Prospects of molecular photoacoustic imaging at 1064 nm wavelength

Laser-activated perfluorocarbon nanodroplets for intracerebral delivery and imaging via blood-brain barrier opening and contrast-enhanced imaging

BACKGROUND

Ultrasound and photoacoustic (US/PA) imaging is a promising tool for in vivo visualization and assessment of drug delivery. However, the acoustic properties of the skull limit the practical application of US/PA imaging in the brain. To address the challenges in targeted drug delivery to the brain and transcranial US/PA imaging, we introduce and evaluate an intracerebral delivery and imaging strategy based on the use of laser-activated perfluorocarbon nanodroplets (PFCnDs).

METHODS

Two specialized PFCnDs were developed to facilitate blood‒brain barrier (BBB) opening and contrast-enhanced US/PA imaging. In mice, PFCnDs were delivered to brain tissue via PFCnD-induced BBB opening to the right side of the brain. In vivo, transcranial US/PA imaging was performed to evaluate the utility of PFCnDs for contrast-enhanced imaging through the skull. Ex vivo, volumetric US/PA imaging was used to characterize the spatial distribution of PFCnDs that entered brain tissue. Immunohistochemical analysis was performed to confirm the spatial extent of BBB opening and the accuracy of the imaging results.

RESULTS

In vivo, transcranial US/PA imaging revealed localized photoacoustic (PA) contrast associated with delivered PFCnDs. In addition, contrast-enhanced ultrasound (CEUS) imaging confirmed the presence of nanodroplets within the same area. Ex vivo, volumetric US/PA imaging revealed PA contrast localized to the area of the brain where PFCnD-induced BBB opening had been performed. Immunohistochemical analysis revealed that the spatial distribution of immunoglobulin (IgG) extravasation into the brain closely matched the imaging results.

CONCLUSIONS

Using our intracerebral delivery and imaging strategy, PFCnDs were successfully delivered to a targeted area of the brain, and they enabled contrast-enhanced US/PA imaging through the skull. Ex vivo imaging, and immunohistochemistry confirmed the accuracy and precision of the approach.

NIR-II-Absorbing TMB Derivative for 1064 nm-Excited Photothermal Immunoassay

Materials exhibiting strong absorption in the NIR-II region are appealing for photothermal conversion-based imaging, diagnosis, and therapy, due to better thermal effect and decreased absorption of water in such a region. 3,3',5,5'-Tetramethylbenzidine (TMB), the typical substrate in ELISA, has been explored in photothermal immunoassay, since its oxidation product (oxTMB) is photothermally active in the NIR region. However, its absorption at 1064 nm (the most often used laser wavelength in photothermal studies) is not appreciable, thus limiting the assay sensitivity. Here, we proposed a derivative of TMB (3,3'-dimethoxy-5,5'-dimethylbenzidine, 2-OCH3) bearing higher NIR-II absorption for 1064 nm-excited photothermal immunoassay. Since electron-donating groups can help decrease the energy gap of molecules (here -CH3 → -OCH3), the oxidation product of 2-OCH3 exhibited substantially red-shifted absorption as compared with oxTMB, leading to a more than twofold higher absorption coefficient at 1064 nm. As a result, 2-OCH3 showed enhanced sensitivity over TMB in a photothermal immunoassay (PTIA), yielding a limit of detection (LOD) of 0.1 ng/mL for prostate-specific antigen (PSA). The feasibility of 2-OCH3-based PTIA for diagnosis was further validated by analyzing PSA in 61 serum samples. Considering its superior photothermal performance, 2-OCH3 can be explored for a broad range of photothermal applications.

Recent Advances in Contrast-Enhanced Photoacoustic Imaging: Overcoming the Physical and Practical Challenges

For decades now, photoacoustic imaging (PAI) has been investigated to realize its potential as a niche biomedical imaging modality. Despite its highly desirable optical contrast and ultrasonic spatiotemporal resolution, PAI is challenged by such physical limitations as a low signal-to-noise ratio (SNR), diminished image contrast due to strong optical attenuation, and a lower-bound on spatial resolution in deep tissue. In addition, contrast-enhanced PAI has faced practical limitations such as insufficient cell-specific targeting due to low delivery efficiency and difficulties in developing clinically translatable agents. Identifying these limitations is essential to the continuing expansion of the field, and substantial advances in developing contrast-enhancing agents, complemented by high-performance image acquisition systems, have synergistically dealt with the challenges of conventional PAI. This review covers the past four years of research on pushing the physical and practical challenges of PAI in terms of SNR/contrast, spatial resolution, targeted delivery, and clinical application. Promising strategies for dealing with each challenge are reviewed in detail, and future research directions for next generation contrast-enhanced PAI are discussed.

Optical Light Sources and Wavelengths within the Visible and Near-Infrared Range Using Photoacoustic Effects for Biomedical Applications

The photoacoustic (PA) effect occurs when sound waves are generated by light according to the thermodynamic and optical properties of the materials; they are absorption spectroscopic techniques that can be applied to characterize materials that absorb pulse or continuous wave (CW)-modulated electromagnetic radiation. In addition, the wavelengths and properties of the incident light significantly impact the signal-to-ratio and contrast with photoacoustic signals. In this paper, we reviewed how absorption spectroscopic research results have been used in applying actual photoacoustic effects, focusing on light sources of each wavelength. In addition, the characteristics and compositions of the light sources used for the applications were investigated and organized based on the absorption spectrum of the target materials. Therefore, we expect that this study will help researchers (who desire to study photoacoustic effects) to more efficiently approach the appropriate conditions or environments for selecting the target materials and light sources.

Ultrasound and Photoacoustic Imaging of Breast Cancer: Clinical Systems, Challenges, and Future Outlook

Presently, breast cancer diagnostic methods are dominated by mammography. Although drawbacks of mammography are present including ionizing radiation and patient discomfort, not many alternatives are available. Ultrasound (US) is another method used in the diagnosis of breast cancer, commonly performed on women with dense breasts or in differentiating cysts from solid tumors. Handheld ultrasound (HHUS) and automated breast ultrasound (ABUS) are presently used to generate reflection images which do not contain quantitative information about the tissue. This limitation leads to a subjective interpretation from the sonographer. To rectify the subjective nature of ultrasound, ultrasound tomography (UST) systems have been developed to acquire both reflection and transmission UST (TUST) images. This allows for quantitative assessment of tissue sound speed (SS) and acoustic attenuation which can be used to evaluate the stiffness of the lesions. Another imaging modality being used to detect breast cancer is photoacoustic tomography (PAT). Utilizing much of the same hardware as ultrasound tomography, PAT receives acoustic waves generated from tissue chromophores that are optically excited by a high energy pulsed laser. This allows the user to ideally produce chromophore concentration maps or extract other tissue parameters through spectroscopic PAT. Here, several systems in the area of TUST and PAT are discussed along with their advantages and disadvantages in breast cancer diagnosis. This overview of available systems can provide a landscape of possible intersections and future refinements in cancer diagnosis.

Towards Transabdominal Functional Photoacoustic Imaging of the Placenta: Improvement in Imaging Depth Through Optimization of Light Delivery

Functional photoacoustic imaging of the placenta could provide an innovative tool to diagnose preeclampsia, monitor fetal growth restriction, and determine the developmental impacts of gestational diabetes. However, transabdominal photoacoustic imaging is limited in imaging depth due to the tissue's scattering and absorption of light. The aim of this paper was to investigate the impact of geometry and wavelength on transabdominal light delivery. Our methods included the development of a multilayer model of the abdominal tissue and simulation of the light propagation using Monte Carlo methods. A bifurcated light source with varying incident angle of light, distance between light beams, and beam area was simulated to analyze the effect of light delivery geometry on the fluence distribution at depth. The impact of wavelength and the effects of variable thicknesses of adipose tissue and muscle were also studied. Our results showed that the beam area plays a major role in improving the delivery of light to deep tissue, in comparison to light incidence angle or distance between the bifurcated fibers. Longer wavelengths, with incident fluence at the maximum permissible exposure limit, also increases fluence within deeper tissue. We validated our simulations using a commercially available light delivery system and ex vivo human placental tissue. Additionally, we compared our optimized light delivery to a commercially available light delivery system, and conclude that our optimized geometry could improve imaging depth more than 1.6×, bringing the imaging depth to within the needed range for transabdominal imaging of the human placenta.

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.

Miniature Hollow Gold Nanorods with Enhanced Effect for In Vivo Photoacoustic Imaging in the NIR-II Window

The miniaturization of gold nanorods exhibits a bright prospect for intravital photoacoustic imaging (PAI) and the hollow structure possesses a better plasmonic property. Herein, miniature hollow gold nanorods (M-AuHNRs) (≈46 nm in length) possessing strong plasmonic absorbance in the second near-infrared (NIR-II) window (1000-1350 nm) are developed, which are considered as the most suitable range for the intravital PAI. The as-prepared M-AuHNRs exhibit 3.5 times stronger photoacoustic signal intensity than the large hollow Au nanorods (≈105 nm in length) at 0.2 optical density under 1064 nm laser irradiation. The in vivo biodistribution measurement shows that the accumulation in tumor of miniature nanorods is twofold as high as that of the large counterpart. After modifying with a tumor-targeting molecule and fluorochrome, in living tumor-bearing mice, the M-AuHNRs group gives a high fluorescence intensity in tumors, which is 3.6-fold that of the large ones with the same functionalization. Moreover, in the intravital PAI of living tumor-bearing mice, the M-AuHNRs generate longer-lasting and stronger photoacoustic signal than the large counterpart in the NIR-II window. Overall, this study presents the fabrication of M-AuHNRs as a promising contrast agent for intravital PAI.

Rapid functional optoacoustic micro-angiography in a burst mode

Optoacoustic microscopy (OAM) can image intrinsic optical absorption contrast at depths of several millimeters where state-of-the-art optical microscopy techniques fail due to intense light scattering in living tissues. Yet, wide adoption of OAM in biology and medicine is hindered by slow image acquisition speed, small field of view (FOV), and/or lack of spectral differentiation capacity of common system implementations. We report on a rapid acquisition functional optoacoustic micro-angiography approach that employs a burst-mode laser triggering scheme to simultaneously acquire multi-wavelength 3D images over an extended FOV covering ${50}\;{\rm mm} \times {50}\;{\rm mm}$50mm×50mm in a single mechanical overfly scan, attaining 28 µm and 14 µm resolution in lateral and axial dimensions, respectively. Owing to an ultrawideband low-noise design featuring a spherically focused polyvinylidene difluoride transducer, we demonstrate imaging of human skin and underlying vasculature at up to 3.8 mm depth when using per-pulse laser energies of only 25 µJ without employing signal averaging. Overall, the developed system greatly enhances performance and usability of OAM for dermatologic and micro-angiographic studies.

Magnetic targeted near-infrared II PA/MR imaging guided photothermal therapy to trigger cancer immunotherapy

Rationale: Photothermal therapy (PTT) alone is easy to cause cancer recurrence and fail to completely resist metastasis, yet recurrence and metastasis are two major difficulties in cancer treatment. Titanium disulfide (TiS₂) nanosheet anchored iron oxide nanoparticles (IO NPs) with strong absorption in the second near-infrared (NIR-II) window and excellent magnetic properties is developed as therapeutic agent for NIR-II photoacoustic (PA) imaging and magnetic resonance (MR) imaging guided NIR-II PTT triggered immunotherapy.

Methods: The TiS₂ nanosheets were prepared through a modified colloidal chemistry approach, and TSIO nanoagents were prepared by using a one pot self-assembly technique. The magnetic targeting capability of TSIO nanoagents were monitored by NIR-II PA, MR and thermal imaging in vivo. The NIR-II PTT combined with immunotherapy effect was investigated in mouse breast cancer tumor-bearing mice.

Results: The TSIO nanoplatform showed enhanced tumor accumulation when a magnetic field was applied and had the ability to real time monitor the treatment process via dual NIR-II PA and MR imaging. In addition, the magnetic targeted NIR-II PA/MR imaging guided PTT provides an effective way to reverse the immunosuppression inside a tumor and to cooperate with immunotherapy to improve therapeutic outcome of the primary, distal and metastatic tumors.

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.

Combined ultrasound and photoacoustic imaging of blood clot during microbubble-assisted sonothrombolysis

Blockage of healthy blood vessels by blood clots can lead to serious or even life-threatening complications. The use of a combined ultrasound (US) and photoacoustic (PA) imaging was explored for blood clot monitoring during microbubble-assisted sonothrombolysis. PA imaging is an emerging hybrid imaging modality that has garnered the attention of the biomedical imaging community in recent years. It enables the study of the composition of a blood clot due to its sensitivity toward optical absorption. Here, in vitro imaging of the side of a blood clot facing the microbubbles was done over time. The US and PA signal-to-noise (SNR) ratio value changes during microbubble-assisted sonothrombolysis were studied for two different local environments: blood clot in deionized water and blood clot in blood. In the first case, US and PA SNR values increased by 4.6% and reduced by 20.8%, respectively after 30 min of sonothrombolysis treatment. After 10 min of sonothrombolysis treatment of the blood clot in blood, the US and PA SNR values increased by 7.7% and 38.3%, respectively. The US and PA SNR value changes were recorded in response to its local environment. This technique can be used to determine the final composition of the blood clot which may, in turn, help in the administration of clot-dissolving drugs.

Beyond 1000 nm Emission Wavelength: Recent Advances in Organic and Inorganic Emitters for Deep-Tissue Molecular Imaging

In vivo second near-infrared (NIR-II, 1.0-1.7 µm) bioimaging , a rapidly expanding imaging tool for preclinical diagnosis and prognosis, is of great importance to afford precise dynamic actions in vivo with high spatiotemporal resolution, deeper penetration, and decreasing light absorption and scattering. In the course of preclinical practices, organic and inorganic emitters with NIR-II signals are indispensable keys to open the invisible biological window. In this review, NIR-II emitters, including but not limited to organic emitters like organic small molecules and copolymers, and inorganic emitters such as lanthanide-based nanocrystals, quantum dots like Ag₂ S dots, and carbon nanotubes, are described, especially regarding their unique optical features and noteworthy functions for animal bioimaging. Along with these existing advances, the challenges and potential spaces for further progress are discussed to offer an approximate direction for future researches.

High-Resolution 3D NIR-II Photoacoustic Imaging of Cerebral and Tumor Vasculatures Using Conjugated Polymer Nanoparticles as Contrast Agent

Exogenous contrast-agent-assisted NIR-II optical-resolution photoacoustic microscopy imaging (ORPAMI) holds promise to decipher wide-field 3D biological structures with deep penetration, large signal-to-background ratio (SBR), and high maximum imaging depth to depth resolution ratio. Herein, NIR-II conjugated polymer nanoparticle (CP NP) assisted ORPAMI is reported for pinpointing cerebral and tumor vasculatures. The CP NPs exhibit a large extinction coefficient of 48.1 L g^-1 at the absorption maximum of 1161 nm, with an ultrahigh PA sensitivity up to 2 µg mL^-1 . 3D ORPAMI of wide-field mice ear allows clear visualization of regular vasculatures with a resolution of 19.2 µm and an SBR of 29.3 dB at the maximal imaging depth of 539 µm. The margin of ear tumor composed of torsional dense vessels among surrounding normal regular vessels can be clearly delineated via 3D angiography. In addition, 3D whole-cortex cerebral vasculatures with large imaging area (48 mm² ), good resolution (25.4 µm), and high SBR (22.3 dB) at a depth up to 1001 µm are clearly resolved through the intact skull. These results are superior to the recently reported 3D NIR-II fluorescence confocal vascular imaging, which opens up new opportunities for NIR-II CP-NP-assisted ORPAMI in various biomedical applications.

Miniature gold nanorods for photoacoustic molecular imaging in the second near-infrared optical window

In photoacoustic imaging, the second near-infrared (NIR-II) window is where tissue generates the least background signal. However, the large size of the few available contrast agents in this spectral range impedes their pharmacokinetics and decreases their thermal stability, leading to unreliable photoacoustic imaging. Here, we report the synthesis of miniaturized gold nanorods absorbing in the NIR-II that are 5-11 times smaller than regular-sized gold nanorods with a similar aspect ratio. Under nanosecond pulsed laser illumination, small nanorods are about 3 times more thermally stable and generate 3.5 times stronger photoacoustic signal than their absorption-matched larger counterparts. These unexpected findings are confirmed using theoretical and numerical analysis, showing that photoacoustic signal is not only proportional to the optical absorption of the nanoparticle solution but also to the surface-to-volume ratio of the nanoparticles. In living tumour-bearing mice, these small targeted nanorods display a 30% improvement in efficiency of agent delivery to tumours and generate 4.5 times greater photoacoustic contrast.

1064 nm acoustic resolution photoacoustic microscopy

Photoacoustic imaging is a noninvasive imaging technique having the advantages of high-optical contrast and good acoustic resolution at improved imaging depths. Light transport in biological tissues is mainly characterized by strong optical scattering and absorption. Photoacoustic microscopy is capable of achieving high-resolution images at greater depth compared to conventional optical microscopy methods. In this work, we have developed a high-resolution, acoustic resolution photoacoustic microscopy (AR-PAM) system in the near infra-red (NIR) window II (NIR-II, eg, 1064 nm) for deep tissue imaging. Higher imaging depth is achieved as the tissue scattering at 1064 nm is lesser compared to visible or near infrared window-I (NIR-I). Our developed system can provide a lateral resolution of 130 μm, axial resolution of 57 μm, and image up to 11 mm deep in biological tissues. This 1064-AR-PAM system was used for imaging sentinel lymph node and the lymph vessel in rat. Urinary bladder of rat filled with black ink was also imaged to validate the feasibility of the developed system to study deeply seated organs.

A 1064 nm excitable semiconducting polymer nanoparticle for photoacoustic imaging of gliomas

Photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (especially at 1064 nm) has the benefits of low background signal, high spatial resolution and deep tissue penetration. Here we report a semiconducting polymer nanoparticle (PDPPTBZ NP) and demonstrate its potential as a contrast agent for PA imaging of orthotopic brain tumors, using a 1064 nm pulsed laser as a light source. PDPPTBZ NPs have maximum absorption at 1064 nm with a mass extinction coefficient of 43 mL mg-1 cm-1, which is the highest value reported so far in this region. The high photothermal conversion efficiency (67%) and near non-fluorescence impart PDPPTBZ NPs with excellent PA properties. We used PDPPTBZ NP-containing agar gel phantoms even at a low concentration (50 μg mL-1) to successfully image to a depth of 4 cm (of chicken-breast tissue), with an ultralow power fluence (4 mJ cm-2). Furthermore, we could clearly visualize a glioma tumor in a mouse at a depth of 3.8 mm below the skull. This study demonstrates that PDPPTBZ NPs display great potential as a NIR-II PA contrast agent for high quality deep tissue imaging.

Listening for the therapeutic window: Advances in drug delivery utilizing photoacoustic imaging

The preclinical landscape of photoacoustic imaging has experienced tremendous growth in the past decade. This non-invasive imaging modality augments the spatiotemporal capabilities of ultrasound with optical contrast. While it has principally been investigated for diagnostic applications, many recent reports have described theranostic delivery systems and drug monitoring strategies using photoacoustics. Here, we provide an overview of the progress to date while highlighting work in three specific areas: theranostic nanoparticles, real-time drug monitoring, and stem cell ("living drug") tracking. Additionally, we discuss the challenges that remain to be addressed in this burgeoning field.

Photoacoustic imaging in the second near-infrared window: a review

Photoacoustic (PA) imaging is an emerging medical imaging modality that combines optical excitation and ultrasound detection. Because ultrasound scatters much less than light in biological tissues, PA generates high-resolution images at centimeters depth. In recent years, wavelengths in the second near-infrared (NIR-II) window (1000 to 1700 nm) have been increasingly explored due to its potential for preclinical and clinical applications. In contrast to the conventional PA imaging in the visible (400 to 700 nm) and the first NIR-I (700 to 1000 nm) window, PA imaging in the NIR-II window offers numerous advantages, including high spatial resolution, deeper penetration depth, reduced optical absorption, and tissue scattering. Moreover, the second window allows a fivefold higher light excitation energy density compared to the visible window for enhancing the imaging depth significantly. We highlight the importance of the second window for PA imaging and discuss the various NIR-II PA imaging systems and contrast agents with strong absorption in the NIR-II spectral region. Numerous applications of NIR-II PA imaging, including whole-body animal imaging and human imaging, are also discussed.

Metabolizable Semiconducting Polymer Nanoparticles for Second Near-Infrared Photoacoustic Imaging

Photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (1000-1700 nm) holds great promise for deep-tissue diagnosis due to the reduced light scattering and minimized tissue absorption; however, exploration of such a noninvasive imaging technique is greatly constrained by the lack of biodegradable NIR-II absorbing agents. Herein, the first series of metabolizable NIR-II PA agents are reported based on semiconducting polymer nanoparticles (SPNs). Such completely organic nanoagents consist of π-conjugated yet oxidizable optical polymer as PA generator and hydrolyzable amphiphilic polymer as particle matrix to provide water solubility. The obtained SPNs are readily degraded by myeloperoxidase and lipase abundant in phagocytes, transforming from nonfluorescent nanoparticles (30 nm) into NIR fluorescent ultrasmall metabolites (≈1 nm). As such, these nanoagents can be effectively cleared out via both hepatobiliary and renal excretions after systematic administration, leaving no toxicity to living mice. Particularly these nanoagents possess high photothermal conversion efficiencies and emit bright PA signals at 1064 nm, enabling sensitive NIR-II PA imaging of both subcutaneous tumor and deep brain vasculature through intact skull in living animals at a low systematic dosage. This study thus provides a generalized molecular design toward organic metabolizable semiconducting materials for biophotonic applications in NIR-II window.

Spectral photoacoustic imaging to estimate in vivo placental oxygenation during preeclampsia

Preeclampsia is a pregnancy-related hypertensive disorder accounting for 14% of global maternal deaths annually. Preeclampsia - maternal hypertension and proteinuria - is promoted by placental ischemia resulting from reduced uteroplacental perfusion. Here, we assess longitudinal changes in placental oxygenation during preeclampsia using spectral photoacoustic imaging. Spectral photoacoustic images were acquired of the placenta of normal pregnant (NP) and preeclamptic reduced uterine perfusion pressure (RUPP) Sprague Dawley rats on gestational days (GD) 14, 16, and 18, corresponding to mid- to late gestation (n = 10 per cohort). Two days after implementation of the RUPP surgical model, placental oxygen saturation decreased 12% in comparison with NP. Proteinuria was determined from a 24-hour urine collection prior to imaging on GD18. Blood pressure measurements were obtained on GD18 after imaging. Placental hypoxia in the RUPP was confirmed with histological staining for hypoxia-inducible factor (HIF)-1α, a cellular transcription regulator which responds to local oxygen levels. Using in vivo, longitudinal imaging methods we determined that the placenta in the reduced uterine perfusion pressure rat model of preeclampsia is hypoxic, and that this hypoxia is maintained through late gestation. Future work will utilize these methods to assess the impact of novel therapeutics on placental ischemia and the progression of preeclampsia.

Photoacoustic imaging and photothermal therapy in the second near-infrared window

This review summarizes the recent progress of PA imaging and PTT agents in the second NIR window.

Materials and design of nanostructured broadband light absorbers for advanced light-to-heat conversion

Light-to-heat conversion systems have been attracting growing research interest in the last few decades, due to their highly intriguing photothermal properties and their wide applications ranging from biomedical applications to solar energy harvesting to mechanical actuators. In general, because the light-to-heat conversion efficiency depends strongly on the absorbing material, significant efforts have been focused on fabricating broadband light absorbers. However, there are several challenges associated with the design and fabrication of light absorbers, such as minimizing heat loss, and optimizing the broadband light absorption and omnidirectional light absorption. Thus, the rational design of enhanced light absorbers is critical to achieve efficient light absorption over a broad wavelength range. In this paper, we introduce the basic theory of light absorption and heat transfer, then summarize fundamental understanding of representative light-to-heat conversion agents including carbon-based, semiconductor-based and plasmonic metal-based materials and structures, and highlight state-of-the-art structural designs towards the development of broadband light absorbers. In addition, the practical applications of these materials and designs are also discussed.

Bi2Se3 nanoplates for contrast-enhanced photoacoustic imaging at 1064 nm

Photoacoustic (PA) imaging is a high-resolution biomedical imaging modality, which can be used to visualize biological tissues located beyond the limited penetration depth of existing optical imaging techniques. An optical wavelength of 1064 nm is of great interest in PA imaging due to low intrinsic absorption at this wavelength. Reduced absorption implies an increased depth of imaging, which enables several new clinical applications such as bladder imaging, gastrointestinal (GI) imaging, and sentinel lymph node (SLN) imaging. In addition, a 1064 nm Nd:YAG laser system enables a high power, cost-effective, and compact laser-based PA imaging system. However, at this wavelength, due to low intrinsic contrast, high absorption exogenous PA contrast agents are necessary for imaging. To this end, we present new Bi2Se3 nanoplates as PA contrast agents at 1064 nm wavelength for PA imaging. We successfully synthesized Bi2Se3 nanoplates and they exhibited relatively strong PA signals at 1064 nm. We confirmed the increased imaging depth of penetration by imaging the Bi2Se3-containing tube located 4.6 cm deep in biological tissues. We present in vivo PA imaging of the bladder, GI tract, and SLN in mice using a Bi2Se3 contrast agent establishing the clinical feasibility of these agents with a clinical photoacoustic/ultrasound imaging system. Our results confirm that Bi2Se3 nanoplates are promising PA contrast agents at 1064 nm that offer a high optical absorbance in the second NIR region providing a high contrast imaging and increased depth of penetration.

Contrast-enhanced photoacoustic imaging in the second near-infrared window using semiconducting polymer nanoparticles

Photoacoustic imaging (PAI) is a fast growing deep-tissue imaging modality. However, light scattering and absorption in biological tissues limit imaging depth. Short near-infrared wavelengths (650 to 950 nm) are widely used for PAI. Using longer near-infrared wavelengths reduces scattering. We demonstrate deep-tissue contrast-enhanced in vivo photoacoustic imaging at a wavelength of 1064 nm. An ultranarrow bandgap semiconducting polymer poly (thienoisoindigo-alt-diketopyrrolopyrrole) (denoted as PIGD) is designed and demonstrated for imaging at 1064 nm. By embedding colloidal nanoparticles (NPs) of PIGD in chicken-breast tissue, an imaging depth of ∼5  cm is achieved. Intravenous injection of PIGD NPs in living rats showed brain vascular images with ∼2 times higher contrast compared with the brain vascular images without any contrast agent. Thus, PIGD NPs as an NIR-II contrast agent opens new opportunities for both preclinical and clinical imaging of deep tissues with enhanced contrast.

Recent progress in photoacoustic molecular imaging

By acoustically detecting the optical absorption contrast, photoacoustic (PA) tomography (PAT) has broken the penetration limits of traditional high-resolution optical imaging. Through spectroscopic analysis of the target's optical absorption, PAT can identify a wealth of endogenous and exogenous molecules and thus is inherently capable of molecular imaging with high sensitivity. PAT's molecular sensitivity is uniquely accompanied by non-ionizing radiation, high spatial resolution, and deep penetration in biological tissues, which other optical imaging modalities cannot achieve yet. In this concise review, we summarize the most recent technological advancements in PA molecular imaging and highlight the novel molecular probes specifically made for PAT in deep tissues. We conclude with a brief discussion of the opportunities for future advancements.

Ingestible roasted barley for contrast-enhanced photoacoustic imaging in animal and human subjects

Photoacoustic computed tomography (PACT) is an emerging imaging modality. While many contrast agents have been developed for PACT, these typically cannot immediately be used in humans due to the lengthy regulatory process. We screened two hundred types of ingestible foodstuff samples for photoacoustic contrast with 1064 nm pulse laser excitation, and identified roasted barley as a promising candidate. Twenty brands of roasted barley were further screened to identify the one with the strongest contrast, presumably based on complex chemical modifications incurred during the roasting process. Individual roasted barley particles could be detected through 3.5 cm of chicken-breast tissue and through the whole hand of healthy human volunteers. With PACT, but not ultrasound imaging, a single grain of roasted barley was detected in a field of hundreds of non-roasted particles. Upon oral administration, roasted barley enabled imaging of the gut and peristalsis in mice. Prepared roasted barley tea could be detected through 2.5 cm chicken breast tissue. When barley tea was administered to humans, photoacoustic imaging visualized swallowing dynamics in healthy volunteers. Thus, roasted barley represents an edible foodstuff that should be considered for photoacoustic contrast imaging of swallowing and gut processes, with immediate potential for clinical translation.

Towards optimized naphthalocyanines as sonochromes for photoacoustic imaging in vivo

In this paper we establish a methodology to predict photoacoustic imaging capabilities from the structure of absorber molecules (sonochromes). The comparative in vitro and in vivo screening of naphthalocyanines and cyanine dyes has shown a substitution pattern dependent shift in photoacoustic excitation wavelength, with distal substitution producing the preferred maximum around 800 nm. Central ion change showed variable production of photoacoustic signals, as well as singlet oxygen photoproduction and fluorescence with the optimum for photoacoustic imaging being nickel(II). Our approach paves the way for the design, evaluation and realization of optimized sonochromes as photoacoustic contrast agents.

Semiconducting Polymer Nanoparticles for Centimeters-Deep Photoacoustic Imaging in the Second Near-Infrared Window

Thienoisoindigo-based semiconducting polymer with a strong near-infrared absorbance is synthesized and its water-dispersed nanoparticles (TSPNs) are investigated as a contrast agent for photoacoustic (PA) imaging in the second near-infrared (NIR-II) window (1000-1350 nm). The TSPNs generate a strong PA signal in the NIR-II optical window, where background signals from endogenous contrast agents, including blood and lipid, are at the local minima. By embedding a TSPN-containing tube in chicken-breast tissue, an imaging depth of more than 5 cm at 1064 nm excitation is achieved with a contrast-agent concentration as low as 40 µg mL^-1 . The TSPNs under the skin or in the tumor are clearly visualized at 1100 and 1300 nm, with negligible interference from the tissue background. TSPN as a PA contrast in the NIR-II window opens new opportunities for biomedical imaging of deep tissues with improved contrast.

Broadband Absorbing Semiconducting Polymer Nanoparticles for Photoacoustic Imaging in Second Near-Infrared Window

Photoacoustic (PA) imaging holds great promise for preclinical research and clinical practice. However, most studies rely on the laser wavelength in the first near-infrared (NIR) window (NIR-I, 650-950 nm), while few studies have been exploited in the second NIR window (NIR-II, 1000-1700 nm), mainly due to the lack of NIR-II absorbing contrast agents. We herein report the synthesis of a broadband absorbing PA contrast agent based on semiconducting polymer nanoparticles (SPN-II) and apply it for PA imaging in NIR-II window. SPN-II can absorb in both NIR-I and NIR-II regions, providing the feasibility to directly compare PA imaging at 750 nm with that at 1064 nm. Because of the weaker background PA signals from biological tissues in NIR-II window, the signal-to-noise ratio (SNR) of SPN-II resulted PA images at 1064 nm can be 1.4-times higher than that at 750 nm when comparing at the imaging depth of 3 cm. The proof-of-concept application of NIR-II PA imaging is demonstrated in in vivo imaging of brain vasculature in living rats, which showed 1.5-times higher SNR as compared with NIR-I PA imaging. Our study not only introduces the first broadband absorbing organic contrast agent that is applicable for PA imaging in both NIR-I and NIR-II windows but also reveals the advantages of NIR-II over NIR-I in PA imaging.

Dual plasmonic gold nanostars for photoacoustic imaging and photothermal therapy

AIM

To fabricate multimodal nanoconstruct that act as a single node for photoacoustic imaging (PAI) and photothermal therapy (PTT) in the fight against cancer.

MATERIALS & METHODS

Dual plasmonic gold nanostars (DPGNS) were chemically synthesized by reducing gold precursor using ascorbic acid and silver ions as shape directing agent. PAI and PTT were performed using commonly available 1064 nm laser source on DPGNS embedded tumor xenografts on mice.

RESULTS & CONCLUSION

Photoacoustic amplitude increase with longer wavelength source and with silica coating of DPGNS. The in vivo photothermal capability of DPGNS resulted in a significant decrease in the tumor cellular area. DPGNS exhibited potential for single node diagnosis and therapy with longer wavelength facilitating deeper imaging and therapy.

Deep tissue photoacoustic computed tomography with a fast and compact laser system

Photoacoustic computed tomography (PACT) holds great promise for biomedical imaging, but wide-spread implementation is impeded by the bulkiness of flash-lamp-pumped laser systems, which typically weigh between 50 - 200 kg, require continuous water cooling, and operate at a low repetition rate. Here, we demonstrate that compact lasers based on emerging diode technologies are well-suited for preclinical and clinical PACT. The diode-pumped laser used in this study had a miniature footprint (13 × 14 × 7 cm³), weighed only 1.6 kg, and outputted up to 80 mJ per pulse at 1064 nm. In vitro, the laser system readily provided over 4 cm PACT depth in chicken breast tissue. In vivo, in addition to high resolution, non-invasive brain imaging in living mice, the system can operate at 50 Hz, which enabled high-speed cross-sectional imaging of murine cardiac and respiratory function. The system also provided high quality, high-frame rate, and non-invasive three-dimensional mapping of arm, palm, and breast vasculature at multi centimeter depths in living human subjects, demonstrating the clinical viability of compact lasers for PACT.

Energy-Looping Nanoparticles: Harnessing Excited-State Absorption for Deep-Tissue Imaging

Near infrared (NIR) microscopy enables noninvasive imaging in tissue, particularly in the NIR-II spectral range (1000-1400 nm) where attenuation due to tissue scattering and absorption is minimized. Lanthanide-doped upconverting nanocrystals are promising deep-tissue imaging probes due to their photostable emission in the visible and NIR, but these materials are not efficiently excited at NIR-II wavelengths due to the dearth of lanthanide ground-state absorption transitions in this window. Here, we develop a class of lanthanide-doped imaging probes that harness an energy-looping mechanism that facilitates excitation at NIR-II wavelengths, such as 1064 nm, that are resonant with excited-state absorption transitions but not ground-state absorption. Using computational methods and combinatorial screening, we have identified Tm(3+)-doped NaYF4 nanoparticles as efficient looping systems that emit at 800 nm under continuous-wave excitation at 1064 nm. Using this benign excitation with standard confocal microscopy, energy-looping nanoparticles (ELNPs) are imaged in cultured mammalian cells and through brain tissue without autofluorescence. The 1 mm imaging depths and 2 μm feature sizes are comparable to those demonstrated by state-of-the-art multiphoton techniques, illustrating that ELNPs are a promising class of NIR probes for high-fidelity visualization in cells and tissue.

Blinking Phase-Change Nanocapsules Enable Background-Free Ultrasound Imaging

Microbubbles are widely used as contrast agents to improve the diagnostic capability of conventional, highly speckled, low-contrast ultrasound imaging. However, while microbubbles can be used for molecular imaging, these agents are limited to the vascular space due to their large size (> 1 μm). Smaller microbubbles are desired but their ultrasound visualization is limited due to lower echogenicity or higher resonant frequencies. Here we present nanometer scale, phase changing, blinking nanocapsules (BLInCs), which can be repeatedly optically triggered to provide transient contrast and enable background-free ultrasound imaging. In response to irradiation by near-infrared laser pulses, the BLInCs undergo cycles of rapid vaporization followed by recondensation into their native liquid state at body temperature. High frame rate ultrasound imaging measures the dynamic echogenicity changes associated with these controllable, periodic phase transitions. Using a newly developed image processing algorithm, the blinking particles are distinguished from tissue, providing a background-free image of the BLInCs while the underlying B-mode ultrasound image is used as an anatomical reference of the tissue. We demonstrate the function of BLInCs and the associated imaging technique in a tissue-mimicking phantom and in vivo for the identification of the sentinel lymph node. Our studies indicate that BLInCs may become a powerful tool to identify biological targets using a conventional ultrasound imaging system.

What is new in nanoparticle-based photoacoustic imaging?

Photoacoustic imaging combines the high temporal and spatial resolution of ultrasound with the good contrast and spectral tuning of optical imaging. Contrast agents are used in photoacoustic imaging to further increase the contrast and specificity of imaging or to image a specific molecular process, e.g., cell-surface proteins or small molecule biomarkers. Nanoparticle-based contrast agents are important tools in photoacoustic imaging because they offer intense and stable signal and can be targeted to specific molecular processes. In this review, we describe some of the most interesting and recent advances in nanoparticle-based photoacoustic imaging including inorganic nanoparticles, organic/polymeric nanoparticles, nanoparticle coatings, multimodality imaging, as well as emerging topics in the field. WIREs Nanomed Nanobiotechnol 2017, 9:e1404. doi: 10.1002/wnan.1404 For further resources related to this article, please visit the WIREs website.

A Phosphorus Phthalocyanine Formulation with Intense Absorbance at 1000 nm for Deep Optical Imaging

Although photoacoustic computed tomography (PACT) operates with high spatial resolution in biological tissues deeper than other optical modalities, light scattering is a limiting factor. The use of longer near infrared wavelengths reduces scattering. Recently, the rational design of a stable phosphorus phthalocyanine (P-Pc) with a long wavelength absorption band beyond 1000 nm has been reported. Here, we show that when dissolved in liquid surfactants, P-Pc can give rise to formulations with absorbance of greater than 1000 (calculated for a 1 cm path length) at wavelengths beyond 1000 nm. Using the broadly accessible Nd:YAG pulse laser emission output of 1064 nm, P-Pc could be imaged through 11.6 cm of chicken breast with PACT. P-Pc accumulated passively in tumors following intravenous injection in mice as observed by PACT. Following oral administration, P-Pc passed through the intestine harmlessly, and PACT could be used to non-invasively observe intestine function. When the contrast agent placed under the arm of a healthy adult human, a PACT transducer on the top of the arm could readily detect P-Pc through the entire 5 cm limb. Thus, the approach of using contrast media with extreme absorption at 1064 nm readily enables high quality optical imaging in vitro and in vivo in humans at exceptional depths.

Photoacoustic imaging driven by an interstitial irradiation source

Photoacoustic (PA) imaging has shown tremendous promise in providing valuable diagnostic and therapy-monitoring information in select clinical procedures. Many of these pursued applications, however, have been relatively superficial due to difficulties with delivering light deep into tissue. To address this limitation, this work investigates generating a PA image using an interstitial irradiation source with a clinical ultrasound (US) system, which was shown to yield improved PA signal quality at distances beyond 13 mm and to provide improved spectral fidelity. Additionally, interstitially driven multi-wavelength PA imaging was able to provide accurate spectra of gold nanoshells and deoxyhemoglobin in excised prostate and liver tissue, respectively, and allowed for clear visualization of a wire at 7 cm in excised liver. This work demonstrates the potential of using a local irradiation source to extend the depth capabilities of future PA imaging techniques for minimally invasive interventional radiology procedures.

A facile synthesis of versatile Cu2-xS nanoprobe for enhanced MRI and infrared thermal/photoacoustic multimodal imaging

A novel type of intelligent nanoprobe by using single component of Cu2-xS for multimodal imaging has been facilely and rapidly synthesized in scale via thermal decomposition followed by biomimetic phospholipid modification, which endows them with uniform and small nanoparticle size (ca.15 nm), well phosphate buffer saline (PBS) dispersity, high stability, and excellent biocompatibility. The as-synthesized Cu2-xS nanoprobes (Cu2-xS NPs) are capable of providing contrast enhancement for T1-weighted magnetic resonance imaging (MRI), as demonstrated by the both in vitro and in vivo imaging investigations for the first time. In addition, due to their strong near infrared (NIR) optical absorption, they can also serve as a candidate contrast agent for enhanced infrared thermal/photoacoustic imaging, to meet the shortfalls of MRI. Hence, complementary and potentially more comprehensive information can be acquired for the early detection and accurate diagnosis of cancer. Furthermore, negligible systematic side effects to the blood and tissue were observed in a relatively long period of 3 months. The distinctive multimodal imaging capability with excellent hemo/histocompatibility of the Cu2-xS NPs could open up a new molecular imaging possibility for detecting and diagnosing cancer or other diseases in the future.

Real-time integrated photoacoustic and ultrasound (PAUS) imaging system to guide interventional procedures: ex vivo study

Because of depth-dependent light attenuation, bulky, low-repetition-rate lasers are usually used in most photoacoustic (PA) systems to provide sufficient pulse energies to image at depth within the body. However, integrating these lasers with real-time clinical ultrasound (US) scanners has been problematic because of their size and cost. In this paper, an integrated PA/US (PAUS) imaging system is presented operating at frame rates >30 Hz. By employing a portable, low-cost, low-pulse-energy (~2 mJ/pulse), high-repetition-rate (~1 kHz), 1053-nm laser, and a rotating galvo-mirror system enabling rapid laser beam scanning over the imaging area, the approach is demonstrated for potential applications requiring a few centimeters of penetration. In particular, we demonstrate here real-time (30 Hz frame rate) imaging (by combining multiple single-shot sub-images covering the scan region) of an 18-gauge needle inserted into a piece of chicken breast with subsequent delivery of an absorptive agent at more than 1-cm depth to mimic PAUS guidance of an interventional procedure. A signal-to-noise ratio of more than 35 dB is obtained for the needle in an imaging area 2.8 × 2.8 cm (depth × lateral). Higher frame rate operation is envisioned with an optimized scanning scheme.

Theranostic probe for simultaneous in vivo photoacoustic imaging and confined photothermolysis by pulsed laser at 1064 nm in 4T1 breast cancer model

Here, we report that polyethylene glycol (PEG)-coated copper(II) sulfide nanoparticles (PEG-CuS NPs) with their peak absorption tuned to 1064 nm could be used both as a contrast agent for photoacoustic tomographic imaging of mouse tumor vasculature and as a mediator for confined photothermolysis of tumor cells in an orthotopic syngeneic 4T1 breast tumor model. PEG-CuS NPs showed stronger photoacoustic signal than hollow gold nanospheres and single-wall carbon nanotubes at 1064 nm. MicroPET imaging of 4T1 tumor-bearing mice showed a gradual accumulation of the NPs in the tumor over time. About 6.5% of injected dose were taken up in each gram of tumor tissue at 24 h after intravenous injection of (64)Cu-labeled PEG-CuS NPs. For both photoacoustic imaging and therapeutic studies, nanosecond (ns)-pulsed laser was delivered with Q-switched Nd:YAG at a wavelength of 1064 nm. Unlike conventional photothermal ablation therapy mediated by continuous wave laser with which heat could spread to the surrounding normal tissue, interaction of CuS NPs with short pulsed laser deliver heat rapidly to the treatment volume keeping the thermal damage confined to the target tissues. Our data demonstrated that it is possible to use a single-compartment nanoplatform to achieve both photoacoustic tomography and highly selective tumor destruction at 1064 nm in small animals.

Near-infrared optical-resolution photoacoustic microscopy

Compared with visible light (380-700 nm), near-infrared light (700-1400 nm) undergoes weaker optical attenuation in biological tissue; thus, it can penetrate deeper. Herein, we demonstrate near-infrared optical-resolution photoacoustic microscopy (NIR-OR-PAM) with 1046 nm illumination. A penetration depth of 3.2 mm was achieved in chicken breast tissue ex vivo using optical fluence within the American National Standards Institute (ANSI) limit (100  mJ/cm2). Beyond ∼0.6  mm deep in chicken breast tissue, NIR-OR-PAM has shown finer resolution than the visible counterpart with 570 nm illumination. The deep imaging capability of NIR-OR-PAM was validated in both a mouse ear and a mouse brain. NIR-OR-PAM of possible lipid contrast was explored as well.

Photoacoustic and ultrasound imaging using dual contrast perfluorocarbon nanodroplets triggered by laser pulses at 1064 nm

Recently, a dual photoacoustic and ultrasound contrast agent-named photoacoustic nanodroplet-has been introduced. Photoacoustic nanodroplets consist of a perfluorocarbon core, surfactant shell, and encapsulated photoabsorber. Upon pulsed laser irradiation the perfluorocarbon converts to gas, inducing a photoacoustic signal from vaporization and subsequent ultrasound contrast from the resulting gas microbubbles. In this work we synthesize nanodroplets which encapsulate gold nanorods with a peak absorption near 1064 nm. Such nanodroplets are optimal for extended photoacoustic imaging depth and contrast, safety and system cost. We characterized the nanodroplets for optical absorption, image contrast and vaporization threshold. We then imaged the particles in an ex vivo porcine tissue sample, reporting contrast enhancement in a biological environment. These 1064 nm triggerable photoacoustic nanodroplets are a robust biomedical tool to enhance image contrast at clinically relevant depths.

Ultrasound-guided photoacoustic imaging: current state and future development

Photoacoustic imaging, frequently coregistered with ultrasonic imaging, can provide functional and cellular/molecular information about tissue within the anatomical landmarks of an imaged region. This review details the fundamentals of photoacoustic imaging and its most promising imaging applications. Particular attention is paid to photoacoustic imaging's relationship with ultrasound, focusing on distinct differences and similarities between the two modalities and highlighting the mutual benefit of using both concurrently in certain preclinical and clinical applications. Much like its origins as an imaging modality were intertwined with ultrasonic imaging (namely, its acoustic transducers and hardware), the future of photoacoustic imaging-particularly in the clinical arena-similarly depends on ultrasound and its time-tested ability to provide real-time visualization of soft tissue.

Silica-coated gold nanoplates as stable photoacoustic contrast agents for sentinel lymph node imaging

A biopsy of the first lymph node to which a tumor drains-the sentinel lymph node (SLN)-is commonly performed to identify micrometastases. Image guidance of the SLN biopsy procedure has the potential to improve its accuracy and decrease its morbidity. We have developed a new stable contrast agent for photoacoustic image-guided SLN biopsy: silica-coated gold nanoplates (Si-AuNPs). The Si-AuNPs exhibit high photothermal stability when exposed to pulsed and continuous wave laser irradiation. This makes them well suited for in vivo photoacoustic imaging. Furthermore, Si-AuNPs are shown to have low cytotoxicity. We tested the Si-AuNPs for SLN mapping in a mouse model where they exhibited a strong, sustained photoacoustic signal. Real-time ultrasound and photoacoustic imaging revealed that the Si-AuNPs quickly drain to the SLN, gradually spreading throughout a large portion of the node.