Research Of Geometric Calibration And Design Of Testing Phantom For Dental Computed Tomography

Dental cone beam CT solves the inherent problems such as image overlap, distortion, etc in the conventional two-dimensional perspective imaging and compared to general CT, it has the advantage of small dose, high spatial resolution, etc. By measuring the accumulation of attenuation coefficient (or named as projection) when the X-rays passing through the tomography of the body from every direction and reconstructing the distribution of X-rays attenuation coefficient in the whole tomography with the computer, the dental cone beam CT can help the doctors diagnose lesion and has become indispensable in dental medical diagnostic aid. But the image quality of dental cone beam CT influences the effect of diagnosis directly. At present, the measurements of the parameters that can be used to evaluate the quality of the image such as the spatial resolution, density resolution, layer thickness, the signal-to-noise ratio, uniformity of and image grey value, etc, all need the help of testing phantom. At the same time, dental cone beam CT is still facing with many problems and challenges in dental clinical applications. The most important ones are the accuracy of the geometrical parameter calibration and geometry artifacts. Based on this, this paper mainly researches on the phantom for the evaluation of dental cone beam CT image quality and geometric correction method. The main contents of this paper are as follows.Firstly, for the geometrical parameter calibration method, this paper implements a geometrical parameter calibration method firstly, then change the calculation method of four feature points in the trajectory to improve the method and compared these two results got by the original method and the improving one. In order to solve the problem of geometrical parameter calibration after the detector bias to increase FOV(Field of View), on the basis of the algorithm, implement the eccentric geometric calibration method with using the characteristic points of the three known ball trajectory to calculate the one of the unknown ball, which makes the calculation of the geometric parameters are more accurate.Secondly, this paper puts forward the geometric artifact correction methods based on the edge sharpness of the circular water module’s image. According to the judge of whether the edge sharpness of the circular water module’s image is in the ideal range, adjust the geometric parameters. By this method, the quality of the image gets improved a lot and solve the problems with inaccurate geometric parameters calibration method.Finally, for the testing phantom, this paper puts forward the one which is exclusively used in the evaluation of the performance of dental cone beam CT. The diameter of this testing phantom is six centimeter and can be present in the scanning visual field of most dental cone beam CT. There are four testing modules in this phantom which make it can detect the five parameters that used to evaluate the performance of dental cone beam CT more reasonably and accurately. In addition, the phantom can also be used for experimental dual-energy subtraction angiography.