Study on Artifact occurredin the X-ray CT Image due to the Motion of the Object

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(1) In the X ray computed tomography (CT), time to scan an object must be allowed to obtain its projection data. If the object is not steady during the scanning, itsu reconstructed image would contain errors called "mottifacts. Recently , two kinds of motion artifact,"blurred image" and "stroboscopic image" , have been found in clinical experience, and their theoretical analyzes have been demanded. Assuming that an object is a rigid body, its motion can be divided into rotary and parallel motions. This paper analyzes the degradiation of reconstructed images and artifacts due to the rotary motion of a rigid object. The degradiation of a CT image caused by rotation of an arbitrarily shaped rigid object is formulated by using a point-spread function. The cjaracteristics of blurred images of a rigid object due to its rotation in a CT and the shape of stroboscopic images also are analyzed. The cause of storoboscopic images is explained by using line source. The theories have been confirmed … More by computer simulation.(2) Compton scattering imaging can demonstrate the electron density of a given object which could be important for medical diagnosis. However, clinical application of the technique has not yet been developed, bacause a clear image can not be obtained for several reasons. The image is seriously contaminated by multiple scattering which is inevitable with Compton scattering. The electron density in a small region of the object is not obtained correctly unless multiple scattering is handled carefully. In the present paper, methods to eliminate multiple scattering in Compton scattering imaging system are discussed. A general purpose Monte Carlo code called EGS4 was used to simulate the processes of an incident photon interacting with a substance inside an object. Simulations were performed using a water sphere phantom with a radius of 10 cm irradiated by a beam of monochromatic X-rays at 100 keV.Scattered photons emitted from the phantom were observed and the ratio of the amount of single Compton scattering from a focal point to that of multiple scatterong was calculated. To improve the image quality, elimination of multiple scattering in Compton scattering is indispensable. To this end , theta and phi-collimators and an energy window were proposed and their effects were estimated quantitatively by simulations. It was shown that by introducing of the proposed methods in the imaging system, the ratio of the Compton scattering signal to the multiple scattering noise, S/N,could be improved from 0.022 to 40. Quantitative results derived from the simulations may be useful for the design of a pratical Compton scattering imaging system. Less