Linearly polarized light that illuminates skin is backscattered by superficial layers and rapidly depolarized by birefringent collagen fibers. It is possible to distinguish such superficially backscattered light from the total diffusely reflected light that is dominated by light penetrating deeply into the dermis. The method involves acquisition of two images through an analyzing linear polarizer in front of the camera, one image (I(par)) acquired with the analyzer oriented parallel to the polarization of illumination and one image (I(per)) acquired with the analyzer oriented perpendicular to the illumination. An image based on the polarization ratio, Pol=(I(par)-I(per))/(I(par)+I(per)), is created. This paper compares normal light images, represented by I(per), and Pol images of various skin pathologies in a pilot clinical study using incoherent visible-spectrum light. Images include pigmented skin sites (freckle, tattoo, pigmented nevi) and unpigmented skin sites [nonpigmented intradermal nevus, neurofibroma, actinic keratosis, malignant basal cell carcinoma, squamous cell carcinoma, vascular abnormality (venous lake), burn scar]. Images of a shadow cast from a razor blade onto a forearm skin site illustrate the behavior of Pol values near the shadow edge. Near the shadow edge, Pol approximately doubles in value because no I(per) photons are superficially scattered into the shadow-edge pixels by the shadow region while I(par) photons are directly backscattered from the superficial layer of these pixels. This result suggests that the point spread function in skin for cross-talk between Pol pixels has a half-width-half-max of about 390 microm.

Imaging skin pathology with polarized light

Fiber optic probes are a key element for biomedical spectroscopic sensing. We review the use of fiber optic probes for optical spectroscopy, focusing on applications in turbid media, such as tissue. The design of probes for reflectance, polarized reflectance, fluorescence, and Raman spectroscopy is illustrated. We cover universal design principles as well as technologies for beam deflecting and reshaping.

/pdf/fiber-optic-probes-for-biomedical-optical-spectroscopy-1ymv9w8tlb.pdf

Fiber optic probes for biomedical optical spectroscopy.

Objective
Polarized light can be used to obtain images of superficial tissue layers such as skin, and some example images are presented. This study presents a study of the transition of linearly polarized light into randomly polarized light during light propagation through tissues.

Study Design/Materials and Methods
The transition of polarization was studied in polystyrene microsphere solutions, in chicken muscle (breast) and liver, and in porcine muscle and skin. The transition is discussed in terms of a diffusion process characterized by an angular diffusivity (radians2/mean free path) for the change in angular orientation of linearly polarized light per unit optical path traveled by the light.

Results
Microsphere diffusivity increased from 0.031 to 0.800 for diameters decreasing from 6.04 μm to 0.306 μm, respectively. Tissue diffusivity varied from a very low value (0.0004) for chicken liver to an intermediate value (0.055) for chicken and porcine muscle to a very high value (0.78) for pig skin.

Conclusion
The results are consistent with the hypothesis that birefringent tissues randomize linearly polarized light more rapidly than nonbirefringent tissues. The results suggest that polarized light imaging of skin yields images based only on photons backscattered from the superficial epidermal and initial papillary dermis because the birefringent dermal collagen rapidly randomizes polarized light. This anatomic region of the skin is where cancer commonly arises. Lasers Surg. Med. 26:119–129, 2000. © 2000 Wiley-Liss, Inc.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/%28SICI%291096-9101%282000%2926%3A2%3C119%3A%3AAID-LSM3%3E3.0.CO%3B2-Y

Imaging superficial tissues with polarized light.

A thorough understanding of optical properties of biological tissues is critical to effective treatment planning for therapies such as photodynamic therapy (PDT). In the last two decades, new technologies, such as broadband diffuse spectroscopy, have been developed to obtain in vivo data in humans that was not possible before. We found that the in vivo optical properties generally vary in the ranges μ(a) = 0.03-1.6 cm⁻¹ and μ'(s) = 1.2-40 cm⁻¹, although the actual range is tissue-type dependent. We have also examined the overall trend of the absorption spectra (for μ(a) and μ'(s)) as a function of wavelength within a 95% confidence interval for various tissues in vivo. The impact of optical properties on light fluence rate is also discussed for various light application geometries including superficial, interstitial, and within a cavity.

/pdf/a-review-of-in-vivo-optical-properties-of-human-tissues-and-4sa3qlende.pdf

A review of in-vivo optical properties of human tissues and its impact on PDT

A detailed study, based on a Monte Carlo algorithm, of polarized light propagation in birefringent turbid media is presented in this paper. Linear birefringence, which results from the fibrous structures, changes the single scattering matrix and alters the polarization states of photons propagating in biological tissues. Some Mueller matrix elements of light backscattered from birefringent anisotropic turbid media present unusual intensity patterns compared with those for nonbirefringent isotropic turbid media. This result is in good agreement with the analytic results based on the double-scattering model. Degree of polarization, Stokes parameters, and diffuse reflectance as functions of linearly birefringent parameters based on numerical results and theoretical analysis are discussed and compared in an effort to understand the essential physical processes of polarized light propagation in fibrous tissues.

/pdf/propagation-of-polarized-light-in-birefringent-turbid-media-4l2tu4580v.pdf

Propagation of polarized light in birefringent turbid media: a Monte Carlo study.

The movement of polarized light through the superficial layers of the skin was visualized using a video camera with a polarizing filter. This study constitutes a description of the impulse response to a point source of incident collimated linearly polarized light. Polarization images reject unwanted diffusely backscattered light from deeper in the tissue and the specular reflectance from the air/tissue interface. Two experiments were conducted: (1) Video polarization reflectometry used a polarized HeNe laser (633 nm) pointing perpendicularly down onto a phantom medium (0.900-micrometer dia. polystyrene spheres in water). The video camera was oriented 10 degrees off the vertical axis and viewed the irradiation site where the laser beam met the phantom. Video images were acquired through a polarizing filter that was either parallel or perpendicular with the reference plane defined by the source, camera, and irradiation site on the phantom medium's surface. The source polarization was parallel to the reference plane. The two images (parallel and perpendicular) were used to calculate a polarization image which indicated the attenuation of polarization as a function of distance between the source and point of photon escape from the phantom. Results indicated a strong polarization pattern within approximately 0.35 cm (approximately 2.2 mfp') from source. [mfp' equals 1/(microna plus microns')]. (2) Optical fiber reflectometry using a polarized diode laser (792 nm) coupled to a polarization-maintaining single-mode fiber, and a multi-mode fiber collector to collect regardless of polarization. Reflectance as a function of fiber separation was measured for the source fiber oriented parallel and perpendicular with the reference plane. Results indicated that the strongest polarization propagated within approximately 0.43 cm (2.2 mfp') from source. The polarization survived approximately 2.2 mfp', which for skin at 630 - 800 nm (mfp' approximately equals 0.066 cm) corresponds to 1.5 mm (or 6.4 ps of travel at the speed of light). Using 6.4 ps as a maximum time of survival, classical paths of photon transport (Feynman paths) were calculated to illustrate the expected depth of interrogation by polarized imaging. The expected mean depth of photons is about 0.36 mm at these longer wavelengths. Shorter wavelengths would result in a shorter mfp' and therefore more superficial imaging of the skin. Polarization images offer an inexpensive approach toward 2-D acquisition of time- gated images based on the early light escaping the tissue. Polarization imaging is an opportunity for a new form of optical image especially useful for dermatology.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

/pdf/polarized-light-transmission-through-skin-using-video-225mr5ew7x.pdf

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

We develop a perturbation theory for diffuse light transport in random media that is applicable to objects with different scattering and absorption than the surrounding medium. The effect of an object with increased absorption is a simple depletion of the surrounding light field. An object with increased scattering causes a similar depletion that can be interpreted as scattering induced absorption. Additionally, a sharp transition of the scattering coefficient at the surface causes a dipole-like surface induced back scattering of light in the direction of the source. In an example we show that the first order 'Born' approximation fails for too strong perturbations, but that an iterative algorithm to calculate higher order approximations can still yield accurate results.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Perturbation theory for optical diffusion theory: general approach for absorbing and scattering objects in tissue

We determined the optical properties of the human prostate, in vivo, over multiple wavelengths for 6 men, in an outpatient setting. Analysis of these data reveal reliable determination of the optical properties in the 650 - 800 nm range, however, values in the shorter wavelengths (400 - 625 nm) were not interpretable due to a low signal to noise ratio. To the best of our knowledge, this is the first report of the optical properties i.e. reduced scattering coefficient [(mu) s' (cm-1)], absorption coefficient [(mu) a (cm-1)], and penetration depth [(delta) (cm)], for the in vivo human prostate over multiple wavelengths in the white light spectrum.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Martin R. Ostermeyer

Papers

Polarized light transmission through skin using video reflectometry: toward optical tomography of superficial tissue layers

Perturbation theory for optical diffusion theory: general approach for absorbing and scattering objects in tissue

Transurethral in-vivo optical properties of the human prostate gland