Photoacoustic Imaging Research Articles

Transparent ultrasonic transducers based on relaxor ferroelectric crystals for advanced photoacoustic imaging

Photoacoustic imaging is a promising non-invasive functional imaging modality for fundamental research and clinical diagnosis. However, achieving capillary-level resolution, wide field-of-view, and high frame rates remains challenging. To address this, we propose a transparent ultrasonic transducer design using our developed transparent Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals. Our fabrication technique incorporates quartz-glass-and-epoxy matching layers with low-resistance indium-tin-oxide electrodes through a brass-ring based structure, enabling a high frequency (28.5 MHz), wide bandwidth (78%), and enhanced pulse-echo sensitivity (2.5 V under 2-μJ pulse excitation). Our Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3-based transparent ultrasonic transducer demonstrates a four-fold enhancement in photoacoustic detection sensitivity when compared to the LiNbO3-based counterpart, leading to a 13 dB improvement of signal-to-noise ratio in microvascular photoacoustic imaging. This enables dynamic monitoring of mouse cerebral cortex microvasculature during seizures at 0.8 Hz frame rates over a 1.5 × 1.5 mm2 field-of-view. Our work paves the way for high-performance and compact photoacoustic imaging systems using advanced piezoelectric materials.

Ultrasound-assisted aberration correction of transcranial photoacoustic imaging based on angular spectrum theory

To correct the refraction aberration induced by the skull in photoacoustic imaging, a method for phase distortion compensation is proposed based on the angular spectrum theory with the aid of ultrasonic signals. This method first updates the speed of sound distribution by iteratively performing aberration correction in the ultrasonic reconstruction. Then the speed of sound distribution obtained with ultrasound-assisted serves as prior knowledge to address phase distortion compensation by adjusting the phase shift factor of the wavefront in different media. Finally, the aberration-corrected ultrasonic-photoacoustic dual-modality image can be obtained. Numerical simulations and phantom experiments confirm the effectiveness of this method. Specifically, in simulations, the position error of the proposed method is reduced from −13.61% to 1.27% in depth compared to the method based on the reconstruction with constant speed. Moreover, a real ex-vivo rabbit skull experiment illustrates the potential biological application of the proposed method in transcranial photoacoustic imaging.

Noninvasive high-resolution deep-brain photoacoustic imaging with a negatively focused fiber-laser ultrasound transducer

Noninvasive high-resolution deep-brain imaging is essential to fundamental cognitive process study and neuroprotective drugs development. Although optical microscopes can resolve fine biological structures with good contrast without exposure to ionizing radiation or a strong magnetic field, the optical scattering limits the penetration depth and hinders its capability for deep-brain imaging. Here, in vivo high-resolution imaging of the whole mouse brain is demonstrated by using a photoacoustic computed tomography system with a negatively focused fiber-laser ultrasound transducer. By leveraging the high flexibility and low bending loss of the optical fiber, a rationally designed negatively focused fiber laser cavity exhibits a low detection limit down to 5.4 Pa and a broad view angle of ∼120 deg, enabling mouse brain imaging with a penetration larger than 7 mm and a nearly isotropic spatial resolution of ∼130 μm. In addition, the negative curvature of the fiber laser reduces the working distance, which facilitates the development of a compact and portable linear scanning imaging system. In vivo imaging of a mouse model with intracerebral hemorrhage is also showcased to demonstrate its capability for potential biomedical and clinical applications. With high spatial resolution and large tissue penetration, the system may provide a noninvasive, user-friendly, and high-performance imaging solution for biomedical research and preclinical/clinical diagnosis.

Sensitive Optomechanical Ultrasound Sensor in an LED-Based, Low Fluence Photoacoustic Imaging System

Sensitive Optomechanical Ultrasound Sensor in an LED-Based, Low Fluence Photoacoustic Imaging System

A hypoxia-activated and microenvironment-remodeling nanoplatform for multifunctional imaging and potentiated immunotherapy of cancer

Activatable theranostic systems combining precise diagnosis and robust immune activation have significant potential in cancer treatment. Herein, we develop a versatile nanoplatform integrating hypoxia-activatable molecular imaging with effective photoimmunotherapy for cancer treatment. Our molecular probe features turn-on near-infrared-II (NIR-II) fluorescence and photoacoustic signals in hypoxic tumor environments. It also induces hypoxia-triggered photodynamic and photothermal effects, promoting immunogenic cell death and activating the STING pathway, engaging both innate and adaptive immunity. The molecular probe is formulated with a vascular disrupting agent to amplify the hypoxia-responsive phototheranostic properties, on which M1-like macrophage membrane is camouflaged to shield against premature release while conferring cancer-targeting affinity. The activatable NIR-II fluorescence and photoacoustic imaging enable precise tumor delineation, while the enhanced phototherapy activates tumor-specific cytotoxic T cells, impeding both primary and distant tumor progression and providing protective immunity against rechallenge in 4T1 tumor-bearing female mice. This work advances activatable theranostic protocols for image-guided immunotherapy.

Ultra-sparse reconstruction for photoacoustic tomography: Sinogram domain prior-guided method exploiting enhanced score-based diffusion model

Photoacoustic tomography, a novel non-invasive imaging modality, combines the principles of optical and acoustic imaging for use in biomedical applications. In scenarios where photoacoustic signal acquisition is insufficient due to sparse-view sampling, conventional direct reconstruction methods significantly degrade image resolution and generate numerous artifacts. To mitigate these constraints, a novel sinogram-domain priors guided extremely sparse-view reconstruction method for photoacoustic tomography boosted by enhanced diffusion model is proposed. The model learns prior information from the data distribution of sinograms under full-ring, 512-projections. In iterative reconstruction, the prior information serves as a constraint in least-squares optimization, facilitating convergence towards more plausible solutions. The performance of the method is evaluated using blood vessel simulation, phantoms, and in vivo experimental data. Subsequently, the transformation of the reconstructed sinograms into the image domain is achieved through the delay-and-sum method, enabling a thorough assessment of the proposed method. The results show that the proposed method demonstrates superior performance compared to the U-Net method, yielding images of markedly higher quality. Notably, for in vivo data under 32 projections, the sinogram structural similarity improved by ∼21 % over U-Net, and the image structural similarity increased by ∼51 % and ∼84 % compared to U-Net and delay-and-sum methods, respectively. The reconstruction in the sinogram domain for photoacoustic tomography enhances sparse-view imaging capabilities, potentially expanding the applications of photoacoustic tomography.

Photoacoustic polydopamine-indocyanine green (PDA-ICG) nanoprobe for detection of senescent cells

Cellular senescence is considered an important tumour suppression mechanism in response to damage and oncogenic stress in early lesions. However, when senescent cells are not immune-cleared and persist in the tumour microenvironment, they can drive a variety of tumour-promoting activities, including cancer initiation, progression, and metastasis. Additionally, there is compelling evidence demonstrating a direct connection between chemo(radio)therapy-induced senescence and the development of drug resistance and cancer recurrence. Therefore, detection of senescent cells in tissues holds great promise for predicting cancer occurrence earlier, assessing tumour progression, aiding patient stratification and prognosis, and informing about the efficacy of potential senotherapies. However, effective detection of senescent cells is limited by lack of biomarkers and readout strategies suitable for in vivo clinical imaging. To this end, a nanoprobe composed of biocompatible polydopamine (PDA) nanoparticle doped with FDA-approved indocyanine green (ICG) dye, namely PDA-ICG, was designed as a contrast agent for senescence detection using photoacoustic imaging (PAI). In an in vitro model of chemotherapy-induced senescence, PDA-ICG nanoprobe showed an elevated uptake in senescent cells relative to cancer cells. In addition to its improved photostability, 2.5-fold enhancement in photoacoustic signal relative to ICG was observed. Collectively, the results indicate that the PDA-ICG nanoprobe has the potential to be used as a contrast agent for senescence detection of chemotherapy-induced senescence using PAI.

Shape and Size Dependence of Pharmacokinetics, Biodistribution, and Toxicity of Gold Nanoparticles.

Gold nanoparticles (AuNPs) are extensively utilized in biomolecular sensing, photothermal therapy, drug delivery, and various imaging techniques like photoacoustic and fluorescent imaging. Despite their diverse applications, inconsistent findings from previous toxicity studies underscore the critical need for standardized methodologies. This study introduces ten distinct types of AuNPs─cubes, stars, rods, dumbbells, and bipyramids at sizes of 50 and 100 nm, to systematically assess their toxicity under controlled conditions both in vitro and in vivo. Our findings reveal a clear correlation between cytotoxicity and the morphology, size, incubation duration, and concentration of AuNPs. Anisotropically shaped nanoparticles, such as nanorods, nanodumbbells, and nanobipyramids, tend to exhibit higher cytotoxicity compared to more spherical forms like nanocubes and nanostars. Interestingly, while in vivo plasma biochemistry parameters show minimal variation, biodistribution, histopathological alterations, and pharmacokinetics are notably influenced by the shape and size of AuNPs. In most instances, smaller and anisotropic AuNPs that remain in the bloodstream for extended periods are observed. This research offers significant insights into the design of AuNPs with specific morphologies and sizes, particularly for their application in drug delivery systems via intravenous injection. These outcomes emphasize the nuanced toxicity profiles of AuNPs, necessitating tailored approaches in preclinical and clinical research.

Mid-infrared photoacoustic brain imaging enabled by cascaded gas-filled hollow-core fiber lasers.

Extending the photoacoustic microscopy (PAM) into the mid-infrared (MIR) molecular fingerprint region constitutes a promising route toward label-free imaging of biological molecular structures. Realizing this objective requires a high-energy nanosecond MIR laser source. However, existing MIR laser technologies are limited to either low pulse energy or free-space structure that is sensitive to environmental conditions. Fiber lasers are promising technologies for PAM for their potential to offer both high pulse energy and robust performance, which however have not yet been used for PAM because it is still at the infant research stage. We aim to employ the emerging gas-filled anti-resonant hollow-core fiber (ARHCF) laser technology for MIR-PAM for the purpose of imaging myelin-rich regions in a mouse brain. This laser source is developed with a high-pulse-energy nanosecond laser at , targeting the main absorption band of myelin sheaths, the primary chemical component of axons in the central nervous system. The laser mechanism relies on two-order gas-induced vibrational stimulated Raman scattering for non-linear wavelength conversion, starting from a 1060-nm pump laser to through the two-stage gas-filled ARHCFs. The developed fiber Raman laser was used for the first time for MIR-PAM of mouse brain regions containing structures rich in myelin. The high peak power of and robust performance of the generated MIR Raman pulse addressed the challenge faced by the commonly used MIR lasers. We pioneered the potential use of high-energy and nanosecond gas-filled ARHCF laser source to MIR-PAM, with a first attempt to report this kind of fiber laser source for PAM of lipid-rich myelin regions in a mouse brain. We also open up possibilities for expanding into a versatile multiwavelength laser source covering multiple biomarkers and being employed to image other materials such as plastics.

High-sensitive Fabry-Perot cavity-enhanced optical resonator for photoacoustic sensing

Highly sensitive broadband acoustic detectors are needed to expand the capabilities of geological exploration, photoacoustic imaging, and industrial inspection techniques. However, while pursuing miniaturization, it is difficult to combine high sensitivity and wide acoustic detection frequency range. Meanwhile, the consistency and mechanical stability of the manufacturing process become important challenges for optical sensors in practical applications. To address this issue, we present a new silicon-based cavity-enhanced Fabry-Pérot interferometer photoacoustic sensor and fully characterize its acoustic performance. Micro-resonant cavity-enhanced photoacoustic sensor with broadband acoustic responses up to 50 Hz-10 k Hz has been fabricated. The detection sensitivity is also impressive, reaching -120.23 dB re rad/µPa @ 1 k Hz, with a noise equivalent pressure (NEP) of 88.7 µPa/√Hz @ 1 k Hz. This approach will help design photoacoustic sensors to improve detection sensitivity and bandwidth with limited fabrication accuracy and size.

NIR Diradical-Featured Croconaine Nanoagent for Cancer Photoacoustic Imaging and Photothermal Therapy.

Radical-featured organic materials usually demonstrate significantly narrower bandgap than that of regular organic materials, which is beneficial for near-infrared (NIR) absorption and non-radiative decay process, making them good candidates for photothermal agents (PTAs) of photothermal therapy (PTT). However, most organic radicals are too short-lived to be separated and stable. Herein, a stable pyrylium-croconaine (CR) dye CR845 with high diradical character was synthesized and assembled into CR845 nanoparticles (NPs) to explore the photoacoustic imaging (PAI) and photothermal therapeutic performance under NIR irradiation in vitroand in vivo. CR845 exhibited a stable NIR absorption at 845 nm (ε= 3.37 × 105L mol-1cm-1) and a relatively high photothermal conversion efficiency (PCE) of 58.8%. When assembled into nanoparticles, CR845 NPs showed good photostability, good biocompatibility, favorable NIR photoacoustic potential, effective cancer cell killing and complete tumor elimination upon 850 nm laser irradiation. The work demonstrates a diradical-featured CR PTA with favorable PAI/PTT effects, which can help to provide new ideas for the development of NIR PTAs, and further promote the application of organic radical materials in the field of biomedicine.

Enhanced dual-mode imaging: Superior photoacoustic and ultrasound endoscopy in live pigs using a transparent ultrasound transducer.

Dual-mode photoacoustic/ultrasound endoscopy (ePAUS) is a promising tool for preclinical and clinical interventions. To be clinically useful, ePAUS must deliver high-performance ultrasound imaging comparable to commercial systems and maintain high photoacoustic imaging performance at long working distances. This requires a transducer with an intact physical aperture and coaxial alignment of acoustic and optical beams within the probe, a challenging integration task. We present a high-performance ePAUS probe with a miniaturized, optically transparent ultrasonic transducer (TUT) called ePAUS-TUT. The 1.8-mm-diameter probe, fitting into standard endoscopic channels, aligns acoustic and optical beams efficiently, achieving commercial-level ultrasound and high-resolution photoacoustic imaging over long distances. These imaging capabilities were validated through in vivo imaging of a rat's rectum and a pig's esophagus. The ePAUS-TUT system substantially enhances feasibility and potential for clinical applications.

4T1 Cell Membrane-Coated Pdots with NIR-II Absorption and Fluorescence Properties for Targeted Phototheranostics of Breast Tumors.

Designing highly biocompatible organic semiconducting conjugated polymer dots (Pdots) with bright fluorescence and superior absorption properties in the second near-infrared window (NIR-II: 1000-1700 nm) remains a huge challenge for tumor phototheranostics. In this study, we constructed 4T1 cell membrane-coated m-PBTQ4F Pdots (CPdots) with enhanced NIR-II photoacoustic (PA) and fluorescence (FL) imaging capability for NIR-II photothermal therapy (PTT) of breast tumors. Our findings demonstrated that CPdots could specifically target breast tumors, leading to enhanced tumor accumulation after systemic administration in living mice. In addition, CPdots can not only serve as contrast agents for NIR-II PA and FL imaging for improved breast tumor detection but also generate more cytotoxic heat to improve PTT efficacy. Therefore, this pilot study opens an option avenue for developing new NIR-II Pdots with homologous targeting capability for enhanced phototheranostics of breast tumors.

Optimizing quantitative photoacoustic imaging systems: the Bayesian Cramér–Rao bound approach

Quantitative photoacoustic computed tomography (qPACT) is an emerging medical imaging modality that carries the promise of high-contrast, fine-resolution imaging of clinically relevant quantities like hemoglobin concentration and blood-oxygen saturation. However, qPACT image reconstruction is governed by a multiphysics, partial differential equation (PDE) based inverse problem that is highly non-linear and severely ill-posed. Compounding the difficulty of the problem is the lack of established design standards for qPACT imaging systems, as there is currently a proliferation of qPACT system designs for various applications and it is unknown which ones are optimal or how to best modify the systems under various design constraints. This work introduces a novel computational approach for the optimal experimental design of qPACT imaging systems based on the Bayesian Cramér-Rao bound (CRB). Our approach incorporates several techniques to address challenges associated with forming the bound in the infinite-dimensional function space setting of qPACT, including priors with trace-class covariance operators and the use of the variational adjoint method to compute derivatives of the log-likelihood function needed in the bound computation. The resulting Bayesian CRB based design metric is computationally efficient and independent of the choice of estimator used to solve the inverse problem. The efficacy of the bound in guiding experimental design was demonstrated in a numerical study of qPACT design schemes under a stylized two-dimensional imaging geometry. To the best of our knowledge, this is the first work to propose Bayesian CRB based design for systems governed by PDEs.

Improving the Theranostic Potential of Magnetic Nanoparticles by Coating with Natural Rubber Latex for Ultrasound, Photoacoustic Imaging, and Magnetic Hyperthermia: An In Vitro Study.

Magnetic nanoparticles (MNPs) have gained attention in theranostics for their ability to combine diagnostic imaging and therapeutic capabilities in a single platform, enhancing targeted treatment and monitoring. Surface coatings are essential for stabilizing MNPs, improving biocompatibility, and preventing oxidation that could compromise their functionality. Natural rubber latex (NRL) offers a promising coating alternative due to its biocompatibility and stability-enhancing properties. While NRL-coated MNPs have shown potential in applications such as magnetic resonance imaging, their effectiveness in theranostics, particularly magnetic hyperthermia (MH) and photoacoustic imaging (PAI), remains underexplored. In this study, iron oxide nanoparticles were synthesized via coprecipitation, using NRL as the coating agent. The samples were labeled by NRL amount used during synthesis: NRL-100 for 100 μL and NRL-400 for 400 μL. Characterization results showed that NRL-100 and NRL-400 samples exhibited improved stability with zeta potentials of -27 mV and -30 mV, respectively and higher saturation magnetization values of 79 emu/g and 88 emu/g of Fe3O4. Building on these findings, we evaluated the performance of these nanoparticles in biomedical applications, including magnetomotive ultrasound (MMUS), PAI, and MH. NRL-100 and NRL-400 samples showed greater displacements and higher contrast in MMUS than uncoated samples (5, 8, and 9 µm) at 0.5 wt%. In addition, NRL-coated samples demonstrated an improved signal-to-noise ratio (SNR) in PAI. SNR values were 24.72 (0.51), 31.44 (0.44), and 33.81 (0.46) dB for the phantoms containing uncoated MNPs, NRL-100, and NRL-400, respectively. Calorimetric measurements for MH confirmed the potential of NRL-coated MNPs as efficient heat-generating agents, showing values of 43 and 40 W/g for NRL-100 and NRL-400, respectively. Overall, NRL-coated MNPs showed great promise as contrast agents in MMUS and PAI imaging, as well as in MH applications.

Full-view volumetric photoacoustic imaging using a hemispheric transducer array combined with an acoustic reflector

Photoacoustic computed tomography (PACT) has evoked extensive interest for applications in preclinical and clinical research. However, the current systems suffer from the limited view provided by detection setups, thus impeding the sufficient acquisition of intricate tissue structures. Here, we propose an approach to enable fast 3D full-view imaging. A hemispherical ultrasonic transducer array combined with a planar acoustic reflector serves as the ultrasonic detection device in the PACT system. The planar acoustic reflector can create a mirrored virtual transducer array, and the detection view range can be enlarged to cover approximately 3.7 π steradians in our detection setup. To verify the effectiveness of our proposed configuration, we present the imaging results of a hair phantom, an in vivo zebrafish larva, and a leaf skeleton phantom. Furthermore, the real-time dynamic imaging capacity of this system is demonstrated by observing the movement of zebrafish within 2 s. This strategy holds great potential for both preclinical and clinical research by providing more detailed and comprehensive images of biological tissues.

Biodegradable lipid bilayer-assisted indocyanine green J- aggregates for photothermal therapy: Formulation, in vitro toxicity and in vivo clearance

Indocyanine green (ICG) J-aggregates (IJA) are a unique form of aggregation that exhibits superior properties to monomeric ICG. Despite their higher photoacoustic (PA) signals for imaging and heating stability during photothermal therapy (PTT), they exhibit low stability under a biological milieu. Our group previously proposed a simple procedure for in-situ preparation of IJA into liposomes, accelerating their formation and optical properties. To comprehend their potential applications, we systematically investigated the effect of the lipid bilayer composition on ICG J-aggregation and stability. Moreover, their in vitro compatibility and photothermal toxicity in monolayers and cancer spheroids, besides their in vivo biodistribution and clearance were evaluated. Our findings revealed the importance of high cholesterol and PEG-lipid content and low charged lipids (∼ 5 mol %) in liposomes to promote a high IJA/ICG ratio and, thus, high heating stability. More importantly, IJA-liposomes revealed high biocompatibility in monolayer and cancer spheroids with efficient photothermal toxicity. Finally, IJA-liposomes were cleared from the body without toxicity. Interestingly, IJA-liposomes mainly showed lower affinity to the liver than monomeric ICG, resulting in higher renal clearance. Overall, our biodegradable IJA-liposomes could be an excellent alternative to gold-based agents suitable for PA imaging and cancer PTT.

Cross-sectional imaging of speed-of-sound distribution using photoacoustic reversal beacons

Photoacoustic tomography (PAT) enables non-invasive cross-sectional imaging of biological tissues, but it fails to map the spatial variation of speed-of-sound (SOS) within tissues. While SOS is intimately linked to density and elastic modulus of tissues, the imaging of SOS distribution serves as a complementary imaging modality to PAT. Moreover, an accurate SOS map can be leveraged to correct for PAT image degradation arising from acoustic heterogeneities. Herein, we propose a method for SOS imaging using scanned photoacoustic beacons excited by short laser pulse with inversion reconstruction. Our method is based on photoacoustic reversal beacons (PRBs), which are small light-absorbing targets with strong photoacoustic contrast. We excite and scan a number of PRBs positioned at the periphery of the target, and the generated photoacoustic waves propagate through the target from various directions, thereby achieve spatial sampling of the internal SOS. By picking up the PRB signal using a graph-based dynamic programing algorithm, we formulate a linear inverse model for pixel-wise SOS reconstruction and solve it with iterative optimization technique. We validate the feasibility of the proposed method through simulations, phantoms, and ex vivo biological tissue tests. Experimental results demonstrate that our approach can achieve accurate reconstruction of SOS distribution. Leveraging the obtained SOS map, we further demonstrate significantly enhanced PAT image reconstruction with acoustic correction.

De novo strategy of organic semiconducting polymer brushes for NIR-II light-triggered carbon monoxide release to boost deep-tissue cancer phototheranostics

The integration of photoacoustic imaging (PAI) and photothermal therapy (PTT) within the second near-infrared (NIR-II) window, offering a combination of high-resolution imaging and precise non-invasive thermal ablation, presents an attractive opportunity for cancer treatment. Despite the significant promise, the development of this noninvasive phototheranostic nanomedicines encounters challenges that stem from tumor thermotolerance and limited therapeutic efficacy. In this contribution, we designed an amphiphilic semiconducting polymer brush (SPB) featuring a thermosensitive carbon monoxide (CO) donor (TDF-CO) for NIR-II PAI-assisted gas-augmented deep-tissue tumor PTT. TDF-CO nanoparticles (NPs) exhibited a powerful photothermal conversion efficiency (43.1%) and the capacity to trigger CO release after NIR-II photoirradiation. Notably, the liberated CO not only acts on mitochondria, leading to mitochondrial dysfunction and promoting cellular apoptosis but also hinders the overexpression of heat shock proteins (HSPs), enhancing the tumor’s thermosensitivity to PTT. This dual action accelerates cellular thermal ablation, achieving a gas-augmented synergistic therapeutic effect in cancer treatment. Intravenous administration of TDF-CO NPs in 4T1 tumor-bearing mice demonstrated bright PAI signals and remarkable tumor ablation under 1064 nm laser irradiation, underscoring the potential of CO-mediated photothermal/gas synergistic therapy. We envision this tailor-made multifunctional NIR-II light-triggered SPB provides a feasible approach to amplify the performance of PTT for advancing future cancer phototheranostics.Graphical abstract

Impact of skin tone on target size detectability in photoacoustic breast imaging

Impact of skin tone on target size detectability in photoacoustic breast imaging