This paper investigates optical coherence tomography (OCT) as an advanced, non-invasive method for 2D and 3D imaging of the surface and subsurface morphology of glass cultural heritage. The OCT system used is a commercial ThorLabs Ganymede II spectral domain-Fourier domain system with a 930 nm center wavelength, axial resolution of 4-6 μm, and lateral resolution of 8 μm. Results from model alkali silicate glass artificially aged at 90°C and 90% RH allow distinction of real features from artifacts produced by the highly reflective glass, and serve as a basis for interpretation of deterioration phenomena. Analytical results from historical glass artifacts are focused on a group of musical glass flutes created in Paris by Claude Laurent between 1807 and 1848. OCT images are compared to results of destructive analysis of the same samples and objects by scanning electron microscopy with backscattered electron imaging of cross-sections, as well as non-invasive light microscopy and NIR fiber optic reflectance spectrometry, the latter of which yields complementary molecular information in terms of water vibrations in hydrated glass.
KEYWORDS: Epilepsy, Brain, Control systems, Functional magnetic resonance imaging, Feature extraction, Statistical analysis, Principal component analysis, Magnetic resonance imaging, Brain activation, Algorithm development
This paper describes the development of novel computer-aided analysis algorithms to identify the language activation
patterns at a certain Region of Interest (ROI) in Functional Magnetic Resonance Imaging (fMRI). Previous analysis
techniques have been used to compare typical and pathologic activation patterns in fMRI images resulting from identical
tasks but none of them analyzed activation topographically in a quantitative manner. This paper presents new analysis
techniques and algorithms capable of identifying a pattern of language activation associated with localization related
epilepsy. fMRI images of 64 healthy individuals and 31 patients with localization related epilepsy have been studied and
analyzed on an ROI basis. All subjects are right handed with normal MRI scans and have been classified into three age
groups (4-6, 7-9, 10-12 years). Our initial efforts have focused on investigating activation in the Left Inferior Frontal
Gyrus (LIFG). A number of volumetric features have been extracted from the data. The LIFG has been cut into slices
and the activation has been investigated topographically on a slice by slice basis. Overall, a total of 809 features have
been extracted, and correlation analysis was applied to eliminate highly correlated features. Principal Component
analysis was then applied to account only for major components in the data and One-Way Analysis of Variance
(ANOVA) has been applied to test for significantly different features between normal and patient groups. Twenty Nine
features have were found to be significantly different (p<0.05) between patient and control groups
Texture analysis for tissue characterization is a current area of optical coherence tomography (OCT) research. We discuss some of the differences between OCT systems and the effects those differences have on the resulting images and subsequent image analysis. In addition, as an example, two algorithms for the automatic recognition of bladder cancer are compared: one that was developed on a single system with no consideration for system differences, and one that was developed to address the issues associated with system differences. The first algorithm had a sensitivity of 73% and specificity of 69% when tested using leave-one-out cross-validation on data taken from a single system. When tested on images from another system with a different central wavelength, however, the method classified all images as cancerous regardless of the true pathology. By contrast, with the use of wavelet analysis and the removal of system-dependent features, the second algorithm reported sensitivity and specificity values of 87 and 58%, respectively, when trained on images taken with one imaging system and tested on images taken with another.
The vast majority of bladder cancers originate within 600 µm of the tissue surface, making optical coherence tomography (OCT) a potentially powerful tool for recognizing cancers that are not easily visible with current techniques. OCT is a new technology, however, and surgeons are not familiar with the resulting images. Technology able to analyze and provide diagnoses based on OCT images would improve the clinical utility of OCT systems. We present an automated algorithm that uses texture analysis to detect bladder cancer from OCT images. Our algorithm was applied to 182 OCT images of bladder tissue, taken from 68 distinct areas and 21 patients, to classify the images as noncancerous, dysplasia, carcinoma in situ (CIS), or papillary lesions, and to determine tumor invasion. The results, when compared with the corresponding pathology, indicate that the algorithm is effective at differentiating cancerous from noncancerous tissue with a sensitivity of 92% and a specificity of 62%. With further research to improve discrimination between cancer types and recognition of false positives, it may be possible to use OCT to guide endoscopic biopsies toward tissue likely to contain cancer and to avoid unnecessary biopsies of normal tissue.
This paper provides an overview of recent research in the use of microelectromechanical systems (MEMS) actuators for beam steering applications, including optical coherence tomography (OCT). Prototype scanning devices have been fabricated out of polyimide substrates using conventional integrated circuit technology. The devices utilize piezoelectric bimorphs to mechanically actuate the torsion mirror structure made of polyimide. The material properties of the polyimide allow very large scan angles to be realized in the devices while using low voltages. Prototype devices have demonstrated optical scan angles of over 80 degrees with applied voltages of only 40V. Different device sizes have also been demonstrated with resonant frequencies between 15-60Hz (appropriate for real-time imaging). Analytical models have been developed that predict resonant frequency of the device as well as the angular displacement of the mirror. Further finite element modeling (FEM) has been done using ANSYS. These models closely reflect measured scan angles of the prototype devices. Based upon these models, further refinements can be made to the design to produce specific resonant frequencies for use in a multitude of applications. These models are currently being used to design and fabricate multiple devices on a single wafer with minimal post processing requirements. The ability to fabricate these devices in bulk will reduce their cost and potentially make them disposable to reduce the cost of their use in numerous applications, including patient care when used in biomedical imaging applications.
We have modeled, fabricated, and tested polyimide amplified piezoelectric bimorph scanning mirrors for application in optical coherence tomography (OCT). These scanning mirrors are fabricated using photolithography using polyimide as a substrate. These devices use bimorph actuators to drive polyimide micromechanical structures at resonance. The forced vibration of these micromechanical structures causes polysilicon gold plated mirrors attached to two torsion hinges to tilt. Operating the device at resonance allows us to achieve very large displacements of the mirror at real-time imaging speeds. The large scan angles and fast imaging speeds give these novel scanning devices the potential to be used to image larger areas of tissue to search for diseases such as mucosal cancers and to guide interventional procedures such as laser ablations and biopsies in real time. The mirror and support structures were modeled using one-dimensional beam theory and fundamental vibration mechanics. The structures were also modeled and simulated using ANSYS, a finite element analysis package. The finite element modeling has also lead to the development of new methods to fabricate the entire devices on a single silicon wafer. Prototype scanning devices have demonstrated optical scan angles up to 97 degrees with applied voltages from 15-60 V at a resonant frequencies ranging from 12-60 Hz, appropriate for real time imaging. These amplified bimorph imaging probes have been integrated into the scanning arm of a Spectral Domain OCT (SD-OCT) imaging system and have been used to generate preliminary in vivo human skin images at frame rates of 25 frames per second.
This paper provides an overview of several years of research in the use of polyimide MEMS actuators for medical imaging applications, including high frequency ultrasound and optical coherence tomography (OCT). These scanning devices are microfabricated out of polyimide substrates using conventional integrated circuit technology. The material properties of the polyimide allow very large scan angles to be realized and also allow the resonant frequencies of the structures to be in the appropriate ranges for real-time imaging. The primary application of these probes is endoscopic and catheter-based imaging procedures. The microfabrication enables the creation of very small devices essential for compact imaging probes. In addition, they can be fabricated in bulk, reducing their cost and potentially making them disposable to reduce the cost of patient care and minimize the potential for patient cross-contamination. Several different scanning geometries and actuators have been investigated for imaging applications, including both forward-viewing and side-scanning configurations. Probes that utilize both electrostatic polyimide actuators and piezoelectric bimorphs to mechanically scan the ultrasound or OCT imaging beams will be presented. These probes have been developed for both use in both ultrasound and OCT imaging systems. Medical applications of these probes include the early detection of cancerous and precancerous conditions in the bladder and other mucosal tissues. These imaging probes will allow the physician to visualize the subsurface microstructure of the tissues and detect abnormalities not visible through the use of conventional endoscopic imaging techniques. Prototype devices have been used to image geometric wire phantoms, in vitro porcine tissue, and in vivo subjects. The progress made over the last several years in the development of these polyimide scanning probes will be presented.
Endoscopic optical coherence tomography (EOCT) is a medical imaging technique that uses infrared light delivered via an endoscope to produce high-resolution images of tissue microstructure of the gastrointestinal tract. A key component of an EOCT system is the method used to scan the infrared beam across the tissue surface. We have begun developing electrostatic MEMS micromirror devices for use in EOCT. These devices consist of 1 mm square gold-plated silicon mirrors on polyimide tables that tilt on 3 micron thick torsion hinges. The MEMS actuator used to tilt the mirror, the integrated forces array (IFA) is a thin (2.2 μm) polyimide membrane consisting of hundreds of thousands of deformable capacitors that can produce strains up to 20% and forces equivalent to 13 mg with applied voltages from 30-120 V. Measurements of optical deflections of these devices range from 18° at low frequencies to more than 120° near the resonant frequencies of the structures (30-60 Hz). The support structures, hinges, and actuators are fabricated from polyimide on silicon using photolithography. These electrostatic MEMS micromirrors were inserted into the scanning arm of an OCT imaging system to take in vitro images of porcine tissue and in vivo images of human skin at frame rates from 4-8 Hz. SLA probe tips were designed and fabricated to align the optics of the device and to protect the fragile polyimide devices during endoscopic imaging. In addition, devices are being fabricated that combine the IFA and mirror structures onto a single silicon wafer, reducing fabrication difficulty.
This paper describes a compact micromirror device for use in the scanning arm of an optical coherence tomography (OCT) system using an electrostatic micromachine (MEMS) actuator. Optical deflections of these MEMS mirror devices range from 18 degrees at low frequencies to more than 140 degrees near the resonant frequencies of the structures (30-60 Hz). These devices consist of gold plated silicon mirrors resting on polyimide tables that tilt on 3 μm thick torsion hinges when pulled on by the micromachine (MEMS) actuator, the integrated force array (IFA). The IFA is a thin (2.2 μm) polyimide membrane consisting of hundreds of thousands of micron scale deformable capacitors, and contracts with strains up to 20% and forces up to 13 dynes. The support structures, hinges, and actuators are fabricated from polyimide on silicon wafers using photolithography, leading to the possibility for integrated fabrication of the devices resulting in highly repeatable and inexpensive scanning arms. These devices were inserted into the scanning arm of a high speed OCT imaging system to acquire in vitro images of porcine eye and colon tissue and in vivo images of human skin at frame rates from 4-8 Hz.
The Integrated Force Array (IFA) is a metallized polyimide actuator made up of a large array of capacitive cells that deform when voltage is applied. The deformations of the individual cells add to produce an overall muscle-like compression of the array. In previously reported work deformations of up to 30% have been realized and the IFAs have been used as mechanical scanners in ultrasound imaging systems. The gaps of the capacitive cells are etched directly into the polyimide and oriented perpendicular to the plane of the array. Metal is deposited on the sidewalls of the etched features in order to form the plates of each capacitor. The force associated with the IFA motion is directly proportional to the height of the sidewall metal and thus to the thickness of the membrane. Until now, the thickness has been 2μm with gap widths of 1μm. In recent work, much higher aspect ratio IFAs (thicker but with the same gap width) have been fabricated in order to produce devices that operate with greater force and are much more robust devices.
There is much interest in the biomedical community in mechanically steering both high frequency ultrasound transducers and various optical beams. We are currently investigating the use of two different types of MEMS actuators, integrated force arrays (IFAs) and spiral wound transducers (SWTs). The IFA is a linear actuator that is a parallel network of hundreds of thousands of flexible capacitors that electrostatically contract, and the SWT is a patterned tape that is wound to form a circular network of flexible capacitors that can be electrostatically compressed to tilt desired structures. Using ANSYS finite element analysis, we have developed tilting polyimide support structures, which are fabricated on silicon wafers. High frequency ultrasound transducers (20-30 MHz) have been built on these structures and IFAs used to tilt them to steer the ultrasound beam in fluids. Prototype structures have produced 20 degree sector scans scanning at frequencies up to 30 Hz. IFAs have also been used along with similar support structures to steer optical laser beams up to 45 degrees at frequencies up to 60 Hz. The SWT is a more recent development that operates with much greater force than the IFA that could steer ultrasound and optical beams for similar applications.
The Spiral Wound Transducer (SWT) is a muscle-like actuator comprising an array of deformable capacitors. The SWT is fabricated as a flat multilayer metallized plastic structure on a silicon wafer. Conventional microelectronic manufacturing techniques are used and will be described. The novel aspect of fabrication occurs after the flat SWT structures (tapes) are lifted off the wafer. In order to form a volumetric three- dimensional actuator the tapes are wound up upon themselves and bounded using a laminating process. This work as well as a characterization of the performance of the SWTs will be presented.
We will discuss our work to build, characterize, and scale- up a metallized plastic muscle-like actuator called a Spiral Wound Transducer (SWT). Prototype SWTs have been built using microelectronics fabrication methods. The prototypes have demonstrated large amplitude motion and analog response. The prototypes, though small, have demonstrated forces equivalent to 12 grams for compressions of more than 15 percent at 30 Hz. The size of the SWTs is essentially unrestricted. Our work with commercially available metallized Mylar films to produce much larger, more powerful, and lower cost SWT devices will also be discussed.
A miniature ultrasound scanner has been constructed using a MEMS actuator called an Integrated Force Array. A second type of actuator called a Spiral Wound Transducer (SWT) is under development and shows significant promise for this application. Both the scanner and SWT will be discussed.
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