Purpose: We investigate the feasibility of slot-scan dual-energy x-ray absorptiometry (DXA) on robotic x-ray platforms capable of synchronized source and detector translation. This novel approach will enhance the capabilities of such platforms to include quantitative assessment of bone quality using areal bone mineral density (aBMD), normally obtained only with a dedicated DXA scanner. Methods: We performed simulation studies of a robotized x-ray platform that enables fast linear translation of the x-ray source and flat-panel detector (FPD) to execute slot-scan dual-energy (DE) imaging of the entire spine. Two consecutive translations are performed to acquire the low-energy (LE, 80 kVp) and high-energy (HE, 120 kVp) data in <15 sec total time. The slot views are corrected with convolution-based scatter estimation and backprojected to yield tiled long-length LE and HE radiographs. Projection-based DE decomposition is applied to the tiled radiographs to yield (i) aBMD measurements in bone, and (ii) adipose content measurement in bone-free regions. The feasibility of achieving accurate aBMD estimates was assessed using a high-fidelity simulation framework with a digital body phantom and a realistic bone model covering a clinically relevant range of mineral densities. Experiments examined the effects of slot size (1 – 20 cm), scatter correction, and patient size/adipose content (waist circumference: 77 – 95 cm) on the accuracy and reproducibility of aBMD. Results: The proposed combination of backprojection-based tiling of the slot views and DE decomposition yielded bone density maps of the spine that were free of any apparent distortions. The x-ray scatter increased with slot width, leading to aBMD errors ranging from 0.2 g/cm2 for a 5 cm slot to 0.7 g/cm2 for a 20 cm slot when no scatter correction was applied. The convolution-based correction reduced the aBMD error to within 0.02 g/cm2 for slot widths <10 cm. Reproducible aBMD measurements across a range of body sizes (aBMD variability <0.1 g/cm2) were achieved by applying a calibration based on DE adipose thickness estimates from peripheral body sites. Conclusion: The feasibility of accurate and reproducible aBMD measurements on an FPD-based x-ray platform was demonstrated using DE slot scan trajectories, backprojection-domain decomposition, scatter correction, and adipose precorrection.
Measurements of skeletal geometries are a crucial tool for the assessment of pathologies in orthopedics. Usually, those measurements are performed in conventional 2-D X-ray images. Due to the cone-beam geometry of most commercially available X-ray systems, effects like magnification and distortion are inevitable and may impede the precision of the orthopedic measurements. In particular measurements of angles, axes, and lengths in spine or limb acquisitions would benefit from a true 1-to-1 mapping without any distortion or magnification.
In this work, we developed a model to quantify these effects for realistic patient sizes and clinically relevant acquisition procedures. Moreover, we compared the current state-of-the-art technique for the imaging of length- extended radiographs, e. g. for spine or leg acquisitions (i. e. the source-tilt technique) with a slot-scanning method. To validate our model we conducted several experiments with physical as well as anthropomorphic phantoms, which turned out to be in good agreement with our model. To this end, we found, that the images acquired with the reconstruction-based slot-scanning technique comprise no magnification or distortion. This would allow precise measurements directly on images without considering calibration objects, which might be beneficial for the quality and workflow efficiency of orthopedic applications.
The acquisition time of cone-beam CT (CBCT) systems is limited by different technical constraints. One important factor is the mechanical stability of the system components, especially when using C-arm or robotic systems. This leads to the fact that today’s acquisition protocols are performed at a system speed, where geometrical reproducibility can be guaranteed. However, from an application point of view faster acquisition times are useful since the time for breath-holding or being restraint in a static position has direct impact on patient comfort and image quality. Moreover, for certain applications, like imaging of extremities, a higher resolution might offer additional diagnostic value. In this work, we show that it is possible to intentionally exceed the conventional acquisition limits by accepting geometrical inaccuracies. To compensate deviations from the assumed scanning trajectory, a marker-free auto-focus method based on the gray-level histogram entropy was developed and evaluated. First experiments on a modified twin-robotic X-ray system (Multitom Rax, Siemens Healthcare GmbH, Erlangen, Germany) show that the acquisition time could be reduced from 14 s down to 9 s, while maintaining the same high-level image quality. In addition to that, by using optimized acquisition protocols, ultra-high-resolution imaging techniques become accessible.
Digital breast tomosynthesis (DBT) is a three-dimensional (3-D) X-ray imaging modality that allows the breast to be viewed in a 3-D format, minimizing the effect of overlapping breast tissue. DBT is commonly known for its high in-plane spatial resolution allowing to detect very small structures inside the breast which makes it a powerful tool in the clinical environment. However, since DBT is a limited angle tomography, artifacts are inevitable. In this paper, we investigate the influence of the angular scanning range as well as the slice thickness, i. e. the distance between two adjacent slices, on the in-plane spatial resolution of calcifications and present an analytic model to describe the imaging process. For the validation of the analytic model, 54 datasets with varying calcification diameter, slice thickness, and angular scanning range, were used and compared to a ray-casting simulation. It could be shown that the relative mean error between the analytic model and the generated ground truth over all datatsets is ε- = 0.0137. The results indicate that both investigated parameters affect the in-plane spatial resolutio
The upsurge in interest of digital tomosynthesis is mainly caused by breast imaging; however, it finds more and more attention in orthopedic imaging as well. Offering a superior in-plane resolution compared to CT imaging and the additional depth information compared to conventional 2-D X-ray images, tomosynthesis may be an interesting complement to the other two imaging modalities. Additionally, a tomosynthesis scan is likely to be faster and the radiation dose is considerably below that of a CT. Usually, a tomosynthetic acquisition focuses only on one body part as the common acquisition techniques restrict the field-of-view. We propose a method which is able to perform full-body acquisitions with a standard X-ray system by shifting source and detector simultaneously in parallel planes without the need to calibrate the system beforehand. Furthermore, a novel aliasing filter is introduced which addresses the impact of the non-isotropic resolution during the reconstruction. We provide images obtained by filtered as well as unfiltered backprojection and discuss the influence of the scanning angle as well as the reconstruction filter on the reconstructed images. We found from the experiments that our method shows promising results especially for the imaging of anatomical structures which are usually obscured by each other since the depth resolution allows to distinguish between these structures. Additionally, as of the high isotropic in-plane spatial resolution of the tomographic volume, it is easily possible to perform precise measurements which are a crucial task, e. g. during the planning of orthopedic surgeries or the assessment of pathologies like scoliosis or subtle fractures.