Optical Absorption Distribution Research Articles

The effects of junction interdiffusion and misfit dislocations on the efficiency of highly mismatched heterojunction photovoltaic devices

A general modelling methodology has been developed to evaluate the effects of chemical interdiffusion and misfit dislocations on the performance of heterojunction solar cells made from highly mismatched materials. Results for the exemplar materials system CdS-CdTe are contrary to the widely held belief that such interdiffusion is beneficial to photovoltaic performance. In the model, recombination is presumed to take place at the cores of misfit dislocations, with the distribution of these dislocations in the interdiffused layer being calculated so as to minimise the total energy (an incidental result shows that the total number of dislocations is independent of the diffusion profile). The model takes calculated chemical profiles, optical absorption, and dislocation distributions from which the photovoltaic performance and recombination losses are evaluated. It was shown that for the realistic case in which the interdiffused region does not extend beyond the space charge region, the photovoltage losses dominate over any photocurrent gains. Methods to engineer mixed junctions that may increase solar conversion efficiency are discussed.

Highly efficient ultrathin-film amorphous silicon solar cells on top of imprinted periodic nanodot arrays

The addressing of the light absorption and conversion efficiency is critical to the ultrathin-film hydrogenated amorphous silicon (a-Si:H) solar cells. We systematically investigate ultrathin a-Si:H solar cells with a 100 nm absorber on top of imprinted hexagonal nanodot arrays. Experimental evidences are demonstrated for not only notable silver nanodot arrays but also lower-cost ITO and Al:ZnO nanodot arrays. The measured external quantum efficiency is explained by the simulation results. The Jsc values are 12.1, 13.0, and 14.3 mA/cm2 and efficiencies are 6.6%, 7.5%, and 8.3% for ITO, Al:ZnO, and silver nanodot arrays, respectively. Simulated optical absorption distribution shows high light trapping within amorphous silicon layer.

Spatially Fourier-encoded photoacoustic microscopy using a digital micromirror device

We have developed spatially Fourier-encoded photoacoustic (PA) microscopy using a digital micromirror device. The spatial intensity distribution of laser pulses is Fourier-encoded, and a series of such encoded PA measurements allows one to decode the spatial distribution of optical absorption. The throughput and Fellgett advantages were demonstrated by imaging a chromium target. By using 63 spatial elements, the signal-to-noise ratio in the recovered PA signal was enhanced by ∼4×. The system was used to image two biological targets, a monolayer of red blood cells and melanoma cells.

Reconstruction of a Piecewise Smooth Absorption Coefficient by an Acousto-Optic Process

The aim of this paper is to tackle the nonlinear optical reconstruction problem. Given a set of acousto-optic measurements, we develop a mathematical framework for the reconstruction problem in the case where the optical absorption distribution is supposed to be a perturbation of a piecewise constant function. Analyzing the acousto-optic measurements, we prove that the optical absorption coefficient satisfies, in the sense of distributions, a new equation. For doing so, we introduce a weak Helmholtz decomposition and interpret in a weak sense the cross-correlation measurements using the spherical Radon transform. We next show how to find an initial guess for the unknown coefficient. Finally, in order to construct the true coefficient we provide a Landweber type iteration and prove that the resulting sequence converges to the solution of the system constituted by the optical diffusion equation and the new equation mentioned above. Our results in this paper generalize the acousto-optic process proposed in [5] for piecewise smooth optical absorption distributions.

Estimating optical absorption, scattering, and Grueneisen distributions with multiple-illumination photoacoustic tomography

While photoacoustic methods offer significant promise for high-resolution optical contrast imaging, quantification has thus far proved challenging. In this paper, a noniterative reconstruction technique for producing quantitative photoacoustic images of both absorption and scattering perturbations is introduced for the case when the optical properties of the turbid background are known and multiple optical illumination locations are used. Through theoretical developments and computational examples, it is demonstrated that multiple-illumination photoacoustic tomography (MI-PAT) can alleviate ill-posedness due to absorption-scattering nonuniqueness and produce quantitative high-resolution reconstructions of optical absorption, scattering, and Gruneisen parameter distributions. While numerical challenges still exist, we show that the linearized MI-PAT framework that we propose has orders of magnitude improved condition number compared with CW diffuse optical tomography.

The physics of biomedical imaging and therapy by interacting light and sound.

Light and sound waves propagating through tissue are coupled in useful and interesting ways. When a beam of sound travels through diffuse light, photons passing through the beam are phase modulated at the ultrasound frequency, yielding information on tissue optical and acoustical properties via the acousto-optic (AO) effect. Alternatively, the absorption of short-pulse laser light induces rapid heating that launches acoustic waves that can then be used to determine the spatial distribution of optical absorption, a phenomenon referred to as the “optoacoustic” or “photoacoustic” (PA) effect. We will present a phenomenological introduction to the emerging field of dual-wave sensing and imaging using light and sound, give a few illustrative examples, and comment on the potential for applying these concepts to tissue imaging. We then transition from imaging mode to therapy mode by introducing a high-intensity focused ultrasound (HIFU) beam that can (1) pump the AO interaction, (2) enhance the level of PA emissions, or (3) promote cavitation-mediated therapy. We include a brief review of the physical and biological effects of HIFU-induced cavitation and will discuss how the addition of light and nanoparticles enhances both the efficacy and targeting accuracy of HIFU therapy in optically diffuse tissues such as breast and brain.

Target detection and quantification using a hybrid hand-held diffuse optical tomography and photoacoustic tomography system

We present a photoacoustic tomography-guided diffuse optical tomography approach using a hand-held probe for detection and characterization of deeply-seated targets embedded in a turbid medium. Diffuse optical tomography guided by coregistered ultrasound, MRI, and x ray has demonstrated a great clinical potential to overcome lesion location uncertainty and to improve light quantification accuracy. However, due to the different contrast mechanisms, some lesions may not be detectable by a nonoptical modality but yet have high optical contrast. Photoacoustic tomography utilizes a short-pulsed laser beam to diffusively penetrate into tissue. Upon absorption of the light by the target, photoacoustic waves are generated and used to reconstruct, at ultrasound resolution, the optical absorption distribution that reveals optical contrast. However, the robustness of optical property quantification of targets by photoacoustic tomography is complicated because of the wide range of ultrasound transducer sensitivity, the orientation and shape of the targets relative to the ultrasound array, and the uniformity of the laser beam. We show in this paper that the relative optical absorption map provided by photoacoustic tomography can potentially guide the diffuse optical tomography to accurately reconstruct target absorption maps.

SU‐GG‐I‐168: Image Reconstruction in an Attenuating Medium with Two Different Attenuation Coefficients Using Photoacoustic Tomography

Purpose: The goal of this research is to incorporate the effect of attenuation of the surrounding medium and accurately reconstruct the optical absorption distribution of the photoacoustic source. Method and Materials: Photoacoustic tomography (PAT) reconstructs an image of the optical absorption of an object using the pressure signals that are generated when the object is illuminated via pulsed electromagnetic radiation. The effect of ultrasonic attenuation can be significant since photoacoustic tomography uses broadband detection and the ultrasonic attenuation varies with frequency. Previous studies have related the detected pressure to the optical absorption coefficient when there is uniform attenuation. In this work, we specifically consider the practically important problem of an photoacoustic source embedded in a medium (such as tissue) with an attenuation coefficient, surrounded by a matching medium (such as water) with a different attenuation coefficient. The photoacoustic pressure that is measured by the detector in this case is not directly related to the optical absorption coefficient of the source. We have derived an analytical expression relating the photoacoustic pressure in the contact plane to the pressure in the detector plane. We can then reconstruct an image using the calculated pressure on the contact plane and the standard image reconstruction techniques for PAT. Results: Preliminary results in 2D suggest that the analytic expression that we derived to relate the photoacoustic pressure on the detection plane with that in the contact plane provides a suitable method for obtaining the photoacoustic source lying in an attenuating medium. Conclusion: The attenuation effect of the surrounding medium have been incorporated into image reconstruction in photoacoustic tomography. This work will be useful to accurately reconstruct the image in PAT when the attenuation coefficient of the object (such as tumor) differs from that of the surrounding medium (healthy tissue).

A Combined Reconstruction Algorithm for Limited-View Multi-Element Photoacoustic Imaging

We present a photoacoustic imaging system with a linear transducer array scanning in limited-view fields and develop a combined reconstruction algorithm, which is a combination of the limited-field filtered back projection (LFBP) algorithm and the simultaneous iterative reconstruction technique (SIRT) algorithm, to reconstruct the optical absorption distribution. In this algorithm, the LFBP algorithm is exploited to reconstruct the original photoacoustic image, and then the SIRT algorithm is used to improve the quality of the final reconstructed photoacoustic image. Numerical simulations with calculated incomplete data validate the reliability of this algorithm and the reconstructed experimental results further demonstrate that the combined reconstruction algorithm effectively reduces the artifacts and blurs and yields better quality of reconstruction image than that with the LFBP algorithm.

High resolution fast digital photoacoustic CT for breast cancer diagnosis

Photoacoustic imaging combines advantages of both ultrasound imaging and optical imaging to provide high ultrasonic resolution and high optical contrast in noninvasive medical images. A photoacoustic computer tomography （PCT） system with a focused digital phased array transducer is proposed. In particular each element of the transducer array has a thin cylindrical ultrasonic lens to select 2D image plane and suppress the out-of-plane signals to realize photoacoustic computed tomography. A fast full-view photoacoustic imaging system for visualizing tissue structures using a linear transducer array with combined scan, whies; detects photoacoustic signals by combined electronic and mechanical scanning. An improved limited-field filtered back projection algorithm with directivity factors was applied to reconstruct the optical absorption distribution and further improve the lateral resolution. In the experiment, the spatial resolution of this imaging system is determined as 0.2mm, and the slice resolution of Z axis is 1.5mm. Mechanical step scanning was performed, leading to a measurement time of 6.4 seconds per position, and a total scan duration of 150 seconds to obtain the two-dimensional photoacoustic image. High spatial resolution of 2D photoacoustic vascular image and quasi-3D photoacoustic CT for breast cancer imaging are achicned. The experimental results demonstrate that this method and imaging system can realize the high resolution photoacoustic tomography wtich can be applied to noninvasive detection of early breast cancer.

Optoacoustics: Can ultrasound become the preferred modality for molecular imaging

Optoacoustics can be used to image the distribution of optical absorption in tissue, combining the specificity and sensitivity of optical imaging with the high resolution and penetration of ultrasound imaging. It represents one of the most promising techniques for molecular imaging because the optical absorption of bioconjugated nanoparticles can greatly exceed that of tissue over a range of wavelengths in which light can penetrate multiple centimeters into the body. We have explored several types of nanoparticles conjugated to a range of antibodies targeted to several important biological systems. Here we discuss two potential applications of bioconjugated gold nanorods, one for cancer cell targeting and the other to identify inflamed endothelial cells signaling the early stages of atherosclerosis. Optoacoustic images of cell cultures and animal models demonstrate the sensitivity and specificity of these nanosystems for molecular imaging. To translate these experimental findings into a clinically acceptable molecular imaging modality, we have also explored integrated optical systems able to deliver the optical pulse for optoacoustic excitation and detect the resultant ultrasonic waves using an all‐optical transducer. The basic operating principles of this device, and the prospects for ultrasound‐based molecular imaging using it, will be discussed.

A maximum likelihood method for obtaining integrated attenuation from ultrasound transmission mode measurements

In photoacoustic imaging applications an image is reconstructed of the optical absorption distribution the imaged object. The photoacoustic measurements however are also dependent on acoustic properties of the imaged object. By estimating the acoustic properties from photoacoustic measurements, we can improve the performance of the optical absorption reconstruction and allow for the imaging of separate acoustic modalities. We derive and evaluate a maximum likelihood estimator for the measurements of integrated acoustic attenuation. This estimator is applicable to media like soft tissue. In this kind of media, the attenuation due to dissipative effects obeys a frequency power law. By measuring the propagation of ultrasound signals through such a medium, the parameters that describe the attenuation can be estimated. In this paper a new method is introduced for estimating the attenuation of ultrasound media by means of transmission mode measurements. The method is based on analyzing the noise characteristics of the received signals and the formulation of a maximum likelihood estimator. The new estimator is compared to existing methods and was found to be a better estimator in terms of the RMS error than previous methods.

Photoacoustic imaging using an ultrasonic Fresnel zone plate transducer

A photoacoustic (PA) imaging system based on an ultrasonic Fresnel zone plate (FZP) transducer is developed for the purpose of imaging biological tissue. This FZP transducer has a two-zone negative zone plate piezoelectric material pattern, and an optical fibre is integrated with the transducer on the symmetric axis of the zone plates to deliver laser pulses to the sample. The focal characteristic of the FZP transducer is analysed by theoretical prediction and experimental measurement, and the measured results are in good agreement with the predicted results. The limited-field back-projection deconvolution algorithm combined with the coherence-factor weighting technique is used to reconstruct the optical absorption distribution. The experiments were performed with phantoms and the blood vessels of chicken embryo chorioallantoic membrane. The results demonstrate that PA imaging using the FZP transducer has the ability to image biological tissue and has potential application in monitoring neovascularization in tumour angiogenesis.

Application of inverse source concepts to photoacoustic tomography

Photoacoustic tomography (PAT), also known as optoacoustic or thermoacoustic tomography, is a hybrid imaging technique that possesses great potential for a wide range of biomedical imaging applications. Image reconstruction in PAT is tantamount to solving an inverse source problem, where the source represents the optical energy absorption distribution in the object that is induced by an interrogating pulsed optical waveform. In this work, we re-examine the PAT image reconstruction problem from a Fourier domain perspective by use of established time-harmonic inverse source concepts. A mathematical relationship between the photoacoustic pressure wavefield data on an aperture that encloses the object and the three-dimensional Fourier transform of the optical absorption distribution evaluated on a collection of concentric spheres is investigated. In addition to providing a framework for deriving both exact and approximate analytic reconstruction formulae, we demonstrate that this mapping provides an intuitive means of understanding certain spatial resolution characteristics of PAT.

Fast full-view photoacoustic imaging by combined scanning with a linear transducer array

We present a fast full-view photoacoustic imaging system for visualizing tissue structures using a linear transducer array with combined scan. In this system, a 128-element linear transducer array was used to detect photoacoustic signals by combined scanning of electronic scan and mechanical scan. An improved limited-field filtered back projection algorithm with directivity factors was applied to reconstruct the optical absorption distribution. The experiments of phantoms and in vivo blood vessels in a rat brain were performed with this system. And a clear view of the curve boundaries of objects and the network of blood vessels of rat's brain were acquired. The experimental results demonstrate the multi-element photoacoustic imaging system has the ability of imaging complicated structures of objects.

Spectroscopic Three-Dimensional Imaging of Light Scattering Medium by Detection of Ultrasonic Velocity Change owing to Light Illumination

An experimental apparatus that automatically detects the ultrasonic velocity change of a phantom due to light absorption was fabricated. Optical three-dimensional images of the phantom were constructed by illumination at a light intensity under the skin exposure limit of a continuous-wave diode laser. It was confirmed that the image of optical absorption distribution at the specified depth of the phantom could be obtained.

A photoacoustic tomography system for imaging of biological tissues

Non-invasive laser-induced photoacoustic tomography (PAT) is a promising imaging modality in the biomedical optical imaging field. This technology, based on the intrinsic optical properties of tissue and ultrasonic detection, overcomes the resolution disadvantage of pure-optical imaging caused by strong light scattering and the contrast and speckle disadvantages of pure ultrasonic imaging. Here, we report a PAT experimental system constructed in our laboratory. In our system, a Q-switched Nd : YAG pulse laser operated at 532 nm with a 8 ns pulse width is used to generate a photoacoustic signal. By using this system, the two-dimensional distribution of optical absorption in the tissue-mimicking phantom is reconstructed and has an excellent agreement with the original ones. The spatial resolution of the imaging system approaches 100 µm through about 4 cm of highly scattering medium.

Po‐Poster ‐ 33: A finite element model for bioluminescence imaging in small animals

Bioluminescence imaging is a powerful technique for visualizing gene expression in small animals but it suffers a serious limitation: the absorption and scattering of light in tissue. Several factors influence the image: source strength and depth, effective numerical aperture of the imaging optics, and attenuation by the tissue between the source and the camera. Our overall goal is to account for these effects and to recover the actual strength and spatial location of the bioluminescence sources in vivo. An essential first step in this research is to develop a physical model that accurately predicts the light reaching the surface of the animal for an arbitrary distribution of sources and optical absorption and scattering coefficients. The calculations must be fast, so that the model can be used eventually in an iterative algorithm to solve the inverse problem. We use the diffusion approximation, valid when scattering dominates absorption and when it is not necessary to calculate the light field close to sources. The diffusion equation expresses the light fluence rate as a function of position and the spatially dependent absorption coefficient, scattering coefficient, and source function. A finite element code called NIRFAST has been developed to generate numerical solutions. NIRFAST has been implemented in MATLAB and uses a 2 or 3 dimensional model to represent the object. The absorption and scattering coefficients are specified at each node of the mesh. We have assigned “reasonable” values of the absorption and scattering coefficients to each node based on tissue identification by x‐ray CT.

Photoacoustic imaging with deconvolution algorithm

The impulse response of the ultrasonic transducer used for detection is crucial for photoacoustic imaging with high resolution. We demonstrate a reconstruction method that allows the optical absorption distribution of a sample to be reconstructed without knowing the impulse response of the ultrasonic transducer. A convolution relationship between photoacoustic signals measured by an ultrasound transducer and optical absorption distribution is developed. Based on this theory, the projection of the optical absorption distribution of a sample can be obtained directly by deconvolving the recorded PA signal originating from a point source out of that from the sample. And a modified filtered back projection algorithm is used to reconstruct the optical absorption distribution. We constructed a photoacoustic imaging system to validate the reconstruction method and the experimental results demonstrated that the reconstructed images agreed well with the original phantom samples. The spatial resolution of the system reaches 0.3 mm.

Three-dimensional laser-induced photoacoustic tomography of mouse brain with the skin and skull intact.

Three-dimensional laser-induced photoacoustic tomography, also referred to as optoacoustic tomography, is developed to image animal brain structures noninvasively with the skin and skull intact. This imaging modality combines the advantages of optical contrast and ultrasonic resolution. The distribution of optical absorption in a mouse brain is imaged successfully. The intrinsic optical contrast reveals not only blood vessels but also other detailed brain structures, such as the cerebellum, hippocampus, and ventriculi lateralis. The spatial resolution is primarily diffraction limited by the received photoacoustic waves. Imaged structures of the brain at different depths match the corresponding histological pictures well.