Kirk W. Gossage

Texture analysis of optical coherence tomography images: feasibility for tissue classification

Optical coherence tomography (OCT) acquires cross-sectional images of tissue by measuring back-reflected light. Images from in vivo OCT systems typically have a resolution of 10 to 15 mm, and are thus best suited for visualizing structures in the range of tens to hundreds of microns, such as tissue layers or glands. Many normal and abnormal tissues lack visible structures in this size range, so it may appear that OCT is unsuitable for identification of these tissues. However, examination of structure-poor OCT images reveals that they frequently display a characteristic texture that is due to speckle. We evaluated the application of statistical and spectral texture analysis techniques for differentiating tissue types based on the structural and speckle content in OCT images. Excellent correct classification rates were obtained when images had slight visual differences (mouse skin and fat, correct classification rates of 98.5 and 97.3%, respectively), and reasonable rates were obtained with nearly identical-appearing images (normal versus abnormal mouse lung, correct classification rates of 64.0 and 88.6%, respectively). This study shows that texture analysis of OCT images may be capable of differentiating tissue types without reliance on visible structures.

Investigating Sun-damaged Skin and Actinic Keratosis with Optical Coherence Tomography: A Pilot Study

Actinic Keratosis (AK) arises from sun-damaged skin and is the first clinical manifestation in the multistep process of skin carcinogenesis to invasive squamous cell carcinoma. Thus, it is an ideal target for chemopreventive efforts. Noninvasive measures of AK severity are needed to assess the efficacy of chemoprevention agents. We performed a pilot study on 20 participants to investigate the OCT appearance of sun-protected skin of the upper inner arm as well as sun-damaged skin and early AKs of the dorsal forearms, and to determine if features or quantitative measures in Optical Coherence Tomography (OCT) images could be used to reliably differentiate between these categories. OCT images of upper inner arm (normal appearing skin) showed skin layers and features (stratum corneum, epidermis, dermis, blood vessels) seen in previous studies; additionally in this participant group the subcutaneous fat layer was usually identified. Sun-damaged skin was characterized by increased signal in the epidermis and rapid attenuation of light. AKs were diverse in appearance but frequently characterized by high surface reflection, the presence of a low-signal band in the stratum corneum, and heterogeneous appearance in the epidermis/dermis. Significant differences were found between skin categories using measures of stratum corneum and epidermal/dermal depths and intensities. The presence of a dark band in the stratum corneum was 79% sensitive and 100% specific for AK. This study indicates that OCT holds promise as a useful technique for identifying and characterizing AKs and monitoring their response to chemoprevention agents.

Customized analog circuit design for fiber-based optical coherence microscopy

Optical coherence microscopy (OCM) is an interferometric method for acquiring high-resolution, depth-resolved, en face images. In this article we demonstrate a fiber-based OCM system with analog fringe generation and signal demodulation. A high power operational amplifier drives a mirrored piezoelectric stack mounted in the reference arm of the interferometer causing a displacement equal to 0.42 times the light source center wavelength. The drive signal is synchronized with the demodulation frequency of two analog lock-in amplifiers which extract the first and second harmonics of the interferometric component of the signal. Four outputs (X and Y components of first and second harmonics) are acquired with a data-acquisition board and combined to eliminate the slow phase drift in the interferometer. A sample image of carrot tap root is presented. High dynamic range images are obtained at acquisition speeds up to 40000pixels∕s.

Speckle image properties in optical coherence tomography

Most identification of tissues in OCT images has relied on the presence or absence of features and layers. However, in some pathologies as well as some normal tissues OCT images appear homogeneous. Examination of these images reveals that they display a characteristic repetitive structure due to speckle. Since speckle is influenced by the local index of refraction mismatches, it may be possible to differentiate between different types of tissues based on analysis of the speckle pattern. The determination of tissue type may be supported as well by local contrast distribution analysis or speckle decorrelation degree, which are widely used in measurement and characterization of surface roughness. In this study we examined three areas: 1) the application of speckle theory based on surface roughness to a three- dimensional media and a short coherence length light source, 2) the effect that the optical system design has on the received speckle distribution, and the optimum optical system geometry for speckle analysis, and 3) the speckle properties of OCT images of tissue phantoms and various tissues such as fat and muscle. Results obtained from two methods of speckle analysis (texture analysis and speckle contrast) were compared for their ability to differentiate between tissue types.

Dynamic Molecular Imaging: Anatomical Co-registration and Dynamic Contrast Enhancement

A new approach to acquiring and analyzing small animal molecular imaging data is presented. By imaging the in-vivo dynamics of a dye or targeted probe, improved contrast and all-optical anatomical co-registration can be achieved.

A low-cost customized analog circuit design for optical coherence microscopy

Optical Coherence Microscopy (OCM) enables the acquisition of high resolution, en face images. Most current OCM systems are based on slow analog or high speed digital demodulation schemes. In this paper we demonstrate a low-cost, high speed analog fringe generation and demodulation method. A high power operational amplifier drives a mirrored piezoelectric stack mounted in the reference arm of the interferometer. The drive signal is synchronized with the demodulation frequency of two analog lock-in amplifiers, which extract the first and second harmonic power of the coherence fringes. Tenth order Bessel low-pass filters (LPFs) allow fast system response and reduce carrier frequency noise. Four outputs (X and Y components of first and second harmonic) are acquired with a low-cost data acquisition board and combined to eliminate the slow phase drift in the interferometer. C# software processes and displays the image, and performs automatic interferometer pathlength matching and adjustment. We present images of Arabidopsis leaf in situ, sections of carrot, and ex vivo rat ovary. Excellent image quality is achieved at acquisition speeds up to 40,000 samples/second.

Texture analysis of speckle in optical coherence tomography images of tissue phantoms

Optical coherence tomography (OCT) is an imaging modality capable of acquiring cross-sectional images of tissue using back-reflected light. Conventional OCT images have a resolution of 10-15μm, and are thus best suited for visualizing tissue layers and structures. OCT images of collagen (with and without endothelial cells) have no resolvable features and may appear to simply show an exponential decrease in intensity with depth. However, examination of these images reveals that they display a characteristic repetitive structure due to speckle. The purpose of this study is to evaluate the application of statistical and spectral texture analysis techniques for differentiating living and non-living tissue phantoms containing various sizes and distributions of scatterers based on speckle content in OCT images. Statistically significant differences between texture parameters and excellent classification rates were obtained when comparing various endothelial cell concentrations ranging from 0 cells/ml to 25 million/ml. Statistically significant results and excellent classification rates were also obtained using various sizes of microspheres with concentrations ranging from 0 microspheres/ml to 500 million microspheres/ml. This study has shown that texture analysis of OCT images may be capable of differentiating tissue phantoms containing various sizes and distributions of scatterers.

Texture Analysis for Tissue Classification of Optical Coherence Tomography Images

Optical coherence tomography (OCT) is a cross-sectional imaging modality capable of acquiring images to depths of a few millimeters at resolutions ranging from 10-15 μm. This makes OCT useful for visualizing layers and structures within the tissue, but not effective for seeing in vivo cellular level detail. Random spatially dependent speckle patterns were seen in our images due to the coherent properties of light utilized in OCT. These speckle patterns are dependent on various optical parameters of the system, including numerical aperture, as well as the size and distribution of light scattering particles within the sample. The purpose of this study is to evaluate the application of statistical and spectral texture analysis techniques for differentiating tissue types based on the structural and speckle content in OCT images. Good correct classification rates were obtained when five different bovine tissues were compared in pairs, averaging 80% correct, and reasonable rates were obtained comparing normal vs. abnormal mouse lung tissue, averaging 64.0% and 88.6%, respectively. This study has shown that texture analysis of OCT images may be capable of differentiating tissue types without reliance on visible structures.

Using optical coherence tomography to evaluate glaucoma implant healing response in rabbit eyes

Glaucoma is a set of diseases that cause optic nerve damage and visual field loss. The most important risk factor for the development of glaucoma is elevated intraocular pressure. One approach used to alleviate the pressure increase is to surgically install glaucoma implants. Two standard Ahmed and ten experimental ePTFE implants were evaluated in this study in rabbit eyes. The implants were imaged with optical coherence tomography (OCT) at 0, 7, 15, 30, and 90 days after implantation. Histology was collected at days 7, 15, 30, and 90 and compared to the OCT images. Preliminary analysis of images indicates that OCT can visualize the development of fibrous encapsulation of the implant, tissue erosion, fibrin accumulation in the implant tube, and tube position in the anterior chamber. A new OCT handheld probe was developed to facilitate in vivo imaging in rabbit eye studies. The OCT probe consists of a mechanical scaffold designed to allow the imaging fiber to be held in a fixed position with respect to the rabbit eye, with minimal anesthesia. A piezo electric lateral scanning device allows the imaging fiber to be scanned across the tissue so that 2D images may be acquired.