Acousto-optic Imaging Research Articles

Acousto-optic Filtration of Interfering Light Beams for Visualization of Amplitude and Phase Structure of Small-size Objects

A technique for acousto-optic image filtration in full-field interferometry is proposed. It is based on the spectral selection of light in the registration channel of the Michelson interferometer by an imaging acousto-optical tunable filter. The technique is applicable for various full-field spectral-domain systems in profilometry and optical coherence tomography. It is experimentally shown that registered spectral signals may be used for 3-dimensional visualization of structure of optically inhomogeneous objects.

Acoustooptic technique of nondestructive quality control of crystals used in acoustoelectronics

Acoustooptic modification of implementation of nondestructive crystals’ quality control technique is proposed. The potentials of the technique for the investigation of the properties of crystals used in acoustoelectronic devices are shown. Acoustooptic Schlieren images technique has been applied. Visualization of acoustic fields’ structure has allowed us to estimate the optical homogeneity of the material and to determine the spatial characteristics of the acoustic waves, the effects of divergence and deviation of propagation direction from the wave normal.

Reconstruction and stability in acousto-optic imaging for absorption maps with bounded variation

The aim of this paper is to propose for the first time a reconstruction scheme and a stability result for recovering from acousto-optic data absorption distributions with bounded variation. The paper extends earlier results in [3] and [5] on smooth absorption distributions. It opens a door for a mathematical and numerical framework for imaging, from internal data, parameter distributions with high contrast in biological tissues.

Receiver operating characteristic analysis for the detectability of malignant breast lesions in acousto-optic transmission ultrasound breast imaging

Conventional B-mode ultrasound imaging has proven to be a valuable supplement to x-ray mammography for the detection of malignant breast lesions in premenopausal women with high breast density. We have developed a high-resolution transmission ultrasound breast imaging system employing a novel acousto-optic (AO) liquid crystal detector to enable rapid acquisition of full-field breast ultrasound images during routine cancer screening. In this study, a receiver operating characteristic (ROC) analysis was performed to assess the diagnostic utility of our prototype system. Using a comprehensive system model, we simulated the AO transmission ultrasound images expected for a 1-mm malignant lesion contained within a dense breast consisting of 75% normal breast parenchyma and 25% fat tissue. A Gaussian noise model was assumed with SNRs ranging from 0 to 30. For each unique SNR, an ROC curve was constructed and the area under the curve (AUC) was computed to assess the lesion detectability of our system. For SNRs in excess of 10, the analysis revealed AUCs greater than 0.8983, thus demonstrating strong detectability. Our results indicate the potential for using an imaging system of this kind to improve breast cancer screening efforts by reducing the high false negative rate of mammography in premenopausal women.

Multi-modal acousto-optic/ultrasound imaging of ex vivo liver tumors at 790 nm using a Sn2 P2 S6 wavefront adaptive holographic setup.

Biological tissues are very strong light-scattering media. As a consequence, current medical imaging devices do not allow deep optical imaging unless invasive techniques are used. Acousto-optic imaging is a light-ultrasound coupling technique that takes advantage of the ballistic propagation of ultrasound in biological tissues to access optical contrast with a millimeter resolution. We have developed a photorefractive-crystal-based system that performs self-adaptive wavefront holography and works within the optical therapeutic window. As it works at an appropriate wavelength range for biological tissues imaging, it was tested on ex vivo liver samples containing tumors as a pre-clinical study. Optical contrast was obtained even if acoustical one was not significant. Ultrasound image (left) and acousto-optic image (right) of a liver biopsy with tumors. Acousto-optic imaging exhibits tumors that are not detected through ultrasound.

Fast wavefront adaptive holography in Nd:YVO4 for ultrasound optical tomography imaging.

Several approaches exist to perform acousto-optic imaging of multiple-scattering media such as biological samples. Up to now, most of the coherent detection methods use holographic setup based on photorefractive crystals such as BSO or SPS. One of the issue of these techniques is the moderate response time compared to the speckle decorrelation time in biological sample. We introduce a new approach for the holographic detection based on two-wave mixing in a Nd:YVO4 gain medium enabling us to perform a fast wavefront adaption (50 μs) of the speckle field from a multiple-scattering sample.

Acousto-optic Bragg imaging of biological tissue.

Acousto-optic Bragg imaging is a technique that uses the interaction of light with ultrasound to optically image obstructions in acoustical fields. Existing reports of acousto-optic Bragg imaging based on transmission of acoustic fields through obstructions exhibit strong acoustic impedance mismatches manifested by poor image quality and missing details of physical structures of obstructions. In this work, the image quality was improved to exhibit detailed physical structures of an object by using an improved Bragg imaging system described in Sec. III below. This paper investigates the possibility of extending an acoustic Bragg imaging technique in transmission mode to image animal or plant tissues; a small azalea leaf is used as an illustration in this case. The Bragg image produced clearly shows the veins of the vascular azalea leaf serving as a proof of concept for cost-effective potential application of acoustic Bragg imaging of biological objects in the medical field. Moreover, acousto-optic Bragg imaging is potentially harmless to biological cells and is sensitive to density and elastic variations in the tissue.

Modelization and optimized speckle detection scheme in photorefractive self-referenced acousto-optic imaging.

A photorefractive BSO single crystal can be used for axially resolved acousto-optic imaging of thick scattering media in absence of a reference beam. This configuration renders the experimental setup easier to realize for imaging through thick scattering media with an improved optical etendue. We present here a model and simulations that explains these results. It is based on the spatial heterogeneity of the speckle pattern incident on the crystal. Optimization of the detector position and of the speckle grain size is confirmed by the model.

Improvement in the synchronization between the radio frequency signal and the image detector in an acousto-optic tunable filter imaging spectrometer.

An improved synchronization between the radio frequency (RF) signal and the image detector in an acousto-optic tunable filter (AOTF) imaging spectrometer is proposed to optimize power consumption and eliminate image smear. The RF signal is controlled on and off alternately to match the exposure of the image sensor. This scheme reduces the RF power and rejects the light illumination on the image sensor in the interval of charge transfer. An experiment using a visible AOTF, a frame transfer charge-coupled device camera, and an incandescent lamp is conducted for demonstration. The average RF power decreases 7.6%, and the image smear is eliminated.

The effect of scattering-medium parameters on signal magnitude under acousto-optic tomography

We have experimentally studied the influence of scattering anisotropy parameter g of a medium on the magnitude of signal S (visualization parameter) at an ultrasonic frequency that is registered upon acoustooptic tomography. Aqueous solutions of mixtures of cream and skimmed milk with different ratios between them were used as scattering media. The optical properties of media (absorption coefficient μa and reduced scattering coefficient μ′S) have been measured on a spectrophotometer (Perkin-Elmer Lambda 950 UV-VIS-NIR) using the inverse adding-doubling technique. As a result of the investigation, we have found that there is a certain correlation between the value of the scattering anisotropy parameter g of aqueous solutions of investigated mixtures and the percentage of the mixture in the aqueous solution, which ensures the required small value of extinction coefficient μ of the scattering medium. An increase in signal S has been revealed with increasing anisotropy parameter g of the medium at a invariable value of extinction coefficient μ. We have concluded that, to solve an inverse problem on the acousto-optic tomography, it is necessary to take into account possible changes in the g factor in scattering media, including biological ones.

Towards acousto-optic tissue imaging with nanosecond laser pulses

We present a way to generate acousto-optical signals in timovssue-like media with nanosecond laser pulses. Our method is based on recording and analyzing speckle patterns formed by interaction of nanosecond laser pulses with tissue, without and with simultaneous application of ultrasound. Stroboscopic application allows visualizing the temporal behavior of speckles while the ultrasound is propagating through the medium. We investigate two ways of quantifying the acousto-optic effect, viz. adding and subtracting speckle patterns obtained at various ultrasound phases. Both methods are compared with the existing speckle contrast method using a 2D scan and are found to perform similarly. Our method gives outlook on overcoming the speckle decorrelation problem in acousto-optics, and therefore brings in-vivo acousto-optic measurements one step closer. Furthermore it enables combining acousto-optics and photoacoustics in one setup with a single laser.

Photoacoustic guided ultrasound wavefront shaping for targeted acousto-optic imaging

To overcome speed of sound aberrations that negatively impact the acoustic focus in acousto-optic imaging, received photoacoustic signals are used to guide the formation of ultrasound wavefronts to compensate for acoustic inhomogeneities. Photoacoustic point sources composed of gold and superparamagnetic iron oxide nanoparticles are used to generate acoustic waves that acoustically probe the medium as they propagate to the detector. By utilizing cross-correlation techniques with the received photoacoustic signal, transmitted ultrasound wavefronts compensate for the aberration, allowing for optimized and configurable ultrasound transmission to targeted locations. It is demonstrated that utilizing a portable commercially available ultrasound system using customized software, photoacoustic guided ultrasound wavefront shaping for targeted acousto-optic imaging is robust in the presence of large, highly attenuating acoustic aberration.

Three-dimensional reconstruction of sound fields based on the acousto-optic effect

The acousto-optic effect can be used to measure the pressure fluctuations in air created by acoustic disturbances (the propagation of light is affected by changes in the medium due to the presence of sound waves). This makes it possible to measure an arbitrary sound field using acousto-optic tomography via scanning the field with a laser Doppler vibrometer. Consequently, the spatial characteristics of the sound field are captured in the measurement, implicitly bearing the potential for a full holographic reconstruction in a three-dimensional space. Recent studies have examined the reconstruction of sound pressure fields from acousto-optic measurements in the audible frequency range, based on Fourier transforms and elementary wave expansion methods. The present study examines the complete reconstruction of the sound field from acousto-optic measurements, recovering all acoustic quantities, and compares the results to the ones obtained from conventional microphone array measurements.

Development of a dual-path system for band-to-band registration of an acousto-optic tunable filter-based imaging spectrometer

Accurate band-to-band registration is crucial when an acousto-optic tunable filter (AOTF)-based imaging spectrometer is used to acquire spectral data on an unstable platform. However, it is difficult to registrate hyperspectral images accurately using traditional image registration algorithms, which will seriously affect the accurate acquisition of spectra and reduce the classification accuracy. Here we demonstrate an optical configuration that acquires the diffracted and undiffracted beam of the AOTF simultaneously. These two beams have identical geometrical optical characteristic and suffer from the same image motion caused by the platform jitter. Compared with the diffracted beam, however, the intensity of the undiffracted beam is changeless when tuning the AOTF. Therefore, the use of undiffracted beams allows accurate measurement of image motion in different bands. The experiment shows that this technique greatly improves the registration accuracy of spectral images with poor correlation and low SNR.

Optical phase conjugation in Nd:YVO_4 for acousto-optic detection in scattering media

Acousto-optic imaging is a technique that maps the optical properties of a thick scattering sample with millimetric resolution. The detection of the acousto-optic signal represents a challenge, because it is very weak among a strong parasitic signal. Various methods based on holography in photorefractive crystals or digital holography have been studied. Here dynamic holography is obtained with the gain medium Nd:YVO(4). We study the experimental feasibility of a detection system based on holography in a gain medium and show acousto-optic results obtained in a 5 mm slice of chicken breast.

Experimental study of z resolution in acousto-optical coherence tomography using random phase jumps on ultrasound and light

Acousto-optical coherence tomography (AOCT) is a variant of acousto-optic imaging (also called ultrasound modulated optical tomography) that makes possible to get resolution along the ultrasound propagation axis z. We present here AOCT experimental results, and we study how the z resolution depends on time step between phase jumps T(φ), or on the correlation length Δz. By working at low resolution, we perform a quantitative comparison of the z measurements with the theoretical point spread function. We also present images recorded with different z resolution, and we qualitatively show how the image quality varies with T(φ), or Δz.

Dynamic ultrasound modulated optical tomography by self-referenced photorefractive holography

Photorefractive Bi(12)SiO(20) single crystal is used for acousto-optic imaging in thick scattering media in the green part of the spectrum, in an adaptive speckle correlation configuration. Light fields at the output of the scattering sample exhibit typical speckle grains of 1 μm size within the volume of the nonlinear crystal. This heterogeneous illumination induces a complex refractive index structure without applying a reference beam on the crystal, leading to a self-referenced diffraction correlation scheme. We demonstrate that this simple and robust configuration is able to perform axially resolved ultrasound modulated optical tomography of thick scattering media with an improved optical etendue.

Monte Carlo Simulation of Scattered Light with Shear Waves Generated by Acoustic Radiation Force for Acousto-Optic Imaging

A Monte Carlo method of multiple scattered coherent light with the information of shear wave propagation in scattering media is presented. The established Monte-Carlo algorithm is mainly relative to optical phase variations due to the acoustic-radiation-force shear-wave-induced displacements of light scatterers. Both the distributions and temporal behaviors of optical phase increments in probe locations are obtained. Consequently, shear wave speed is evaluated quantitatively. It is noted that the phase increments exactly track the propagations of shear waves induced by focus-ultrasound radiation force. In addition, attenuations of shear waves are demonstrated in simulation results. By using linear regression processing, the shear wave speed, which is set to 2.1 m/s in simulation, is estimated to be 2.18 m/s and 2.35 m/s at time sampling intervals of 0.2 ms and 0.5 ms, respectively.

Mathematical modeling of acousto-optical tomography and its level set simulations

Acousto-optical tomography is modeled by using a linear integral equation and an inverse problem involving a diffusion equation in n spatial dimensions, n = 2, 3. Based on measured data, the optical absorption coefficient µ is reconstructed inside a given domain. We make a two-step mathematical model. First, we solve a linear integral equation. Assuming the energy fluence rate has been recovered from the previous equation, we then reconstruct the absorption coefficient µ by solving an inverse problem. Numerical experiments are presented for the case n = 2. We used the level set method for the numerical experiments. At the end, the advantages and the disadvantages of those two methods are mentioned.

Time resolved three-dimensional acousto-optic imaging of thick scattering media

Acousto-optic imaging is based on light interaction with focused ultrasound in a scattering medium. Thanks to photorefractive holography combined with pulsed ultrasound, we perform a time-resolved detection of ultrasound-modulated photons in the therapeutic window (780 nm). A high-gain SPS:Te crystal is used for this purpose and enables us to image through large optical thickness (500 mean free paths). We are able to generate three-dimensional (3D) acousto-optic images by translating a multielement ultrasound probe in only one direction. A 3D absorbing object is imaged through a 3 cm thick phantom.