Dual-angle optical coherence tomography for index of refraction estimation using rigid registration and cross-correlation

Abstract

The index of refraction (n) of materials and/or tissues depends on their physical properties and serves as a source of optical contrast in imaging. The variations of the index of refraction have also been investigated for diagnostic purposes in various fields, such as hematology, oncology, etc., since they can signify disease and cell dynamic changes. Optical coherence tomography (OCT) has been used in the past to measure the index ex vivo. However, most methodologies described in the literature are not appropriate for in vivo imaging since they require either a mirror below the sample or a complicated imaging setup and algorithms. We describe a technique that uses two images, obtained at different angles, to estimate the index of refraction and can, thus, also be applied in vivo. The index of refraction is calculated from the path-length difference observed by the OCT beam at the two different angles. When a reflector is not available, the path-length difference can be estimated using image registration and the cross-correlation of adjacent A-scans. The proposed technique was validated experimentally using both clear and scattering samples. The resulting values of the index of refraction were within ∼1% of the expected. The main limitation of this technique is the effect of misalignment on the results, requiring the precision provided by an angular-resolved OCT system. These very promising results provide evidence that the dual-angle method should be further investigated and validated on human tissues so that it can be developed into a clinically useful diagnostic tool in the future.

Extracting Morphological and Sub-Resolution Features from Optical Coherence Tomography Images, a Review with Applications in Cancer Diagnosis

“…This dual-angle method has been validated experimentally using both clear and scattering samples (Figure 11). The resulting measurements of n were within a mean of ~1% of the expected values [166]. C,E).…”

“…Novel techniques which measure the GVD in vivo have been proposed, have been demonstrated in the diagnoses of colon and esophageal cancer, and have the potential to provide diagnostically useful biomarkers [136,139]. The variations of the index of refraction, as well as the size of the cell nuclei, extracted from OCT data, have also been investigated for diagnostic purposes since they can reflect disease and cell dynamic changes [166,181]. However, large and detailed clinical studies are still required to prove the correlation of these features to both relevant biological changes but also to predictors of disease outcome.…”

“…C,E). The arrow points to the first maximum after the zero lag (not shown here for better visualization of the peaks) [166].…”

“…Utilizing the width of the autocorrelation central lobe, the contrast of the OCT C,E). The arrow points to the first maximum after the zero lag (not shown here for better visualization of the peaks) [166].…”

From digital holographic microscopy to optical coherence tomography – separate past and a common goal

“…This solution, however, required axial scanning of the sample to perform tomography. Moving the sampleactually rotating to two angular positions was also used to assess the refractive index of the sample in FD-FF-OCT based on differences in the optical path length (sensitive to reconstruction misalignment) [27]. The method based on comparing optical path lengths of the sample to retrieve the refractive index of a layer was also possible without rotating the sample, but not without additional measurement modalityin this case OCT was supplemented with multiphoton microscopy, which unfortunately requires staining to retrieve second optical path length [28].…”