Stationary-fiber rotary probe with unobstructed 360° view for optical coherence tomography.

A side-scanning fiber probe is a critical component for optical coherence tomography in medical imaging and diagnosis. We propose and fabricate an on-axis rotating probe that performs in situ, circumferential scanning that is shadow-free (not susceptible to shadow effects caused by the motor's wires). A miniature motor that incorporates a bored-out shaft for the optical fiber is located at the distal end of the probe, which results in a more stable and uniform circumferential scan, free from wire-shadow interference effects. More importantly, this design, novel to our knowledge, compared to other probes avoids the insertion losses introduced by optical coupling components and the multitude of optical interfaces, which is very important for sensing weak signals backscattered from structures deep in the tissue.

Simultaneous dual-wavelength-band common-path swept-source optical coherence tomography with single polygon mirror scanner.

We report a novel (to the best of our knowledge) simultaneous 1310/1550 two-wavelength band swept laser source and dual-band common-path swept-source optical coherence tomography (SS-OCT). Synchronized dual-wavelength tuning is performed by using two laser cavities and narrowband wavelength filters with a single dual-window polygonal scanner. Measured average output powers of 60 and 27 mW have been achieved for the 1310 and 1550 nm bands, respectively, while the two wavelengths were swept simultaneously from 1227 to 1387 nm for the 1310 nm band and from 1519 to 1581 nm for the 1550 nm band at an A-scan rate of 65 kHz. Broadband wavelength-division multiplexing is used for coupling two wavelengths into a common-path single-mode GRIN-lensed fiber probe to form dual-band common-path SS-OCT. Simultaneous OCT imaging at 1310 and 1550 nm is achieved. This technique allows for in vivo high-speed OCT imaging with potential application in functional (spectroscopic) investigations.

Optical coherence tomography: fundamental principles, instrumental designs and biomedical applications.

The advances made in the last two decades in interference technologies, optical instrumentation, catheter technology, optical detectors, speed of data acquisition and processing as well as light sources have facilitated the transformation of optical coherence tomography from an optical method used mainly in research laboratories into a valuable tool applied in various areas of medicine and health sciences. This review paper highlights the place occupied by optical coherence tomography in relation to other imaging methods that are used in medical and life science areas such as ophthalmology, cardiology, dentistry and gastrointestinal endoscopy. Together with the basic principles that lay behind the imaging method itself, this review provides a summary of the functional differences between time-domain, spectral-domain and full-field optical coherence tomography, a presentation of specific methods for processing the data acquired by these systems, an introduction to the noise sources that plague the detected signal and the progress made in optical coherence tomography catheter technology over the last decade.

Signal attenuation and box-counting fractal analysis of optical coherence tomography images of arterial tissue.

The sensitivity of optical coherence tomography images to sample morphology is tested by two methods. The first method estimates the attenuation of the OCT signal from various regions of the probed tissue. The second method uses a box-counting algorithm to calculate the fractal dimensions in the regions of interest identified in the images. Although both the attenuation coefficient as well as the fractal dimension correlate very well with the anatomical features of the probed samples; the attenuation method provides a better sensitivity. Two types of samples are used in this study: segments of arteries collected from atherosclerosis-prone Watanabe rabbits (WHHL-MI) and healthy segments of porcine coronary arteries.

3x3 Mach-Zehnder interferometer with unbalanced differential detection for full-range swept-source optical coherence tomography.

Quadrature interferometry based on 3x3 fiber couplers could be used to double the effective imaging depth in swept-source optical coherence tomography. This is due to its ability to suppress the complex conjugate artifact naturally. We present theoretical and experimental results for a 3x3 Mach-Zehnder interferometer using a new unbalanced differential optical detection method. The new interferometer provides simultaneous access to complementary phase components of the complex interferometric signal. No calculations by trigonometric relationships are needed. We demonstrate a complex conjugate artifact suppression of 27 dB obtained in swept-source optical coherence tomography using our unbalanced differential detection. We show that our unbalanced differential detection has increased the signal-to-noise ratio by at least 4 dB compared to the commonly used balanced detection technique. This is due to better utilization of optical power.

Statistics of the depth-scan photocurrent in time-domain optical coherence tomography.

We derive the time-variant second-order statistics of the depth-scan photocurrent in time-domain optical coherence tomography (TD-OCT) systems using polarized thermal light sources and superluminescent diodes (SLDs). Since the asymptotic-joint-probability-distribution function (JPDF) of the photocurrent due to polarized thermal light is Gaussian and the signal-noise-ratio in TD-OCT is typically high (>80 dB), the JPDF of the depth-scan photocurrent could be approximated as a Gaussian random process that is completely determined by its second-order statistics. We analyze both direct and differential light detection schemes and include the effect of electronic thermal fluctuations. Our results are a necessary prerequisite for future development of statistical image processing techniques for TD-OCT.

Graded-index fiber lens proposed for ultrasmall probes used in biomedical imaging.

The quality and parameters of probing optical beams are extremely important in biomedical imaging systems both for image quality and light coupling efficiency considerations. For example, the shape, size, focal position, and focal range of such beams could have a great impact on the lateral resolution, penetration depth, and signal-to-noise ratio of the image in optical coherence tomography. We present a beam profile characterization of different variations of graded-index (GRIN) fiber lenses, which were recently proposed for biomedical imaging probes. Those GRIN lens modules are made of a single mode fiber and a GRIN fiber lens with or without a fiber spacer between them. We discuss theoretical analysis methods, fabrication techniques, and measured performance compared with theory.

Image enhancement for multilayer information retrieval by using full-field optical coherence tomography.

When a full-field optical coherence tomography (OCT) system is used to extract tomographic images from a multilayer information carrier, the resulting images may suffer from interlayer modulations and parasitic patterns derived from interference fringes. We describe and analyze these negative influences that degrade the quality of extracted tomographic images and propose practical algorithms and methods to minimize them. The emphasis of the discussion will be the removal of the parasitic fringes produced by the imperfection of a CCD camera. The simulative and experimental results of image enhancement for multilayer tomography extraction using full-field OCT are provided.

Pattern recognition with generalized centroids and subcentroids.

We propose a class of generalized moment functions (GMFs) that can be used to determine a set of geometric points, namely, generalized centroids (G centroids), within an object. Based on a linear GMF, a mass centroid and its subcentroids can be defined and extracted, which provide information on the location and orientation of an object. Similar to traditional moment functions, GMFs can also be used to describe the global shape of an object, including symmetry and fullness. However, GMFs, along with G centroids and subcentroids, can further serve to construct a feature vector of an object, which is critical for image registration and invariant pattern recognition. One can extract more distinguishing features from the same object by changing the combination of GMFs. We present results that show simulations of pattern recognition from uniform backgrounds.

Bifocal optical system for distant object tracking.

A new bifocal optical system used for distant object tracking is proposed. This system combines a birefringent element with a conventional glass lens so that the spot image size and its variation with the axial distance can be controlled according to the requirement of a distant object tracker. The lens design for the tracking application is discussed and an example is given. The new lens system provides a more uniform spot image size and an extended focal depth compared to a conventional lens with one focus.

Cemented doublet lens with an extended focal depth.

A method of designing a lens with an extended focal depth is studied. The lens is a cemented doublet composed of a birefringent lens and a conventional lens. The crystal optical axis of the birefringent lens is perpendicular to the axis of the optical system. By properly selecting the parameters of the birefringent lens and the conventional lens, we can flexibly configure an imaging system that simultaneously has a large focal depth and high resolution. We also provide a theoretical analysis which shows that the focal depth of the lens is approximately 1.5 times that of a conventional lens.

Information coding and retrieving using fluorescent semiconductor nanocrystals for object identification.

The spectral features, i.e., wavelength and intensity, of fluorescence generated from semiconductor nanocrystals (quantum dots) can be used for coding information. Unlike the 1-D and 2-D barcodes, the information carrier is applied to a very small area and hardly visible. The information retrieving by a fluorospectrometer is not subjected to the changes of rotation and scale. A de-convolution-based algorithm is used to separate the overlapped spectral profiles. This technology can be applied to small products labeling, document security and object identification.

Cryptography based on the absorption/emission features of multicolor semiconductor nanocrystal quantum dots.

Further to the optical coding based on fluorescent semiconductor quantum dots (QDs), a concept of using mixtures of multiple single-color QDs for creating highly secret cryptograms based on their absorption/emission properties was demonstrated. The key to readout of the optical codes is a group of excitation lights with the predetermined wavelengths programmed in a secret manner. The cryptograms can be printed on the surfaces of different objects such as valuable documents for security purposes.

Optical system having a large focal depth for distant object tracking.

We analyze an optical system after inserting a simple quartic phase plate in its pupil plane to extend the focal depth. The system is used specifically to track distant objects like stars. We design an optimum quartic phase plate for a real lens system which has an effective focal length of 29 mm, an F-number of 1.6, a field of view of 20 degrees, and a working wavelength range of 0.5~0.75 ?m. By introducing the quartic phase plate, we enhance the focal depth of the system more than threefold as compared to a system having no phase plate.