Photoacoustic Waves Research Articles

Photoacoustic eigen-spectrum from light-absorbing microspheres and its application in noncontact elasticity evaluation

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.

Time-resolved light scattering by photoexcited V2O3

ABSTRACTUsing ultrafast angle-resolved light scattering technique, we were able to trigger photoinduced phase transition processes in V2O3 film grown on a glass substrate. The phase transition is caused by photoacoustic wave in the film and appears as coherent oscillations of scattering signal at various time scales. These processes strongly depend on the size of microstructures constituting the V2O3 film. One of the key findings of our study is the presence of a size dependent phase transition threshold for V2O3 microstructures, where small size structures ($\tilde <$ 0.5μm) have lowest contribution to the phase transition. The presence of this threshold can be well described by considering uneven internal strain in the films which is one of the key parameters controlling phase transition dynamics in various vanadium oxides.

Taking advantage of acoustic inhomogeneities in photoacoustic measurements.

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.

Ultrasonic detection based on polarization-dependent optical reflection.

Owing to their extremely wide bandwidths, pure optical ultrasonic detection methods are gaining increasing interest. In this Letter, we proposed a simple ultrasonic detector that is based on the polarization-dependent optical reflection. When the acoustic wave reaches the liquid-glass interface, the acoustic pressure changed the relative refractive index between two media, leading to perturbations in the reflectance of the optical probe beam in glass. Unlike previous studies that detected the modulations in the intensity of the reflected beam, our method, named "polarization-dependent reflection ultrasonic detection (PRUD)," detects the intensity difference between two polarization components of the same probe beam. The PRUD significantly increased the sensitivity. Besides a phantom study, we also successfully detect weak photoacoustic waves in an in vivo animal experiment. This novel method can provide a simple way for ultrasonic detection, which will have great potential for ultrasound and photoacoustic imaging and sensing.

A computer-based simulator for intravascular photoacoustic images

Intravascular photoacoustic (IVPA) is a newly developed catheter-based imaging technique for the diagnosis of arterial atherosclerosis. A framework of simulating IVPA transversal images from a cross-sectional vessel model with given optical and acoustic parameters was presented. The light illumination and transportation in multi-layered wall and atherosclerotic plaque tissues were modeled through Monte Carlo (MC) simulation. The generation and transmission of photoacoustic (PA) waves in the acoustically homogeneous medium were modeled through the PA wave equation, which is solved explicitly with a finite difference time domain (FDTD) algorithm in polar coordinates. Finally, a series of cross-sectional gray-scale images displaying the distribution of the deposited optical energy were reconstructed from the time-dependent acoustic pressure series with a time-reversal based algorithm. Experimental results demonstrate a good correlation between the simulated IVPA images and the optical absorption distribution profiles. The simulator provides a powerful tool for generating IVPA image data sets, which are used to improve the imaging catheter and to test the performance of image post-processing algorithms.

Near-infrared light-responsive liposomal contrast agent for photoacoustic imaging and drug release applications.

Photoacoustic imaging has become an emerging tool for theranostic applications. Not only does it help in <italic<in vivo</italic<, 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 near-infrared 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.

Laser-generated focused ultrasound for arbitrary waveforms

Transducers for laser generated focused ultrasound can achieve photoacoustic waves with several hundred bars positive pressure in water. Previous designs employed concave glass substrates decorated with catalytically grown carbon nanotubes. Here, we show that arbitrarily shaped surfaces made of polymers and printed with 3d printers allow the generation of waveforms with complex temporal and spatial shape. We first present three different polymer materials together with a simplified deposition technique. This is achieved by painting layers of carbon-nanotube powder and polydimethylsiloxane. Together with a clear resin (Formlabs Photopolymer Clear Resin), pressure amplitudes of 300 bar peak positive were obtained. With the flexibility of polymer substrates, complex waveforms can be generated. This is demonstrated with a stepped surface which launches two waves separated by 0.8 μs. Detailed pressure measurements are supported with shadowgraphy images and simulations of the wave.

Photoacoustic imaging for ultrasound tissue characterization and treatment monitoring

Photoacoustic imaging relies on generation of ultrasound waves from optically absorbing structures. The ultrasound produced in photoacoustic imaging can be analyzed by methods developed to analyze ultrasound backscatter signals for ultrasound tissue characterization, but the interpretation of the analysis is based on the physics of photoacoustic wave generation. In the absence of exogenous absorbers, blood is one of the dominant optically absorbing tissues. Hemoglobin in red blood cells is the main endogenous chromophore in blood. The spatial distribution of red blood cells in tissue determines the spectral characteristics of the ultrasound signals produced. We are interested in cancer treatment monitoring. Tumor blood vessels have distinct organizational structure compared to normal blood vessels: normal vessel networks are hierarchically organized, with vessels that are evenly distributed to ensure adequate oxygen and nutrient delivery. Tumor vessels are structurally different: they are torturous and hyperpermeable. Therapies that target the vasculature can induce changes in the vascular networks that in principle should be detected using photoacoustic imaging. In this work, we will show how the frequency content of the photoacoustic signals encodes information about the size, concentration, and spatial distribution of non-resolvable blood vessels that can be used to assess treatment response.

2-D image reconstruction of photoacoustic endoscopic imaging based on time-reversal

BackgroundsPhotoacoustic endoscopic (PAE) imaging is a rapidly emerging interventional imaging modality for identification and characterization of intraluminal pathological tissues. Since the scanning aperture of PAE is enclosed in the lumen, image reconstruction techniques used in photoacoustic tomography (PAT) can not be directly applied. ObjectiveThe purpose of this work is to design an image reconstruction method based on time-reversal (TR) for a PAE imaging catheter equipped with a single-element NFU transducer with circular scanning. MethodsFirstly, the back-propagation of photoacoustic waves emitted from the tissue absorbers was modeled and simulated. Then, two-dimensional (2-D) grayscale images of the acoustic pressure distribution were obtained displaying the morphological structure of luminal cross-sections. A computer-simulated vessel phantom embedded with atherosclerotic plaques was used to validate and quantitatively evaluate the method. ResultThe structural similarity (SSIM) of the images reconstructed with TR is comparable to algebraic reconstruction technique (ART), which is at least 65% higher than filtered back-projection (FBP). The time cost of TR is about 16 times that of FBP and 1/4 of ART under the same test condition. ConclusionThe reconstructed image quality may degrade when the photoacoustic data are incomplete due to sparse measuring locations and limited-view scanning. The spline interpolation can be used to improve the image quality and eliminate artifacts.

Indocyanine green loaded graphene oxide for high-efficient photoacoustic tumor therapy

Photoacoustic therapy, using the photoacoustic effect of agents for selectively killing tumor cells, has shown promising for treating tumor. Utilization of high optical absorption probes can help to effectively improve the photoacoustic therapy efficiency. Herein, we report a novel high-absorption photoacoustic probe that is composed of indocyanine green (ICG) and graphene oxide (GO), entitled GO-ICG, for photoacoustic therapy. The attached ICG with narrow absorption spectral profile has strong optical absorption in the infrared region. The absorption spectrum of the GO-ICG solution reveals that the GO-ICG particles exhibited a 10-fold higher absorbance at 780[Formula: see text]nm (its peak absorbance) as compared with GO. Importantly, ICG’s fluorescence is quenched by GO via fluorescence resonance energy transfer. As a result, GO-ICG can high-efficiently convert the absorbed light energy to acoustic wave under pulsed laser irradiation. We further demonstrate that GO-ICG can produce stronger photoacoustic wave than the GO and ICG alone. Moreover, we conjugate this contrast agent with integrin [Formula: see text] mono-clonal antibody to molecularly target the U87-MG human glioblastoma cells for selective tumor cell killing. Finally, our results testify that the photoacoustic therapy efficiency of GO-ICG is higher than the existing photoacoustic therapy agent. Our work demonstrates that GO-ICG is a high-efficiency photoacoustic therapy agent. This novel photoacoustic probe is likely to be an available candidate for tumor therapy.

Simulating photoacoustic waves from individual nanoparticle of various shapes using k-Wave

Photoacoustic imaging techniques have been widely discussed for their potential applications in the biomedical field. Current attempts at understanding the photoacoustic signal characteristic of various shapes of nanoparticles is mostly performed through numerical simulations. They require complex mathematical formulations prior to the simulation. Moreover, computationally they are expensive. In this work, k-Wave simulation toolbox was used to simulate the photoacoustic signal generated from various shapes of nanoparticles. Seven shapes of nanoparticles are simulated here: sphere, cylinder, hollow cylinder, cube, hollow cube, triangle, and nano star. A point sensor (ultrasound detector) is used to detect the photoacoustic signal. The simulated photoacoustic signal is presented both in time and frequency domain and used for understanding the impact of the various shapes on the photoacoustic signal characteristics. The detector was placed at various locations to see how the photoacoustic signal characteristics changes with transducer positions. There is a distinguishable signature of the nanoparticle shape on the frequency spectrum of the photoacoustic waves. The simulation time for each nano particle is as short as 40 s, which is significant improvement over previously reported simulation time of few hours using analytical methods.

Photoacoustic clutter reduction by inversion of a linear scatter model using plane wave ultrasound measurements.

Photoacoustic imaging aims to visualize light absorption properties of biological tissue by receiving a sound wave that is generated inside the observed object as a result of the photoacoustic effect. In clinical applications, the strong light absorption in human skin is a major problem. When high amplitude photoacoustic waves that originate from skin absorption propagate into the tissue, they are reflected back by acoustical scatterers and the reflections contribute to the received signal. The artifacts associated with these reflected waves are referred to as clutter or skin echo and limit the applicability of photoacoustic imaging for medical applications severely. This study seeks to exploit the acoustic tissue information gained by plane wave ultrasound measurements with a linear array in order to correct for reflections in the photoacoustic image. By deriving a theory for clutter waves in k-space and a matching inversion approach, photoacoustic measurements compensated for clutter are shown to be recovered.

In vivo acoustic and photoacoustic focusing of circulating cells.

In vivo flow cytometry using vessels as natural tubes with native cell flows has revolutionized the study of rare circulating tumor cells in a complex blood background. However, the presence of many blood cells in the detection volume makes it difficult to count each cell in this volume. We introduce method for manipulation of circulating cells in vivo with the use of gradient acoustic forces induced by ultrasound and photoacoustic waves. In a murine model, we demonstrated cell trapping, redirecting and focusing in blood and lymph flow into a tight stream, noninvasive wall-free transportation of blood, and the potential for photoacoustic detection of sickle cells without labeling and of leukocytes targeted by functionalized nanoparticles. Integration of cell focusing with intravital imaging methods may provide a versatile biological tool for single-cell analysis in circulation, with a focus on in vivo needleless blood tests, and preclinical studies of human diseases in animal models.

Assessment of lifetime resolution limits in time-resolved photoacoustic calorimetry vs. transducer frequencies: setting the stage for picosecond resolution.

Time-resolved photoacoustic calorimetry (PAC) gives access to lifetimes and energy fractions of reaction intermediates by deconvolution of the photoacoustic wave of a sample (E-wave) with that of the instrumental response (T-wave). The ability to discriminate between short lifetimes increases with transducer frequencies employed to detect the PAC waves. We investigate the lifetime resolution limits of PAC as a function of the transducer frequencies using the instrumental response obtained with the photoacoustic reference 2-hydroxybenzophenone in toluene or acetonitrile. The instrumental response was obtained for a set of transducers with central frequencies ranging from 0.5 MHz up to 225 MHz. The simulated dependence of the lifetime resolution with the transducer frequencies was anchored on experimental data obtained for the singlet state of naphthalene with a 2.25 MHz transducer. The shortest lifetime resolved with the 2.25 MHz transducer was 19 ns and our modelling of the transducer responses indicates that sub-nanosecond lifetimes of photoacoustic transients can be resolved with transducers of central frequencies above 100 MHz.

Microcantilever Actuation by Laser Induced Photoacoustic Waves.

We present here a combined theoretical and experimental investigation on effective excitation of microcantilever by using photoacoustic waves. The photoacoustic waves arose from a vibrating Al foil induced by an intensity-modulated laser. We demonstrate that, superior to photothermal excitation, this new configuration avoids direct heating of the microcantilever, thus minimizing undesired thermal effects on the vibration of microcantilever, while still keeps the advantage of being a remote, non-contact excitation method. We also measured the vibration amplitude of the microcantilever as a function of distance between the microcantilever and the Al foil and found that the amplitudes decay gradually according to the inverse distance law. This method is universal and can be adopted in bio-microelectromechanical systems (BioMEMs) for the detection of small signals where detrimental thermal effects must be avoided.

Photoacoustic shock wave emission and cavitation from structured optical fiber tips

Photoacoustic waves generated at the tip of an optical fiber consist of a compressive shock wave followed by tensile diffraction waves. These tensile waves overlap along the fiber axis and form a cloud of cavitation bubbles. We demonstrate that shaping the fiber tip through micromachining alters the number and direction of the emitted waves and cavitation clouds. Shock wave emission and cavitation patterns from five distinctively shaped fiber tips have been studied experimentally and compared to a linear wave propagation model. In particular, multiple shock wave emission and generation of strong tension away from the fiber axis have been realized using modified fiber tips. These altered waveforms may be applied for novel microsurgery protocols, such as fiber-based histotripsy, by utilizing bubble-shock wave interaction.

Non-Contact Photoacoustic Imaging Using a Commercial Heterodyne Interferometer.

Most current photoacoustic imaging (PAI) systems employ piezoelectric transducers to receive photoacoustic signals, which requires coupling medium to facilitate photoacoustic wave propagation and are not favored in many applications. Here, we report an all-optical non-contact PAI system based on a commercial heterodyne interferometer working at 1550 nm. The interferometer remotely detects ultrasound-induced surface vibration and does not involve any physical contact with the sample. The theoretically predicated and experimentally measured noise equivalent detection limits of the optical sensor are about 4.5 and 810 Pa over 1.2 MHz bandwidth. Using a raster-scan PAI system equipped with the non-contact design, stereotactic boundaries of an artificial tumor in a pig brain were accurately delineated. The non-contact design also enables the tomographic PAI of biological tissue samples in a non-invasive manner. The preliminary results and analyses reveal that the heterodyne interferometer-based non-contact PAI system holds good potential in biomedical imaging.

Detection of Photoacoustic Signals from Blood in Dental Pulp

Photoacoustic waves from hemoglobin solutions in dental roots are detected by using a 1064-nm laser and an ultrasonic soft probe based on a composite transducer on the tooth surface. The high-frequency ultrasonic waves are detected from a tooth with a hemoglobin solution in the pulp cavity due to the large heat transfer coefficient and absorption coefficient of hemoglobin. The spectral intensities of frequency components higher than 1 MHz show good correlation with the hemoglobin solution concentrations, and maps of frequency spectra calculated by taking short-time Fourier transforms clearly exhibit the effect of absorbance in dental pulp.

Momentum Transfer on Underwater Shock Generation Induced by Pulsed Laser Irradiation with Thin Metal

The present paper has investigated the momentum transport phenomena of underwater shock wave generation in terms of photo-acoustic wave with a very thin metal. The shock wave was induced by a pulsed-laser irradiation. In order to clarify the momentum transport mechanics in this phenomenon, we have been considered the momentum and energy transport from laser to metal, and metal to water. A numerical solution of thermo-elastic wave in metal has been obtained to estimate a fundamental gain of the longitudinal wave. Then, the underwater shock wave phenomena have been analyzed by adapting compressible fluid dynamics with suitable boundary condition between the solid and liquid. We had performed an experiment as well and observed the shock wave with optical system. The aim of the research is to estimate the underwater shock wave strength theoretically. The metal region was calculated by Laplace transformation of heat conduction and wave equations. The water region was simulated by MacCormack’s method. Some of boundary conditions have been examined and the acceleration condition has been adopted at the interface. The simulated results show a good agreement with experimental result, consequently the momentum transfer mechanism from longitudinal wave to underwater shock wave has been cleared in the present report.

Finite-difference time-domain solution of second-order photoacoustic wave equation

Finite-difference time-domain solution of second-order photoacoustic wave equation