Dieter Hahn

Quantitative X-ray phase-contrast computed tomography at 82 keV

Potential applications of grating-based X-ray phase-contrast imaging are investigated in various fields due to its compatibility with laboratory X-ray sources. So far the method was mainly restricted to X-ray energies below 40 keV, which is too low to examine dense or thick objects, but a routine operation at higher energies is on the brink of realisation. In this study, imaging results obtained at 82 keV are presented. These comprise a test object consisting of well-defined materials for a quantitative analysis and a tooth to translate the findings to a biomedical sample. Measured linear attenuation coefficients ? and electron densities ?e are in good agreement with theoretical values. Improved contrast-to-noise ratios were found in phase contrast compared to attenuation contrast. The combination of both contrast modalities further enables to simultaneously assess information on density and composition of materials with effective atomic numbers Z? > 8. In our biomedical example, we demonstrate the possibility to detect differences in mass density and calcium concentration within teeth.

Statistical iterative reconstruction algorithm for X-ray phase-contrast CT

Grating-based phase-contrast computed tomography (PCCT) is a promising imaging tool on the horizon for pre-clinical and clinical applications. Until now PCCT has been plagued by strong artifacts when dense materials like bones are present. In this paper, we present a new statistical iterative reconstruction algorithm which overcomes this limitation. It makes use of the fact that an X-ray interferometer provides a conventional absorption as well as a dark-field signal in addition to the phase-contrast signal. The method is based on a statistical iterative reconstruction algorithm utilizing maximum-a-posteriori principles and integrating the statistical properties of the raw data as well as information of dense objects gained from the absorption signal. Reconstruction of a pre-clinical mouse scan illustrates that artifacts caused by bones are significantly reduced and image quality is improved when employing our approach. Especially small structures, which are usually lost because of streaks, are recovered in our results. In comparison with the current state-of-the-art algorithms our approach provides significantly improved image quality with respect to quantitative and qualitative results. In summary, we expect that our new statistical iterative reconstruction method to increase the general usability of PCCT imaging for medical diagnosis apart from applications focused solely on soft tissue visualization.

Grating-based X-ray Dark-field Computed Tomography of Living Mice

Changes in x-ray attenuating tissue caused by lung disorders like emphysema or fibrosis are subtle and thus only resolved by high-resolution computed tomography (CT). The structural reorganization, however, is of strong influence for lung function. Dark-field CT (DFCT), based on small-angle scattering of x-rays, reveals such structural changes even at resolutions coarser than the pulmonary network and thus provides access to their anatomical distribution. In this proof-of-concept study we present x-ray in vivo DFCTs of lungs of a healthy, an emphysematous and a fibrotic mouse. The tomographies show excellent depiction of the distribution of structural – and thus indirectly functional – changes in lung parenchyma, on single-modality slices in dark field as well as on multimodal fusion images. Therefore, we anticipate numerous applications of DFCT in diagnostic lung imaging. We introduce a scatter-based Hounsfield Unit (sHU) scale to facilitate comparability of scans. In this newly defined sHU scale, the pathophysiological changes by emphysema and fibrosis cause a shift towards lower numbers, compared to healthy lung tissue.

Evaluation of the potential of phase-contrast computed tomography for improved visualization of cancerous human liver tissue

PURPOSE Phase-contrast X-ray computed tomography (PCCT) is currently investigated and developed as a potentially very interesting extension of conventional CT, and can offer several advantages for specific indications in diagnostic imaging. Current absorption-based computed tomography (CT) without the application of contrast material is limited in the detection of minor density differences in soft-tissue. The purpose of this study is to test whether PCCT can improve soft tissue contrast in healthy and tumorous human liver specimens. MATERIALS AND METHODS Two specimens of human liver (one healthy and one metastasized liver sample) were imaged with brilliant X-ray beam at the synchrotron radiation source ESRF in Grenoble, France. For correlation the same specimens were imaged with a magnetic resonance imaging system at 1.5 T. The histopathology confirmed our findings in the corresponding sections of the specimens. RESULTS In the phase-contrast CT images we observed a significantly enhanced soft-tissue contrast when compared to simultaneously recorded standard absorption CT measurements. Further, we found that the pathological and morphological information in the PCCT reconstructions show significant improvement when compared to those performed on MRI. Based on matching of prominent features, a good correlation between PCCT and the histological section is demonstrated; especially the tumor capsule and the surrounding vascular structures are visible in PCCT. In addition, our study revealed the ability of PCCT to visualize the blood vessels structure in the tumorous liver without the need of any contrast agents. CONCLUSION Grating-based PCCT significantly improves the soft-tissue contrast in ex-vivo liver specimens and holds the potential to overcome the need of contrast materials for visualization of the tumor vascularization.

Simulated Cystic Renal Lesions: Quantitative X-ray Phase-Contrast CT—An in Vitro Phantom Study

PURPOSE To determine if grating-based x-ray phase-contrast computed tomography (CT) can allow differentiation of simulated simple, protein-rich, hemorrhagic, and enhancing cystic renal lesions in an in vitro phantom. MATERIALS AND METHODS An in vitro phantom specifically designed to simulate simple, protein-rich, hemorrhagic, and enhancing renal cysts was scanned with an experimental grating-based phase-contrast CT setup consisting of a Talbot-Lau interferometer with a rotating anode x-ray tube and a single photon counting detector. Various combinations of serum and saline (100% and 0% to 0% and 100%), blood and saline, blood and serum (100% and 0% to 6.25% and 93.75% for both), and an iodinated contrast agent and saline (7.6-1.6 mg per milliliter of saline) were used to reproduce the chemical composition of the different types of cysts. A thickened solution of an iodinated contrast agent calibrated with a clinical multidetector CT scanner served as contrast agent-enhanced renal parenchyma (195 HU at 80 kVp, 400 mAs and 98 HU at 140 kVp, 200 mAs). Standard attenuation- and phase-contrast images were reconstructed from the raw projection data. Quantitative values for attenuation and phase contrast and image noise were determined. Contrast-to-noise ratios were calculated. Simulated lesions were assessed for visual differentiability by means of pairwise comparison of the attenuation- and phase-contrast images and both images simultaneously. RESULTS Attenuation-contrast imaging showed large differences in Hounsfield units with increasing concentrations of iodine (118.9 HU for 1.6 mg/mL vs 331.4 HU for 7.6 mg/mL). Values for phase-contrast imaging were substantially distinguishable for saline, serum, and blood (7.9, 23.7, and 52.8 HU, respectively). Both imaging modalities combined allowed differentiation of all four types of simulated cysts (100% saline, 100% serum, 100% blood, and 1.6-7.6 mg of iodine per milliliter of saline) with one imaging acquisition. CONCLUSION Grating-based phase-contrast CT allows differentiation of simulated simple, protein-rich, hemorrhagic, and enhancing renal cysts in an in vitro phantom through simultaneous assessment of their distinct attenuation- and phase-contrast signal.

Numerical comparison of x-ray differential phase contrast and attenuation contrast

We present a numerical tool to compare directly the contrast-to-noise-ratio (CNR) of the attenuation- and differential phase-contrast signals available from grating-based X-ray imaging for single radiographs. The attenuation projection is differentiated to bring it into a modality comparable to the differential phase projection using a Gaussian derivative filter. A Relative Contrast Gain (RCG) is then defined as the ratio of the CNR of image values in a region of interest (ROI) in the differential phase projection to the CNR of image values in the same ROI in the differential attenuation projection. We apply the method on experimental data of human breast tissue acquired using a grating interferometer to compare the two contrast modes for two regions of interest differing in the type of tissue. Our results indicate that the proposed method can be used as a local estimate of the spatial distribution of the ratio δ/β, i.e., real and imaginary part of the complex refractive index, across a sample.

TH‐A‐213CD‐04: A Bone Artifact Reduction Algorithm for Differential Phase‐Contrast CT Based On Statistical Iterative Reconstruction

Purpose: The purpose of this work is a reduction of the influence of dense materials, e.g. bones, on the reconstruction of grating‐based differential phase‐contrast computed tomography (PCCT). These strongly absorbing materials introduce streaking artifacts, in look and feel similar to metal artifacts in conventional absorption CT.Methods: An iterative reconstruction algorithm, which statistically models the differential phase‐ contrastimaging process, was developed. It allows for individually introducing a statistical weight for each sinogram pixel, i.e. its influence on the resulting reconstruction. In grating‐based imaging the absorption and phase‐contrast signals are simultaneously retrieved from the same set of raw projection images and are thus inherently registered. This can be utilized for getting information on the position of highly dense materials, e.g. bones, from the absorption signal. The statistical weights are then modified according to this information to reduce the influence of sinogram pixels that contain dense material and are likely to cause artifacts. This concept was applied to a tomographic dataset of a mouse acquired with a gratinginterferometer at the European Synchrotron Radiation Facility in Grenoble, France. Reconstructed with a standard filtered backprojection, the sample dataset suffers from very strong streaking artifacts in the region surrounding the spine. Detail contrast in the soft tissue around the spine is lost due to these streaks. Results: Comparing reconstructions performed with a standard filtered backprojection and with the newly developed algorithm shows a significant reduction of streaking artifacts and a strong improvement in soft tissue contrast in regions influenced by dense material.Conclusions: This work significantly expands the potential of grating‐based differential phase‐contrast computed tomography. It is now possible to offer the high soft‐tissue contrast of phase‐contrast imaging in cases where dense materials such as bones, are present. D.H., P.T., M.B. and F.P. acknowledge financial support through the DFG Cluster of Excellence Munich‐Centre for Advanced Photonics and the European Research Council (FP7, Starting grant #240142).

Improved diagnostic differentiation of renal cystic lesions with phase-contrast computed tomography (PCCT)

The diagnostic quality of phase-contrast computed tomography (PCCT) is one the unexplored areas in medical imaging; at the same time, it seems to offer the opportunity as a fast and highly sensitive diagnostic tool. Conventional computed tomography (CT) has had an enormous impact on medicine, while it is limited in soft-tissue contrast. One example that portrays this challenge is the differentiation between benign and malignant renal cysts. In this work we report on a feasibility study to determine the usefulness of PCCT in differentiation of renal cysts. A renal phantom was imaged with a grating-based PCCT system consisting of a standard rotating anode x-ray tube (40 kV, 70 mA) and a Pilatus II photoncounting detector (pixel size: 172 μm). The phantom is composed of a renal equivalent soft-tissue and cystic lesions grouped in non-enhancing cyst and hemorrhage series and an iodine enhancing series. The acquired projection images (absorption and phase-contrast) are reconstructed with a standard filtered backprojection algorithm. For evaluation both reconstructions are compared in respect to contrast-to-noise ratio (CNR), signal-to-noise ratio (SNR), and subjective image quality. We found that with PCCT a significantly improved differentiation between hemorrhage renal cysts from contrast enhancing malignant cysts is possible. If comparing PCCT and CT with respect to CNR and SNR, PCCT shows significant improvements. In conclusion, PCCT has the potential to improve the diagnostics and characterization of renal cysts without using any contrast agents. These results in combination with a non-synchrotron setup indicate a future paradigm shift in diagnostic computed tomography.

SU‐E‐I‐162: Quantitative Analysis of Human Soft Tissue Using Grating‐Based X‐ Ray Phase Contrast

Purpose: In the last decade, grating‐based x‐ray phase‐contrast imaging was developed as a novel and very promising imaging modality. It is demonstrated to not only significantly increase the soft tissuecontrast compared to conventional absorption‐ based imaging, but also to provide quantitative information about the tissue. In this work we report on the results of our study to determine quantitative CT numbers in phase contrast (corresponding to Hounsfield Units (HU) in absorption CT) for various healthy and cancerous human soft tissue, and to correlate these numbers to the HUs in absorption contrast. Methods: The study concentrates on two types of human soft tissue, breast and livertissue, as the imaging of theses tissue types is already a great demand on the conventional absorption CT. The specimens are fixated in formalin and analyzed tomographically with grating‐based imaging at a synchrotron radiationsource. This benchmarking study used monochromatic synchrotron radiation to ensure a high accuracy of the measured values. Results: Our study shows that the grating‐based imaging modality significantly enhances the soft‐tissue contrast and allows clearly distinguishing between healthy and canceroustissue. From the gray values the CT numbers can be calculated directly by averaging over different regions and geometrical corrections, as the values are measured relative to those of water. Conclusions: With this work we demonstrate the potential of grating‐based phase‐contrast imaging to determine quantitative, tissue specific values for human soft tissue. We strongly believe that these results will enhance the quality of the tumor detection in both, human breast and liver, as the tissue‐specific values can directly be used for numerical simulations and optimization of the setup, as well as for phase‐contrast imaging dedicated phantoms design.