X-ray computed tomography (CT) has been established as a daily tool in clinical diagnostics and has been continuously refined by more recent innovations in the last years. These systems are, however, limited by fundamental constraints since they are only capable of mapping X-ray attenuation differences in the tissue. Phase-contrast and dark-field imaging provide complementary contrast, which originates from physically different interaction processes of X-rays with matter. Particularly the dark-field signal is considered to have significant diagnostic potential since it is capable to retrieve micro-structural information below the actual resolution limit of the imaging system. This was demonstrated in various laboratory setups and recently also in the fist study with human patients in a clinical radiography system based on a grating interferometer. In a recent work, we presented the first implementation of such an X-ray interferometer into a clinical CT gantry. Upscaling and adapting this technology for a rotating CT gantry involves several challenges and tradeoffs ranging from limitations in interferometer design over fast, continuous signal acquisition requirements to tolerances in applied patient dose. In this work we discuss the performance of the first clinical dark-field CT prototype. For this purpose, we present results of our phantom studies which were designed to evaluate whether and how the dark-field contrast generated by the system is capable to provide additional structural sample information. The key aspects include the possibility of quantitative imaging and a gradual approach to simulate results that come as close as possible to a real application in a human patient.
Grating-based phase-contrast and dark-field X-ray imaging is a promising technology for improving the diagnosis and imaging capabilities of breast cancer and lung diseases. While traditional X-ray techniques only consider the attenuation coefficient, phase-contrast and dark-field imaging are also capable of measuring the refractive index decrement and the so-called linear diffusion coefficient, a measure of a sample’s small-angle scattering strength. Consequently, the technique provides additional information about the micro-structure of a sample. While it is already possible to perform human chest dark-field radiography, it is assumed that its diagnostic value increases when performed in a tomographic setup. The thereby acquired three-dimensional mappings of the three modalities yield detailed information about morphological changes without being obscured by overlaying structures. This work presents the sample data processing and reconstruction pipeline of the first human-sized clinical dark-field CT system. In this novel setting we require a processing concept which is (1) compatible with continuous rotation, (2) can compensate for perturbances induced by system vibrations, and (3) still enables short processing and reconstruction times. An advanced sliding window approach was chosen for the sample data extraction to meet requirements (1) and (3). Furthermore, we present the corrective measures that have to be applied in the employed processing and reconstruction algorithms to mitigate the effects of vibrations and deformations of the interferometer gratings. The developed techniques are shown to successfully reduce the emergence of artefacts in the reconstructed images.
Computed tomography (CT) is a foundation of modern clinical diagnostics but it presently only retrieves information from X-rays attenuation. However, it is known that micro structural texture or porosity – which is well below the spatial resolution of CT – can be revealed by grating-based dark-field imaging. Diagnostic value of this sub-resolution tissue information has been demonstrated in pre-clinical studies on small-animal disease models and recently also in a first clinical radiography system.1 These studies show that dark-field imaging is particularly useful for early detection and staging of lung diseases. While dark-field CT is regularly realized in laboratory environment, the transfer to human scale and bringing it to clinical application poses several technical challenges. Switching from a step-and-shoot acquisition to a mode where the gantry and acquisition operate continuously as well as reducing scan times to below seconds and ensuring stability against vibrations are key concerns when it comes to the translation of the established laboratory dark-field technology to full-body medical CT. In,2 we recently demonstrated the first dark-field CT implementation, which collectively solves these roadblocks and therefore is a milestone in the development of clinical CT imaging. The prototype we present allows to reconstruct the attenuation and dark-field channels of a human thorax phantom from a one second long acquisition and covers a 45 cm diameter field of view. In this work, we present how the first dark-field CT prototype works and focus particularly on the technical design, optimized design of the gratings for CT application and the first characterization of the interferometer in the rotating gantry. We discuss which steps where particularly important for the realization and where we see potential for further improvements. These results provide key insights for future dark-field CT implementations.
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