Polarization Sensitive Spatial Frequency Domain Imaging System And Phantom Verification

Polarization-sensitive optical imaging is a non-labeled,non-destructive detection technique that is sensitive to subwavelength microstructural changes and can provide a wealth of structural and optical information about the tissue.In the visible wavelength range,biological tissues are strongly scattering media,and light propagating in tissues will undergo multiple scattering,changing the phase and polarization information originally carried,and characterizing the physiological parameters of pathological tissues by analyzing the change of polarization information.In this study,a polarization spatial frequency domain imaging system was built.In order to quickly and accurately measure the Mueller matrix polarization information of biological tissues,a liquid crystal phase retarder was chosen to replace the quarter-wave plate in conventional polarized light imaging,and the calibration value of the gray scale plate was used as the reference value for the calibration of the polarization spatial frequency domain imaging system.The depth of scattering of polarized light in the tissue is particularly important in polarized light imaging.However,in conventional polarized light imaging methods,the scattering depth of polarized light in biological tissues cannot be controlled,and the polarization information sought is an average of unknown depths in biological tissues,which cannot accurately measure the polarization information in pathological tissue regions.Take skin diseases as an example,most of the skin diseases have their pathological tissue areas in the superficial layer of the skin,and the depth of polarized light penetration in traditional polarized light imaging is greater than the depth of the pathological tissue area,so the polarization information obtained is not accurate and does not have medical reference value.The system of polarization spatial frequency domain imaging built in this study consists of three main parts: stripe projection,polarization modulation and imaging detection.The stripe projection part uses a digital micromirror device to project the stripe pattern.In this part,the illumination light generated by the light source is filtered by a liquid crystal tunable filter to a single wavelength(670 nm)and irradiated to the surface of the digital micromirror device,which is spatially modulated to form a wide-field sinusoidal pattern and projected onto the tissue surface of the sample to be measured.The polarization modulation section consists of four sets of liquid crystal phase delayers and two sets of polarizers.In the first half of the optical path,the polarizers and the two liquid crystal phase delayers form a polarizer,which is used to modulate the illumination light to different polarization states.The second half is a polarizer,which separates the scattered light of different polarization states and is detected by the CCD camera.The CCD camera is used in the image detection section to acquire diffuse reflectance images of the sample tissue.The images are recorded with the light intensity values of the different polarization states and then solved and analyzed using a calculation program written in MATLAB.The fast axes of the liquid crystal phase delayers in the deflector are 45° and 90° from the horizontal direction,and the fast axes of the liquid crystal phase delayers in the deflector are 90° and 45° from the horizontal direction,respectively.The whole set is controlled by a set of customized QT programs.After the system calibration is completed,the polarization spatial frequency domain imaging system is examined using fat milk,glass plate,and wave sheet.The fat milk concentration,glass plate tilt angle,and waveplate type(i.e.,phase delay)were varied by the controlled variable method.The experimental results showed that the deconvolution coefficient tended to be proportional to the fat milk volume fraction.The two-way attenuation coefficient increases with increasing two-way attenuation due to the two-way attenuator,and the accurate measurement of the phase delay of quarter-wave and full-wave plates indicates that the system can accurately measure the sample polarization parameters.Comparing uniform light field illumination and polarization-sensitive spatial frequency domain imaging,it is found that polarization-sensitive spatial frequency domain imaging effectively controls the depth and accurately measures the shallow Mueller matrix of the sample.In summary,the results of this study are expected to effectively improve the accuracy of superficial tissue polarization characteristics detection and promote early tumor detection and protection.