Polarimetrical imaging is a noninvasive optical technique of great interest in biophotonics since it has the capability of obtaining relevant information of biological samples, being useful, for instance, for the early detection of diseases or the classification of biological structures, both on animal and vegetal tissues. Different structures produce different outcomes when interacting with light due to their polarimetric properties such as depolarization, dichroism or retardance. An exhaustive polarimetric analysis of these characteristics can unveil the relation between the tissue inherent characteristics and its polarimetric response, enabling us to find the most appropriate polarimetric parameters to describe or study a sample. These polarimetric characteristics can be obtained through the experimental measurement of the Mueller matrix (M) of a sample, from which a range of different polarimetric observables, giving physical interpretation, can be deduced. By taking advantage of these parameters, we propose a study of the suitability of different groups of metrics for the contrast enhancement in biological tissues imaging, taking special attention on some depolarization metrics and some physical parameters such as the wavelength or the angle of incidence of the illumination light. The results obtained suggest the convenience of certain parameters which may be of interest in multiple biomedical scenarios such as pathology early detection or enhanced visualization of different structures for clinical applications.
Complete characterization of biological samples is of potential interest in different industrial and research areas, as for instance, in biomedical applications, for the recognition of organic structures or for the early detection of some diseases. During the last decades, polarimetric methods are experiencing an increase of attention in the study of biomedical tissues, and they are nowadays used in such framework to provide qualitative (polarimetric imaging) and quantitative (data processing) information for the studied samples. Polarimetric methods are based on the analysis of polarization modifications produced by light-matter interactions which can be triggered by a number of complex internal processes but can be roughly understood as the result of the combination of three pure polarimetric features of the sample: its diattenuation, retardance and depolarization. For the analysis of the depolarization content, we propose the use of the Indices of Polarimetric Purity (IPP) to describe the sample behavior. Related with the randomness of the scattering processes, IPPs provide more information of depolarizing systems than the widely used depolarization index (p▵), which further synthetize the depolarization content of samples. Moreover, certain combinations of IPP parameters leads to p▵. As a result, IPPs allow the revelation of some structures from tissue samples hidden in regular intensity images of even in the p▵ channel, leading to better tissue classification results. In this work, we present different applications of IPPs in biomedical tissue that show its potential, which are not restricted to the biomedical framework as relevant results in plants characterization are also presented.
Light interaction with material systems may introduce depolarization to the incident light. This phenomenon comes from multiple scattering processes that take place inside the media and strongly depends on the particle characteristics. In the case of botany, plant leaves can be understood as depolarizing systems. A non-contact method to analyze these samples consist of illuminating them with well-known polarized light and study the scattered light to retrieve the physical characteristics of the sample. This physical study can be done by measuring the Mueller matrix of samples, in which the physical information of samples is encoded in their 16 elements and further mathematical treatment is required to extract the information. In the case of scattering systems, the depolarization content carries very valuable information but it is usually not inspected in the botanic field. A way to study depolarized content is by determining the so-called depolarization index PΔ, which gives an overall measure of the degree of depolarization of a system but it does not measure possible anisotropic dependence of the depolarization. For instance, a depolarizer equally depolarizing any fully polarized input polarization or a depolarizer that depolarizes them in a strongly heterogenous way, may lead to the same PΔ value. In contrast, the Indices of Polarimetric Purity (IPP) are a group of metrics that further synthesize the depolarizing content, taking into account the anisotropic depolarization. In this work, we describe the main physical characteristics of samples achieved by using these IPP through plant samples. Moreover, we show how IPP highlights some structures hidden in regular intensity measurements, highlighting the potential of these metrics for botanical applications.
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