Recovery of angular scattering profiles through a flexible multimode fiber

Abstract

Endoscopic angle-resolved light scattering methods have been developed for early cancer detection but they typically require multi-element coherent fiber optic bundles to recover scattering distributions from tissues. Recent work has focused on using a single multimode fiber (MMF) to measure angle resolved scattering but this approach has practical limitations to overcome before clinical translation. Here we address these limitations by proposing an MMF-based endoscope capable of measuring angular scattering patterns suitable for determining structure. Significantly, this approach implements a spectrally resolved detection scheme to reduce speckle and leverages the azimuthal symmetry of the angular scattering patterns to enable measurements that are robust to fiber bending. This results in a unique method that does not require matrix inversion or machine learning to measure a transmitted scattering distribution. The MMF utilized here is 1000 mm in length with a 200 µm core and is demonstrated to recover angular scattering distributions even with bending displacements of up to 30 cm. This advance has a significant impact on the clinical translation of biomedical endoscopic diagnostic techniques that use angular scattering to determine the size of cell nuclei to detect early cancer.

Fig. 1.

(Left) Instrumentation setup for reconstructing angular scattering profiles with spectroscopic information through a single MMF. The gavlo-mirror is scanned along the x-dimension and the beam is imaged through the imaging spectrometer sequentially, and reconstruction methods are used to fully obtain the total three-dimensional information of the beam, including the spatial and the spectroscopic information. This system is designed to operate in two modes, where backscattered light from a sample or digital micromirror device (DMD) is transmitted through the MMF. OBJ: Objective lens. BS: Beam Splitter. (Right) Schematic of the detection scheme, illustrating how the galvanometer scans the received angular distribution across the entrance slit of the spectrometer (adapted from [28]).

Fig. 2.

Image processing pipeline for recovering the 1D scattering profile from scattering phantoms of size 10µm in a mixture of water and glycerol (n = 1.36). The full beam profile including the spectroscopic information is first acquired using the galvo-mirror and the imaging spectrometer. The reconstructed beam profiles are then normalized, averaged, radially integrated and polynomial subtracted to form the 1D scattering profile, shown as the red curve on the right figure. This profile is then compared to the profiles generated using Mie theory to find the best fit based on χ² analysis, shown as the blue curve on the right figure.

Fig. 3.

Mie fitting results of scattering phantoms with size 4.0 µm, 6.0 µm and 10.1 µm in air. The red curves correspond to the reconstructed 1D angular scattering profile, and the blue curve corresponds to the Mie fitting results based on χ2 analysis. Note that the difference in scattering distribution for the 10.1 µm beads compared to Fig. 2 is due to the difference in the surrounding medium refractive index.

Fig. 4.

Calibration curve demonstrating scattering size measurement capability of the system. The red symbols correspond to the analyzed sizes from the scattering phantoms with polystyrene microspheres in air, and the blue symbols correspond to the determined sizes from the scattering phantoms with microspheres in a mixture of water and glycerol. The error bar indicates the variability in fitting across 5 different measurements. The criteria for subwavelength accuracy is shown as the light blue region bounded by the two dashed lines.

Fig. 5.

Displacement of multimode fiber during measurements of scattering distribution (up to 30 cm).

Fig. 6.

Calibration curve demonstrating generalizability across measurements with fiber bending. The red symbols correspond to the analyzed sizes from the scattering phantoms with polystyrene microspheres in air, and the blue symbols correspond to the determined sizes from the scattering phantoms with microspheres in a mixture of water and glycerol. The error bar indicates the variability in fitting across 5 different measurements. The criteria for subwavelength accuracy is shown as the light blue region bounded by the two dashed lines.