We report imaging studies on site-specific peptide-targeting in tumor tissues using newly developed optical peptide
probes and spectral-domain optical coherence tomography (SD-OCT). The system used two broadband superluminescent light emission diodes with different central wavelengths. An electro-optic modulation in the reference beam was used to get full-range deep imaging inside tumor tissues. The optical probes were based on Bombesin (BBN) that is a fourteen amino acid peptide. BBN has high binding affinity to gastrin-releasing peptide (GRP) receptors overexpressed on several human cancer cell lines. Fluorescence BBN probes were developed by conjugating the last eight residues of BBN, -Q-W-A-V-G-H-L-M-(NH2), with Alexa Flour 680 or Alexa Fluor 750 dye molecules via amino acid linker -G-G-G. The SD-OCT imaging can identify normal tissue and tumor tissue through the difference in scattering coefficient, and trace the BBN conjugate probes through the absorption of the dye molecules using the twowavelength algorithm. We performed the specific uptake and receptor-blocking experiments of the optical BBN probes in severely compromised immunodeficient mouse model bearing human PC-3 prostate tumor xenografts. Tumor and muscle tissues were collected and used for SD-OCT imaging. The SD-OCT images showed fluorescence traces of the BBN probes in the peptide-targeted tumor tissues. Our results demonstrated that SD-OCT is a potential tool for preclinical and clinical early cancer detection.
Photoconductors based on wide band gap semiconductors are potential devices for UV light detection due to internal
photoelectrical gain and fabrication simplicity. Photoresponses of photoconductors based on GaN and ZnO show high
values in UV range under large biases and relatively low values in visible range. Although photoresponse of ZnO
photoconductors is similar to that of GaN-based photoconductors, mechanisms of photoconductance between two
materials are very different. This difference can be found in optical power dependence of photocurrent and I-V
characteristics, and has an impact on device design. In this paper we report experimental studies of photoresponse for
newly developed ZnO photoconductors. The ZnO film was grown on a 6H-SiC substrate by hybrid beam deposition. The
photoconductor device is formed with interdigitated finger-shaped Ti/Au ohmic contacts on the ZnO film. Electrical
characteristics, spectral photoresponse, and persistence properties were studied for the device under variable biases. We
find that there are at least three mechanisms involved in the device. At low biases and low incident light power, the
photoresponse is mainly due to photocreation. At higher light power and lower biases, the space charge regions are
responsible for the photocurrent. At higher biases, the contribution from surface states is dominant.
Among wide bandgap materials that are sensitive to photons in the ultraviolet (UV) region, ZnO is a promising photonic
material because of its unique optoelectronic properties. Based on the lateral interdigitated back-to-back Schottky
contact structure on ZnO film, metal-semiconductor-metal (MSM) photodetectors have substantially lower parasitic
capacitance compared with vertical p-i-n photodetectors, which leads to a very high speed photodetection. In this paper,
we report optical characteristics of MSM ZnO UV photodetectors for which ZnO films were fabricated by hybrid beam
deposition. An annealing process was used in oxygen ambient. The MSM ZnO photodetector consists of two
interdigitated electrodes both with Ti/Au metals on an n-type ZnO thin film. The electrodes on the photodetector are
finger-shaped. We found that the annealing process decreases contact resistance and photoresponse time. The possible
mechanism of annealing process is the removal of surface defects created in the fabrication process. A sublinear power
dependence of photocurrent reveals the existence of a light induced space charge region inside the ZnO film. The device
displays fast pulse response with a very short rise time and a relatively long relaxation time with applied bias. The
exponential decay tail indicates an RC type time response.
We report a new method to detect coherence waves by using photoconductivity in semi-insulating semiconductors. The
method is based on a low coherence interferometry that uses a superluminescence light emission diode as the light
source. In a two-wave mixing configuration, a photorefractive multiple quantum wells (PRQW) device serves as the
beam combiner. When the signal beam and the reference beam interfere in the PRQW, it forms an intensity fringe
pattern along the device surface. In a transverse geometry of the PRQW, photocarriers in the bright region of the
intensity fringe move to the dark region. The space charge distribution causes changes of local electric field in the
PRQW. It leads to the changes in absorption and index of refraction as well as photoconductivity. Conventional
coherence domain imaging using PRQW is based on the diffraction of one of the beams in multi-wave mixing, such as
two-wave mixing and four-wave mixing. Our innovation is to use the photoconductivity to detect coherent signals. We
tested the concept by changing the optical delay and measuring the photocurrent. The change of the local electric field
causes the change of the photocurrent in PRQW even when the totally incident light density stays the same. We studied
the photocurrent under various externally applied electric fields and incident light densities. The relative change of
photocurrent is about 10 times higher than the relative change of diffraction in two-wave mixing, which is the highest
diffraction efficiency in multi-wave mixing configurations. The change of photocurrent is also proportional to the
incident signal light density while the reference intensity keeps the same and the total intensity is relatively low. This
method provides a potential solution of coherent signal detection using PRQW for biomedical optical imaging
applications.
High pulse energy continuum generation in conventional multimode optical fibers has been studied for potential
applications to a holographic optical coherence imaging system. As a new imaging modality for the biological tissue
imaging, high-resolution holographic optical coherence imaging requires a broadband light source with a high
brightness, a relatively low spatial coherence and a high stability. A broadband femtosecond laser can not be used as the
light source of holographic imaging system since the laser creates a lot of speckle patterns. By coupling high peak power
femtosecond laser pulses into a multimode optical fiber, nonlinear optical effects cause a continuum generation that can
be served as a super-bright and broadband light source. In our experiment, an amplified femtosecond laser was coupled
into the fiber through a microscopic objective. We measured the FWHM of the continuum generation as a function of
incident pulse energy from 80 nJ to 800 μJ. The maximum FWHM is about 8 times higher than that of the input pulses.
The stability was analyzed at different pump energies, integration times and fiber lengths. The spectral broadening and
peak position show that more than two processes compete in the fiber.
We have developed a series of new dye bombesin conjugates for site-specific absorption and fluorescence imaging of human prostate and breast cancers. Bombesin (BBN), an amphibian analog to the endogenous ligand, binds to the gastrin releasing peptide (GRP) receptors with high specificity and affinity. Previously, we developed an Alexa Fluor 680-GGG-BBN peptide conjugate which demonstrated high binding affinity and specificity for breast cancer cells in the in vitro and in vivo tests (Ref: Ma et al., Molecular Imaging, vol. 6, no. 3, 2007: 171-180). This probe can not be used as an
absorption probe in near-infrared imaging because its absorption peak is in the visible wavelength range. In addition, site specific longer wavelength fluorescent probe is desired for in vivo molecular imaging because long wavelength photons penetrate deeper into tissue. The new absorption and fluorescent probe we developed is based on the last eight-residues of BBN, -Q-W-A-V-G-H-L-M-(NH2), and labeled with AlexaFluor750 through a chemical linker, beta-alanine. The new probe, Alexa Fluor 750-BetaAla-BBN(7-14)NH2, exhibits optimal pharmacokinetics for specific targeting and optical
imaging of the GRP receptor over-expressing cancer cells. Absorption spectrum has been measured and showed absorption peaks at 690nm, 720nm and 735nm. Fluorescent band is located at 755nm. In vitro and in vivo investigations have demonstrated the effectiveness of the new conjugates to specifically target human cancer cells overexpressing GRP receptors and tumor xenografts in severely compromised immunodeficient mouse model.
Multimodality molecular imaging that combines anatomical and functional information has shown promise in
development of tumor-targeted pharmaceuticals for cancer detection or therapy. Most multimodality imaging techniques
are based on nuclear imaging modalities and MRI or CT. Fluorescence molecular tomography (FMT) is an emerging
optical modality for non-invasive functional imaging and early diagnosis of carcinoma. Three-dimensional FMT can
differentiate tissue physiological changes in vivo to provide functional information when used in conjunction with cancer
cell selectively targeted probes. In this study, we present the design of such a system for multimodality molecular
imaging. A frequency domain radio frequency technique based on commercial amateur radio equipment has been
developed. A heterodyne method is used to transfer a low frequency oscillation into a single-side-band at radio
frequency. The difference in phase, caused by fluorescence photon density wave, is detected between a transmitting
fiber and a receiving fiber bundle, and then measured at lower frequency after demodulation. To achieve multimodality
molecular imaging, a new fluorescent labeled tumor-targeting probe, fluorescent bombesin conjugates, has been
developed with high affinity and specificity for targeting breast cancer cells. The developed multimodality fusion
strategy will provide increased sensitivity/specificity for cancer cells, with respect to any single imaging modality.
This paper reviews the physical basis of holographic optical coherence imaging (OCI) applied in image-domain holography (IDH) and Fourier-domain holography (FDH). Holographic OCI is a multi-spatial-channel direct imaging approach that is closely related to short-coherence speckle interferometry and speckle holography, drawing in addition from laser-ranging concepts and techniques of optical coherence tomography (OCT). It produces a series of en face images at successive depths that can be presented in a so-called video "fly-through". Interchannel cross-talk is described as multichannel spatial heterodyne that produces image-bearing speckle. The speckle holograms are proposed to relate to specific structure in the tissue and may be useful as a clinical diagnostic. For instance, sub-cellular motility (a metric of the vitality of a cell and a means to quantify the response to inter-cellular signaling) can be detected with wide field of view without the need for cellular-scale optical resolution. This can be applied across biologically significant areas of tissue with potential for intraoperative applications to asses the state of health beneath the surface of broad areas of excised tissue.
Time-dependent speckle holograms from inside tumor spheroids using short-coherence near-infrared light provide quantitative measures of the state of health of the tumor tissue. Holographic optical coherence imaging (OCI) records full-frame en face images from successive depths inside a tumor in a so-called “flythrough”. When the flythrough is stopped at a specified depth, the holographic features can be classified as variable (relating to cell motility and Brownian motion) or persistent (arising from specific structure such as necrosis inside the tumor) depending on their temporal variations. A strong trend is observed in the ratio of variable-to-persistent features in tumors that are healthy, metabolically poisoned, or chemically cross-linked. Autocorrelation times also reflect this trend. Depth-gated speckle holography provides a means to sample biologically significant areas without the need for cellular-scale spatial resolution, with possible relevance for intraoperative applications.
Holographic optical coherence imaging (OCI) has been used to acquire depth resolved images in tumor spheroids. OCI is a coherence-domain imaging technique that uses dynamic holography as the coherence gate. The technique is full-frame (en face) and background free, allowing real-time acquisition to a digital camera without motional reconstruction artifacts. We describe the method of operation of the holographic OCI on highly scattering specimens of tumor spheroids. Because of the sub-resolution structure in the sample, the holograms consist primarily of speckle fields. We present two kinds of volumetric data acquisition. One is uses fly-throughs with a stepping reference delay. Another is static holograms at a fixed reference delay with the coherence gate inside the tumor spheroids. At a fixed reference delay, the holograms consist of time-dependent speckle patterns. The method can be used to study cell motility inside tumor spheroids when metabolic or cross-linking poisons are delivered to the specimens.
We report the first results of optical coherence imaging (OCI) of rat tumor spheroids. OCI is a full-frame variant of optical coherence tomography (OCT). The coherent image spatially modulates a high-sensitivity dynamic holographic film composed of a photorefractive quantum well (PRQW). Full-frame readout out of the hologram is observed in real time on a video camera. This system may be considered generally as a video camera with a coherence filter on the lens. Tumor spheroids are small (100-1000 m) balls of tumor cells that are cultured in vitro. Larger spheroids have increasingly complex inner structure. Necrosis and calcification form and expand, reminiscent of structure in malignant cysts in human tumors. In addition, rafts of tumor cells become separated by fluid-filled voids. These features are within the resolution limit of the OCI system, and produce highly structured coherent images.
Optical coherence imaging (OCI) is an autocorrelation imaging technique that uses short-coherence light and holographic recording and reconstruction to perform laser-ranging into translucent media. OCI is a full-frame variant of OCT and shares excellent discrimination against scattered light from heterogeneous media. We present the first use of OCI to image into a heterogeneous translucent media: sandstone. There are two motivations for studying sandstone. First, it is an excellent example of a heterogeneous translucent medium on which to study the effects of holographic reconstruction in the presence of static scattered speckle. Second, it is of intrinsic interest for energy production as an excellent example of an oil or gas reservoir rock. Using Optical Coherence Imaging (OCI) we have imaged several layers of grains in a sandstone sample. Information on grain geometry was obtained as deep as 400 microns into the sample.
Adaptive laser-based ultrasound detection uses laser homodyne detection in an adaptive interferometer to detect ultrasound displacements. This technique has been applied to non-destructive evaluation for detection of surface displacements. However, in biomedical ultrasound applications, ultrasound displacements of interest may be inside tissue and the detection light beam may need to propagate through turbid media. Here we report laser-based adaptive ultrasonic detection that can detect ultrasound displacements inside or through turbid media using a fsec-laser as the light source in an adaptive optical coherence detection scheme. The use of an interferometer and a femtosecond laser gates out the unwanted scatter while still allowing homodyne detection. In our laser-based ultrasound system, an adaptive Mach-Zehnder interferometer is based upon two-wave mixing in semiconductor quantum-well films. It provides depth information inside the sample by adjusting the optical delay in the reference beam arm. Homodyne detection was experimentally studied for low intensity and highly wavefront distortion caused by turbid media. Using this system, ultrasonic homodyne signals through 11 MFP turbid media have been successfully detected.
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