Search

The work presents the results of a research on the photoacoustic spectra of thin surface layers of Cd1-xBexTe crystals formed by grinding and polishing their surfaces. As a result of matching the theoretical and experimental photoacoustic spectra, thermal and optical parameters of these layers were determined. Thermal parameters of the surface layers, such as thermal conductivity and thermal diffusivity, turned out to be much worse than the analogous parameters of the substrate. The increase in the optical absorption of surface layers for photon energies below Eg was also determined. [ABSTRACT FROM AUTHOR]

Although the photoacoustic effect is typically generated by pulsed or amplitude modulated optical beams, it is clear from examination of the wave equation for pressure that motion of an optical source in space will result in the production of sound as well. Here, the properties of the photoacoustic effect generated by moving sources in one dimension are investigated. The cases of a moving Gaussian beam, an oscillating delta function source, and an accelerating Gaussian optical sources are reported. The salient feature of one-dimensional sources in the linear acoustic limit is that the amplitude of the beam increases in time without bound. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate remarkable photoacoustic emission from self-supported Au nanoparticle/ dielectric/Ag mirror structures. They were prepared by depositing thin films on both sides of a 50-nm-thick silicon nitride membrane. The photoacoustic emission generated by the self-supported sample is more than 2 orders of magnitude larger than that of a similar structure supported by a Si wafer at a laser modulation frequency of 1 kHz. In this frequency region, the thermal diffusion length becomes much greater than the thickness of the multilayers, so that the photoacoustic amplitude is determined mainly by the thermal conductivity of the substrate. Therefore, the large enhancement of the photoacoustic emission can be understood in terms of the reduced thermal conductivity of the substrate. [ABSTRACT FROM AUTHOR]

Room-temperature photoacoustic spectra and jet-cooled action spectra of the regions of the first and second C-H stretch overtones of pyrrole were measured with the goal of gaining new insight on the vibrational patterns and the intramolecular energy flow out of the initially excited vibrational states. The rotational cooling of the action spectra helped in observing hitherto unresolved features, assisting determination of the existing multiple bands and their positions in each region. These bands were analyzed by building vibrational Hamiltonian matrices related to a simplified joint local-mode/normal-mode (LM/NM) model, accounting for two types of C-H stretches and their Fermi resonances with the CCH deformation modes. The diagonalization of the LM/NM vibrational Hamiltonians and the fitting of the eigenvalues to the band positions revealed model parameters, enabling assignment of the observed bands. The time dependences of the survival probabilities of the C-H stretches in the region of the first and second overtones, deduced from the vibrational Hamiltonian, show quantum beats due to the couplings to the deformations and decays driven by weaker interactions to the bath states. The C-H stretches, although somewhat lower in energy, show stronger coupling than the N-H stretches. [ABSTRACT FROM AUTHOR]

Abstract In this study, we investigated the thermal transport in porous silicon nanostructures and silicon nanowires fabricated by electrochemical and metal-assisted chemical etching processes, respectively. In particular, the thermal conductivity values for the silicon-based nanomaterials were estimated based on photoacoustic measurements obtained from piezoelectric recordings. The amplitude–frequency and phase–frequency dependencies of the photoacoustic responses of the nanostructured silicon samples were obtained, and simulated successfully with a theoretical model. Correlations were established between the thermal conductivities and etching parameters. Highlights • Silicon nanostructured by metal-assisted chemical and electrochemical etching. • Photoacoustic signals recorded in piezoelectric detection configuration. • Thermal conductivity evaluated by fitting amplitude–frequency characteristics. • Thermal conductivities of porous silicon consistent with previous reports. • Thermal conductivity dependent on thickness of silicon nanowire arrays. [ABSTRACT FROM AUTHOR]

In this work, a method of causing solid-phase adhered particles to detach and move via photoacoustic resonance was studied. A laser micro-resonator was designed for excitation of the photoacoustic resonance. Both simulation and experimental results showed that a sound field was formed due to transient photoacoustic interactions between the laser and the resonator. A fundamental resonance was found at 18.9 kHz when the laser harmonized with the Eigen-frequency of the resonator. For the 100 μJ/pulse laser energy, the maximum centrifugal acceleration of 3.6 × 105 m/s2 was acquired by the fundamental photoacoustic resonance. The micro-resonator performed competently for the detachment of adhered particles larger than 5 μm. Particle motion could be controlled with an acceleration or constant speed by manipulating the laser frequency and energy. This photoacoustic manipulation of microscopic objects may have applications in separation and fixation of cells, giant molecules, and dusts in lab-on-a-chip systems. [ABSTRACT FROM AUTHOR]

Precision phototheranostics, including photoacoustic imaging and photothermal therapy, requires stable photothermal agents. Developing such agents with high stability and high photothermal conversion efficiency (PTCE) remains a considerable challenge. Herein, we introduce a new photothermal agent based on water‐soluble quaterrylenediimide (QDI) that can self‐assemble into nanoparticles (QDI‐NPs) in aqueous solution. Incorporating polyethylene glycol (PEG) into the QDI core significantly enhances both physiological stability and biocompatibility of QDI‐NPs. The highly photostable QDI‐NPs offer advantages including intense absorption in the near‐infrared (NIR) and high PTCE of up to 64.7±4 %. This is higher than that of commercial indocyanine green (ICG). Their small size (ca. 10 nm) enables sustained retention in deep tumor sites and also proper clearance from the body. QDI‐NPs allow high‐resolution photoacoustic imaging and efficient 808 nm laser‐triggered photothermal therapy of cancer in vivo. Red light white heat: The poly(ethylene glycol)‐functionalized quaterrylenediimide (QDI) can self‐assemble into QDI‐nanoparticles (QDI‐NPs) with sizes of approximately 10 nm in aqueous solution. QDI‐NPs exhibit high‐resolution photoacoustic imaging and efficient photothermal cancer therapy at 808 nm NIR laser irradiation. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging (PAI) is a rapidly growing field which offers high spatial resolution and high contrast for deep‐tissue imaging in vivo. PAI is nonionizing and noninvasive and combines the optical resolution of fluorescence imaging with the spatial resolution of ultrasound imaging. In particular, the development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation ranging from small molecules to macromolecular proteins, polymeric and inorganic nanoparticles. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented and a review of prior reports spanning from early‐to‐current literature is put forth. The development of exogenous PA contrast agents has gained significant momentum of late with a vastly expanding complexity of dye materials under investigation. The goal of this review is to survey the current state of the art in molecular photoacoustic contrast agents (MPACs) for applications in biomedical imaging. The fundamental design principles of MPACs are presented, and a review of prior reports spanning from early‐to‐current literature is put forth. [ABSTRACT FROM AUTHOR]

Photothermo-chemotherapy, as a new strategy for cancer treatment, incorporates the complementary advantages of photothermal therapy and chemotherapy. In this study, a pH-sensitive diblock copolymer poly(aspartic acid-butanediamine)-poly(2-(diisopropylamino)ethyl methacrylate) (PAsp(DAB)-PDPA) was synthesized and self-assembled into doxorubicin-loaded micelle, which was further used as a template to form a gold nanoshell. After further modification with poly(ethylene glycol), the resulting nanoplatform provided good biocompatibility and desirable photo-thermal conversion efficiency to facilitate photothermal therapy. Meanwhile the nanoparticle also exhibited pH sensitivity, which prevented drug loss while circulating in the blood but enabled rapid drug release after endocytosis. An improved effect was achieved with the combination of photothermal therapy and chemotherapy. In addition, systemic delivery of the nanoplatform could be monitored by photoacoustic tomography. Thereby, this multifunctional nanoplatform would be highly potential for the diagnosis and therapy of cancer. [ABSTRACT FROM AUTHOR]

An ultrasonic tomographic imaging technique is proposed for measurement of the distribution of Young's modulus in an isotropic plate. This technique is based on velocity mapping which is performed by applying full waveform inversion on the ultrasonic signals captured by transducers around the stiffness variations. The resulting wave velocity maps are then converted to Young's modulus maps by the known dispersion relation of selected guided modes. Finite element simulations were carried out to investigate the reconstruction performance of A 0 mode propagating through various smoothly varying stiffness defects in a plate. It is shown that for selected cases an average through-thickness Young's modulus can be accurately reconstructed and its potential to describe flexural stiffness of the plate is discussed. The model was validated by experiments, where Young's modulus was varied in a steel plate via heating. The map of the Young's modulus was reconstructed from temperature measurements and ultrasound data and results from the two methods showed excellent agreement. [ABSTRACT FROM AUTHOR]

Photoactive noble metal nanoparticles are of increasing importance toward personalized cancer therapy in the field of precision nanomedicine. A critical challenge remains in the exploration of clinically potential noble metal nanoparticles for highly efficient cancer theranostics. Here, we introduce albumin-coordinated assembly of clearable Pt nanodots (Pt-NDs) with monodisperse nanostructure as high-performance theranostic agents for imaging-guided photothermal tumor ablation. We precisely manipulate the reduction and growth of tetravalent Pt ions into ultrasmall nanodots through albumin-directed growth kinetics, thereby leading to the synthesis of monodisperse 6.7 nm Pt-NDs with albumin molecules as the corona. Pt-NDs exhibit the surface plasmon resonance at 225 nm with enhanced near-infrared (NIR) absorbance, ideal resistance to photo-bleaching, distinct photoacoustic and X-ray signals, as well as remarkable photothermal effect through non-radiative relaxation under NIR light irradiation. In particular, Pt-NDs possess preferable tumor accumulation, and effective in vivo excretory capability. Thus, these nanodots promote preferable in vivo microscopic photoacoustics and spatially anatomic CT imaging with enhanced contrast, as well as potent hyperthermia-mediated tumor ablation. These findings represent a facile and general approach to fabricate high-performance noble metal nanostructures with clinical potential for cancer theranostics. [ABSTRACT FROM AUTHOR]

A key feature of optoacoustic imaging is the ability to illuminate tissue at multiple wavelengths and therefore record images with a spectral dimension. While optoacoustic images at single wavelengths reveal morphological features, in analogy to ultrasound imaging or X-ray imaging, spectral imaging concedes sensing of intrinsic chromophores and externally administered agents that can reveal physiological, cellular and subcellular functions. Nevertheless, identification of spectral moieties within images obtained at multiple wavelengths requires spectral unmixing techniques, which present a unique mathematical problem given the threedimensional nature of the optoacoustic images. Herein we discuss progress with spectral unmixing techniques developed for multispectral optoacoustic tomography. We explain how different techniques are required for accurate sensing of intrinsic tissue chromophores such as oxygenated and deoxygenated haemoglobin versus extrinsically administered photo-absorbing agents and nanoparticles. Finally, we review recent developments that allow accurate quantification of blood oxygen saturation (sO2) by transforming and solving the sO2 estimation problem from the spatial to the spectral domain. [ABSTRACT FROM AUTHOR]

Pneumonia is an acute lung infection that takes life of many young children in developing countries. Early stage (red hepatization) detection of pneumonia would be pragmatic to control mortality rate. Detection of this disease at early stages demands the knowledge of pathology, making it difficult to screen noninvasively. We propose photoacoustic spectral response (PASR), a noninvasive elasticity-dependent technique for early stage pneumonia detection. We report the quantitative red hepatization detection of pneumonia through median frequency, spectral energy, and variance. Significant contrast in spectral parameters due to change in sample elasticity is found. The tissue-mimicking phantom study illustrates a 39% increase in median frequency for 1.5 times the change in density. On applying to formalin-fixed pneumonia-affected goat lungs, it provides a distinct change in spectral parameters between pneumonia affected areas and normal lungs. The obtained PASR results were found to be highly correlating to standard histopathology. The proposed technique therefore has potential to be a regular diagnostic tool for early pneumonia detection. [ABSTRACT FROM AUTHOR]

Graphene-based nanomaterials (GBNs) are quickly revolutionizing modern electronics, energy generation and storage, clothing and biomedical devices. Due to GBN's variety of physical and chemical parameters that define their toxicity and their aggregation in suspension, interpreting its toxicology without accurate information on graphene's distribution and behavior in live organisms is challenging. In this work, we present a laser-based optical detection methodology for noninvasive detection and pharmacokinetics analysis of GBNs directly in blood flow in mice using in vivo photoacoustic (PA) flow cytometry (PAFC). PAFC provides unique insight on how chemical modifications of GBNs affect their distribution in blood circulation and how quickly they are eliminated from the flow. Overall, PAFC provided unique data crucial for understanding GBN toxicity through real-time detection of GBNs using their intrinsic light absorption contrast. Copyright © 2017 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Voltage-sensitive dyes (VSDs) are designed to monitor membrane potential by detecting fluorescence changes in response to neuronal or muscle electrical activity. However, fluorescence imaging is limited by depth of penetration and high scattering losses, which leads to low sensitivity in vivo systems for external detection. By contrast, photoacoustic (PA) imaging, an emerging modality, is capable of deep tissue, noninvasive imaging by combining near-infrared light excitation and ultrasound detection. Here, we show that voltagedependent quenching of dye fluorescence leads to a reciprocal enhancement of PA intensity. We synthesized a near-infrared photoacoustic VSD (PA-VSD), whose PA intensity change is sensitive to membrane potential. In the polarized state, this cyanine-based probe enhances PA intensity while decreasing fluorescence output in a lipid vesicle membrane model. A theoretical model accounts for how the experimental PA intensity change depends on fluorescence and absorbance properties of the dye. These results not only demonstrate PA voltage sensing but also emphasize the interplay of both fluorescence and absorbance properties in the design of optimized PA probes. Together, our results demonstrate PA sensing as a potential new modality for recording and external imaging of electrophysiological and neurochemical events in the brain. [ABSTRACT FROM AUTHOR]

Optical resolution photoacoustic microscopy (ORPAM) is a high-resolution hybrid imaging modality having potential for microscale in vivo imaging. Optical diffraction limits the lateral resolution of ORPAM. A photonic nanojet (PNJ) was used to break this diffraction limit. A single round microsphere can generate a PNJ with subwavelength waist, but its short axial length limits its applications to surface imaging only. We investigate different sphere designs to achieve ultralong nanojets that will make the nanojet more viable in far-field applications, such as photoacoustic imaging. The PNJ properties, including effective length, waist size, working distance, and peak intensity, can be tuned and controlled by changing the sphere design and its refractive index. A truncated multilayer microsphere design could generate an ultraelongated PNJ with length larger than ~172λ ð~138 μmÞ while retaining a large working distance ~32λ ð~26 μmÞ. Through simulation study, we observed ~11-fold enhancement in lateral resolution with 5 μm round sphere (refractive index 2.2) when used in a conventional ORPAM setup with NA = 0.1 and λ = 800 nm. [ABSTRACT FROM AUTHOR]

Photoacoustic computed tomography (PACT) is an emerging computed imaging modality that exploits optical contrast and ultrasonic detection principles to form images of the absorbed optical energy density within tissue. If the object possesses spatially variant acoustic properties that are unaccounted for by the reconstruction method, the estimated image can contain distortions. While reconstruction methods have recently been developed to compensate for this effect, they generally require the object's acoustic properties to be known a priori. To circumvent the need for detailed information regarding an object's acoustic properties, we previously proposed a half-time reconstruction method for PACT. A half-time reconstruction method estimates the PACT image from a data set that has been temporally truncated to exclude the data components that have been strongly aberrated. However, this method can be improved upon when the approximate sizes and locations of isolated heterogeneous structures, such as bones or gas pockets, are known. To address this, we investigate PACT reconstruction methods that are based on a variable data truncation (VDT) approach. The VDT approach represents a generalization of the half-time approach, in which the degree of temporal truncation for each measurement is determined by the distance between the corresponding ultrasonic transducer location and the nearest known bone or gas void location. Computer-simulated and experimental data are employed to demonstrate the effectiveness of the approach in mitigating artifacts due to acoustic heterogeneities. [ABSTRACT FROM AUTHOR]

Photoacoustic computed tomography (PACT) and ultrasound computed tomography (USCT) are emerging modalities for breast imaging. As in all emerging imaging technologies, computer-simulation studies play a critically important role in developing and optimizing the designs of hardware and image reconstruction methods for PACT and USCT. Using computer-simulations, the parameters of an imaging system can be systematically and comprehensively explored in a way that is generally not possible through experimentation. When conducting such studies, numerical phantoms are employed to represent the physical properties of the patient or object to-be-imaged that influence the measured image data. It is highly desirable to utilize numerical phantoms that are realistic, especially when task-based measures of image quality are to be utilized to guide system design. However, most reported computer-simulation studies of PACT and USCT breast imaging employ simple numerical phantoms that oversimplify the complex anatomical structures in the human female breast. We develop and implement a methodology for generating anatomically realistic numerical breast phantoms from clinical contrast-enhanced magnetic resonance imaging data. The phantoms will depict vascular structures and the volumetric distribution of different tissue types in the breast. By assigning optical and acoustic parameters to different tissue structures, both optical and acoustic breast phantoms will be established for use in PACT and USCT studies. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging (PAI) may have the ability to reveal the composition and the anatomical structure of carotid plaques, which determines its mechanical properties and vulnerability. We used PAI and plane wave ultrasound (PUS) imaging to obtain three-dimensional (3-D) images of endarterectomy samples ex vivo and compared the results with histology to investigate the potential of PAI-based identification of intraplaque hemorrhage. Seven carotid plaque samples were obtained from patients undergoing carotid endarterectomy and imaged with a fully integrated hand-held photoacoustic (PA) probe, consisting of a pulsed diode laser (t PULSE = 130 ns, EPULSE = 1 mJ, λ = 808 nm) and a linear array transducer (f c = 7.5 MHz). The samples were rotated 360 deg with 10 deg steps, and data were spatially compounded to obtain complete 3-D images of the plaques. Areas of high absorption in the 3-D datasets were identified and compared to histological data of the plaques. Data in six out of seven endarterectomy samples revealed the presence of intraplaque hemorrhages that were not visible in the PUS images. Due to the noninvasive nature of PAI, this ex vivo study may elucidate preclinical studies toward the in vivo, noninvasive, vulnerability assessment of the atherosclerotic carotid plaque. [ABSTRACT FROM AUTHOR]

Biomedical imaging systems incorporating both photoacoustic (PA) and ultrasound capabilities are of interest for obtaining optical and acoustic properties deep in tissue. While most dual-modality systems utilize piezoelectric transducers, all-optical systems can obtain broadband high-resolution data with hands-free operation. Previously described reflection-mode all-optical laser-ultrasound (LUS) systems use a confocal source and detector; however, angle-dependent raypaths are lost in this configuration. As a result, the overall imaging aperture is reduced, which becomes increasingly problematic with depth. We present a reflection-mode nonconfocal LUS and PA imaging system that uses signals recorded on all-optical hardware to create angledependent images. We use reverse-time migration and time reversal to reconstruct the LUS and PA images. We demonstrate this methodology with both a numerical model and tissue phantom experiment to image a steep-curvature vessel with a limited aperture 2-cm beneath the surface. Nonconfocal imaging demonstrates improved focusing by 30% and 15% compared to images acquired with a single LUS source in the numerical and experimental LUS images, respectively. The appearance of artifacts is also reduced. Complementary PA images are straightforward to acquire with the nonconfocal system by tuning the source wavelength and can be further developed for quantitative multiview PA imaging. [ABSTRACT FROM AUTHOR]

Assessment of microvasculature and tissue perfusion can provide diagnostic information on local or systemic diseases. Photoacoustic (PA) imaging has strong clinical potential because of its sensitivity to hemoglobin. We used a hand-held PA probe with integrated diode lasers and examined its feasibility and validity in the detection of increasing blood volume and (sub) dermal vascularization. Blood volume detection was tested in custom-made perfusion phantoms. Results showed that an increase of blood volume in a physiological range of 1.3% to 5.4% could be detected. The results were validated with power Doppler sonography. Using a motorized scanning setup, areas of the skin were imaged at relatively short scanning times (<10 s/cm2) with PA. Threedimensional visualization of these structures was achieved by combining the consecutively acquired crosssectional images. Images revealed the epidermis and submillimeter vasculature up to depth of 5 mm. The geometries of imaged vasculature were validated with segmentation of the vasculature in high-frequency ultrasound imaging. This study proves the feasibility of PA imaging in its current implementation for the detection of perfusion-related parameters in skin and subdermal tissue and underlines its potential as a diagnostic tool in vascular or dermal pathologies. [ABSTRACT FROM AUTHOR]

We have enhanced photoacoustic computed tomography with dry acoustic coupling that eliminates water immersion anxiety and wrinkling of the animal and facilitates incorporating complementary modalities and procedures. The dry acoustic coupler is made of a tubular elastic membrane enclosed by a closed transparent water tank. The tubular membrane ensures water-free contact with the animal, and the closed water tank allows pressurization for animal stabilization. The dry coupler was tested using a whole-body small-animal ring-shaped photoacoustic computed tomography system. Dry coupling was found to provide image quality comparable to that of conventional water coupling. [ABSTRACT FROM AUTHOR]

Although the drug-eluting stent (DES) has dramatically reduced the rate of coronary restenosis, it still occurs in up to 20% of patients with a DES. Monitoring drug delivery could be one way to decrease restenosis rates. We demonstrate real-time photoacoustic imaging and spectroscopy (PAIS) using a wavelength-tunable visible laser and clinical ultrasound scanner to track cardiac drug delivery. The photoacoustic signal was initially calibrated using porcine myocardial samples soaked with a known concentration of a drug surrogate (DiI). Next, an in situ coronary artery was perfused with DiI for 20 min and imaged to monitor dye transport in the tissue. Finally, a partially DiI-coated stent was inserted into the porcine brachiocephalic trunk for imaging. The photoacoustic signal was proportional to the DiI concentration between 2.4 and 120 μg/ml, and the dye was detected over 1.5 mm from the targeted coronary vessel. Photoacoustic imaging was also able to differentiate the DiI-coated portion of the stent from the uncoated region. These results suggest that PAIS can track drug delivery to cardiac tissue and detect drugs loaded onto a stent with sub-mm precision. Future work using PAIS may help improve DES design and reduce the probability of restenosis. [ABSTRACT FROM AUTHOR]

This paper proposes a method for improving the localization and the quantification of the optical parameters in photoacoustic (PA) tomography of biological tissues that are intrinsically heterogeneous in both optical and acoustic properties. It is based on the exploitation of both the PA signal, generated by the heterogeneous optical structures, and the secondary acoustic echoes due to the interaction between a primary PA wave generated near the tissue surface and the heterogeneous acoustic structures. These secondary echoes can also be collected through proper measurements of the PA signals. The experimental procedure is presented along with the method to filter the signal and the reconstruction algorithm that includes the account of the acoustic information. [ABSTRACT FROM AUTHOR]

Fabry-Pérot sensors have been used to produce in-vivo photoacoustic images of exquisite quality. However, for ease of construction and interrogation, they are produced in a planar form. Planar arrays suffer from a limited detection aperture, which leads to artifacts in the reconstruction of the initial pressure distribution. Here, an L-shaped detection geometry is described that allows a greater field of view by placing a second planar array orthogonal to the first. This captures data from the deeper lying regions of interest and mitigates the limited view, thus reducing artifacts in the reconstructed initial pressure distribution. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging has become an emerging tool for theranostic applications. Not only does it help in in vivo, noninvasive imaging of biological structures at depths but it can also be used for drug release and therapeutic applications. We explore near-infrared light-sensitive liposomes coated with gold nanostars (AuNSs) for both imaging and drug release applications using a photoacoustic imaging system. Being amphiphilic, the liposomes lipid bilayer and the aqueous core enable encapsulation of both hydrophobic and hydrophilic drugs. The AuNSs on the surface of the liposomes act as photon absorbers due to their intrinsic surface plasmon resonance. Upon excitation by laser light at specific wavelength, AuNSs facilitate rapid release of the contents encapsulated in the liposomes due to local heating and pressure wave formation (photoacoustic wave). Herein, we describe the design and optimization of the AuNSs-coated liposomes and demonstrate the release of both hydrophobic and hydrophilic model drugs (paclitaxel and calcein, respectively) through laser excitation at nearinfrared wavelength. The use of AuNSs-coated liposomes as contrast agents for photoacoustic imaging is also explored with tissue phantom experiments. In comparison to blood, the AuNSs-coated liposomes have better contrast (approximately two times) at 2-cm imaging depth. [ABSTRACT FROM AUTHOR]

This work explores light delivery optimization for photoacoustic-guided minimally invasive surgeries, such as the endonasal transsphenoidal approach. Monte Carlo simulations were employed to study threedimensional light propagation in tissue, comprising one or two 4-mm diameter arteries located 3 mm below bone, an absorbing metallic drill contacting the bone surface, and a single light source placed next to the 2.4-mm diameter drill shaft with a 2.9-mm diameter spherical drill tip. The optimal fiber distance from the drill shaft was determined from the maximum normalized fluence to the underlying artery. Using this optimal fiber-to-drill shaft distance, Zemax simulations were employed to propagate Gaussian beams through one or more 600 micron-core diameter optical fibers for detection on the bone surface. When the number of equally spaced fibers surrounding the drill increased, a single merged optical profile formed with seven or more fibers, determined by thresholding the resulting light profile images at 1/e times the maximum intensity. We used these simulations to inform design requirements, build a one to seven multifiber light delivery prototype to surround a surgical drill, and demonstrate its ability to simultaneously visualize the tool tip and blood vessel targets in the absence and presence of bone. The results and methodology are generalizable to multiple interventional photoacoustic applications. [ABSTRACT FROM AUTHOR]

Photoacoustic imaging is an emerging hybrid imaging modality that can provide multicontrast, multiscale imaging of biological features ranging from organelles to organs. The three major embodiments of photoacoustic imaging are microscopy, endoscopy, and computed tomography. Photoacoustic tomography (PAT) or photoacoustic computed tomography allows deep-tissue imaging, and hence it is more suitable for whole body preclinical/clinical imaging applications. Due to fast-growing laser technology and ultrasound detector technology, PAT is evolving rapidly, leading to a quicker translation into clinical trials. We review the recent developments of PAT systems and their applications in preclinical and clinical practices. [ABSTRACT FROM AUTHOR]

Translating photoacoustic (PA) imaging into clinical setup is a challenge. We report an integrated PA and ultrasound imaging system by combining the light delivery to the tissue with the ultrasound probe. First, Monte Carlo simulations were run to study the variation in absorbance within tissue for different angles of illumination, fiber-to-probe distance (FPD), and fiber-to-tissue distance (FTD). This is followed by simulation for different depths of the embedded sphere (object of interest). Several probe holders were designed for different light launching angles. Phantoms were developed to mimic a sentinel lymph node imaging scenario. It was observed that, for shallower imaging depths, the variation in signal-to-noise ratio (SNR) values could be as high as 100% depending on the angle of illumination at a fixed FPD and FTD. Results confirm that different light illumination angles are required for different imaging depths to get the highest SNR PA images. The results also validate that one can use Monte Carlo simulation as a tool to optimize the probe holder design depending on the imaging needs. This eliminates a trial-and-error approach generally used for designing a probe holder. [ABSTRACT FROM AUTHOR]

Frequency characteristics of ultrasound detectors used in optoacoustic tomography have a major impact on imaging performance. It is common practice to select transducers based on their sensitivity at the central frequency and under normal incidence. However, the bandwidth and angular sensitivity play an equally important role in establishing the quality and accuracy of the reconstructed images. Here, we developed a calibrated optoacoustic characterization method specifically tailored for broadband measurements of the angular transducer sensitivity (directivity). Ultrawideband omnidirectional optoacoustic responses were generated by uniformly illuminating thin absorbing sutures with nanosecond laser pulses and characterized with a needle hydrophone. This calibrated optoacoustic source was used to characterize the frequency dependence of the angular response by a conventional piezoelectric transducer (PZT) and a capacitive micromachined ultrasonic transducer (cMUT) with similar size and central frequency. Furthermore, both transducers had no preamplification electronics directly attached to the detection elements. While the PZT presented a 7.8 dB sensitivity advantage at normal incidence, it was able to provide detectable signal-to-noise levels only at incidence angles of up to 20 deg whereas the cMUT maintained reasonable sensitivity levels and broadband response at incidence angles of 40 deg and beyond. We further experimentally showcase a reduction in the limited-view image artifacts resulting from the broader acceptance angle of the cMUT. [ABSTRACT FROM AUTHOR]

Aerosol source apportionment is a major challenge for air quality monitoring. Due to the frequently observed correlation between the chemical properties of aerosol particles and the wavelength dependence of their optical absorption coefficient, the photoacoustic (PA) method (similarly to alternative optical absorption measurement techniques) offers the unique possibility of real time source apportionment under certain circumstances. However, so far, the selection of lasers into a multi-wavelength PA system for source apportionment was based only on rule of thumb type estimations (like “one should use the highest available light powers with the widest possible wavelength separation”). Here we introduce a simulation method to estimate the accuracy of source apportionment and to be used for the optimised selection of the light sources of a dual wavelength PA system in cases, when light absorbing carbonaceous aerosol (LAC) originates from two distinct sources and dominates the atmospheric aerosol load. The method has two phases and it can be applied either on actual measurement results or on simulated datasets. In the first “calibration” phase the parameters of a non-linear calibration curve are determined, which describes the correlation between the ratio of optical absorption coefficients ( OAC s) measured photoacoustically at two wavelengths and the ratio of the source specific segregated mass concentrations (mass concentration ratio, MCR ) measured by an independent calibration method. During the second “measurement” phase these calibration parameters are used to estimate the MCRs solely from the photoacoustically measured OAC ratios. The relative uncertainty of this estimation is used to grade dual wavelength PA systems. Executions of this simulation on different MCR , OAC datasets have proved that with a fairly simple PA system (which consists of a 405 and a 1064 nm diode laser having modulated light power of 150 and 300 mW, respectively) MCR determination is feasible with at least 10% relative accuracy whenever the mass concentration of LAC exceeds 1 µg/m 3 . [ABSTRACT FROM AUTHOR]

Formation of blood clots, called thrombus, can happen due to hyper-coagulation of blood. Thrombi, while moving through blood vessels can impede blood flow, an important criterion for many critical diseases like deep vein thrombosis and heart attacks. Understanding mechanical properties of clot formation is vital for assessment of severity of thrombosis and proper treatment. However, biomechanics of thrombus is less known to clinicians and not very well investigated. Photoacoustic (PA) spectral response, a non-invasive technique, is proposed to investigate the mechanism of formation of blood clots through elasticity and also differentiate clots from blood. Distinct shift (increase in frequency) of the PA response dominant frequency during clot formation is reported. In addition, quantitative differentiation of blood clots from blood has been achieved through parameters like dominant frequency and spectral energy of PA spectral response. Nearly twofold increases in dominant frequency in blood clots compared to blood were found in the PA spectral response. Significant changes in energy also help in quantitatively differentiating clots from blood, in the blood. Our results reveal that increase in density during clot formation is reflected in the PA spectral response, a significant step towards understanding the mechanobiology of thrombus formation. Hence, the proposed tool, in addition to detecting thrombus formation, could reveal mechanical properties of the sample through quantitative photoacoustic spectral parameters. [ABSTRACT FROM AUTHOR]

Although sulfuryl fluoride (SOF) is an efficient fumigant that does not react with the surface of indoor materials and does not reduce the stratospheric ozone shield, there are some concerns about its use. It is a toxic gas that attacks the central nervous system, and its global warming potential (GWP) value is 4780 for 100 years' time. Therefore, it is a clear necessity of implementing detection methods for tracing such a molecule. In this work a sensitive photoacoustic setup was built to detect SOF at concentrations of parts per billion by volume (ppbv). The symmetric S-O stretching mode was excited by a continuous-wave quantum cascade laser with radiation wavenumber ranging from 1275.7 to 1269.3 cm. The photoacoustic signal was generated by modulating the laser wavenumber at the first longitudinal mode of the photoacoustic cell with amplitude depth of 5 × 10 cm. The detection of a minimum SOF concentration of 20 ppbv was achieved. [ABSTRACT FROM AUTHOR]

Noncontact evaluation of elastic properties of a microstructure is still a challenge in turbid media. In this Letter, we present the observation of a phenomenon-the photoacoustic eigen-spectrum from light-absorbing objects. Analysis and experiments demonstrate that the eigen-vibration information of a microstructure is imprinted in its photoacoustic coda waves after it is exposed to a laser pulse illumination. The spectral lines in the time-frequency map of photoacoustic coda waves correspond to the eigen-frequencies of the light-absorber. This phenomenon provides a physical basis for noncontact evaluation of elastic properties of a microstructure in turbid media. Elastic parameters can be accurately inversed from the measured photoacoustic eigen-spectrum. [ABSTRACT FROM AUTHOR]

A double antinode excited quartz-enhanced photoacoustic spectroscopy (DAE-QEPAS) spectrophone, employing a custom-made quartz tuning fork (QTF) and operating at the 1stt overtone resonance mode is reported. The signal phase variation along the QTF prong was investigated, and a piezoelectric transducer was introduced to compensate the phase shift between two QTF separated 1st overtone antinode points. Two sets of acoustic micro-resonators were optimized and assembled at two antinode points to improve the spectrophone performance. With the two antinodes excited by one laser source, the DAE-QEPAS spectrophone attained a sensitivity gain factor of ∼100 times and ∼3 times with respect to the 1st overtone resonances of a bare custom QTF and a standard onbeam QEPAS spectrophone, respectively. H2O was selected as the target analyte and a detection limit of ∼230 ppb was obtained by the DAE-QEPAS spectrophone for a 1 s integration time, corresponding to a normalized noise equivalent absorption coefficient of 1.73×10-9cm-1·W·Hz-1/2. [ABSTRACT FROM AUTHOR]

Metallic diffusion-limited aggregate (DLA) films are well-known to exhibit near-perfect broadband optical absorption. We demonstrate that such films also manifest a substantial and relatively material-independent photoacoustic response, as a consequence of their random nanostructure. We theoretically and experimentally analyze the photoacoustic phenomena in DLA films and show that they can be used to create broadband air-coupled acoustic sources. These sources are inexpensive and simple to fabricate and work into the ultrasonic regime. We illustrate the device possibilities by building and testing an optically addressed acoustic phased array capable of producing virtually arbitrary acoustic intensity patterns in air. [ABSTRACT FROM AUTHOR]

This paper presents results of experimental and theoretical studies of the plasma wave and the drum effect in the photoacoustic (PA) spectra of CdSe and CdMgSe samples. The PA experimental amplitude and phase frequency characteristics of these samples have been shown. The experimental results have been interpreted in two different PA models: in the model of a drum effect and in the plasma wave model. The obtained results have been discussed and presented in this paper. They indicate that the values of extracted parameters depend on the model of the PA signal. To distinguish between the plasma and the drum model the aluminium foil test is proposed. From the results obtained one can conclude that the plasma model used for interpretation of the PA characteristics yields overestimated values of the thermal diffusivity and poor PA phase fittings. The results obtained using the drum model agree well with other measurements conducted on the investigated samples. [ABSTRACT FROM AUTHOR]

In view of the high sensitivity and high selectivity of photoacoustic technology, a compound fire alarm system used for detecting smoke particles and carbon monoxide was designed on the basis of the photoacoustic spectrometry principle. The system uses an infrared laser with a central wavelength of 1579 nm as the light source, adopts wavelength modulation technology to modulate the continuous light source, and moves the inspection frequency to high frequency with lower noise by integrating harmonic detection technology, thus realizing a measurement of the photoacoustic signal. Concurrently, the system obtains the CO concentration and extinction coefficient of smoke particles via inversion by using the measured scattered light intensity data combined with a certain algorithm. A test was carried out on the system by using three small-scale experiments, namely, cotton rope smoldering fire, polyurethane foam fire and sandalwood smoldering fire. The results show that the CO gas concentrations and extinction coefficient of smoke particles produced by these three experiments are well detected. The CO detection limit of the system was 2.98 ppm. The test results also show that pure powder (used for simulating particles) or pure CO gas will not trigger the system alarm, so unnecessary alarms caused by traditional fire detectors can be avoided and the reliability of the fire alarm can be improved. [ABSTRACT FROM AUTHOR]

For a circular scanning geometry in photoacoustic tomography, the axial/radial resolution is spatially invariant and is not affected by the ultrasound transducer (UST, detector) aperture. However, the tangential resolution is dependent on the detector aperture size and it varies spatially. Many techniques were proposed to improve the tangential resolution, such as attaching a concave lens in front of the nonfocused transducer or using a virtual point detector. Both of these methods have difficulties. Therefore, a modified delay-and-sum reconstruction algorithm has been proposed which can be used together with a standard ultrasound detector (nonfocused) to improve the tangential resolution. In this work, we validate the modified delay-and-sum algorithm experimentally for both flat and cylindrically focused USTs. More than threefold improvement in tangential resolution is observed. It is also shown that the object shape is recovered with this modified algorithm, which is very helpful for diagnosis and treatment purposes. [ABSTRACT FROM AUTHOR]

A purely optical setup for the coregistration of photoacoustic (PA), ultrasound (US), and speed-ofsound (SOS) section images is presented. It extends a previously developed method for simultaneous PA and laser-US (LUS) pulse-echo imaging with a LUS transmission imaging setup providing two-dimensional (2-D) SOS maps. For transmission imaging, the sound waves traversing the investigated object are generated instantaneously by illuminating optically absorbing targets that are arranged at various distances in front of the sample. All signals are recorded by an optical beam which is part of a Mach-Zehnder interferometer that integrates the acoustic field along its path. Due to the cascaded arrangement of LUS sources, a single-recorded signal yields information for a projection of the SOS distribution. After collection of data from all directions, an inverse Radon transform is applied to this set of projections to obtain a 2-D SOS image. The setup is characterized and its performance is tested on phantom experiments. In addition to providing additional contrast, it is also shown that the resolution of the coregistered PA and LUS images can be improved by implementing the knowledge of the SOS distribution in the reconstruction. [ABSTRACT FROM AUTHOR]

We report the THz generation using optical rectification process in an indigenously grown organic salt 4-N, N -dimethylamino-4′-N′-methyl-stilbazoliumtoyslate (DAST). A 60 fs, 1 kHz optical parametric amplifier, tunable between 0.8 and 1.6 μm range was employed to generate THz radiation. It is observed that the THz signal is accompanied with weak second harmonic signal of pump wavelength. The conversion efficiency (η) of generated THz is 0.0161%. The radiation generated from DAST crystal is only THz signal, which is confirmed theoretically by choosing the crystal thickness is equivalent to be its optimum coherence length. In addition, we have selected two rotational lines i.e. J = 19, 58 of pure N 2 O gas by using 0.5 and 1.5 THz band pass filters for recording the time resolved photoacoustic (PA) spectra at 1 atm pressure for the first time. [ABSTRACT FROM AUTHOR]

Absorption coefficients of the ethanol vapors at atmospheric pressure and room temperature were measured by photoacoustic technique using a cw, line-tunable, frequency-stabilized CO 2 laser as radiation source. The spectrum of the employed CO 2 laser includes 54 lines with wavelengths in the infrared region of 9.2–10.8 μm and power levels up to 4.7 W. Measurements revealed a predominant absorption for ethanol within 9.4 μm band of the CO 2 laser spectrum, where the highest values of the absorption coefficients were recorded: 3.68 cm − 1 atm − 1 at 9R(20) line and 3.65 cm − 1 atm − 1 at 9R(22) line. The estimated detection range covers six orders of magnitude, from a minimum of 30 ppbV to a maximum of 4% concentration of ethanol in nitrogen, which proves the suitability of the photoacoustic technique for accurate measurements of the ethanol concentration in various applications. [ABSTRACT FROM AUTHOR]

Photoacoustic (PA) spectrum is a promising method in SF6 decomposition product detection. To develop an on-line monitoring device, a gas detection system based on PA effect was introduced. Three kinds of gases, SO2, CO2, and CF4, were chosen to test the device performance. Some experiments were conducted to get the relationship of PA signal and gas concentration. The minimum detection limit of each gas was also measured. After that, the influence of temperature on PA detection was studied under 30 °C−50 °C. It is found that the PA signals of SO2 (1360 cm ^-1 ), CO2 (2347 cm ^-1 ), and CF4 (1285 cm ^-1 ) all have negative temperature characteristics within that temperature range. Then, a temperature linear correction model was proposed. The maximum error between correction result and standard PA signal under reference temperature was <4%. The temperature correction method lays a foundation for manufacturing SF6 decomposition components on-line monitoring device. [ABSTRACT FROM PUBLISHER]

A recent study showed that ferritin is a suitable endogenous contrast agent for photoacoustic molecular imaging in cultured mammalian cells. We have therefore tested whether this imaging technique can be used for in vivo quantification of iron in mouse livers. To verify this hypothesis, we used multispectral optoacoustic tomography (MSOT) to image albino CD1 mice before and after experimental iron loading. Postmortem assays showed that the iron treatment caused a 15-fold increase in liver iron and a 40-fold increase in liver ferritin levels, while in vivo longitudinal analysis using MSOT revealed just a 1.6-fold increase in the ferritin/iron photoacoustic signal in the same animals. We conclude that MSOT can monitor changes in ferritin/iron levels in vivo, but its sensitivity is much lower than that of ex vivo iron assays. [ABSTRACT FROM AUTHOR]

We characterized the thermoacoustic and photoacoustic properties of large-area, few-layer graphene by pulsed microwave and optical excitations. Due to its high electric conductivity and low heat capacity per unit area, graphene lends itself to excellent microwave and optical energy absorption and acoustic signal emanation due to the thermoacoustic effect. When exposed to pulsed microwave or optical radiation, distinct thermoacoustic and photoacoustic signals generated by the few-layer graphene are obtained due to microwave and laser absorption of the graphene, respectively. Clear thermoacoustic and photoacoustic images of large-area graphene sample are achieved. A numerical model is developed and the simulated results are in good accordance with the measured ones. This characterization work may find applications in ultrasound generator and detectors for microwave and optical radiation. It may also become an alternative characterization approach for graphene and other types of two-dimensional materials. [ABSTRACT FROM AUTHOR]