The heterodyne detection technique, on which the coherent detection imaging (CDI) method founds, can discriminate and select very weak, highly directional forward scattered, and coherence retaining photons that emerge from scattering media in spite of their complex and highly scattering nature. That property enables us to reconstruct tomographic images using the same reconstruction technique as that of X-Ray CT, i.e., the filtered backprojection method. Our group had so far developed a transillumination laser CT imaging method based on the CDI method in the visible and near-infrared regions and reconstruction from projections, and reported a variety of tomographic images both in vitro and in vivo of biological objects to demonstrate the effectiveness to biomedical use. Since the previous system was not optimized, it took several hours to obtain a single image. For a practical use, we developed a prototype CDI-based imaging system using parallel fiber array and optical switches to reduce the measurement time significantly. Here, we describe a prototype transillumination laser CT imaging system using fiber-optic based on optical heterodyne detection for early diagnosis of rheumatoid arthritis (RA), by demonstrating the tomographic imaging of acrylic phantom as well as the fundamental imaging properties. We expect that further refinements of the fiber-optic-based laser CT imaging system could lead to a novel and practical diagnostic tool for rheumatoid arthritis and other joint- and bone-related diseases in human finger.
A reconstruction method for transillumination laser CT using optical heterodyne detection is proposed. Laser CT does not obey the Radon transform due to surface effects, which occur as results of refractive index mismatch and roughness of surface, and bring about annulus artifact and impair quantitative accuracy of a reconstructed image. Optical attenuation with surface effects is described using a model function with respect to an incident angle. This reconstruction produces a least squares problem incorporating the surface effect, which is solved via the conjugate gradient method. The method is applied to data from a acrylic physical phantom containing Intralipid-10% solution in various concentrations for demonstrating the effectiveness of the proposed method in terms of morphological and quantitative information. The CDI method is proved to have an excellent quantitative accuracy.
Tomographic images of biological materials, for example, phantoms of Polyethylene, Polycarbonate, Nylon and Plexiglas with 10 mm diameter with internal bores of 2 to 5 mm diameter and length containing Intralipid-10% solution in various concentrations are obtained. The concentration is defined as the percentage of the volume of the Intralipd-10% against that of water, and the region of interest. In addition to these, the phantoms are filled with few other soft materials (external shell with internal soft material) and the images are obtained. The data acquisition and image reconstruction procedures of the laser CT system are similar to those of X-ray CT. The images are acceptably distinct. Based on experimental observation of the images, the present system can reconstruct images of small objects and soft materials in the trans-illumination mode. In addition, we have studied partially the dynamical properties of the embedded biological soft tissue of a snail.
In this research, we investigate the imaging properties of tansillumination laser CT system based on the coherent detection imaging (CDI) method, for early diagnosis of rheumatoid arthritis, from the viewpoints of morphological and quantitative imaging. First, to investigate the morphological imaging ability, the index finger PIP joint of a healthy volunteer was imaged with the laser CT, X-ray CT, and MRI. By comparison of these images, we present that the laser CT delineates the bone region with high spatial resolution of sub-millimeters. Next, to investigate the quantitative imaging, three kinds of the physical phantoms simulating bone, made from polyurethane including different concentrations of CaCO3, were imaged. We present that quantitative measurement is possible by demonstrating a satisfactory linear relationship between the averaged pixel value of the reconstructed images and the actual concentrations. These results demonstrate the feasibility of early diagnosis for rheumatoid arthritis.
So far we have shown, through various preliminary imaging experiments with small-animal ankle’s and human finger’s joints both healthy and joint-diseased, that early diagnosis for joint disease such as rheumatoid arthritis (RA) is feasible using a transillumination laser CT. For a practical purpose, we have recently proposed and developed a transillumination laser CT imaging system using optical fibers based on the optical heterodyne detection method for a clinical use. In the proposed system, motion-artifact free images can be obtained because measurements can be performed with the object fixed. In addition, use of fiber-optics enables portability, and robustness against environmental changes in a room, such as variable temperature, air-flow shifts, and unexpected vibrations. The imaging system has the following sensing properties: spatial resolution of 500 μm, a dynamic range of approximately 120 dB, and a minimum-detectable-optical power of 10-14 W as a result of the excellent properties of the heterodyne detection technique. In the present paper, we describe a prototype laser CT imaging system using optical fibers for early diagnosis of joint disease such as rheumatoid arthritis by demonstrating the first in vivo tomographic image of a volunteer’s index finger joint as well as the fundamental imaging properties.
This paper considers the interior problem of CT reconstruction in which outer data are deficient in each projection. It is effective to this problem to restrict the parameters, i.e., the pixels, to be estimated to the region in which an object exists. We investigate this problem using the minimum description length principle proposed by Rissanen which is the amount of information required to describe a model based on information theory. Reconstruction algorithm and the data structure for this model to reduce amounts of calculation and memory are proposed. Finally, its effectiveness is shown by simulation.
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