Abstract
.The index of refraction () 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 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.
Highlights
The index of refraction (n) is an important intrinsic optical parameter of tissue that has been recently exploited for various applications
Studies have shown that the distribution of n in tissue, using data from medical systems such as computed tomography and optical coherence tomography (OCT), could be used as a biomarker for medical diagnosis.[1,2]
Microbiology, hematology, and infectious disease studies show that n distribution can provide valuable information about cellular growth and division and bacteria identification.[3,4,5]
Summary
The index of refraction (n) is an important intrinsic optical parameter of tissue that has been recently exploited for various applications. Using OCT, a common method to estimate n is based on measuring the path-length change from a sample placed over a reflector.[9] this method is only appropriate for ex vivo measurements, its accuracy and simplicity make it ideal for the verification of newly developed techniques This method exploits the fact that the optical path-length measured by OCT is proportional to the index of refraction. Other methods that have been proposed depend on very complicated experimental setups and algorithms and are very hard to apply in vivo They either require specific hardware to precisely track the focus and calculate the path length and/ or require iterative methods to calculate the index of refraction by fitting to assumed models, which are processing-intensive and time-consuming. Focus tracking method and optical path shifting methods were evaluated for simultaneously calculating refractive index and thickness, in samples with different scattering properties These methods required specialized hardware and multiple measurements. Such a technique could be developed into a useful in vivo diagnostic tool in the future
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