Investigation of the Ablation and Implosion Phases in 1 MA Wire Array Z-Pinches with UV and X-ray Diagnostics

Z pinches are a class of plasma configuration in which a large electrical current pulse magnetically compresses and confines a cylindrical plasma column. Z-Pinches are the most powerful laboratory producer of X-ray power and energy in the world. They are unstable and inhomogeneous plasma formation, and subject to strong instabilities. Plasma conditions during the ablation and implosion stages can determine the quality of the stagnating Z-Pinch and radioactive properties. New plasma diagnostics were fielded to study the ablation and implosion stages of the Z-Pinch. Experiments were performed using the 1 MA Zebra pulsed power generator and 50 TW Leopard laser at the Nevada Terawatt Facility and the University of Nevada, Reno. Ultraviolet (UV) laser diagnostics at the wavelength of 266 nm were applied to study the ablation and implosion stages of the wire array Z-Pinch. UV interferometry with an air-wedge differential interferometer was used to measures electron density during the ablation and implosion stage of a wire array Z-Pinch, and measured electron densities up to (1-3) x 1020 cm-3. Faraday rotation was used to measure magnetic fields and derive current distribution in the Z-Pinch during the ablation stage. X-ray imaging was also fielded to study the wire cores during the ablation stage. The high penetration of the X-rays allows the diagnostic to image the dense solid-liquid wire cores inside the ablating plasma columns in wire array Z-Pinches. Wire cores were backlit by silica He-α spectral line with a wavelength of 6.65 Å, and then imaging with a spherically bent quartz 1011 crystal. Fielding the X-ray imaging couple with UV shadowgraphy and interferometry allows for the study of Z-Pinch plasma in a wide range of electron density. X-ray absorption spectroscopy was used to study the electron temperature, ionization stage, and areal density of the plasma in Zebra-Leopard coupled shots. A single ray in aluminum star-like wire arrays was studied during the ablation stage. A samarium backlighting target was struck with the Leopard laser, producing a quasi-continuum emission of X-rays in the 8-9 Å range used for backlighting the wire ray. Two focusing conical spectrometers with mica crystals recorded absorption and reference spectra onto X-ray sensitive film. Absorption spectra was visible in the region of 8.2 - 8.4 Å. Electron temperature was determined using atomic kinetic codes and a two-temperature model of plasma

Whole-body x-ray dark-field radiography of a human cadaver

Background!#!Grating-based x-ray dark-field and phase-contrast imaging allow extracting information about refraction and small-angle scatter, beyond conventional attenuation. A step towards clinical translation has recently been achieved, allowing further investigation on humans.!##!Methods!#!After the ethics committee approval, we scanned the full body of a human cadaver in anterior-posterior orientation. Six measurements were stitched together to form the whole-body image. All radiographs were taken at a three-grating large-object x-ray dark-field scanner, each lasting about 40 s. Signal intensities of different anatomical regions were assessed. The magnitude of visibility reduction caused by beam hardening instead of small-angle scatter was analysed using different phantom materials. Maximal effective dose was 0.3 mSv for the abdomen.!##!Results!#!Combined attenuation and dark-field radiography are technically possible throughout a whole human body. High signal levels were found in several bony structures, foreign materials, and the lung. Signal levels were 0.25 ± 0.13 (mean ± standard deviation) for the lungs, 0.08 ± 0.06 for the bones, 0.023 ± 0.019 for soft tissue, and 0.30 ± 0.02 for an antibiotic bead chain. We found that phantom materials, which do not produce small-angle scatter, can generate a strong visibility reduction signal.!##!Conclusion!#!We acquired a whole-body x-ray dark-field radiograph of a human body in few minutes with an effective dose in a clinical acceptable range. Our findings suggest that the observed visibility reduction in the bone and metal is dominated by beam hardening and that the true dark-field signal in the lung is therefore much higher than that of the bone

Contrast-to-noise ratios and thickness-normalized, ventilation-dependent signal levels in dark-field and conventional in vivo thorax radiographs of two pigs

Lung tissue causes significant small-angle X-ray scattering, which can be visualized with grating-based X-ray dark-field imaging. Structural lung diseases alter alveolar microstructure, which often causes a dark-field signal decrease. The imaging method provides benefits for diagnosis of such diseases in small-animal models, and was successfully used on porcine and human lungs in a fringe-scanning setup. Micro- and macroscopic changes occur in the lung during breathing, but their individual effects on the dark-field signal are unknown. However, this information is important for quantitative medical evaluation of dark-field thorax radiographs. To estimate the effect of these changes on the dark-field signal during a clinical examination, we acquired in vivo dark-field chest radiographs of two pigs at three ventilation pressures. Pigs were used due to the high degree of similarity between porcine and human lungs. To analyze lung expansion separately, we acquired CT scans of both pigs at comparable posture and ventilation pressures. Segmentation, masking, and forward-projection of the CT datasets yielded maps of lung thickness and logarithmic lung attenuation signal in registration with the dark-field radiographs. Upon correlating this data, we discovered approximately linear relationships between the logarithmic dark-field signal and both projected quantities for all scans. Increasing ventilation pressure strongly decreased dark-field extinction coefficients, whereas the ratio of lung dark-field and attenuation signal changed only slightly. Furthermore, we investigated ratios of dark-field and attenuation noise levels at realistic signal levels via calculations and phantom measurements. Dark-field contrast-to-noise ratio (CNR) per lung height was 5 to 10% of the same quantity in attenuation. We conclude that better CNR performance in the dark-field modality is typically due to greater anatomical noise in the conventional radiograph. Given the high physiological similarity of human and porcine lungs, the presented thickness-normalized, ventilation-dependent values allow estimation of dark-field activity of human lungs of variable size and inspiration, which facilitates the design of suitable clinical imaging setups

Dark-field computed tomography reaches the human scale

X-ray computed tomography (CT) is one of the most commonly used three-dimensional medical imaging modalities today. It has been refined over several decades, with the most recent innovations including dual-energy and spectral photon-counting technologies. Nevertheless, it has been discovered that wave-optical contrast mechanisms—beyond the presently used X-ray attenuation—offer the potential of complementary information, particularly on otherwise unresolved tissue microstructure. One such approach is dark-field imaging, which has recently been introduced and already demonstrated significantly improved radiological benefit in small-animal models, especially for lung diseases. Until now, however, dark-field CT could not yet be translated to the human scale and has been restricted to benchtop and small-animal systems, with scan durations of several minutes or more. This is mainly because the adaption and upscaling to the mechanical complexity, speed, and size of a human CT scanner so far remained an unsolved challenge. Here, we now report the successful integration of a Talbot–Lau interferometer into a clinical CT gantry and present dark-field CT results of a human-sized anthropomorphic body phantom, reconstructed from a single rotation scan performed in 1 s. Moreover, we present our key hardware and software solutions to the previously unsolved roadblocks, which so far have kept dark-field CT from being translated from the optical bench into a rapidly rotating CT gantry, with all its associated challenges like vibrations, continuous rotation, and large field of view. This development enables clinical dark-field CT studies with human patients in the near future

Energy sensitive X-ray phase contrast imaging with a CdTe-Timepix3 detector

The Timepix3 is a photon counting semiconductor detector that enables to simultaneously measure the energy and time of arrival of each incident X- ray photon. These properties, along with the high spatial resolution and high efficiency, due to the CdTe sensor material, can be exploited for several imaging applications, such as X-ray phase contrast imaging (XPCI). XPCI relies on the phase shift suffered by X-rays when traversing the sample. This study focuses on the free-space propagation XPCI and single mask edge illumination XPCI methods, which are two approaches that are well suited for laboratory implementations. Since both techniques are highly sensitive to charge-sharing, the Timepix3 energy and time information for each photon are used to minimize this effect by using pixel clustering methods. In addition, the performance of both XPCI techniques across a 30kVp source spectrum is studied using the energy-resolving capabilities of the detector. In both cases, the phase contrast and signal-to-noise ratio (SNR) are assessed as a function of different energy. Finally, it is demonstrated that phase contrast enhancement is feasible with pixel clustering and energy-selection for both XPCI techniques