Optoacoustic Signal Research Articles

Reliability assessment for blood oxygen saturation levels measured with optoacoustic imaging.

Significance: Quantitative optoacoustic (OA) imaging has the potential to provide blood oxygen saturation () estimates due to the proportionality between the measured signal and the blood’s absorption coefficient. However, due to the wavelength-dependent attenuation of light in tissue, a spectral correction of the OA signals is required, and a prime challenge is the validation of both the optical characterization of the tissue and the .Aim: We propose to assess the reliability of levels retrieved from spectral fitting by measuring the similarity of OA spectra to the fitted blood absorption spectra.Approach: We introduce a metric that quantifies the trends of blood spectra by assigning a pair of spectral slopes to each spectrum. The applicability of the metric is illustrated with in vivo measurements on a human forearm.Results: We show that physiologically sound values do not necessarily imply a successful spectral correction and demonstrate how the metric can be used to distinguish values that are trustworthy from unreliable ones.Conclusions: The metric is independent of the methods used for the OA data acquisition, image reconstruction, and spectral correction, thus it can be readily combined with existing approaches, in order to monitor the accuracy of quantitative OA imaging.

In vivo optoacoustic monitoring of percutaneous laser ablation of tumors in a murine breast cancer model

Laser ablation (LA) is a promising approach for minimally invasive cancer treatments. Its in vivo applicability is often impeded by the lack of efficient monitoring tools that can help to minimize collateral tissue damage and aid in determining the optimal treatment end-points. We have devised a new, to the best of our knowledge, hybrid LA approach combining simultaneous volumetric optoacoustic (OA) imaging to monitor the lesion progression accurately in real time and 3D. Time-lapse imaging of laser ablation of solid tumors was performed in a murine breast cancer model in vivo by irradiation of subcutaneous tumors with a 100 mJ short-pulsed (${\sim}{5}\;{\rm ns}$∼5ns) laser operating at 1064 nm and 100 Hz pulse repetition frequency. Local changes in the OA signal intensity ascribed to structural alterations in the tumor vasculature were clearly observed, while the OA volumetric projections recorded in vivo appeared to correlate with cross sections of the excised tumors.

Toward whole-brain in vivo optoacoustic angiography of rodents: modeling and experimental observations.

Cerebrovascular imaging of rodents is one of the trending applications of optoacoustics aimed at studying brain activity and pathology. Imaging of deep brain structures is often hindered by sub-optimal arrangement of the light delivery and acoustic detection systems. In our work we revisit the physics behind opto-acoustic signal generation for theoretical evaluation of optimal laser wavelengths to perform cerebrovascular optoacoustic angiography of rodents beyond the penetration barriers imposed by light diffusion in highly scattering and absorbing brain tissues. A comprehensive model based on diffusion approximation was developed to simulate optoacoustic signal generation using optical and acoustic parameters closely mimicking a typical murine brain. The model revealed three characteristic wavelength ranges in the visible and near-infrared spectra optimally suited for imaging cerebral vasculature of different size and depth. The theoretical conclusions are confirmed by numerical simulations while in vivo imaging experiments further validated the ability to accurately resolve brain vasculature at depths ranging between 0.7 and 7 mm.

Short-wavelength optoacoustic spectroscopy based on water muting

Infrared (IR) optoacoustic spectroscopy can separate a multitude of molecules based on their absorption spectra. However, the technique is limited when measuring target molecules in aqueous solution by strong water absorption at IR wavelengths, which reduces detection sensitivity. Based on the dependence of optoacoustic signal on the temperature of the probed medium, we introduce cooled IR optoacoustic spectroscopy (CIROAS) to mute water contributions in optoacoustic spectroscopy. We showcase that spectral measurements of proteins, lipids, and glucose in the short-wavelength IR region, performed at 4 °C, lead to marked sensitivity improvements over conventional optoacoustic or IR spectroscopy. We elaborate on the dependence of optoacoustic signals on water temperature and demonstrate polarity changes in the recorded signal at temperatures below 4 °C. We further elucidate the dependence of the optoacoustic signal and the muting temperature on sample concentration and demonstrate that changes in these dependences enable quantification of the solute concentration. We discuss how CIROAS may enhance abilities for molecular sensing in the IR.

Ultrasound Guided Optoacoustic Tomography in Assessment of Tumor Margins for Lumpectomies

PURPOSE: To determine the accuracy of a handheld ultrasound-guided optoacoustic tomography (US-OT) probe developed for human deep-tissue imaging in ex vivo assessment of tumor margins postlumpectomy. METHODS: A custom-built two-dimensional (2D) US-OT–handheld probe was used to scan 15 lumpectomy breast specimens. Optoacoustic signals acquired at multiple wavelengths between 700 and 1100 nm were reconstructed using model linear algorithm, followed by spectral unmixing for lipid and deoxyhemoglobin (Hb). Distribution maps of lipid and Hb on the anterior, posterior, superior, inferior, medial, and lateral margins of the specimens were inspected for margin involvement, and results were correlated with histopathologic findings. The agreement in tumor margin assessment between US-OT and histopathology was determined using the Bland–Altman plot. Accuracy, sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of margin assessment using US-OT were calculated. RESULTS: Ninety margins (6 × 15 specimens) were assessed. The US-OT probe resolved blood vessels and lipid up to a depth of 6 mm. Negative and positive margins were discriminated by marked differences in the distribution patterns of lipid and Hb. US-OT assessments were concordant with histopathologic findings in 87 of 89 margins assessed (one margin was uninterpretable and excluded), with diagnostic accuracy of 97.9% (kappa = 0.79). The sensitivity, specificity, PPV, and NPV were 100% (4/4), 97.6% (83/85), 66.7% (4/6), and 100% (83/83), respectively. CONCLUSION: US-OT was capable of providing distribution maps of lipid and Hb in lumpectomy specimens that predicted tumor margins with high sensitivity and specificity, making it a potential tool for intraoperative tumor margin assessment.

An Unsymmetrical Squaraine-Based Activatable Probe for Imaging Lymphatic Metastasis by Responding to Tumor Hypoxia with MSOT and Aggregation-Enhanced Fluorescent Imaging.

Optoacoustic imaging has great potential for preclinical research and clinical practice, and designing robust activatable optoacoustic probes for specific diseases is beneficial for its further development. Herein, an activatable probe has been developed for tumor hypoxia imaging. For this probe, indole and quinoline were linked on each side of an oxocyclobutenolate core to form an unsymmetrical squaraine. A triarylamine group was incorporated to endow the molecule with the aggregation enhanced emission (AEE) properties. In aqueous media, the squaraine chromophore aggregates into the nanoprobe, which specifically responds to nitroreductase and produces strong optoacoustic signals due to its high extinction coefficient, as well as prominent fluorescence emission as a result of its AEE feature. The nanoprobe was used to image tumor metastasis via the lymphatic system both optoacoustically and fluorescently. Moreover, both the fluorescence signals and three-dimensional multispectral optoacoustic tomography signals from the activated nanoprobe allow us to locate the tumor site and to map the metastatic route.

Nanoaggregate Probe for Breast Cancer Metastasis through Multispectral Optoacoustic Tomography and Aggregation-Induced NIR-I/II Fluorescence Imaging.

An activatable nanoprobe for imaging breast cancer metastases through near infrared-I (NIR-I)/NIR-II fluorescence imaging and multispectral optoacoustic tomography (MSOT) imaging was designed. With a dihydroxanthene moiety serving as the electron donor, quinolinium as the electron acceptor and nitrobenzyloxydiphenylamino as the recognition element, the probe can specifically respond to nitroreductase and transform into an activated D-π-A structure with a NIR emission band extending beyond 900 nm. The activated nanoprobe exhibits NIR emission enhanced by aggregation-induced emission (AIE) and produces strong optoacoustic signal. The nanoprobe was used to detect and image metastases from the orthotopic breast tumors to lymph nodes and then to lung in two breast cancer mouse models. Moreover, the nanoprobe can monitor the treatment efficacy during chemotherapeutic course through fluorescence and MSOT imaging.

Nanoaggregate Probe for Breast Cancer Metastasis through Multispectral Optoacoustic Tomography and Aggregation‐Induced NIR‐I/II Fluorescence Imaging

AbstractAn activatable nanoprobe for imaging breast cancer metastases through near infrared‐I (NIR‐I)/NIR‐II fluorescence imaging and multispectral optoacoustic tomography (MSOT) imaging was designed. With a dihydroxanthene moiety serving as the electron donor, quinolinium as the electron acceptor and nitrobenzyloxydiphenylamino as the recognition element, the probe can specifically respond to nitroreductase and transform into an activated D‐π‐A structure with a NIR emission band extending beyond 900 nm. The activated nanoprobe exhibits NIR emission enhanced by aggregation‐induced emission (AIE) and produces strong optoacoustic signal. The nanoprobe was used to detect and image metastases from the orthotopic breast tumors to lymph nodes and then to lung in two breast cancer mouse models. Moreover, the nanoprobe can monitor the treatment efficacy during chemotherapeutic course through fluorescence and MSOT imaging.

Quantitative techniques for extraction of blood oxygenation from multispectral optoacoustic measurements

We report on the comparison of two approaches to the reconstruction of oxygen saturation (StO2) within blood vessels from multispectral optoacoustic (OA) measurements. The spatially-resolving approach is based on determination of the optical absorption coefficient µa from OA signal amplitudes, while the calibration-free approach is based on evaluation of the effective optical attenuation coefficient µeff derived from the in-depth OA signal decay. Both approaches were verified in phantom experiments as well as in in vitro and in vivo measurements. The results demonstrate good agreement of µa spectra derived from OA signal amplitudes with the published data indicating this approach to be preferable for the in vivo determination of StO2. Reconstruction of µa spectra from the in vivo measured OA signal amplitudes in a rat yields StO2 values of 0.57 ± 0.08 and 0.50 ± 0.07 for two veins of the thoracic spine. We demonstrate that for cost-effective measurements a dual-probing wavelength scheme can be employed instead of multiple wavelength probing. We found that the probing wavelength of 700 nm combined with a wavelength from the range of 850–1069 nm provides reconstructed StO2 values close to the reference ones derived from multiple wavelength measurements in the entire range of 658‒1069 nm.

Experimental studies of optoacoustic effect on the model of erythrocytes in the presence of carbon nanoparticles

Experimental model has been developed to study optoacoustic signal from model blood cells presented by polystyrene microspheres with nanoparticles. It was found out that nanoparticles due to their strong absorption of light significantly affect the coefficient of cellular optical absorption, while the thermophysical parameters, namely the coefficient of thermal expansion, compressibility and isobaric specific heat of cells remain unchanged, since nanoparticles occupy a small intracellular volume compared to the cell volume. Optoacoustic signals were obtained using model solutions at various concentrations of cells and nanoparticles using 1064 nm laser. The results of experimental measurements using LIMO 100–532/1064-U system based on Nd:YAG showed that the amplitude of the optoacoustic signal increased without increasing the temperature in the laser area.

Optoacoustic properties of Doxorubicin - A pilot study.

Doxorubicin (DOX) is a widely used chemotherapeutic anticancer drug. Its intrinsic fluorescence properties enable investigation of tumor response, drug distribution and metabolism. First phantom studies in vitro showed optoacoustic property of DOX. We therefore aimed to further investigate the optoacoustic properties of DOX in biological tissue in order to explore its potential as theranostic agent. We analysed doxorubicin hydrochloride (Dox·HCl) and liposomal encapsulated doxorubicin hydrochloride (Dox·Lipo), two common drugs for anti-cancer treatment in clinical medicine. Optoacoustic measurements revealed a strong signal of both doxorubicin substrates at 488 nm excitation wavelength. Post mortem analysis of intra-tumoral injections of DOX revealed a detectable optoacoustic signal even at three days after the injection. We thereby demonstrate the general feasibility of doxorubicin detection in biological tissue by means of optoacoustic tomography, which could be applied for high resolution imaging at mesoscopic depths dictated by effective penetration of visible light into the biological tissues.

Diagnosing Drug-Induced Liver Injury by Multispectral Optoacoustic Tomography and Fluorescence Imaging Using a Leucine-Aminopeptidase-Activated Probe.

Drug-induced liver injury (DILI) is a frequent cause of hepatic dysfunction as well as the single most frequent reason for removing approved medications from the market, and multispectral optoacoustic tomography (MSOT) is an emerging and noninvasive imaging modality for diagnosing and monitoring diseases. Herein, we report an activatable optoacoustic probe for imaging DILI through detecting the activity of leucine aminopeptidase (LAP). In this probe, an N-terminal leucyl moiety serving as the LAP recognition element is linked with a chromene-benzoindolium chromophore via 4-aminobenzylalcohol group. The elevated expression of hepatic LAP as a result of DILI cleaves the leucyl moiety and causes the red-shift of the probe's absorption band, thereby generating prominent optoacoustic signals for MSOT imaging. During this process, the probe also exhibits prominent NIR fluorescence, which can be utilized for fluorescent imaging. More importantly, by rendering stacks of cross-sectional images as maximal intensity projection (MIP) images, we could precisely locate the focus of drug-induced liver injury in mice. This probe is expected to serve a powerful tool for studying physiological and pathological processes related to LAP.

Synthetic data framework to estimate the minimum detectable concentration of contrast agents for multispectral optoacoustic imaging of small animals.

The concentrations of contrast agents for optoacoustic imaging of small animals must usually be optimized through extensive pilot experiments on a case-by-case basis. The present work describes a streamlined approach for determining the minimum detectable concentration (MDC) of a contrast agent given experimental conditions and imaging system parameters. The developed Synthetic Data Framework (SDF) allows estimation of MDCs of various contrast agents under different tissue conditions without extensive animal experiments. The SDF combines simulated optoacoustic signals from exogenously administered contrast agents with in vivo experimental signals from background tissue to generate realistic synthetic multispectral optoacoustic images. In this paper, the SDF is validated with in vivo measurements and demonstrates close agreement between SDF synthetic data and experimental data in terms of both image intensity and MDCs. Use of the SDF to estimate MDCs for fluorescent dyes and nanoparticles at different tissue depths and for imaging lesions of different sizes is illustrated.

Method of Modeling Optoacoustic Signals in Composites Transparent Matrix – Metal Nanoparticles

Method of modeling optoacoustic signals initiated by a laser pulse in composites transparent matrix – metal nanoparticles taking into account melting processes has been developed and tested. The method consists in calculating the function of pressure sources depending on time and coordinate and its convolution with the Green’s function of the one-dimensional wave equation. Testing has been performed for composites of pentaerythritol tetranitrate with 50 nm radius aluminum nanoparticles that are important for practical applications. The melting is characterized by an increase in the specific volume and leads to an increase in the amplitude of the maximum of the source function and the appearance of the area of its negative values. The dependences have been calculated of the effective growth constant of the optoacoustic signal and its amplitude on the pulse energy density that must be taken into account in this method. The results are important for the development of methods of nondestructive testing and prediction of functioning of photonic devices and optical detonators containing nanoparticles.

Modeling of Optoacoustic Signals in a System with Elastic Scattering and Light Absorption

We propose a method for simulating optoacoustic signals initiated by a laser pulse in an absorbing medium with elastic radiation scattering and carry out corresponding calculations. It is shown that the use of the method of spherical harmonics allows obtaining an analytical expression for the optoacoustic signal. The deviation of the absorption profile from the Bouguer type near the sample surface leads to a strong influence of the medium parameters on the position of the signal maximum. The growth of the albedo of onefold scattering leads to a decrease in the effective signal growth constant and its amplitude.

Phototrophic purple bacteria as optoacoustic in vivo reporters of macrophage activity

Τhe morphology, physiology and immunology, of solid tumors exhibit spatial heterogeneity which complicates our understanding of cancer progression and therapy response. Understanding spatial heterogeneity necessitates high resolution in vivo imaging of anatomical and pathophysiological tumor information. We introduce Rhodobacter as bacterial reporter for multispectral optoacoustic (photoacoustic) tomography (MSOT). We show that endogenous bacteriochlorophyll a in Rhodobacter gives rise to strong optoacoustic signals >800 nm away from interfering endogenous absorbers. Importantly, our results suggest that changes in the spectral signature of Rhodobacter which depend on macrophage activity inside the tumor can be used to reveal heterogeneity of the tumor microenvironment. Employing non-invasive high resolution MSOT in longitudinal studies we show spatiotemporal changes of Rhodobacter spectral profiles in mice bearing 4T1 and CT26.WT tumor models. Accessibility of Rhodobacter to genetic modification and thus to sensory and therapeutic functions suggests potential for a theranostic platform organism.

Bioengineered bacterial vesicles as biological nano-heaters for optoacoustic imaging

Advances in genetic engineering have enabled the use of bacterial outer membrane vesicles (OMVs) to deliver vaccines, drugs and immunotherapy agents, as a strategy to circumvent biocompatibility and large-scale production issues associated with synthetic nanomaterials. We investigate bioengineered OMVs for contrast enhancement in optoacoustic (photoacoustic) imaging. We produce OMVs encapsulating biopolymer-melanin (OMVMel) using a bacterial strain expressing a tyrosinase transgene. Our results show that upon near-infrared light irradiation, OMVMel generates strong optoacoustic signals appropriate for imaging applications. In addition, we show that OMVMel builds up intense heat from the absorbed laser energy and mediates photothermal effects both in vitro and in vivo. Using multispectral optoacoustic tomography, we noninvasively monitor the spatio-temporal, tumour-associated OMVMel distribution in vivo. This work points to the use of bioengineered vesicles as potent alternatives to synthetic particles more commonly employed for optoacoustic imaging, with the potential to enable both image enhancement and photothermal applications.

Multispectral optoacoustic imaging of dynamic redox correlation and pathophysiological progression utilizing upconversion nanoprobes

Precise and differential profiling of the dynamic correlations and pathophysiological implications of multiplex biological mediators with deep penetration and highly programmed precision remain critical challenges in clinics. Here we present an innovative strategy by tailoring a powerful multispectral optoacoustic tomography (MSOT) technique with a photon-upconverting nanoprobe (UCN) for simultaneous visualization of diversely endogenous redox biomarkers with excellent spatiotemporal resolution in living conditions. Upon incorporating two specific radicals-sensitive NIR cyanine fluorophores onto UCNs surface, such nanoprobes can orthogonally respond to disparate oxidative and nitrosative stimulation, and generate spectrally opposite optoacoustic signal variations, which thus achieves compelling superiorities for reversed ratiometric tracking of multiple radicals under dual independent wavelength channels, and significantly, for precise validating of their complex dynamics and correlations with redox-mediated pathophysiological procession in vivo.

Laser Initiation of PETN with Inclusions of Aluminum Nanoparticles under Static Pressure

This paper describes the experimental measurement of thresholds of explosive decomposition of PETN with inclusions of aluminum nanoparticles (an average particle diameter of 100 nm) with a static pressure of 0–0.288 GPa applied to the samples under the action of the first harmonic of a pulsed (14 ns) neodymium laser. Amplitudes of optoacoustic signals as a function of concentration of inclusions in the samples with a fixed density of laser initiation energy are measured. There is a significant decrease in the initiation threshold, which is due to the fact that a gas-dynamic unload is blocked and the sample density increases.

Effect of Variations in the Gas-Dynamic Unloading on the Laser Initiation of the PETN–Aluminum Composite

Thresholds of explosive decomposition Hcr under irradiation using the first harmonic of a pulsed neodymium laser (14 ns) are experimentally determined for pentaerythritol tetranitrate (PETN) with aluminum nanoparticles (100 nm) at concentrations ranging from 0.05 to 1 wt %. Amplitudes of optoacoustic signals are measured versus concentrations of impurities in the sample at a fixed laser fluence. The experiments are performed in two regimes. In the first one, the irradiated surface of the sample is covered with a finite-weight glass plate that impedes the gas-dynamic unloading. In the second regime, an external pressure of no less than 107 Pa is applied to the glass plate to prevent the gas-dynamic unloading. The interpretation of the experimental effects is proven by the results of theoretical simulation.