Development of X-ray phase-contrast imaging techniques for medical diagnostics

摘要

The X-Ray phase-contrast techniques are innovative imaging methods allowing overtaking the limitations of classic radiology. In addition to the differential X-ray absorption on which standard radiology relies, in phase-contrast imaging the contrast is given by the effects of the refraction of X-rays inside the tissues. The combination of phase-contrast with quantitative computer tomography (CT) allows for a highly accurate reconstruction of the tissues’ index of refraction. Thanks to the high sensitivity of the method, tomographic images can be obtained at clinically compatible dose. For all these reasons phase-contrast imaging is a very promising approach, which can potentially revolutionize diagnostic X-Ray imaging. Several techniques are classified under the name of X-Ray phase-contrast imaging. This Thesis focused on the so-called analyzer-based imaging (ABI) method. ABI uses a perfect crystal, placed between the sample and the detector, to visualize the phase effects occurred within the sample. The quantitative reconstruction of the refraction index from CT data is not trivial and before this Thesis work it was documented only for small size objects. This Thesis has focused on two main scientific problems: (1) the development of theoretical and calculation strategies to determine the quantitative map of the refraction index of large biological tissues/organs (>10 cm) using the ABI technique; and (2) the preparation of accurate and efficient tools to estimate and simulate the dose deposited in CT imaging of large samples.udFor the determination of the refraction index, two CT geometries were considered and studied: the out-of-plane and the in-plane configurations. The first one, the most used in the works reported in the literature, foresees that the rotation axis of the sample occurs in a plane parallel to that of the sensitivity of the analyzer crystal; while, in the second CT geometry, the rotation axis is perpendicular to that plane. The theoretical study, technical design and experimental implementation of the in-plane geometry have been main tasks of this Thesis. A first experiment has been performed in order to compare the results obtained with in-plane quantitative phase contrast CT with the absorption-based CT ones. An improved accuracy and a better agreement with the theoretical density values have been obtained by exploiting the refraction effect while keeping the dose to sample low.udA second campaign of experiments has been performed on large human breasts to investigate the efficiency of the in-plane and out-of-plane CT geometries and the performances of the associated image reconstruction procedures. The same experimental conditions were also studied by numerical simulations and the results were compared. This analysis shows that the in-plane geometry allows producing more accurate quantitative three dimensional maps of the index of refraction, while the out-of-plane case is preferable for qualitative investigations.udA study for developing advanced procedures for improving the quality of the obtained CT images has been also conducted. As a result, a two-step procedure has been tested and identified: first the noise level of the experimental images is reduced by applying a wavelet decomposition algorithm and then a deconvolution procedure. The obtained images show an enhanced sharpness of the interfaces and of the object edges and high signal to noise ratio values are preserved.udThe second problem of this Thesis was to find strategies to calculate, in a fast way, the delivered dose in CT imaging of complex biological samples. For this purpose an acceleration method to speed-up the convergence of Monte Carlo simulations based on the Track Length Estimator method has been computed and included in the open-source software GATE. Results show that this method can lead to the same accuracy of conventional Monte Carlo methods while reducing the required computation time of up to two orders of magnitude, with the respect to the considered geometry. A database of dose curves for the case of monochromatic breast CT has been produced: it allows for a quick estimation of the delivered dose. A way to choose the best energy and the optimal photon flux was also proposed, which leads to a significant reduction of the delivered dose without any loss in terms of image quality.udMost of the experimental and data reconstruction methods developed within this Thesis work can be applied also to other phase-contrast techniques. This Thesis shows that high resolution three dimensional diagnostic imaging of large and complex biological organs can, in principle, be performed at clinical compatible doses; this is the most significant contribution of the Thesis towards the clinical implementation of phase-contrast CT.