Dark-field imaging with x-rays is a novel interferometric technique that enables visualization of the alveolar structure. In the clinical context, the technique has been limited to radiographic applications and has not yet been implemented for computed tomography (CT) imaging. Initial studies have already demonstrated that dark-field imaging complements and improves conventional radiography of the thorax. Dark-field computed tomography, which is capable of yielding unobstructed 3D views, has only recently been brought to the human scale: With a first prototype system, we demonstrated the feasibility to implement a Talbot-Lau interferometer on a clinical CT system to perform dark-field imaging with clinical acquisition time and field of view. Until now, this prototype was limited to axial scans. In this work, we present our advancements in extending the setups capabilities to also support other modes of acquisition, namely surview and helical scans. The new capabilities of the updated dark-field CT scanner are demonstrated using an anthropomorphic thorax phantom.
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.
X-ray computed tomography (CT) is an invaluable imaging technique for non-invasive medical diagnosis. However, for soft tissue in the human body the inherent small difference in attenuation limits its significance. Grating-based X-ray phase-contrast is a relatively novel imaging method which detects additional interaction mechanisms between photons and matter, namely refraction and small-angle scattering, to generate additional images with different contrast. The experimental setup involves a Talbot-Lau interferometer whose susceptibility to mechanical vibrations hindered acquisition schemes suitable for clinical routine in the past. We present a processing pipeline to identify spatially and temporally variable fluctuations occurring in the first interferometer installed on a continuously rotating clinical CT gantry. The correlations of the vibrations in the modular grating setup are exploited to identify a small number of relevant vibration modes, allowing for an artifact-free reconstruction of a sample.
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