Showing papers on "Photoacoustic spectroscopy published in 2010"
TL;DR: This work focuses on the current state of microvascular imaging and characterization based on photoacoustics, and discusses the methods used to characterize important functional parameters, such as total hemoglobin concentration, hemoglobin oxygen saturation, and blood flow.
Abstract: Photoacoustic (optoacoustic) tomography, combining optical absorption contrast and highly scalable spatial resolution (from micrometer optical resolution to millimeter acoustic resolution), has broken through the fundamental penetration limit of optical ballistic imaging modalities-including confocal microscopy, two-photon microscopy, and optical coherence tomography-and has achieved high spatial resolution at depths down to the diffusive regime. Optical absorption contrast is highly desirable for microvascular imaging and characterization because of the presence of endogenous strongly light-absorbing hemoglobin. We focus on the current state of microvascular imaging and characterization based on photoacoustics. We first review the three major embodiments of photoacoustic tomography: microscopy, computed tomography, and endoscopy. We then discuss the methods used to characterize important functional parameters, such as total hemoglobin concentration, hemoglobin oxygen saturation, and blood flow. Next, we highlight a few representative applications in microvascular-related physiological and pathophysiological research, including hemodynamic monitoring, chronic imaging, tumor-vascular interaction, and neurovascular coupling. Finally, several potential technical advances toward clinical applications are suggested, and a few technical challenges in contrast enhancement and fluence compensation are summarized.
377 citations
TL;DR: In this paper, the microresonator of a spectrophone for photo-acoustic spectroscopy was optimized based on an experimental study to achieve a normalized noise-equivalent absorption coefficient (1σ) of 3.3×10−9 cm−1W/Hz1/2 for C2H2 detection at atmospheric pressure.
Abstract: The impact of design parameters of a spectrophone for quartz-enhanced photoacoustic spectroscopy on its performance was investigated. The microresonator of spectrophone is optimized based on an experimental study. The results show that a 4.4 mm-long tube with 0.6 mm inner diameter yields the highest signal-to-noise ratio, which is ∼30 times higher than that of a bare QTF at gas pressures between 400 and 800 Torr. The optimized configuration demonstrates a normalized noise-equivalent absorption coefficient (1σ) of 3.3×10−9 cm−1W/Hz1/2 for C2H2 detection at atmospheric pressure. The effect of the changing carrier gas composition is studied. A side-by-side sensitivity comparison between QEPAS and conventional photoacoustic spectroscopy technique is reported.
294 citations
TL;DR: In this paper, the optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation.
Abstract: The optical band-gap energy of a nanostructured tungsten trioxide film is determined using the photoacoustic spectroscopy method under continuous light excitation. The mechanism of the photoacoustic signal generation is discussed. The band-gap energy is also computed by other methods. The absorption coefficient as well as the band-gap energy of three different crystal structures of tungsten trioxide is calculated by a first-principles Green’s function approach using the projector augmented wave method. The theoretical study indicates that the cubic crystal structure shows good agreement with the experimental data.
274 citations
TL;DR: The hemodynamics within the entire cerebral cortex of a mouse were studied by using photoacoustic tomography (PAT) in real time to demonstrate that PAT is a powerful imaging modality that can be potentially used to study small animal neurofunctional activities.
Abstract: For the first time, the hemodynamics within the entire cerebral cortex of a mouse were studied by using photoacoustic tomography (PAT) in real time. The PAT system, based on a 512-element full-ring ultrasound array, received photoacoustic signals primarily from a slice of 2-mm thickness. This system can provide high-resolution brain vasculature images. We also monitored the fast wash-in process of a photoacoustic contrast agent in the mouse brain. Our results demonstrated that PAT is a powerful imaging modality that can be potentially used to study small animal neurofunctional activities.
133 citations
TL;DR: In this paper, a gas sensor based on quartz-enhanced photoa- coustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric ox- ide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm −1.
Abstract: A gas sensor based on quartz-enhanced photoa- coustic detection and an external cavity quantum cascade laser was realized and characterized for trace nitric ox- ide monitoring using the NO R(6.5) absorption doublet at 1900.075 cm −1 . Signal and noise dependence on gas pressure were studied to optimize sensor performance. The NO concentration resulting in a noise-equivalent signal was found to be 15 parts per billion by volume, with 100 mW optical excitation power and a data acquisition time of 5 s.
132 citations
TL;DR: A novel dual-modal contrast agent-encapsulated-ink poly(lactic-co-glycolic acid) (PLGA) microbubbles and nanobubbles-for photoacoustic and ultrasound imaging can be a valuable tool for intraoperative assessment of tumor boundaries and therapeutic margins.
Abstract: We develop a novel dual-modal contrast agent-encapsulated-ink poly(lactic-co-glycolic acid) (PLGA) microbubbles and nanobubbles-for photoacoustic and ultrasound imaging. Soft gelatin phantoms with embedded tumor simulators of encapsulated-ink PLGA microbubbles and nanobubbles in various concentrations are clearly shown in both photoacoustic and ultrasound images. In addition, using photoacoustic imaging, we successfully image the samples positioned below 1.8-cm-thick chicken breast tissues. Potentially, simultaneous photoacoustic and ultrasound imaging enhanced by encapsulated-dye PLGA microbubbles or nanobubbles can be a valuable tool for intraoperative assessment of tumor boundaries and therapeutic margins.
109 citations
TL;DR: In this article, a gas sensor based on quartz-enhanced photoacoustic spectroscopy and using near-infrared, fiber-coupled diode lasers as an excitation source was developed for chemical analysis of gas mixtures containing H2S, CO2, and CH4 at concentrations from 0 to 100%.
Abstract: A gas sensor based on quartz-enhanced photoacoustic spectroscopy and using near-infrared, fiber-coupled diode lasers as an excitation source was developed for chemical analysis of gas mixtures containing H2S, CO2, and CH4 at concentrations from 0 to 100%. Analysis of physical phenomena affecting the sensor operation is performed, the sensor performance is evaluated, and simple algorithms are developed to derive concentrations of the gases from detected electrical signals.
88 citations
TL;DR: Preliminary results demonstrating the potential of photoacoustic lifetime imaging (PALI) for noninvasive, 3-D imaging of tissue oxygen are presented and possible implementations of the technique in clinical settings and combining it with photodynamic therapy (PDT) for real-time therapy monitoring are discussed.
Abstract: Measuring distribution of dissolved oxygen in biological tissue is of prime interest for cancer diagnosis, prognosis, and therapy optimization. Tumor hypoxia indicates poor prognosis and resistance to radiotherapy. Despite its major clinical significance, no current imaging modality provides direct imaging of tissue oxygen. We present preliminary results demonstrating the potential of photoacoustic lifetime imaging (PALI) for noninvasive, 3-D imaging of tissue oxygen. The technique is based on photoacoustic probing of the excited state lifetime of methylene blue (MB) dye. MB is an FDA-approved water soluble dye with a peak absorption at 660 nm. A double pulse laser system (pump probe) is used to excite the dye and probe its transient absorption by detecting photoacoustic emission. The relaxation rate of MB depends linearly on oxygen concentration. Our measurements show high photoacoustic signal contrast at a probe wavelength of 810 nm, where the excited state absorption is more than four times higher than the ground state absorption. Imaging of a simple phantom is demonstrated. We conclude by discussing possible implementations of the technique in clinical settings and combining it with photodynamic therapy (PDT) for real-time therapy monitoring.
76 citations
TL;DR: A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity.
Abstract: A gas sensor based on off-beam quartz enhanced photoacoustic spectroscopy was developed and optimized Specifically, the length and diameter of the microresonator tube were optimized, and the outer tube shape is modified for enhancing the trace gas detection sensitivity The impact of the distance between the quartz tuning fork and an acoustic microresonator on the sensor performance was experimentally investigated The sensor performance was evaluated by determining the detection sensitivity to H(2)O vapor in ambient air at normal atmospheric pressure A normalized noise equivalent absorption coefficient (1σ) of 62×10(-9) cm(-1) W/Hz(1/2) was achieved
75 citations
TL;DR: By exploiting pulsed laser excitation and raster motor scanning, three-dimensional structure and flow velocity can be imaged simultaneously and the flow direction can be determined with bidirectional scanning.
Abstract: We propose a new method to measure transverse flow ve- locity based on photoacoustic Doppler bandwidth broadening, which is determined by the geometry of the probe-beam and the velocity of the transverse flow. By exploiting pulsed laser excitation and raster motor scanning, three-dimensional structure and flow velocity can be imaged simultaneously. In addition, the flow direction can be deter- mined with bidirectional scanning. In a flowing suspension of red- dyed microspheres diameter: 6 m, transverse flow speeds ranging from 0 to 2.5 mm/s as well as flow direction were measured. A cross- sectional flow image was also obtained with the tube laid in a zigzag pattern. © 2010 Society of Photo-Optical Instrumentation Engineers.
70 citations
TL;DR: The optical properties of absorbing compounds that are often used to construct tissue phantoms for quantitative photoacoustic spectroscopy and imaging are investigated in this article, where the wavelength dependence of the optical absorption of inorganic chromophores, such as copper and nickel chloride, and organic chromophore such as cyanine-based near infrared dyes, was measured using transmittance spectrograms.
Abstract: In this paper, the optical properties of absorbing compounds that are often used to construct tissue phantoms for quantitative photoacoustic spectroscopy and imaging are investigated. The wavelength dependence of the optical absorption of inorganic chromophores, such as copper and nickel chloride, and organic chromophores, such as cyanine-based near infrared dyes, was measured using transmittance spectroscopy and compared with that determined using photoacoustic spectroscopy. In addition, the relative change in the Gruneisen coefficient of these solutions with concentration was determined. The sound speed of aqueous gels and lipid emulsions as a function of concentration was also measured. It was found that copper and nickel chloride are suitable chromophores for the construction of photoacoustic tissue phantoms due to their photostability. By contrast, organic dyes were found unsuitable for quantitative photoacoustic measurements due to optically induced transient changes to their absorption spectrum and permanent oxidative photobleaching.
TL;DR: Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers (OF-CERPAS), a "zero-background" method which increases selectivity and sensitivity, and its sensitivity scales with laser power.
Abstract: Cavity-enhanced resonant photoacoustic spectroscopy with optical feedback cw diode lasers (OF-CERPAS) is introduced as a novel technique for ultratrace gas analysis and high-resolution spectroscopy. In the scheme, a single-mode cw diode laser (3 mW, 635 nm) is coupled into a high-finesse linear cavity and stabilized to the cavity by optical feedback. Inside the cavity, a build-up of laser power to at least 2.5 W occurs. Absorbing gas phase species inside the cavity are detected with high sensitivity by the photoacoustic effect using a microphone embedded in the cavity. To increase sensitivity further, coupling into the cavity is modulated at a frequency corresponding to a longitudinal resonance of an organ pipe acoustic resonator (f=1.35 kHz and Q approximately 10). The technique has been characterized by measuring very weak water overtone transitions near 635 nm. Normalized noise-equivalent absorption coefficients are determined as alpha approximately 4.4x10(-9) cm(-1) s(1/2) (1 s integration time) and 2.6x10(-11) cm(-1) s(1/2) W (1 s integration time and 1 W laser power). These sensitivities compare favorably with existing state-of-the-art techniques. As an advantage, OF-CERPAS is a "zero-background" method which increases selectivity and sensitivity, and its sensitivity scales with laser power.
TL;DR: A mid-infrared quantum cascade laser was used to determine detection limits for acetic acid, acetone, 1,4-dioxane, and vinyl acetate, allowing for the collection of photoacoustic vibrational spectra for these gases.
Abstract: We report on the development of a microelectromechanical systems (MEMS)-scale photoacoustic sensor for the detection of trace gases. A mid-infrared quantum cascade laser (QCL) was used to determine detection limits for acetic acid, acetone, 1,4-dioxane, and vinyl acetate. The source was continuously tunable from 1015 cm -1 to 1240 cm -1 , allowing for the collection of photoacoustic vibrational spectra for these gases. Exceptional agreement between the measured photoacoustic spectra and the infrared spectra for acetic acid, acetone, 1,4-dioxane, and vinyl acetate was observed. Partial least-squares (PLS) regression was used to develop an algorithm for classification of these compounds based solely on photoacoustic spectra. Keywords: photoacoustic spectroscopy; sensor; quantum cascade laser; MEMS; chemometrics 1. Introduction Monitoring trace gases is of great importance in a wide range of applications. The Global War on Terror has made rapid detection and identification of chemical and biological agents a priority for
TL;DR: Three-dimensional images of phantoms and insects using a glass-fiber-based Fabry-Perot interferometer as integrating line detector are presented.
Abstract: For photoacoustic imaging, usually point-like detectors are used. As a special sensing technology for photoacoustic imaging, integrating detectors have been investigated that integrate the acoustic pressure over an area or line that is larger than the imaged object. Different kinds of optical fiber-based detectors are compared regarding their sensitivity and resolution in three-dimensional photoacoustic tomography. In the same type of interferometer, polymer optical fibers yielded much higher sensitivity than glass fibers. Fabry-Perot glass-fiber interferometers in turn gave higher sensitivity than Mach-Zehnder-type interferometers. Regarding imaging resolution, the single-mode glass fiber showed the best performance. Last, three-dimensional images of phantoms and insects using a glass-fiber-based Fabry-Perot interferometer as integrating line detector are presented.
TL;DR: In this paper, a quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combina- tion with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196-1281 cm −1 spectral range.
Abstract: A quartz enhanced photoacoustic spectroscopy sensor designed for fast response was used in combina- tion with a pulsed external cavity quantum cascade laser to rapidly acquire gas absorption data over the 1196- 1281 cm −1 spectral range. The system was used to measure concentrations of water vapor, pentafluoroethane (freon- 125), acetone, and ethanol both individually and in com- bined mixtures. The precision achieved for freon-125 con- centration in a single 1.1 s long spectral scan is 13 ppbv.
TL;DR: The prolongation of high-contrast imaging, and biocompatibility and easy surface functionalization for targeting ligands afforded by amorphous silica, suggest GNR-Si to be potentially significant for the clinical translation of PAT.
Abstract: Photoacoustic tomography (PAT) has garnered much attention for its high contrast and excellent spatial resolution of perfused tissues. Gold nanorods (GNRs) have been employed to further enhance the imaging contrast of PAT. However, the photon fluences typically needed for PA wave induction often also result in GNR shape changes that significantly reduce the efficiency of acoustic wave generation. In this work, we propose, synthesize, and evaluate amorphous silica-coated gold nanorods (GNR-Si) in an effort to improve contrast agent stability and ameliorate efficiency loss during photoacoustic (PA) wave induction. TEM and optical absorption spectra measurements of GNR and GNR-Si show that encasing GNRs within amorphous silica provides substantial protection of nanorod conformation from thermal deformation. PA signals generated by GNR-Si demonstrate considerably greater resistance to degradation of signal intensity with repetitive pulsing than do uncoated GNRs, thereby enabling much longer, high-contrast imaging sessions than previously possible. The prolongation of high-contrast imaging, and biocompatibility and easy surface functionalization for targeting ligands afforded by amorphous silica, suggest GNR-Si to be potentially significant for the clinical translation of PAT.
TL;DR: A photoacoustic correlation technique, inspired by its optical counterpart-the fluorescence correlation spectroscopy (FCS)-was tested for the first time, to the authors' knowledge, to demonstrate the feasibility of low-speed flow measurement based on photoac acoustic signal detection.
Abstract: A photoacoustic correlation technique, inspired by its optical counterpart—the fluorescence correlation spectroscopy (FCS)—was tested for the first time, to our knowledge, to demonstrate the feasibility of low-speed flow measurement based on photoacoustic signal detection. A pulsed laser was used to probe the flow of light-absorbing beads. A photoacoustic correlation system of 0.8 s temporal resolution was built and flow speeds ranging from 249 to 14.9 μm/s with corresponding flow times from 4.42 to 74.1 s were measured. The experiment serves as a proof of concept for photoacoustic correlation spectroscopy, which may have many potential applications similar to the FCS.
TL;DR: In this article, a microelectromechanical system-scale photoacoustic chemical sensor is described, where both a pulsed and a modulated continuous wave quantum cascade laser are used to determine detection limits for dimethyl methylphosphonate (DMMP), a standard nerve gas simulant.
Abstract: The development of a microelectromechanical systems-scale photoacoustic chemical sensor is described. Specifically, both a pulsed and a modulated continuous wave quantum cascade laser were used to determine detection limits for dimethyl methylphosphonate (DMMP), a standard nerve gas simulant. These sources were continuously tunable from 9.3 to 10 ?m. We report a minimum detection level of 20 parts-per-billion (ppb) and exceptional agreement between the measured photoacoustic vibrational spectrum and the IR spectrum of DMMP. The results support the continued development of a miniaturized photoacoustic sensor.
TL;DR: In situ imaging of mouse ears shows the feasibility of resolving individual red blood cells in microvessels using the current system, which has a transverse resolution of 2 microm, the highest to the authors' knowledge among all photoacoustic imaging systems.
Abstract: We present a new-generation optical-resolution confocal photoacoustic microscope, consisting of a 0.25-numerical aperture optical microscope objective and a 75-MHz center-frequency spheri- cally focused ultrasonic transducer. Experiments verified that this mi- croscope has a transverse resolution of 2 m, which is the highest to our knowledge among all photoacoustic imaging systems. In situ im- aging of mouse ears shows the feasibility of resolving individual red blood cells in microvessels using the current system. © 2010 Society of
TL;DR: It is suggested that silver nanosystems can be used as multifunctional agents capable of augmenting image-guided therapy techniques, and shown to be nontoxic in vitro at concentrations of silver up to 2 mgml.
Abstract: Due to their optical absorption properties, metallic nanoparticles are excellent photoacoustic imaging contrast agents. A silver nanosystem is presented here as a potential contrast agent for photoacoustic imaging and image-guided therapy. Currently, the nanosystem consists of a porous silver layer deposited on the surface of spherical silica cores ranging in diameter from 180 to 520 nm. The porous nature of the silver layer will allow for release of drugs or other therapeutic agents encapsulated in the core in future applications. In their current PEGylated form, the silver nanosystem is shown to be nontoxic in vitro at concentrations of silver up to 2 mgml. Furthermore, the near-infrared absorbance properties of the nanosystem are demonstrated by measuring strong, concentration-dependent photoacoustic signal from the silver nanosystem embedded in an ex vivo tissue sample. Our study suggests that silver nanosystems can be used as multifunctional agents capable of augmenting image-guided therapy techniques.
TL;DR: Developing compact, passively Q-switched fiber and microchip laser sources with high repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where real-time imaging performance is essential.
Abstract: Optical-resolution photoacoustic microscopy (OR-PAM) is a novel imaging technology for visualizing optically absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, OR-PAM imaging speed is limited by both scanning speed and laser pulse repetition rate. Unfortunately, lasers with high repetition rates and suitable pulse durations and energies are not widely available and can be cost-prohibitive and bulky. We are developing compact, passively Q-switched fiber and microchip laser sources for this application. The properties of these lasers are discussed, and pulse repetition rates up to 100 kHz are demonstrated. OR-PAM imaging was conducted using a previously developed photoacoustic probe, which enabled flexible scanning of the focused output of the lasers. Phantom studies demonstrate the ability to image with lateral spatial resolution of 7±2 µm with the microchip laser system and 15±5 µm with the fiber laser system. We believe that the high pulse repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where real-time imaging performance is essential.
TL;DR: The ability of the photoacoustic tomography device to create 3-D images with constant resolution throughout the reconstruction volume is demonstrated experimentally using a human hair phantom to show the applicability for preclinical and biological research.
Abstract: A device for three-dimensional (3-D) photoacoustic tomography with resolution in the range of tens of micrometers is presented that uses a light beam for interferometric detection of acoustic waves. Reconstruction of the 3-D initial pressure distribution from the signals representing line integrals of the acoustic field is a two-step process. It uses an inversion of 2-D wave propagation to obtain line projections of the initial pressure distribution and the inverse Radon transform. The light beam, propagating freely in a water bath, is scanned either in an πarc- or box-shaped curve around the object. Simulations are performed to compare the two scanning procedures. The projection images are obtained either using the filtered back projection algorithm for the -arc scanning mode or the frequency domain algorithm for the box scanning mode. While the former algorithm provides slightly better image quality, the latter is about 20 times faster. The ability of the photoacoustic tomography device to create 3-D images with constant resolution throughout the reconstruction volume is demonstrated experimentally using a human hair phantom. These measurements revealed a 3-D resolution below 100 µm. In a second experiment, 3-D imaging of an isolated mouse heart is demonstrated to show the applicability for preclinical and biological research.
Patent•
01 Nov 2010
TL;DR: In this paper, a quantum cascade laser (QCL) system that better achieves single mode, continuous, mode-hop free tuning for use in L-PAS (laser photoacoustic spectroscopy) by independently coordinating gain chip current, diffraction grating angle and external cavity length is described.
Abstract: Methods and apparatus for broad tuning of single wavelength quantum cascade lasers and the use of light output from such lasers for highly sensitive detection of trace gases such as nitrogen dioxide, acetylene, and vapors of explosives such as trinitrotoluene (TNT) and triacetone triperoxide (TATP) and TATP's precursors including acetone and hydrogen peroxide. These methods and apparatus are also suitable for high sensitivity, high selectivity detection of other chemical compounds including chemical warfare agents and toxic industrial chemicals. A quantum cascade laser (QCL) system that better achieves single mode, continuous, mode-hop free tuning for use in L-PAS (laser photoacoustic spectroscopy) by independently coordinating gain chip current, diffraction grating angle and external cavity length is described. An all mechanical method that achieves similar performance is also described. Additionally, methods for improving the sensor performance by critical selection of wavelengths are presented.
TL;DR: In this article, an entangled cavity doubly resonant optical parametric oscillator (ECOPO) was developed to provide tunable narrow line width (<100 MHz) pulsed (8 ns) radiation over the 3.8-4.3μm spectral range at a multi-kilohertz repetition rate with up to 100-W peak power.
Abstract: An entangled cavity doubly resonant optical parametric oscillator (ECOPO) has been developed to provide tunable narrow line width (<100 MHz) pulsed (8 ns) radiation over the 3.8–4.3 μm spectral range at a multi-kilohertz repetition rate with up to 100-W peak power. We demonstrate that coarse single mode tuning is obtained over the full spectral range of oscillation (300 cm−1), while automated mode-hop-free fine tuning is carried out over more than 100 GHz. High-resolution spectra of main greenhouse gases (CO2, N2O, SO2 and CH4) have been obtained in good agreement with calculated spectra from the HITRAN database. These experiments outline the unique capabilities of the ECOPO for multi-gas sensing based on direct absorption as well as photoacoustic spectroscopy.
TL;DR: In this article, the most promising laser photothermal methods of standoff detection of trace explosive residues on surfaces are considered. But, the analysis is restricted to the most successful methods: photoacoustic spectroscopy, deflection spectrography, and IR photothermal imaging of objects under resonant irradiation.
Abstract: Laser photothermal methods of standoff detection of trace explosive residues on surfaces are considered. The analysis is restricted to the most promising methods: photoacoustic spectroscopy, deflection spectroscopy, and IR photothermal imaging of objects under resonant irradiation. Particular attention is paid to the choice of radiation sources and detectors. Comparative analysis of the existing standoff detection methods for explosive particles on the object surface is performed. Prospects of laser photothermal spectroscopy in this field are discussed.
TL;DR: It is shown that for photoacoustic spectroscopy (PAS) of the O(2) A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O( 2).
Abstract: We model and measure the absolute response of an intensity-modulated photoacoustic spectrometer comprising a 10 cm long resonator and having a Q-factor of approximately 30. We present a detailed theoretical analysis of the system and predict its response as a function of gas properties, resonance frequency, and sample energy transfer relaxation rates. We use a low-power continuous wave laser to probe O2 A-band absorption transitions using atmospheric, humidified air as the sample gas to calibrate the system. This approach provides a convenient and well-characterized method for calibrating the absolute response of the system provided that water-vapor-mediated relaxation effects are properly taken into account. We show that for photoacoustic spectroscopy (PAS) of the O2 A-band, the maximum conversion efficiency of absorbed photon energy to acoustic energy is approximately 40% and is limited by finite collision-induced relaxation rates between the two lowest-lying excited electronic states of O2. PAS also sh...
Journal Article•
TL;DR: In this article, photoacoustic spectroscopy is used to measure the absorbed energy of electromagnetic radiation by detecting pressure fluctuations in the form of sound waves or shock pulses, which can be used for data acquisition relating to absorption and material science studies.
Abstract: Based on photoacoustic effect, photoacoustic spectroscopy is an unusual form of spectroscopy which uses both light and sound and is based on the absorption of electromagnetic radiation by analyte molecule. The absorbed energy is measured by detecting pressure fluctuations in the form of sound waves or shock pulses. A photoacoustic spectrum consists of a plot of the intensity of the acoustic signal detected by a microphone or a piezoelectric detector, against the excitation wavelength or another quantity related to the photon energy of the modulated excitation. It is a valuable source for data acquisition relating to absorption and material science studies.
TL;DR: The trans-lycopene content of fresh tomato homogenates was assessed by means of the laser photoacoustic spectroscopy, the laser optothermal window, micro-Raman spectroscopic, and colorimetry; none of these methods require the extraction from the product matrix prior to the analysis.
Abstract: The trans-lycopene content of fresh tomato homogenates was assessed by means of the laser photoacoustic spectroscopy, the laser optothermal window, micro-Raman spectroscopy, and colorimetry; none of these methods require the extraction from the product matrix prior to the analysis. The wet chemistry method (high-performance liquid chromatography) was used as the absolute quantitative method. Analytical figures of merit for all methods were compared statistically; best linear correlation was achieved for the chromaticity index a* and chroma C*.
TL;DR: The saturation effect, which describes the violation of the linearity between the measured photoac acoustic amplitude and the object's optical absorption coefficient in functional photoacoustic imaging when the optical absorption in the object increases, is investigated.
Abstract: We investigate the saturation effect, which describes the violation of the linearity between the measured photoacoustic amplitude and the object's optical absorption coefficient in functional photoacoustic imaging when the optical absorption in the object increases. We model the optical energy deposition and photoacoustic signal generation and detection in a semi-infinite optical absorbing object. Experiments are carried out by measuring photoacoustic signals generated from an ink-filled plastic tube. The saturation effect is studied by varying the optical absorption coefficient in the model and the ink concentration in the photoacoustic experiments. By changing the center frequency of the ultrasonic detector, the requirement to minimize the saturation effect in functional photoacoustic imaging is established.
TL;DR: T tone-burst excitation and time-gated spectral analysis are used for photoacoustic Doppler mapping of flow in an unperturbed vessel phantom and in a vessel with a spatially varying lumen to demonstrate the potential use of the method for characterization of flow irregularities.
Abstract: We demonstrate the use of tone-burst excitation and time-gated spectral analysis for photoacoustic Doppler mapping of flow in an unperturbed vessel phantom and in a vessel with a spatially varying lumen. The method, which mimics pulsed Doppler ultrasound, enables simultaneous measurement of axial position and flow as well as complete characterization of the Doppler spectrum over a wide range of mean velocities (3.5 to 200 mm∕s). To generate the required optical excitation, a continuous cw laser source followed by an external electro-optic modulator is used. Stenoses at various levels are emulated in a C-flex tube with a flowing suspension of micrometer-scale carbon particles. Two-dimensional maps of spectral content versus axial position at different points along the vessel and for various levels of perturbations demonstrate the potential use of the method for characterization of flow irregularities.