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Influence du mouvement des organes et des paramètres d'acquisition sur la qualité de l'image en scanographie : application à la détermination de la composition chimique des calculs rénaux

Abstract : Despite all the improvements known by the Computed Tomography (CT) during the last 20 years, the CT image quality remains heavily influenced by the voluntary or involuntary patient motion. Since the CT is an irradiating imaging modality based on X-rays, it is impossible to study the influence of the organs motion on the image quality by using healthy subjects. Indeed, the unjustified exposure to radiation is prohibited. Consequently, phantoms have to be used to quantify the image quality. Such phantoms are commercially available but none is mobile. We have so designed and built a dynamic platform allowing the simulation of organs motion. This platform allowed us to show that the influence of the motion on image quality depended on the size, the shape and the contrast of the studied object. The motion led to a deformation, a resizing, and to a decreasing of the CT attenuation values of the studied object. The more the speed and the amplitude of the motion were, the more the loss of the information about the object was. The use of CT scanners of different generations, different technologies and different builders allowed us to study the influence of CT acquisition parameters on the image quality with and without motion. This study has highlighted the fact that the choice of the right CT parameters (pitch, kV, mAs, gantry rotation time...) was very important on the determination of the image quality. The influence of theses parameters were not the same with and without motion applied during the acquisition. Once these results obtained, we have tried to develop a technique of determination of the chemical composition of renal stones in CT. This technique is based on-the CT-attenuation values of the renal stones. It allows to characterize rapidly the chemical composition of the renal stone into choose as quickly as possible the best diagnosis and the best treatment (surgery, NPLC...). With the largest number (n = 241) of stones so far described, to our knowledge and with a home made jelly phantom respecting the absorption and the X-ray attenuation of the kidney, we have develop a protocol allowing to determine the chemical nature of the major component of a stone, with a 95% precision. Our dual-energy method allows to identify each stone based on the CT-attenuation values obtained at 80 kV and at 120 kV. Sadly, this method is no more applicable when the phantom moves during the acquisition. Indeed the motion induces decreasing of CT-attenuations values and consequently the scale of values obtained with no motion is no more usable. Our characterization technique works only if absolutely no motion occurs during the acquisition.
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Romain Grosjean. Influence du mouvement des organes et des paramètres d'acquisition sur la qualité de l'image en scanographie : application à la détermination de la composition chimique des calculs rénaux. Médecine humaine et pathologie. Université Henri Poincaré - Nancy 1, 2008. Français. ⟨NNT : 2008NAN10041⟩. ⟨tel-01777280⟩

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