Non-contact Photoacoustic Imaging Research Articles

Non-contact photoacoustic imaging with a silicon photonics-based Laser Doppler Vibrometer

Photoacoustic imaging has emerged as a powerful, non-invasive modality for various biomedical applications. Conventional photoacoustic systems require contact-based ultrasound detection and expensive, bulky high-power lasers for the excitation. The use of contact-based detectors involves the risk of contamination, which is undesirable for most biomedical applications. While other non-contact detection methods can be bulky, in this paper, we demonstrate a proof-of-concept experiment for compact and contactless detection of photoacoustic signals on silicone samples embedded with ink-filled channels. A silicon photonics-based Laser Doppler Vibrometer (LDV) detects the acoustic waves excited by a compact pulsed laser diode. By scanning the LDV beam over the surface of the sample, 2D photoacoustic images were reconstructed of the sample.

High Stability Dual Beam Interferometer Sensor for Non-contact Photoacoustic Imaging

As a non-contact photoacoustic sensor, the reflector of one arm of dual beam interferometer sensor is usually composed of the upper surface of the ultrasonic coupling liquid coated on the surface of the sample. The vibration of the outside and the scanning motor cause the liquid level to shake, causing the reflected probe light path to deviate from the incident light path, which then reduces the detection stability of the sensor. In view of the above problems, a novel high stability dual beam interferometer sensor is introduced in this paper, in which the reflected probe light is derived from the original optical path by using the combination of PBS and quarter wave plate, and the reflected probe light is coupled into 50/50 beam splitting fibre by using aspheric mirror group. The influence of the radius and focal length of each aspheric mirror and the numerical aperture of the optical fibre on the detection stability of the sensor is theoretically analyzed. The results show that the structural design greatly reduces the effect of the vibration of the outside and the scanning motor on the detection stability of the sensor, and provides a theoretical basis for the development and performance optimization of the sensor.

Noncontact photoacoustic lipid imaging by remote sensing on first overtone of the C-H bond

Lipid imaging by conventional photoacoustic microscopy subjects to direct contact sensing with relatively low detection bandwidth and sensitivity, which induces superficial imaging depth and low signal-to-noise ratio (SNR) in practical imaging scenarios. Herein, we present a photoacoustic remote sensing microscopy for lipid distribution mapping in bio-tissue, featuring noncontact implementation, broad detection bandwidth, deep penetration depth, and high SNR. A tailored high-energy pulsed laser source with a spectrum centered at 1750 nm is used as the excitation beam, while a cofocused 1550 nm continuous-wave beam is used as the probe signal. The pump wavelength is selected to overlap the first overtone of the C-H bond in response to the intensive absorption of lipid molecules, which introduces a much-enhanced SNR (55 dB) onto photoacoustic remote sensing (PARS) signals. Meanwhile, the optical sensing scheme of the photoacoustic signals provides broadband detection compared to the acoustic transducer and refrains the bio-samples from direct contact operations by eliminating the ultrasonic coupling medium. Taking merits of the high detection sensitivity, deep penetration depth, broadband detection, and high resolution of the PARS system, high-quality tissue scale lipid imaging is demonstrated in a model organism and brain slice.

In-vivo functional and structural retinal imaging using multiwavelength photoacoustic remote sensing microscopy

Many important eye diseases as well as systemic disorders manifest themselves in the retina. Retinal imaging technologies are rapidly growing and can provide ever-increasing amounts of information about the structure, function, and molecular composition of retinal tissue in-vivo. Photoacoustic remote sensing (PARS) is a novel imaging modality based on all-optical detection of photoacoustic signals, which makes it suitable for a wide range of medical applications. In this study, PARS is applied for in-vivo imaging of the retina and estimating oxygen saturation in the retinal vasculature. To our knowledge, this is the first time that a non-contact photoacoustic imaging technique is applied for in-vivo imaging of the retina. Here, optical coherence tomography is also used as a well-established retinal imaging technique to navigate the PARS imaging beams and demonstrate the capabilities of the optical imaging setup. The system is applied for in-vivo imaging of both microanatomy and the microvasculature of the retina. The developed system has the potential to advance the understanding of the ocular environment and to help in monitoring of ophthalmic diseases.

Double-Clad Optical Fiber-Based Multi-Contrast Noncontact Photoacoustic and Fluorescence Imaging System

A noncontact photoacoustic and fluorescence dual-modality imaging system is proposed, which integrates a fiber-based fluorescence imaging system with noncontact photoacoustic imaging using a specially fabricated double-cladding fiber (DCF) coupler and a DCF lens. The performance of the DCF coupler and lens was evaluated, and the feasibility of this new imaging system was demonstrated using simple tubing phantoms with black ink and fluorophore. Our imaging results demonstrated that the multimodal imaging technique can simultaneously acquire photoacoustic and fluorescence images without coming into contact with the sample. Consequently, the developed method is the first noncontact scheme among multimodal imaging systems that is integrated with a photoacoustic imaging system, which can provide varied and complementary information about the sample.

Progress and Biomedical Application of Non-Contact Photoacoustic Imaging

Progress and Biomedical Application of Non-Contact Photoacoustic Imaging

Label-free, non-contact, in vivo ophthalmic imaging using photoacoustic remote sensing microscopy.

We present, to the best of our knowledge, the first label-free, non-contact, in vivo imaging of the ocular vasculature using photoacoustic remote sensing (PARS) microscopy. Both anterior and posterior segments of a mouse eye were imaged. Vasculature of the iris, sclera, and retina tissues were clearly resolved. To the best of our knowledge, this is the first study showing non-contact photoacoustic imaging conducted on in vivo ocular tissue. We believe that PARS microscopy has the potential to advance the diagnosis and treatment of ocular diseases.

Non-contact photoacoustic imaging using laser speckle contrast analysis

A novel method for non-contact and continuous detection of photoacoustic signals is presented and experimentally demonstrated. The approach is based on analysis of the contrast of time-varying speckle patterns, and suggests a more robust alternative in respect to interferometric and refractometric available solutions.

Noncontact photoacoustic imaging based on optical quadrature detection with a multiport interferometer.

A noncontact photoacoustic imaging method based on optical quadrature detection is proposed. The photo-induced acoustic signal is detected by an optical method without contacting the specimen. By utilizing the intrinsic phase difference of a multiport optical interferometer, the quadrature signal of a conventional interferometric signal could be obtained. With this quadratic signal pair, we could reconstruct the photoacoustic signal without suffering from the initial phase drift that usually occurs in a conventional interferometric system. The performance of the proposed system is verified by imaging human hairs embedded in a polydimethylsiloxane resin block. The system's lateral and axial resolutions are measured to be 84 and 86μm at a 1.5mm depth of a PDMS resin block, respectively. The experimental result is good enough to distinguish the hairs staggered in depth.

Noncontact photoacoustic angiography with an air-coupled ultrasonic transducer for evaluation of burn injury

In vivo identification of viable and thermally coagulated blood in a burn wound can be used to profile the type, extent, and site of the burn, thus assisting the removal of necrotic tissue in preparation for skin grafts. Recently, intensive research on photoacoustic imaging (PAI) has been conducted to evaluate burn injury. However, most PAI systems usually take the approach of direct physical contact of an acoustic detector or an impedance matching medium with the vulnerable burned skin, which exposes significant limitations on the diagnosis of burn injuries. Here, we present a noncontact PAI system, based on a custom designed air-coupled ultrasonic transducer, enabling in vivo noncontact vasculature to be imaged without contrast agents. Using the natural difference in light absorption between coagulated and non-coagulated blood at the wavelength of 532 nm, this PAI system possesses an immanent advantage to discriminate viable and thermally coagulated blood in burned tissues. Phantom experiments and burned rabbit's skin imaging have been implemented to demonstrate that the noncontact PAI technique could be valuable in the adjuvant diagnosis and observation of burns.

Noncontact holographic detection for photoacoustic tomography

A holographic method for high-speed, noncontact photoacoustic tomography is introduced and evaluated. Relative changes of the object's topography, induced by the impact of thermoelastic pressure waves, were determined at nanometer sensitivity without physical contact. The object's surface was illuminated with nanosecond laser pulses and imaged with a high-speed CMOS camera. From two interferograms measured before and after excitation of the acoustic wave, surface displacement was calculated and then used as the basis for a tomographic reconstruction of the initial pressure caused by optical absorption. The holographic detection scheme enables variable sampling rates of the photoacoustic signal of up to 50MHz. The total acquisition times for complete volumes with 230MVoxel is far below 1s. Measurements of silicone and porcine skin tissue phantoms with embedded artificial absorbers, which served as a model for human subcutaneous vascular networks, were possible. Three-dimensional reconstructions of the absorbing structures show details with a diameter of 310 μm up to a depth of 2.5mm. Theoretical limitations and the experimental sensitivity, as well as the potential for in vivo imaging depending on the detection repetition rate, are analyzed and discussed.

Non-contacting photoacoustic tomography in biological samples

In photoacoustic imaging the ultrasonic signals are usually detected by contacting transducers. For some applications, contact with the tissue should be avoided, e.g. in those of brain functional imaging. As alternatives to contacting transducers interferometric techniques can be used to acquire photoacoustic signals remotely. Here, a system for non-contact photoacoustic tomography imaging (NCPAT) has been established. This approach enables NCPAT not to exceed laser exposure safety limits. The stimulated source of NCPAT utilized a laser with center wavelength of 532 nm and output intensity of 17.5 mJ/cm 2, and a laser heterodyne interferometry was used to receive the photoacoustic signals. The NCPAT was used to implement on a rotational imaging geometry for photoacoustic tomography with a real-tissue phantom. The photoacoustic imaging was obtained by applying a reconstruction algorithm to the data acquired for NCPAT. Experiments results showed that the NCPAT system with detection 15 dB bandwidth of 2.25 MHz could resolve spherical optical inclusions with dimension of 500 μm and multi-layered structure with optical contrast in strongly scattering medium. The method could expand the scope of photoacoustic and ultrasonic technology to in-vivo biomedical applications where contact is impractical.

A Problem of a Displacement Calculation of Tissue Surface in Non-Contact Photoacoustic Tomography

Background. Photoacoustic tomography (PAT) is a relatively new imaging modality,which allows e.g. visualizing the vascular network in biological tissue noninvasively. This tomographic method has an advantage in comparison to pure optical/acoustical methods due to high optical contrast and low acoustic scattering in deep tissue. The common PAT methodology, based on measurements of the acoustic pressure by piezoelectric sensors placed on the tissue surface, limits its practical versatility. A novel, completely non-contact and full-field PAT system is described. In noncontact PAT the measurement of surface displacement induced by the acoustic pressure at the tissue/air border is researched. Objective. To solve a simulation problem of the displacement calculation based on the medium pressure, which consists in deriving a formula for recalculating the pressure in the surface displacement based on the momentum conservation law, developing a simulation technique, and comparing the error of the proposed technique with the earlier used one. Methods. Comparing the experimental data with simulated pressure data in the k-Wave toolbox. The criterion of comparison is the relative quadratic error. Results. The simulation results of the displacement based on a new approach are more consistent with the experimental data than previous. The quadratic error numerical value of the new approach is 18 % and the previous is 71 %. Conclusions. The theoretical features of the surface displacement simulation are investigated and the solution of this problem is proposed based on momentum conservation law. The implementation of the proposed methodology has a four times smaller simulation error compared to the previous technique, so it can be implemented in the non-contact PAT. The residual error can be caused by the properties of the tissue, which are not taken into account in the model, which requires further research.

Low-power noncontact photoacoustic microscope for bioimaging applications

An inexpensive noncontact photoacoustic (PA) imaging system using a low-power continuous wave laser and a kilohertz-range microphone has been developed. The system operates in both optical and PA imaging modes and is designed to be compatible with conventional optical microscopes. Aqueous coupling fluids are not required for the detection of the PA signals; air is used as the coupling medium. The main component of the PA system is a custom designed PA imaging sensor that consists of an air-filled sample chamber and a resonator chamber that isolates a standard kilohertz frequency microphone from the input laser. A sample to be examined is placed on the glass substrate inside the chamber. A laser focused to a small spot by a 40 × objective onto the substrate enables generation of PA signals from the sample. Raster scanning the laser over the sample with micrometer-sized steps enables high-resolution PA images to be generated. A lateral resolution of 1.37 ?? ? m was achieved in this proof of concept study, which can be further improved using a higher numerical aperture objective. The application of the system was investigated on a red blood cell, with a noise-equivalent detection sensitivity of 43,887 hemoglobin molecules ( 72.88 × 10 ? 21 ?? mol or 72.88 zeptomol). The minimum pressure detectable limit of the system was 19.1 ?? ? Pa . This inexpensive, compact noncontact PA sensor is easily integrated with existing commercial optical microscopes, enabling optical and PA imaging of the same sample. Applications include forensic measurements, blood coagulation tests, and monitoring the penetration of drugs into human membrane.

An All-Fiber-Optic Combined System of Noncontact Photoacoustic Tomography and Optical Coherence Tomography.

We propose an all-fiber-based dual-modal imaging system that combines noncontact photoacoustic tomography (PAT) and optical coherence tomography (OCT). The PAT remotely measures photoacoustic (PA) signals with a 1550-nm laser on the surface of a sample by utilizing a fiber interferometer as an ultrasound detector. The fiber-based OCT, employing a swept-source laser centered at 1310 nm, shares the sample arm of the PAT system. The fiber-optic probe for the combined system was homemade with a lensed single-mode fiber (SMF) and a large-core multimode fiber (MMF). The compact and robust common probe is capable of obtaining both the PA and the OCT signals at the same position without any physical contact. Additionally, the MMF of the probe delivers the short pulses of a Nd:YAG laser to efficiently excite the PA signals. We experimentally demonstrate the feasibility of the proposed dual-modal system with a phantom made of a fishing line and a black polyethylene terephthalate fiber in a tissue mimicking solution. The all-fiber-optic system, capable of providing complementary information about absorption and scattering, has a promising potential in minimally invasive and endoscopic imaging.

全光非接触光声层析及光学相干层析双模成像

全光非接触光声层析及光学相干层析双模成像

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.

Speckle-based off-axis holographic detection for non-contact photoacoustic tomography

Abstract A very fast innovative holographic off-axis non-contact detection method for Photoacoustic Tomography (PAT) is introduced. It overcomes the main problems of most state-of-the-art photoacoustic imaging approaches that are long acquisition times and the requirement of acoustic contact. In order to increase the acquisition speed significantly, the surface displacements of the object, caused by the photoacoustic pressure waves, are measured interferometrically in two dimensions. Phase alterations in the observed speckle field are used to identify changes in the object’s topography. A sampling rate of up to 80 MHz is feasible, which reduces the occurrence of motion artefacts. This approach was validated with a silicone phantom with cylindrical absorbers that are similar to the shape of blood vessels. A tomographic reconstruction leads to the three dimensional location of the absorbers. A reliable reconstruction proves the ability of the method.

Full-field speckle interferometry for non-contact photoacoustic tomography

A full-field speckle interferometry method for non-contact and prospectively high speed Photoacoustic Tomography is introduced and evaluated as proof of concept. Thermoelastic pressure induced changes of the objects topography are acquired in a repetitive mode without any physical contact to the object. In order to obtain high acquisition speed, the object surface is illuminated by laser pulses and imaged onto a high speed camera chip. In a repetitive triple pulse mode, surface displacements can be acquired with nanometre sensitivity and an adjustable sampling rate of e.g. 20 MHz with a total acquisition time far below one second using kHz repetition rate lasers. Due to recurring interferometric referencing, the method is insensitive to thermal drift of the object due to previous pulses or other motion. The size of the investigated area and the spatial and temporal resolution of the detection are scalable. In this study, the approach is validated by measuring a silicone phantom and a porcine skin phantom with embedded silicone absorbers. The reconstruction of the absorbers is presented in 2D and 3D. The sensitivity of the measurement with respect to the photoacoustic detection is discussed. Potentially, Photoacoustic Imaging can be brought a step closer towards non-anaesthetized in vivo imaging and new medical applications not allowing acoustic contact, such as neurosurgical monitoring or burnt skin investigation.

Multimodal noncontact photoacoustic and optical coherence tomography imaging using wavelength-division multiplexing.

We present multimodal noncontact photoacoustic (PA) and optical coherence tomography (OCT) imaging. PA signals are acquired remotely on the surface of a specimen with a Mach-Zehnder interferometer. The interferometer is realized in a fiber-optic network using a fiber laser at 1550 nm as the source. In the same fiber-optic network, a spectral-domain OCT system is implemented. The OCT system utilizes a supercontinuum light source at 1310 nm and a spectrometer with an InGaAs line array detector. Light from the fiber laser and the OCT source is multiplexed into one fiber using a wavelength-division multiplexer; the same objective is used for both imaging modalities. Reflected light is spectrally demultiplexed and guided to the respective imaging systems. We demonstrate two-dimensional and three-dimensional imaging on a tissue-mimicking sample and a chicken skin phantom. The same fiber network and same optical components are used for PA and OCT imaging, and the obtained images are intrinsically coregistered.