In this talk, a spectral domain polarization sensitive optical coherence tomography (SDPS-OCT) system has been developed so as to obtain high scan speed, high dynamic range and high sensitivity, and simultaneously get birefringence contrast of some biological tissue. To reduce corruption of the DC and autocorrelation terms to images, we introduce the two phase method. The stocks vectors (I, Q, U, and V) of the backscattered light from the specimen have been reconstructed by processing the signals from the two channels which are responsible for detecting the vertical and horizontal polarization state light separately. Further, the phase retardation between the two orthogonal polarization states has been acquired. The results from rabbit eye show that SDPS-OCT system based on the two phase method has great potential to imaging biological tissue.
A spectral domain Polarization sensitive optical coherence tomography (SDPS-OCT) system has been developed to acquire depth images of biological tissues such as porcine tendon, rabbit eye. The Stocks vectors (I, Q, U, and V) of the backscattered light from the biological tissues have been reconstructed. Further, the phase retardation and polarization degree between the two orthogonal polarizing states have been computed. Reconstructed images, i.e. birefringence images, from Stokes parameters, retardation and polarization degree of biological tissues show significant local variations in the polarization state. And the birefringence contrast of biological tissue possibly changes by some outside force. In addition, the local thickness of the birefringence layer determined with our system is significant. The results presented show SDPS-OCT is a potentially powerful technique to investigate tissue structural properties on the basis of the fact that any fibrous structure with biological tissues can influence the polarization state of light.
In photoacoustic (PA) tomography, a piezoelectrical signal of inner characteristic of interesting object is mainly acquired by a hydrophone. Every piezoelectrical signal as output signal is the convolution of the original input signal that denotes the ultrasonic signal emitting from the substance and the system transfer function. The undistorted input signal is the very physical quantity that we want actually. Therefore an original input signal is computed with the deconvolution of the system transfer function and the output signal. While most practical deconvolution problems are called as blind deconvolution because the system transfer function and the input signal are both unknown and estimated from the output signal in the same time. In common, the deconvolution problem has an important property that it is called ill-condition, which is a special and intractable difficulty that both the theoretic analysis and the numerical computation would meet. For the sake of getting the solution of the deconvolution problem reasonable in physics and responsible for the gained data continually, a package of theory method called regularization to cure the ill-conditioned problems is applied in the PA signal processing.
Noninvasive photoacoustic tomography (PAT) is a novel technique with great potential in biomedical image applications
for it combines the merits and most compelling features of light and sound, and has the advantages of providing high
contrast and high resolution images in moderate depth below the surface. When the image depth is on the scale of
centimeter, the millimeter-scale resolution images still can be obtained. Thus it is a powerful tool for the early-stage
breast cancer sensing. In this paper, photoacoustic tomography is studied by using the simulation method. The results
show that: (1) the contrast of image increases linearly with respect to the number of measurement position (NMP); (2)
the contrast increases exponentially with respect to noise-to-signal ratio.
The photoacoustic (PA) effect refers to the generation of acoustic waves by the modulated optical radiation. A novel
tissue imaging technique, photoacoustic imaging is using the acoustic waves made in the PA effect to reconstruct the
sample. It whose characteristic is combining the advantages of pure optical imaging and pure ultrasound imaging can
map the high contrast and high spatial resolution tissue image. The PA imaging reconstmction algorithm performing the
signal filtering operation first and then to reduce the signal data to the image, has the important influence on the quality
of the image made by the experiment of PA. With the laser appearance in 1960s, the PA imaging technique made great
advance and the reconstruction algorithm gains quick development. The prevalent PA reconstruction algorithms include
Kruger's 3D inverse Radon transform, Frenz's Fourier transform, Lihong Wang's method based on the analytic solution.
Nowadays, PA imaging technique develops to the real-time PA image. So to choose a suitable fast algorithm is
significant to the PA imaging system's application. We will review the current PA imaging reconstruction algorithm and
compare them in the aspect of the tissue imaging spatial resolution and so on.
Photo-acoustic tomography(PAT) is a new ultrasound-mediated biomedical imaging technology which combines the
advantages of high optical contrast and high ultrasonic resolution. In theory, PAT can image object embedded several
centimeters under the surface of sample with the resolution of tens of microns. In this paper, several representative image
reconstruction algorithms are discussed. Because the PA signal is wide band signal, it is hard to get the whole frequency
spectrum due to the tremendous calculation needed. Therefore, the most applicable reconstruction algorithms are all
performed in time domain such as "delay-and-sum" and "back projection". The current research methods have been
focused on optical detecting and piezoelectric detecting. The optical method has the advantage of high spatial sensitivity
due to the short wavelength of the probe laser beam. PA signal detecting using piezoelectric sensor has two main modes
i.e. using unfocused transducer or transducer array or using focused transducer array or linear transducer array. When a
focused transducer array is used, the "delay-and-sum" method is often used for image reconstruction. The advantage of
the method is that its data acquisition time can be reduced to several minutes or even several seconds by employing the
phase control linear scan technique. The future development in PAT research and its potential clinic application is also
presented.
Optical Coherence Tomography (OCT) has been developed for more than one decade. With the optimum of system
configuration such as light source, the imaging elements, the imaging quality has been improved to a higher level.
However, many ideal assumptions including dispersion cancellation. in the study of OCT system have become
inapplicable. Actually, dispersion, can lead to a wavelength dependent phase distortion in sample arm, and finally result
in a degrading in image resolution. Therefore, many dispersion compensation methods have been presented by
researchers to correct the distorted image. In this paper, the principle of dispersion in OCT imaging system is discussed,
and we demonstrate how it affects image quality. Then, with respect to the compensation methods as our knowledge, we
classify them into hardware compensation and software compensation and present the detailed procedures and their
characteristics, respectively. At last, a detailed discussion has been made to conclude that novel algorithms which can
perform higher order compensation with depth variant are necessary and uniform evaluating criteria as well.
Spectral OCT (SOCT), with high acquisition speed and high dynamic range, has been implemented by many research
groups in the world. However, SOCT image inherently has virtual image, including auto-correlation noise and mirror
image. The existence of the virtual image may deteriorate the quality of the image. In order to eliminate those virtual
images, some methods have been demonstrated effective, such as differential SOCT and complex SOCT. In this paper, a
novel method is proposed i.e. three-phase shifting method. The pathlength of the reference arm is changed for certain
distance by PZT controller. Three phase shifted coherence spectra are recorded for A-line. The reconstruction algorithm
can eliminate both auto-correlation noise and minor image, thus improve the signal-to-noise ratio of the SOCT image.
Furthermore, this method is also able to amplify the measuring range of SOCT by a factor of 2. An intact porcine cornea
tissue in vitro is further used to show the potential of this method for high-resolution biological imaging.
Photoacoustic tomography (PAT), which reconstructs the distribution of light-energy deposition in the tissue, is becoming an increasingly powerful imaging tool. For example, the technique has potential applications in the earlystage breast cancer sensing and the functional imaging of small animal brain. In PAT, the system signal-to-noise ratio (SNR) and the number of measurement positions (NMP) are the two main factors which affect the quality of final reconstructed image. Undoubtedly, the increase of SNR or the numbers of measurement positions will improves image quality. However, one has to pay a cost on the imaging speed for such improvement of image quality. In this paper, the factors influencing the imaging performance of PAT are investigated by means of computer simulations. The result shows that the increase of the number of averaging times in acquiring of acoustic signal and the number of measurement positions are efficient ways to improve image quality. However, there exists a turning point at which the further increase of NMP and averaging times makes the improvement of imaging performance negligible. Thus a tradeoff should be made to achieve the optimal reconstructed image according to the system SNR.
Photoacoustic tomography (PAT) is a powerful medical imaging technique for medicinal diagnosis in that it combines the merits and most compelling features of light and sound to the biological tissue. It can be potentially used for the detection of the first-stage breast cancers and the blood vessel net-works in the deep depth of tissue. In this paper, a PAT experimental system constructed in our laboratory is presented by the use of 532nm wavelength light as an excitation source. By using this system, we demonstrated that it is feasible to image blood vessel networks in highly scattering ex vivo and in vivo tissue samples.
A three-phase shifting method is introduced to spectral OCT to eliminate mirror image and autocorrelation noise inherent in spectral OCT signal, thus improves the measurable range of spectral OCT by a factor of 2. The feasibility of such method is demonstrated using a mirror like object. Comparison between two-phase and three-phase shifting methods is performed using a 1mm slab as the object. An intact porcine cornea tissue in vitro is also used to show the potential of this method for biological imaging.
Extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors can construct one of most promising sensor group for simultaneous temperature and strain measurement in structure health monitoring. The demodulation of the sensor group is critical. Here, a parallel demodulation system used for the goal is presented, which is based on a Michelson interferometer and combines the method of low coherence interference of EFPI sensor and Fourier transform spectrum of FBG sensor. The parallel demodulation method is modeled with Fourier transform spectrum theory, which shows the common base that existing between low coherence interference and Fourier transform spectrum. Three data processing methods are proposed according to different application situations, and separation method for EFPI and FBG overlapping signals is emphasized. The design of system is described, and the optical path difference (OPD) scanning and sampling method without reference light are discussed in detail. Then a series of experiments were carried out. The results showed the parallel demodulation system has good performance of spectrum demodulation, low coherence interference demodulation and parallel demodulation. It can realize simultaneous strain and temperature measurement while keep the whole system configuration less complex.
Optical coherence tomography (OCT) is a new modality used to image biological tissues that weakly scatter and absorb light. It was demonstrated that this technique provides image with micrometer resolution in a noncontact and noninvasive way. Traditional OCT is time domain OCT (TDOCT). In this method the length of the reference arm in an interferometer is rapidly scanned over a distance corresponding to the imaging depth range. The mechanism of scanning largely limits the acquisition speed and makes real-time imaging impossible. In recent years a new model OCT based on Fourier domain interferometry is emerged, we called it spectral OCT (SOCT) or Fourier domain OCT (FDOCT). SOCT can avoid scanning of the reference, thus can reach very high acquisition speed. In this paper, spectral OCT related theories and techniques are reviewed. This paper consists of three sections: principle of SOCT, different sep-ups, recent progress and advance.
Optical coherence tomography (OCT) is a new imaging modality that is being actively used in a variety of medical applications. Optical coherence tomography performs cross sectional imaging by measuring the time delay and magnitude of optical echoes at different transverse positions, essentially by the use of a low coherence interferometry to obtain the depth resolved information of a sample. The interference can occur only when the optical path lengths of light in both the sample arm and reference arm are matched to within the coherence length of light source. The most commonly used light sources in the current OCT systems are the superluminescent diodes (SLD). However, the coherence lengths of SLD are typically 10-30 microns that are not sufficient to achieve the resolution required for many medical applications. In the meantime, the moderate irradiance offered by the SLD limits the real time applications for OCT system, which usually require a power with an order of at least 10 milliwatts. Recently the diode-pumped superfluorescent optical fibers sources has been used in a variety of communication and sensor applications. The superfluorescent rare-earth doped optical fibers source is also the very good OCT systematic light source, because of that have a wide bandwidth of fluorescence and high emission power.
Optical coherence tomography (OCT) is a new imaging modality that is being actively used in a variety of medical applications. Currently, most of the OCT systems operate in the time domain, which requires scanning the optical path length in the reference arm in order to obtain the in-depth profile, i.e. A scan. This however limits the system scanning speed. To avoid the axial scanning and therefore improve the system scanning speed, a novel OCT system is recently proposed by a number of groups that operates in the frequency domain, i.e. the spectral OCT. In this paper, we report the spectral OCT system being constructed at Tianjin University. The system has a dynamic range at 78dB and is capable of scanning speed at 12 seconds per image, largely limited by the bottleneck of data transferring from the CCD camera currently employed to the computer. The SOCT imaging results obtained from the animal tissues (cornea from an intact porcine eye) in vitro will be presented.
Recent progress in optoacoustic tomography has shown its enormous potential in biomedical imaging of biological samples in vivo. The greatest advantage in this imaging modality is that it can offer the imaging contrast comparable to the optical techniques and the imaging resolution similar to the ultrasonic techniques. However, its sectioning capability is largely dependent on the scattering properties of the targeted subject. Therefore, the understanding of how the tissue optical properties affect the imaging performance of optoacoustic tomography would be important. In this paper, we systematically investigate the influence of absorption and scattering coefficients of tissue on the optoacoustic imaging resolution, depth and contrast. In addition, we also investigate the influence of spreading photon diffusion in the tissue on its sectioning capability. In the experiments, tissue phantoms were constructed with a range of optical properties. We used the intralipid solution to control the scattering properties, and the indocynane green to control the absorption property of the phantom. The result shows that the scattering coefficient is a major factor that affects the imaging depth and imaging contrast. It also influences greatly the thickness of tomographic imaging slice due to the light broadening inside tissue caused by the scattering property.
Non-invasive laser-induced photoacoustic tomography is attracting more and more attentions in the biomedical optical imaging field. This imaging modality takes the advantages in that the tomography image has the optical contrast similar to the optical techniques while enjoying the high spatial resolution comparable to the ultrasound. Currently, its biomedical applications are mainly focused on breast cancer diagnosis and small animal imaging. In this paper, we report in detail a photoacoustic tomography experiment system constructed in our laboratory. In our system, a Q-switched ND:YAG pulse laser operated at 532nm with a 10ns pulse width is employed to generate photoacoustic signal. A tissue-mimicking phantom was built to test the system. When imaged, the phantom and detectors were immersed in a water tank to facilitate the acoustic detection. Based on filtered back-projection process of photoacoustic imaging, the two-dimension distribution of optical absorption in tissue phantom was reconstructed.
Optical coherence tomography (OCT) is a new biomedical imaging technique in resent year. It has some good qualities, such as non-ionizing radiation, non-invasive, high resolution and sensitivity. High-speed OCT imaging is very important for obtaining the cross-sectional images of the internal microstructure of living tissue. Increasing the imaging speed can produce imaging real time. To study high speed OCT, a new method of OCT imaging technique has been designed in this paper--replacing the point-focus mode with line-focus mode in the sample arm. Cylindrical lens can be used for focusing the incident light into a line in the sample. And a 2D OCT imaging can be obtained in one dimension scanning. In the paper we analyze the interference principle of line-focus imaging mode.
Watermark bar code is one of the latest anti-counterfeiting technologies, which is applicable to the security of various financial documents, especially banknotes. With the machine-readable watermark bar code, we can not only distinguish the genuine from the fake, but also recognize different face values of banknotes. So it is necessary to develop an effective method to read it. With watermark bar codes embedded in euro banknotes as samples, we design a system for watermark bar code recognition based on light transmission theory. An 808nm LD serves as light source. By receiving the transmitted light from behind the banknote with a high-speed photodetector, light transmission curves in different parts of different face value euro banknotes are obtained and analyzed in order to get the unique characteristic of watermark bar code. The correlation coefficient is then introduced for two reasons. For one thing, the high correlation coefficient derived from the light transmission curves shows the coherence of vertically placed watermark bar code area in different horizontal parts of the same face value banknote, which testifies the consistency and repeatability of the experiment. For another, the low correlation coefficient shows the irrelevance of watermark bar code area and other area of banknotes. What’s more, it also shows the difference of watermark bar code area of different face value banknotes. As a result, the machine-readable watermark bar code is made the unique characteristic of certain face value banknotes, which can distinguish them from both the counterfeit ones and other face value genuine banknotes.
The action of slot pipe is much importance in petroleum exploitation. Despite the method to defend sand contain variety in course of artesian well and over well, It is still the most in common method to use slot pipe to over well and defend the sand. Use laser process slot pipe is function superiority and price cheap. Not only is this type of slot pipe relatively inexpensive but also durable, easily installed, and efficiently developed. So many oil-fields are positive to use it.
During design and experiments, multi-diode-laser -module was employed to pump a double AO switched Quasi-CW YAG laser with KTP or LBO crystal as frequency doubler. Thermal lensing effect in laser rod and frequency doubling crystal were both considered. Cavity design with ABCD law and crystal thermal compensation given in detail by numerical calculation. High average power output was achieved.
A high power intracavity frequency doubled Nd:YAG laser with KTP crystal and A-O Q-switcher pumped by 1600 Watt-808 nm laser diodes and its thermal effect are discussed. Also we proved that the title angle of KTP crystal can be to compensate for the phase mismatching and to solve the problem of the drop of green laser output power along with the increasing temperature of KTP crystal. Then based on optical parametric oscillator (KTP-OPO) pumped by 532 nm laser and their frequency doubling (with KTP and BBO) a Watt-level red and blue laser system which would be provided as RGB laser projection display are described.
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