We have developed a real-time multi-wavelength spatial frequency domain (SFD) diffuse optical tomography (DOT) to characterize the optical properties of biological tissues, using single-pixel imaging based on lock-in photon-counting. In our approach, three laser diodes at wavelengths of 450 nm, 520 nm and 635 nm, are intensity-modulated by square waves of three frequencies (temporal-encoding), respectively, and focused into the first digital micromirror device (DMD) to generate a sinusoidal illumination pattern at selected spatial frequencies. The reflected light from the surface of turbid medium are spatially integrated by the second DMD, successively using sampling patterns based on the two- dimensional discrete cosine transform (DCT) bases around the spatial modulation frequencies (spatial-compressing). The temporally encoded and spatially compressed multi-wavelength signals that are detected from photomultiplier tube are firstly demodulated by a highly-sensitive lock-in photon-counting module for temporal-decoding, and then uncompressed into the spatial frequency domain images by the inverse DCT, from which tomographic images of the absorption coefficient are finally reconstructed using the first-order Rytov approximation of the diffusion equation. The phantom experiments show that the proposed method can achieve a reconstruction error within 10%, and a temporal resolution of less than 10 s.
Spatial frequency domain (SFD) imaging offers a wide-field modality to effectively characterize the optical properties (absorption and scattering coefficients), and furthermore to calculate the chromophore concentrations from multiwavelength measurements, in biological tissues. Previous SFD imaging systems mostly capture the two-dimensional reflected light using an expensive charge-coupled device camera that requires switching between the multi-wavelength collections. With recent proliferation in low-cost and technology we present herein a highly-sensitive novel single-pixel SFD imaging system for simultaneous and acquisition of multi-wavelength images. In the approach, three LED-sources at 455-nm, 530-nm and 660-nm wavelengths are temporally modulated at different frequencies, and all focused to the first digital micromirror device (DMD) to generate a wide-field sinusoidal illumination on tissues. The reflected signal is spatially integrated by the second DMD that is coded according to the transform matrix, and fed into a lock-in photoncounting module and temporally demodulated to extract the signals at each wavelength. The SFD images at each wavelength are recovered by single-pixel imaging algorithm, respectively, and then used to calculate the modulation transfer function for extraction of the optical properties. The proposed system is experimentally validated on phantoms, demonstrating the system stability, measurement linearity, negligible inter-wavelength crosstalk, and recovery effectiveness.
Photodynamic therapy (PDT) has shown superiorities of noninvasiveness and high-efficiency in the treatment of early-stage skin cancer. Rapid and accurate determination of spatially distributed photon fluence in turbid tissue is essential for the dosimetry evaluation of PDT. It is generally known that photon fluence can be accurately obtained by Monte Carlo (MC) methods, while too much time would be consumed especially for complex light source mode or online real-time dosimetry evaluation of PDT. In this work, a method to rapidly calculate spatially distributed photon fluence in turbid medium is proposed implementing a classical perturbation and iteration theory on mesh Monte Carlo (MMC). In the proposed method, photon fluence can be obtained by superposing a perturbed and iterative solution caused by the defects in turbid medium to an unperturbed solution for the background medium and therefore repetitive MMC simulations can be avoided. To validate the method, a non-melanoma skin cancer model is carried out. The simulation results show the solution of photon fluence can be obtained quickly and correctly by perturbation algorithm.
We present a novel approach for single snapshot determination of absorption coefficient based on multi-frequency modulation transfer function (MTF) characterization from measurement in spatial frequency domain. The adopted Fourier transform domain analysis enables simultaneous extraction of information at multiple applied frequencies and excellent reduction of noise. Simulations were conducted for respectively verifying the feasibility of the MTF based approach and the performance of single snapshot determination of absorption coefficient using multi-frequency measurements. Phantom experiments without reference measurement demonstrated the high accuracy of absolute absorption coefficient determination with a maximum reconstruction error of 0.002 mm-1.
We present a wide-field fluorescence tomography with epi-illumination of sinusoidal pattern. In this scheme, a DMD projector is employed as a spatial light modulator to generate independently wide-field sinusoidal illumination patterns at varying spatial frequencies on a sample, and then the emitted photons at the sample surface were captured with a EM-CCD camera. This method results in a significantly reduced number of the optical field measurements as compared to the point-source-scanning ones and thereby achieves a fast data acquisition that is desired for a dynamic imaging application. Fluorescence yield images are reconstructed using the normalized-Born formulated inversion of the diffusion model. Experimental reconstructions are presented on a phantom embedding the fluorescent targets and compared for a combination of the multiply frequencies. The results validate the ability of the method to determine the target relative depth and quantification with an increasing accuracy.
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