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.
We presented a novel dual-wavelength diffuse optical imaging system which can perform 2-D or 3-D imaging fast and high-sensitively for monitoring the dynamic change of optical parameters. A newly proposed lock-in photon-counting detection method was adopted for week optical signal collection, which brought in excellent property as well as simplified geometry. Fundamental principles of the lock-in photon-counting detection were elaborately demonstrated, and the feasibility was strictly verified by the linearity experiment. Systemic performance of the prototype set up was experimentally accessed, including stray light rejection and inherent interference. Results showed that the system possessed superior anti-interference capability (under 0.58% in darkroom) compared with traditional photon-counting detection, and the crosstalk between two wavelengths was lower than 2.28%. For comprehensive assessment, 2-D phantom experiments towards relatively large dimension model (diameter of 4cm) were conducted. Different absorption targets were imaged to investigate detection sensitivity. Reconstruction image under all conditions was exciting, with a desirable SNR. Study on image quality v.s. integration time put forward a new method for accessing higher SNR with the sacrifice of measuring speed. In summary, the newly developed system showed great potential in promoting detection sensitivity as well as measuring speed. This will make substantial progress in dynamically tracking the blood concentration distribution in many clinical areas, such as small animal disease modeling, human brain activity research and thick tissues (for example, breast) diagnosis.
Diffuse optical tomography was recognized as one of the most potential methods to in-vivo imaging due to its advantages of non-invasiveness, high sensitivity and excellent specificity etc. This modality aims at portraying the concentration distribution of oxy-hemoglobin and deoxy-hemoglobin statically or dynamically by resolving the optical properties at multiple wavelengths. To further improve the instantaneity and sensitivity of the method, we have developed a continuous-wave diffuse optical tomography system based on lock-in photon-counting technique, which can perform dual-wavelength measurement simultaneously at ultra-high sensitivity. The system was configured by modulating the laser sources at different wavelengths with different frequencies and adopting a single photon-counting block based on the digital lock-in detection for the data demodulation. Phantom experiments were conducted to evaluate the capability of the method. Results have shown that the absorption contrast can be commendably reconstructed, and the system we proposed provides a promising tool for in-vivo imaging.
At present, the most widely accepted forward model in diffuse optical tomography (DOT) is the diffusion equation,
which is derived from the radiative transfer equation by employing the P1 approximation. However, due to its validity
restricted to highly scattering regions, this model has several limitations for the whole-body imaging of small-animals,
where some cavity and low scattering areas exist. To overcome the difficulty, we presented a Graphic-Processing-
Unit(GPU) implementation of Monte-Carlo (MC) modeling for photon migration in arbitrarily heterogeneous turbid
medium, and, based on this GPU-accelerated MC forward calculation, developed a fast, universal DOT image
reconstruction algorithm. We experimentally validated the proposed method using a continuous-wave DOT system in the
photon-counting mode and a cylindrical phantom with a cavity inclusion.
In biomedical optics, the Monte Carlo (MC) simulation is widely recognized as a gold standard for its high accuracy and
versatility. However, in fluorescence regime, due to the requirement for tracing a huge number of the consecutive events
of an excitation photon migration, the excitation-to-emission convention and the resultant fluorescent photon migration
in tissue, the MC method is prohibitively time-consuming, especially when the tissue has an optically heterogeneous
structure. To overcome the difficulty, we present a parallel implementation of MC modeling for fluorescence propagation
in tissue, on the basis of the Graphics Processing Units (GPU) and the Compute Unified Device Architecture (CUDA)
platform. By rationalizing the distribution of blocks and threads a certain number of photon migration procedures can be
processed synchronously and efficiently, with the single-instruction-multiple-thread execution mode of GPU. We have
evaluated the implementation for both homogeneous and heterogeneous scenarios by comparing with the conventional
CPU implementations, and shown that the GPU method can obtain significant acceleration of about 20-30 times for
fluorescence modeling in tissue, indicating that the GPU-based fluorescence MC simulation can be a practically effective
tool for methodological investigations of tissue fluorescence spectroscopy and imaging.
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