Analyzing the influence of oblique incidence on quantitative backscattering tissue polarimetry: a pilot ex vivo study

Abstract

Significance: Among the available polarimetric techniques, backscattering Mueller matrix (MM) polarimetry provides a promising non-contact and quantitative tool for in vivo tissue detection and clinical diagnosis. To eliminate the surface reflection from the sample cost-effectively, the non-collinear backscattering MM imaging setup always has an oblique incidence. Meanwhile, for practical organ cavities imaged using polarimetric gastrointestinal endoscopy, the uneven tissue surfaces can induce various relative oblique incidences inevitably, which can affect the polarimetry in a complicated manner and needs to be considered for detailed study.

Aim: The purpose of this study is to systematically analyze the influence of oblique incidence on backscattering tissue polarimetry.

Approach: We measured the MMs of experimental phantom and ex vivo tissues with different incident angles and adopted a Monte Carlo simulation program based on cylindrical scattering model for further verification and analysis. Meanwhile, the results were quantitatively evaluated using the Fourier transform, basic statistics, and frequency distribution histograms.

Results: Oblique incidence can induce different changes on non-periodic, two-periodic, and four-periodic MM elements, leading to false-positive and false-negative polarization information for tissue polarimetry. Moreover, a prominent oblique incidence can bring more dramatic signal variations, such as phase retardance and element transposition.

Conclusions: The findings presented in this study give some crucial criterions of appropriate incident angle selections for in vivo polarimetric endoscopy and other applications and can also be valuable references for studying how to minimize the influence further.

Keywords: Monte Carlo simulation; Mueller matrix; endoscopy; polarimetry; scattering imaging.

Fig. 1

Schematics of setups and samples: (a) oblique incidence induced by tissue surface in gastrointestinal endoscopy; (b) concentrically aligned silk phantom, porcine liver, and human gastric muscularis tissues; and (c) backscattering DRR experimental setup and flowchart of MM imaging acquisition. L1 and L2, lenses; P1 and P2, polarizers; R1 and R2, rotating quarter-wave plates; $\theta$, the oblique incident illumination angle. During each measurement, R1 and R2 rotate with constant rates ($\omega$ for R1 and $5\omega$ for R2).

Fig. 2

Azimuthal dependent curves of $4\times 4$ MM elements under oblique incidences of 5 deg, 10 deg, 15 deg, 20 deg, 30 deg, 40 deg, and 50 deg, respectively. The horizontal axis shows the azimuth angle of the silk fibers, the vertical axis is the value of MM elements. All the elements are normalized by M11.

Fig. 3

M24, M34, M42, and M43 images of porcine liver tissue under obliquely incident illuminations of 10 deg, 20 deg, and 30 deg, respectively.

Fig. 4

MM images and analysis of concentrically aligned silk phantom and porcine liver tissue sample under different obliquely incident illuminations: (a) M11 of the silk phantom; (b) M44 of the silk phantom; (c) mean and kurtosis of the silk phantom. (d1) M44 of porcine liver tissue at 10 deg incidence; (d2) M44 of porcine liver tissue at 20 deg incidence; (d3) M44 of porcine liver tissue at 30 deg incidence. (e) FDH for the M44 of porcine liver tissue.

Fig. 5

MM images and analysis of concentrically aligned silk phantom under different obliquely incident illuminations: (a) M12; (b) M21; (c) baseline and peak-to-valley value, calculated by the M12 and M21 in 1 period between 0 deg and 180 deg; and (d) amplitude of the third harmonic component after FFT, calculated by the M12 and M21, M13, and M31, respectively.

Fig. 6

M12 and M21 images, and the corresponding FDH curves of human gastric muscularis tissue sample under obliquely incident angles at 10 deg, 20 deg, and 30 deg.

Fig. 7

MM images and analysis of concentrically aligned silk phantom under different obliquely incident illuminations. (a) Images of M22, M33, M23, and M32 from above to below, respectively. (b)–(e) Comparison of $P-P$ value represented by blue dots and $P-V$ value represented by red squares of M22, M23, M32, and M33. The peak and valley values are taken from the first 2 periods of 0 deg to 180 deg. (f) Relative locations and gap between peak and valley in the 0 deg to 180 deg period of M33. The period length has been normalized.

Fig. 8

M22 and M33 images, and the corresponding FDH curves of human gastric muscularis tissue sample under obliquely incident angles at 10 deg, 20 deg, and 30 deg.

Fig. 9

MM images and analysis of concentrically aligned silk phantom under different obliquely incident illuminations. (a) Images of the M11, M44, M12, M21, M22, M23, M32, and M33 under oblique incidences of 10 deg, 15 deg, 20 deg, 60 deg, and 70 deg from above to below, respectively. (b) and (c) Experimental and Monte Carlo simulated azimuthal dependent curves of $4\times 4$ MM elements under oblique incidences of 10 deg, 15 deg, 20 deg, 60 deg, and 70 deg, respectively. All the elements are normalized by M11.