Contrasts enhancement in optical coherence tomography

Research project P 14103 Contrast Enhancement in Optical Coherence Tomography Christoph K. HITZENBERGER 24.01.2000 Optical coherence tomography (OCT) is a new, non invasive technique for recording crosssectional images of transparent and semi-transparent tissues. OCT is similar to ultrasound B-mode imaging, however, the intensity of near-infrared light is measured rather than acoustical waves. The resolution of standard OCT is on the order of 10 pm, its penetration depth into scattering tissue is - 2 mm. Therefore, OCT is an ideal intermediate between confocal microscopy and high-frequency ultrasound. Standard OCT measures the intensity of backscattered light and images the distribution of the backscattering potential within a sample. However, as is well known from microscopy, many samples, especially many tissues, show only poor contrast if imaged on a pure intensity basis. These samples may, however, change other properties of the light which can be used to improve image contrast and even to perform quantitative measurements. Among the light properties that can be used for improving image contrast are polarization state, phase, and wavelength. It is the purpose of the proposed research project to develop and investigate advanced OCT techniques which take advantage of these light properties to improve image contrast in different tissues. Two OCT instruments shall be developed and constructed to investigate and optimize these contrast enhancement techniques in different tissues. The first instrument is designed for polarization- and phase contrast OCT. Based on preliminary results, these contrasting techniques shall be optimized by investigating and imaging well defined tissue phantoms of varying optical parameters (scattering coefficient, anisotropy factor, refractive index, birefringence). Then the different contrasting modes shall be evaluated in real tissues with different scattering behavior. An interesting example of a transparent tissue is the cornea which is birefringent and has a non-uniform refractive index. This makes it interesting for both imaging modes. Normal and diseased corneal tissue shall be investigated. An interesting example of scattering tissue is dental enamel. It is birefringent, the birefringence, and hence the refractive index, changes with the processes associated with carious lesions. The suitability of the two imaging modes to diagnose and monitor caries shall be evaluated. The second instrument is designed for differential absorption contrast. With two wavelengths in the mid-infrared, one wavelength corresponding to a water absorption band, the other lying outside that band, the distribution of water content in superficial tissue shall be imaged. The diagnostic potential and the limits of this technique shall be evaluated in phantoms and tissues.

The purpose of the project was to develop new contrasting techniques for optical coherence tomography (OCT). OCT has been developed in the 1990`s as a non invasive technique for recording cross-sectional images of transparent and semi-transparent tissues. The technology is in some respects similar to ultrasound imaging, however, the intensity of near-infrared light is measured rather than acoustical waves. Since the resolution of OCT is superior to ultrasound, it has a great potential for medical diagnostics of ocular tissue and superficial scattering tissue (skin, teeth, mucosa). Conventional OCT measures the intensity of backscattered light which is used to create cross sectional images. However, as is well known from microscopy, many samples, especially many tissues, show only poor contrast if imaged on a pure intensity basis. These samples may, however, change other properties of the light which can be used to improve image contrast and to perform quantitative measurements. During the course of the project, three advanced contrasting techniques were developed and investigated. 1. Differential absorption contrast: An OCT instrument was developed that records images at two infrared wavelengths simultaneously: one wavelength is chosen so that it is absorbed by water (and therefore by tissue with high water concentration), the other wavelength is only weakly absorbed by water. The difference between the two images allows to measure and image the water concentration in tissue. The instrument was used to image water concentration in human cornea. Possible applications are the study of edema and inflammatory processes. 2. Phase contrast: Many types of cells are transparent, their interior (organelles) is rather invisible in intensity based images. However, their components have different refractive indices. Phase contrast can be used to make refractive indices visible. During the project, an OCT instrument was developed that transfers the phase contrast concept from microscopy to OCT. With this instrument it was possible to image individual cells in cell cultures. Furthermore, it was demonstrated that phase contrast imaging is possible through scattering layers which goes beyond the capabilities of conventional phase contrast microscopy. 3. Polarization contrast: Several tissues are birefringent, they change the polarization state of light. Among these are, e.g., fibrous tissues: muscle, tendon, cornea, retina. We developed an OCT instrument that is sensitive to the light polarization state and can measure and image two parameters of birefringence: its amount and orientation (related to the tissue fiber orientation). We acquired three-dimensional images with this technology which helped to explain the birefringence of human cornea in detail. Possible applications are diagnostics of corneal and retinal diseases.