Mueller matrix imaging has shown promise for sensing tissue alignment, and spatial frequency domain imaging is touted for adding depth sensitivity in turbid media. Combining the two techniques would seem to be straightforward. It is found that the spatial frequency domain Mueller matrix does not have the same behavior as a real-space Mueller matrix: it can be complex-valued, and it is not a convex sum of non-depolarizing Mueller matrices. In this presentation, the theory of polarized spatial frequency imaging will be reviewed and results from measurements will be presented.
SignificanceMueller matrix polarimetry (MMP) and spatial frequency domain imaging (SFDI) are wide-field optical imaging modalities that differentiate tissue primarily by structure alignment and photon transport coefficient, respectively. Because these effects can be related, combining MMP and SFDI may enhance tissue differentiation beyond the capability of each modality alone.AimAn instrument was developed to combine MMP and SFDI with the goal of testing whether it enhances contrast of features in reflection mode.ApproachThe instrument was constructed using liquid crystal elements for polarization control, a digital light processing projector for generating sinusoidal illumination patterns, and a digital camera for imaging. A theoretical analysis shows that the SFD Mueller matrix is complex-valued and does not follow the same behavior as a regular Mueller matrix. Images were acquired from an anisotropic tissue phantom, an optical fiber bundle, and cerebellum, thalamus, and cerebrum tissues.ResultsThe measurement results suggest that singly scattered, few scattered, and diffusely scattered photon paths can be distinguished in some of the samples investigated. The combined imaging modality yields additional spatial frequency phase information, which highlights paths having only a few scattering events.ConclusionsThe combination of MMP and SFDI offers contrast mechanisms inaccessible by each modality used alone.
Alignment of tissue fibers, such as collagen, can be disrupted by some cancers. Mueller matrix polarimetry (MMP) is often correlated to this tissue orientation, as seen in cases such as cervical imaging. However, its bulk assessment of a sample’s polarimetric properties prevents it from distinguishing specific structures at different depths, as there would be in any non-uniform or multilayered sample. Spatial Frequency Domain Imaging (SFDI) is a well-documented technique that can be used to manipulate depth of penetration of an investigating light source through use of different sinusoidal frequency patterns. We have combined the two imaging modalities to investigate and differentiate between anisotropic samples with varied depth-dependent structure and demonstrate its use with various tissue phantoms. We also discuss some issues that arise with the combined methodology.
We combine Mueller matrix polarimetry (MMP) with spatial frequency domain imaging (SFDI) to create a technique that is sensitive to near-surface material anisotropy. We demonstrate this imaging modality with scattering and absorbing phantoms and with a fiber optic bundle. Images of depolarization show reduced depolarization under high spatial frequency illumination and demodulation and in some cases, reduced contrast to deeper features. The images of a fiber optic bundle show marked differences between illumination modulations that are aligned with the fibers versus those crossed with the fibers, demonstrating the impact of polarization on scattering direction.
Mueller matrix polarimetry (MMP) is a technique capable of determining a material’s effect on polarized light. However, there are limiting factors that can be optimized to improve the quality of information that is collected using traditional MMP. One such issue lies in the full-field assessment of samples which gives a bulk assessment of a sample’s polarimetric properties and disallows specific structures to be distinguished between different depths as there would be in any non-uniform or multilayered sample. Spatial Frequency Domain Imaging (SFDI) is a well-documented technique that can be used to manipulate depth of penetration of an investigating light source through use of different [sinusoidal] frequency patterns. Higher frequency patterns can be used to restrict the depth of penetration for more superficial structure. The combined technique was used to investigate and differentiate between birefringent samples with varied depth-dependent structure.
Calibration, quantification, and standardization of the polarimetric instrumentation, as well as interpretation and understanding of the obtained data, require the development and use of well-calibrated phantoms and standards. We reviewed the status of tissue phantoms for a variety of applications in polarimetry; more than 500 papers are considered. We divided the phantoms into five groups according to their origin (biological/nonbiological) and fundamental polarimetric properties of retardation, depolarization, and diattenuation. We found that, while biological media are generally depolarizing, retarding, and diattenuating, only one of all the phantoms reviewed incorporated all these properties, and few considered at least combined retardation and depolarization. Samples derived from biological tissue, such as tendon and muscle, remain extremely popular to quickly ascertain a polarimetric system, but do not provide quantifiable results aside from relative direction of their principal optical axis. Microspheres suspensions are the most utilized phantoms for depolarization, and combined with theoretical models can offer true quantification of depolarization or degree of polarization. There is a real paucity of birefringent phantoms despite the retardance being one of the most interesting parameters measurable with polarization techniques. Therefore, future work should be directed at generating truly reliable and repeatable phantoms for this metric determination. Diattenuating phantoms are rare and application-specific. Given that diattenuation is considered to be low in most biological tissues, the lack of such phantoms is seen as less problematic. The heterogeneity of the phantoms reviewed points to a critical need for standardization in this field. Ultimately, all research groups involved in polarimetric studies and instruments development would benefit from sharing a limited set of standardized polarimetric phantoms, as is done earlier in the round robin investigations in ellipsometry.
KEYWORDS: Collagen, Monte Carlo methods, Cervix, Polarimetry, Polarized microscopy, Microscopy, In vivo imaging, Signal attenuation, Light scattering, Scattering
Polarized light microscopy and polarimetry has been used to assess changes in cervical structure by targeting its collagen. 75% of the human cervix is in fact composed of highly arranged collagen which is birefringent. Recently we have used Mueller Matrix polarimetry to image the human cervix in-vivo to determine loss of collagen arrangement associated with later stages of pregnancy. In an effort to improve our system capability and better determine the provenance of the polarized signal we have developed a Polarized Light Monte Carlo model capable of characterizing polarized light interaction with a birefringent, scattering, and absorbing medium such as the cervix. We have utilized this model to investigate the effect of cervical collagen arrangement typical of late stage in pregnancy on polarized light. In this talk we will illustrate the model framework, its validations, and provide several test cases in model cervices.
Annually, about 15 million preterm infants are born in the world. Of these, due to complications resulting from their premature birth, about 1 million would die before the age of five. Since the high incidence of preterm birth (PTB) is partially due to the lack of effective diagnostic modalities, methodologies are needed to determine risk of PTB. We propose a noninvasive tool based on polarized light imaging aimed at measuring the organization of collagen in the cervix. Cervical collagen has been shown to remodel with the approach of parturition. We used a full-field Mueller matrix polarimetric colposcope to assess and compare cervical collagen content and structure in nonpregnant and pregnant women in vivo. Local collagen directional azimuth was used and a total of eight cervices were imaged.
Mueller Matrix polarimetry can provide useful information about the function and structure of the extracellular matrix. Mueller Matrix systems are sophisticated and costly optical tools that have been used primarily in the laboratory or in hospital settings. Here we introduce a low-cost snapshot Mueller Matrix polarimeter that that does not require external power, has no moving parts, and can acquire a full Mueller Matrix in less than 50 milliseconds. We utilized this technology in the study of cervical cancer in Mysore India, yet the system could be translated in multiple diagnostic applications.
Preterm birth (PTB) presents a serious medical heath concern throughout the world and maintains a high incidence rate in both developed and developing countries ranging between 11-15%, respectively. PTB can be caused by many different morbidities and ultimately results in the disorganization of cervical collagen and the premature alteration of the cervix mechanical properties. Changes in cervical collagen orientation and distribution may prove to be a predictor of PTB. Polarization imaging is an effective means to measure optical anisotropy in birefringent materials such as those rich in collagen. Non-invasive, in-vivo full-field Mueller Matrix polarimetry (MMP) imaging was conducting using a modified colposcope in a clinical study comparing collagen orientation and distribution between non-pregnant and pregnant patients. Six patients threatening PTB were imaged at the Jackson Memorial Hospital Triage Unit and six non-pregnant patients were image at Florida International University STAR center. In pregnant women collagen distributions changed depending on patient age and number of pregnancies in the non-pregnant population age played an important role in collagen organization.
Mueller matrix polarimetry (MMP) can be utilized to determine optical anisotropy in birefringent materials. Many factors must be optimized to improve the quality of information collected from MMP of biological samples. As part of a study of pre-term birth (PTB) that relied on measurement of the orientation and distribution of collagen in the cervix, an optimal wavelength for MMp to allow more accurate characterization of collagen in cervical tissue was sought. To this end, we developed a multispectral Mueller matrix polarimeter and conducted experiments on ex-vivo porcine cervix samples preserved in paraffin. The Mueller matrices obtained with this system were decomposed to generate orientation and retardation images. Initial findings indicate that wavelengths below 560 nm offer a more accurate characterization of collagen anisotropy in the porcine cervix.
Preterm birth (PTB) presents a serious medical health concern throughout the world. There is a high incidence of PTB in both developed and developing countries ranging from 11% to 15%, respectively. Recent research has shown that cervical collagen orientation and distribution changes during pregnancy may be useful in predicting PTB. Polarization imaging is an effective means to measure optical anisotropy in birefringent materials, such as the cervix’s extracellular matrix. Noninvasive, full-field Mueller matrix polarimetry (MMP) imaging methodologies, and optical coherence tomography (OCT) imaging were used to assess cervical collagen content and structure in nonpregnant porcine cervices. We demonstrate that the highly ordered structure of the nonpregnant porcine cervix can be observed with MMP. Furthermore, when utilized ex vivo, OCT and MMP yield very similar results with a mean error of 3.46% between the two modalities.
Scars can be debilitating and cause serious functional limitations, significantly reduced physical function and loss of ability to perform normal daily activities. Scar formation is not fully understood and the treatment options have been hampered by the lack of an objective diagnostic tool to assess scars. Presently, assessment of hypertrophic scars has been based on subjective clinician rankings using a four-parameter scale called the Vancouver Scar Scale (VSS) or the Patient Observer Scar Assessment Scale (POSAS) but no objective, standardized tool for quantifying scar severity is available, despite known inadequacies of the subjective scales. We have developed a hand-held multi modal system consisting of a combined Spatial Frequency Domain Imager (SFDI) used for the assessment of tissue molecular components and a polarimeter for structural measurements. The SFDI capability is provided by an Arduino board controlled spectrally and polarimetric diverse Light Emitting Diodes (LED) ring illuminator. For SFDI imagery, the LEDs are combined with sinusoidal patterns. A single pattern snapshot SFDI approach is used to observe and quantify the biological components in the scar tissue including: oxygenated and de oxygenated hemoglobin, water, and melanin. The SFDI system is integrated with a reduced Mueller Matrix polarimetric system, whose illumination is also included in the LED’s ring, and providing for the assessment of collagen orientation through Mueller Matrix decomposition. The design of the system and experimental work on phantoms will be presented.
The primary effect of scarring is the loss of function in the affected area. Scarring also leads to physical and psychological problems that could be devastating to the patient’s life. Currently, scar assessment is highly subjective and physician dependent. The examination relies on the expertise of the physician to determine the characteristics of the scar by touch and visual examination using the Vancouver scar scale (VSS), which categorizes scars depending on pigmentation, pliability, height and vascularity. In order to establish diagnostic guidelines for scar formation, a quantitative, accurate assessment method needs to be developed. An instrument capable of measuring all categories was developed; three of the aforementioned parameters will be explored. In order to look at pliability, a durometer which measures the amount of resistance a surface exerts to prevent the permanent indentation of the surface is used due to its simplicity and quantitative output. To look at height and vascularity, a profilometry system that collects the location of the scar in three-dimensions and laser speckle imaging (LSI), which shows the dynamic changes in perfusion, respectively, are used. Gelatin phantoms were utilized to measure pliability. Finally, dynamic changes in skin perfusion of volunteers’ forearms undergoing pressure cuff occlusion were measured, along with incisional scars.
Preterm birth (PTB) presents a serious medical heath concern throughout the world. There is a high incidence of
PTB in both developed and developing countries ranging from 11%-15%, respectively. Studies have shown there
may be numerous precursors to PTB including infections, genetic predisposition, nutrition and various other
morbidities which all lead to a premature disorganization in the cervical collagen resulting in the weakening of the
structure designed to keep the fetus in utero. The changes in cervical collagen orientation and distribution may prove
to be a predictor of PTB. Polarization imaging is an effective means to measure optical anisotropy in birefringent
materials such as those rich in collagen as the cervix is. Non-invasive, full-field Mueller Matrix polarimetry (MMP)
imaging methodologies and ex-vivo second harmonic generation (SHG) imaging were used to assess cervical
collagen content and structure in non-pregnant porcine cervices. The SHG microscopy was used to verify the
efficacy of the MMP in assessing changes in collagen orientation.
Preterm birth (PTB) presents a serious medical heath concern in both economically developed
and developing nations, with incidence rate from 15%-11% respectively. Changes in cervical
collagen bundle orientation and distribution may prove to be a predictor of PTB. Polarization
imaging is an effective means to measure optical anisotropy in birefringent biological tissue such
as those rich in collagen. Non-invasive, full-field Mueller Matrix polarimetry (MMP) imaging
methodologies, optical coherence tomography (OCT), and second harmonic generation (SHG)
microscopy were used to assess cervical collagen content and structure in non-pregnant cervices.
In vivo studies using a Mueller Matrix colposcope are underway. Further studies of cervical
collagen orientation throughout pregnancy are needed to understand if Mueller matrix
polarimetry can effectively identify at-risk conditions for PTB.
Full-field polarized light imaging provides the capability of investigating the alignment and density of birefringent tissue such as collagen abundantly found in scars, the cervix, and other sites of connective tissue. These can be indicators of disease and conditions affecting a patient. Two-dimensional polarized light Monte Carlo simulations which allow the input of an optical axis of a birefringent sample relative to a detector have been created and validated using optically anisotropic samples such as tendon yet, unlike tendon, most collagen-based tissues is significantly less directional and anisotropic. Most important is the incorporation of three-dimensional structures for polarized light to interact with in order to simulate more realistic biological environments. Here we describe the development of a new polarization sensitive Monte Carlo capable to handle birefringent materials with any spatial distribution. The new computational platform is based on tissue digitization and classification including tissue birefringence and principle axis of polarization. Validation of the system was conducted both numerically and experimentally.
Knowledge of skin surface topography is of great importance when establishing environmental and age related skin damage. Furthermore an effective treatment protocol cannot be established without a quantitative measuring tool that is able to establish significant improvement in skin texture. We utilized an out-of-plane polarimeter for the characterization of skin surface profile non-invasively. The system consists of an imaging Stokes vector polarimeter where the light source and imaging apparatus are arranged at an angle equal to forty degrees with respect to the tissue normal. The light source is rotated at various azimuth angles about the tissue normal. For each position of the incident beam the principal angle of polarization is calculated. This parameter relates indirectly to surface profile and architecture. The system was used to image the forehead and hands of healthy volunteers between eighteen and sixty years of age. A clear separation appeared among different age groups, establishing out-of-plane polarimetry as a promising technique for skin topography quantification.
The extra-cellular space in connective tissue of animals and humans alike is comprised in large part of collagen. Monitoring of collagen arrangement and cross-linking has been utilized to diagnose a variety of medical conditions and guide surgical intervention. For example, collagen monitoring is useful in the assessment and treatment of cervical cancer, skin cancer, myocardial infarction, and non-arteritic anterior ischemic optic neuropathy. We have developed a suite of tools and models based on polarized light transfer for the assessment of collagen presence, cross-linking, and orientation in living tissue. Here we will present some example of such approach applied to the human cervix. We will illustrate a novel Mueller Matrix (MM) imaging system for the study of cervical tissue; furthermore we will show how our model of polarized light transfer through cervical tissue compares to the experimental findings. Finally we will show validation of the methodology through histological results and Second Harmonic imaging microscopy.
Preterm birth is a worldwide health issue, as the number one cause of infant mortality and neurological disorders.
Although affecting nearly 10% of all births, an accurate, reliable diagnostic method for preterm birth has, yet, to be
developed. The primary constituent of the cervix, collagen, provides the structural support and mechanical strength
to maintain cervical closure, through specific organization, during fetal gestation. As pregnancy progresses, the
disorganization of the cervical collagen occurs to allow eventual cervical pliability so the baby can be birthed
through the cervical opening. This disorganization of collagen affects the mechanical properties of the cervix and,
if the changes occur prematurely, may be a significant factor leading to preterm birth. The organization of collagen
can be analyzed through the use of Mueller Matrix Polarimetric imaging of the characteristic birefringence of
collagen. In this research, we have built a full Mueller Matrix Polarimetry attachment to a standard colposcope to
enable imaging of human cervixes during standard prenatal exams at various stages of fetal gestation. Analysis of
the polarimetric images provides information of quantity and organization of cervical collagen at specific gestational
stages of pregnancy. This quantitative information may provide an indication of risk of preterm birth.
Mueller matrix polarimetry and polarization-sensitive optical coherence tomography (PS-OCT) are two emerging techniques utilized in the assessment of tissue anisotropy. While PS-OCT can provide cross-sectional images of local tissue birefringence through its polarimetric sensitivity, Mueller matrix polarimetry can be used to measure bulk polarimetric properties such as depolarization, diattenuation, and retardance. To this day true quantification of PS-OCT data can be elusive, partly due to the reliance on inverse models for the characterization of tissue birefringence and the influence of instrumentation noise. Similarly for Mueller matrix polarimetry, calculation of retardance or depolarization may be influenced by tissue heterogeneities that could be monitored with PS-OCT. Here, we propose an instrument that combines Mueller matrix polarimetry and PS-OCT. Through the co-registration of the two systems, we aim at achieving a better understanding of both modalities.
Assessing collagen alignment is of interest when evaluating a therapeutic strategy and evaluating its outcome in scar
management. In this work we introduce a theoretical and experimental methodology for the quantification of collagen
and birefringent media alignment based on polarized light transport. The technique relies on the fact that these materials
exhibit directional anisotropy. A polarized Monte Carlo model and a spectro-polarimetric imaging system were devised
to predict and measure the impact of birefringence on an impinging polarized light beam. Experiments conducted on
birefringent phantoms, and biological samples consisting of highly packed parallel birefringent fibers, showed a good
agreement with the analytical results.
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