Reagent for Developing Targeted Photoacoustic Imaging Agents.

A new photoacoustic (PA) dye was developed as a simple-to-use reagent for creating targeted PA imaging agents. The lead molecule was prepared via an efficient two-step synthesis from an inexpensive commercially available starting material. With the dye's innate albumin-binding properties, the resulting tetrazine-derived dye is capable of localizing to tumor and exhibits a biological half-life of a few hours, allowing for an optimized distribution profile. The presence of tetrazine in turn makes it possible to link the albumin-binding optoacoustic signaling agent to a wide range of targeting molecules. To demonstrate the utility and ease of use of the platform, a novel PA probe for imaging calcium accretion was generated using a single-step bioorthogonal coupling reaction where high-resolution PA images of the knee joint in mice were obtained as early as 1 h post injection. Whole-body distribution was subsequently determined by labeling the probe with 99mTc and performing tissue counting following necropsy. These studies, along with tumor imaging and in vitro albumin binding studies, revealed that the core PA contrast agent can be imaged in vivo and can be easily linked to targeting molecules for organ-specific uptake.

RETRACTED ARTICLE: KRLODPLSMR-GCV3DC—improving contrast-based photoacoustic imaging based on model reconstruction

Contrast-based photoacoustic imaging is an emerging imaging technology that gold nanoparticles (a kind of photoacoustic contrast agent with a size of 100 nm) are injected into blood vessels for photoacoustic imaging. Image reconstruction is a critical step in photoacoustic imaging, due to the fact that limitations and incompleteness of photoacoustic imaging data of image reconstruction are equivalent to the linear inverse problems. The photoacoustic data is transferred to IOT based on the platform of MATLAB and is quickly processed through the regularization method of Krylov space (the widely used method is LSQR). In order to solve the problem of Krylov space uncertainty, Krylov space dimension selection, and the minimum mismatch of the LSQR objective function (the limitation of LSQR algorithm) of LSQR algorithm, a KRLODPLSMR-GCV3DC algorithm is proposed to solve the above problems in this paper.In this study, the raw photoacoustic data was obtained by injecting gold nanoparticles into capillaries and subcutaneous veins of the palm, and the KRLODPLSMR-GCV3DC algorithm was verified to solve the limitations of LSQR algorithm through four groups of experiments.

Comparing the spectral properties of the laser-induced acoustic responses from blood and cancer cells in vitro

Background ― The treatment of the cancer, especially in more aggressive metastatic forms is more effective at early disease stage. However, existing diagnostic techniques are not sensitive enough for early cancer detection. An alternative, perspective diagnostic approach can be based on photoacoustic (PA) method of irradiation of cancer cells in biotissue, blood and lymph by laser pulses. The fast thermal expansion of heated zones into cells associated with intrinsic or artificial PA contrast agents leads to generation of acoustic waves detected with ultrasound transducers. In particular, melanoma cells with melanin as a PA marker are darker than normal red blood cells and, therefore, produce greater acoustic responses. This technique can theoretically detect even a single cancer cell in the tissue and blood background; however, a robust algorithm of automated response detection is yet to be developed. Objective ― The main aim is to develop the approach for data pre-analysis that can improve the sensitivity and noise resistance of the automated in individual cancer cell detection algorithm, based on estimation of the amplitude of the acoustic responses. Methods ― Acoustic responses were obtained from a round polyurethane tube with human blood, or solution of the mouse melanoma cells in 10 mol/L concentration. In control experiments the laser was blocked by an opaque film. Many (up to 1000) acoustic responses were obtained from normal blood cells and pigmented cancer cells. Spectral analysis of the acoustic responses was used to find the spectral ranges that provide valuable diagnostic information with the sufficient signal-to-noise ratio. Results ― It was estimated that relevant diagnostics information in the acoustic responses is limited to the 0-12 MHz frequency band. Application of the 8th order low-pass Butterwort filter with 12 MHz cut-off frequency improved the signal-to-noise ratio from 21.14±10.39 to 110.81±56.94 for the cancer-related responses, and from 1.04±0.1 to 2.23±0.33 for the normal blood responses. Conclusions ― Adoption of low-pass filtering during the pre-analysis of acoustic responses results in better sensitivity of automated cancer cells detection algorithm.

Near‐Infrared‐III‐Absorbing and ‐Emitting Dyes: Energy‐Gap Engineering of Expanded Porphyrinoids via Metallation

AbstractThe synthesis of organometallic complexes of modified 26π‐conjugated hexaphyrins with absorption and emission capabilities in the third near‐infrared region (NIR‐III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis‐PdII and bis‐PtII complexes of hexaphyrin via N‐confusion modification led to substantial metal dπ–pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO–LUMO energy gap. A remarkable long‐wavelength shift of the lowest S0→S1 absorption beyond 1700 nm was achieved with the bis‐PtII complex, t‐Pt2‐3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t‐Pt2‐3 potentially useful as a NIR‐III PA contrast agent. The rigid bis‐PdII complexes, t‐Pd2‐3 and c‐Pd2‐3, are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR‐III‐emissive and photoacoustic‐response capabilities.

Near-Infrared-III-Absorbing and -Emitting Dyes: Energy-Gap Engineering of Expanded Porphyrinoids via Metallation.

The synthesis of organometallic complexes of modified 26π-conjugated hexaphyrins with absorption and emission capabilities in the third near-infrared region (NIR-III) is described. Symmetry alteration of the frontier molecular orbitals (MOs) of bis-PdII and bis-PtII complexes of hexaphyrin via N-confusion modification led to substantial metal dπ -pπ interactions. This MO mixing, in turn, resulted in a significantly narrower HOMO-LUMO energy gap. A remarkable long-wavelength shift of the lowest S0 →S1 absorption beyond 1700 nm was achieved with the bis-PtII complex, t-Pt2 -3. The emergence of photoacoustic (PA) signals maximized at 1700 nm makes t-Pt2 -3 potentially useful as a NIR-III PA contrast agent. The rigid bis-PdII complexes, t-Pd2 -3 and c-Pd2 -3, are rare examples of NIR emitters beyond 1500 nm. The current study provides new insight into the design of stable, expanded porphyrinic dyes possessing NIR-III-emissive and photoacoustic-response capabilities.

Optical clearing for photoacoustic lympho- and angiography beyond conventional depth limit in vivo

Photoacoustic (PA) imaging (PAI) is an emerging powerful tool for noninvasive real-time mapping of blood and lymphatic vessels and lymph nodes in vivo to diagnose cancer, lymphedema and other diseases. Among different PAI instruments, commercially available raster-scanning optoacoustic mesoscopy (RSOM) (iThera Medical GmbH., Germany) is useful for high-resolution imaging of different tissues with high potential of clinical translation. However, skin light scattering prevents mapping vessels and nodes deeper than 1–2 mm, that limits diagnostic values of PAI including RSOM. Here we demonstrate that glycerol-based tissue optical clearing (TOC) overcomes this challenge by reducing light scattering that improves RSOM depth penetration. In preclinical model of mouse limb in vivo, the replacement of conventional acoustic coupling agents such as water on the mixture of 70 % glycerol and 30 % ultrasound (US) gel resulted in the increase of tissue imaging depth in 1.5–2 times with 3D visualization of vessels with diameter down to 20 μm. To distinguish blood and lymphatic networks, we integrated label-free PA angiography (i.e., imaging of blood vessels), which uses hemoglobin as endogenous contrast agent, with PA lymphography based on labeling of lymphatic vessels with exogenous PA contrast agents. Similar to well-established clinical lymphography, contrast agents were injected in tissue and taken up by lymphatic vessels within a few minutes that provided quick RSOM lymphography. Furthermore, co-injection of PA contrast dye and multilayer nanocomposites as potential low-toxic drug-cargo showed selective prolonged accumulation of nanocomposites in sentinel lymph nodes. Overall, our findings open perspectives for deep and high resolution 3D PA angio- and lymphography, and for PA-guided lymphatic drug delivery using new RSOM & TOC approach.

Ellagic acid–Fe nanoscale coordination polymer with higher longitudinal relaxivity for dual-modality T1-weighted magnetic resonance and photoacoustic tumor imaging

Dual-modality contrast agents for T1-weighted magnetic resonance imaging (MRI) and photoacoustic imaging have attracted substantial attention as they combine the advantages of unlimited penetration depth and high sensitivity. However, most of the reported agents are Gd-based materials that exhibit nephrotoxicity, and few studies have focused on Fe-based materials owing to their lower relaxivity. This work describes the development of an ellagic acid (EA)–Fe nanoscale coordination polymer with high longitudinal relaxivity and strong near-infrared absorption for dual-modality T1-weighted MRI and photoacoustic imaging. The longitudinal relaxivity (r1) of the prepared EA-Fe@BSA nanoparticles was 2.54 mM−1 s−1, an increase of 185% compared with previously reported gallic acid-Fe nanoparticles. Furthermore, in vitro and in vivo experiments demonstrate that the EA-Fe@BSA NPs are an excellent T1-weighted MRI and photoacoustic dual-modality contrast agent with the advantages of convenient synthesis and low toxicity, exhibiting great potential for clinical use in tumor imaging.

Pickering Bubbles as Dual-Modality Ultrasound and Photoacoustic Contrast Agents.

Microbubbles (MBs) stabilized by particle surfactants (i.e., Pickering bubbles) have better thermodynamic stability compared to MBs stabilized by small molecules as a result of steric hindrance against coalescence, higher diffusion resistance, and higher particle desorption energy. In addition, the use of particles to stabilize MBs that are typically used as an ultrasound (US) contrast agent can also introduce photoacoustic (PA) properties, thus enabling a highly effective dual-modality US and PA contrast agent. Here, we report the use of partially reduced and functionalized graphene oxide as the sole surfactant to stabilize perfluorocarbon gas bubbles in the preparation of a dual-modality US and PA agent, with high contrast in both imaging modes and without the need for small-molecule or polymer additives. This approach offers an increase in loading of the PA agent without destabilization and increased thickness of the MB shell compared to traditional systems, in which the focus is on adding a PA agent to existing MB formulations.

Bis-Metal Complexes of Doubly N-Confused Dioxohexaphyrins as Potential Near-Infrared-II Photoacoustic Dyes.

We investigated the detailed photophysical properties of a series of bis-metal (Zn and Cu) dioxohexaphyrin complexes as potential second near-infrared (NIR-II)-light responsive dyes. A cisoid-configured 28π-electron-conjugated dioxohexaphyrin analogue (c-3a) containing two peculiar "confused pyrrole" moieties in the framework is identified as a reduced isomer derivative of a transoid 26π-dioxohexaphyrin (t-2a). The symmetry-altered structure of c-3a affords a heteroleptic inner environment within the NNNN/NNOO donor core, which imparts its highly flexible electronic features and nonplanar geometry. The macrocycle c-3a can be transformed into the corresponding 26π-electron congener (c-2a) having a coplanar rectangular structure by unique solvent-mediated redox reactivity. Furthermore, upon metal complexation, saddle-distorted bis-metal complexes (c-M2-2a) were formed as the 26π-conjugated structural isomer of the trans-dioxohexaphyrin species (i.e., t-M2-2a). These isoelectronic dioxohexaphyrins demonstrate precise geometry-dependent photophysical properties. Broad tailing NIR-II absorption, weak emissive character, and rapid-decay of the S1 state are observed for c-Zn2-2a. In contrast, the coplanar t-M2-2a exhibits efficient photoacoustic response upon laser excitation with NIR-II light (λ > 1000 nm). To the best of our knowledge, this is the first example of an expanded porphyrin-based photoacoustic contrast agent responsive to NIR-II light.

Recent advances in photoacoustic contrast agents for in vivo imaging.

Photoacoustic imaging (PAI) is a noninvasive hybrid imaging modality offering rich optical contrast and high depth-to-resolution ratio deep-tissue imaging. Endogenous chromophores present in the body such as hemoglobin, lipid, melanin, and so on provide strong photoacoustic contrast due to their strong light absorption in certain optical window. To enhance the performance of PAI further, researchers have developed several exogenous contrast agents such as metallic nanoparticles, carbon-based nanomaterials, quantum dots, organic small molecules, semiconducting polymer nanoparticles, and so on. These exogenous contrast agents not only help improving the imaging contrast, but also make targeted molecular imaging possible. In this review article, we first discuss the state-of-the-art PAI techniques with endogenous contrast mechanism. Later, we provide an overview of recent progress in the development of exogenous photoacoustic contrast agents for in vivo imaging applications. Finally, we present the pros/cons of the existing PA contrast agents along with future challenges of contrast agent-based PAI for biomedical applications. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vivo Nanodiagnostics and Imaging.

Poly(ionic liquid)-Gated CuCo2S4 for pH-/Thermo-Triggered Drug Release and Photoacoustic Imaging.

A novel hybrid drug nanocarrier is developed with CuCo2S4 nanoparticles as the core to be encapsulated by poly(ionic liquid) (PIL), that is, poly(tetrabutylphosphonium styrenesulfonate) (P[P4,4,4,4][SS]), as the shell. Doxorubicin (DOX) is loaded onto the PIL shell via electrostatic attraction involving amine in DOX and styrenesulfonate in PIL. pH- and thermal-responsive characteristics of P[P4,4,4,4][SS] endow the multifunctional hybrid nanocarrier system DOX-CuCo2S4@PIL with sensitive dual-stimuli-triggered drug release behaviors. The CuCo2S4 core converts near-infrared (NIR) irradiation into thermal energy to trigger the shrinkage of the PIL shell, which subsequently promotes drug release, and the pH-responsive release of DOX involves pH-sensitive electrostatic interaction of the PIL shell with DOX. A favorable controlled release of 90.5% is achieved under pH/thermo dual stimuli. In vitro experiments with MCF-7 cells well demonstrated that the drug release is controlled by the acidic intracellular environment with NIR irradiation. The CuCo2S4 core also serves as a photoacoustic (PA) imaging contrast agent, as demonstrated by in vivo treatment of the MCF-7-carrying mice.

Noninvasive monitoring of liver metastasis development via combined multispectral photoacoustic imaging and fluorescence diffuse optical tomography.

Rationale: In vivo molecular imaging in preclinical animal models is a tool of choice for understanding the pathophysiological mechanisms involved in cancer development and for conducting drug development research. Moreover, combining several imaging modalities can provide multifaceted, complementary and cross-validated information. Photoacoustic imaging (PAI) is a promising imaging modality that can reflect blood vasculature and tissue oxygenation as well as detect exogenous molecules, but one shortcoming of PAI is a lack of organic photoacoustic contrast agents capable of providing tumor contrast.Methods: In the present study, we designed an animal model of liver metastases from colon cancer and monitored metastasis development by in vivo bioluminescence and X-ray microcomputed tomography. Contrast-agent-free PAI was used to detect the respective amounts of oxy- and deoxyhemoglobin and, thus, liver tissue oxygenation. two contrast agents, Angiostamp800 and indocyanin green (ICG), respectively with and without tumor targeting specificity, were then evaluated for their dual fluorescence and photoacoustic detectability and were then used for combined PAI and fluorescence diffuse optical tomography (fDOT) at various disease development stages.Findings: Contrast-agent-free PAI reflected tumor angiogenesis and gradual hypoxia during metastasis development. Multispectral PAI enabled noninvasive real-time monitoring of ICG blood pharmacokinetics, which demonstrated tumor-related liver dysfunction. Both PAI and fluorescence ICG signals were clearly modified in metastasis-bearing livers but did not allow for differentiation between different disease stages. In contrast, there was a significant improvement achieved by using the tumor-specific marker Angiostamp800, which provided gradually increasing PAI and fDOT signals during metastasis development.Conclusion: We demonstrated for the first time the value of using Angiostamp800 as a bimodal tumor-targeting contrast agent for combined PAI and fluorescence imaging of liver metastasis progression in vivo.

Deep tissue photoacoustic imaging of nickel(II) dithiolene-containing polymeric nanoparticles in the second near-infrared window.

Photoacoustic imaging is gaining great attention in the medical world due to its significant potential for clinical translation. Light excitation in the second near-infrared (NIR-II) window (1000-1350 nm) has resolution and penetration depth suitable for several clinical applications. However, the significant challenge exists for clinical translation because of the absence of notable intrinsic chromophores in this clinically significant optical range to generate diagnostic images.Methods: We present newly developed a biocompatible nickel dithiolene-based polymeric nanoparticle (NiPNP), which have a strong and sharp absorption peak at 1064 nm, as a photoacoustic contrast agent to boost specific absorbance in the NIR-II window for in vivo deep tissue imaging.Results: We confirm the enhanced PA signal by NiPNP's strong light absorption in the NIR-II window (287% higher than that of NIR-I) and deep tissue imaging capability (~5.1 cm) through in vitro experiment. We have successfully acquired diagnostic-quality in vivo photoacoustic images in deep tissue (~3.4 cm) of sentinel lymph nodes, gastrointestinal tracts, and bladders of live rats by using clinically viable imaging system.Conclusions: Our results prove that with strong absorption in the NIR-II window and with deeper imaging depth, the clinical translation of photoacoustic imaging with NiPNP is feasible for preclinical studies and thus would facilitate further clinical investigations.