The valence shell excitations of argon were investigated by an angle-resolved fast-electron energy-loss spectrometer at an incident electron energy of 2500 eV, and the transition multipolarities for the excitations of 3p → 3d, 4d, 5s, and 5p were elucidated with the help of the calculated intermediate coupling coefficients using the COWAN code. The generalized oscillator strengths for the excitations to 3p 5 ͑3d ,3dЈ͒, 3p 5 ͑5p ,5pЈ͒, and 3p 5 ͑5s ,4d͒ were measured, and the profiles of these generalized oscillator strength were analyzed. Furthermore, although the present experimental positions of the maxima for the electric-monopole and electricquadrupole excitations in 3p → 5p are in agreement with the theoretical calculations ͓Amusia et al., Phys. Rev. A 67, 022703 ͑2003͔͒, the generalized oscillator strength profiles show obvious differences. In addition, the experimental generalized oscillator strength ratios for the electric-octupole transitions in 3p → 3d are different from the theoretical prediction calculated by the COWAN code.

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Purpose: The image quality of dedicated cone beam breast CT (CBBCT) is limited by substantial scatter contamination, resulting in cupping artifacts and contrast-loss in reconstructed images. Such effects obscure the visibility of soft-tissue lesions and calcifications, which hinders breast cancer detection and diagnosis. In this work, we propose a library-based software approach to suppress scatter on CBBCT images with high efficiency, accuracy, and reliability. Methods: The authors precompute a scatter library on simplified breast models with different sizes using the 4-based Monte Carlo (MC) toolkit. The breast is approximated as a semiellipsoid with homogeneous glandular/adipose tissue mixture. For scatter correction on real clinical data, the authors estimate the breast size from a first-pass breast CT reconstruction and then select the corresponding scatter distribution from the library. The selected scatter distribution from simplified breast models is spatially translated to match the projection data from the clinical scan and is subtracted from the measured projection for effective scatter correction. The method performance was evaluated using 15 sets of patient data, with a wide range of breast sizes representing about 95% of general population. Spatial nonuniformity (SNU) and contrast to signal deviation ratio (CDR) were used as metrics for evaluation. Results: Since the time-consuming MC simulation for library generation is precomputed, the authors' method efficiently corrects for scatter with minimal processing time. Furthermore, the authors find that a scatter library on a simple breast model with only one input parameter, i.e., the breast diameter, sufficiently guarantees improvements in SNU and CDR. For the 15 clinical datasets, the authors' method reduces the average SNU from 7.14% to 2.47% in coronal views and from 10.14% to 3.02% in sagittal views. On average, the CDR is improved by a factor of 1.49 in coronal views and 2.12 in sagittal views. Conclusions: The library-based scatter correction does not require increase in radiation dose or hardware modifications, and it improves over the existing methods on implementation simplicity and computational efficiency. As demonstrated through patient studies, the authors' approach is effective and stable, and is therefore clinically attractive for CBBCT imaging. C 2016 American Association of Physicists in Medicine. [http://dx

The inelastic x-ray scattering technique has been used to study the excitation mechanism of atomic helium in this work. The squares of the form factor for the single excitations of 1s 2 → 1snl (n 6) of helium have been determined by the high resolution (70 meV) inelastic x-ray scattering. The present squares of the form factor measured by inelastic x-ray scattering provide an independent cross-check to the ones measured by high-energy electron energy loss spectroscopy. It is found that the squares of the form factor for the single excitations of 1s 2 → 1snl (n 6) of helium measured by inelastic x-ray scattering are in excellent agreement with the ones measured by high-energy electron energy loss spectroscopy, which confirms the quality of these two experimental methods. The excellent agreement of the present measurements with the theoretical calculations also gives a rigorous test of the theoretical method. This work demonstrates that inelastic x-ray scattering is a powerful tool to study the excitation mechanism in atomic or molecular systems at the third-generation synchrotron radiation light source.

X-ray cone-beam computed tomography (CBCT) is widely used nowadays, mainly for its large volume coverage and hardware compatibility with open-gantry x-ray imaging systems. As the size of x-ray illumination increases, an inevitable and adverse effect is the boost of scatter contamination on the x-ray images, which becomes one of the fundamental limitations of CBCT imaging. The large scatter signals in CBCT cause severe streaking and cupping artifacts in the CT images and greatly hamper the applications of CBCT due to its degraded image quality as compared to that of the conventional x-ray CT scanner. Research on scatter correction has gained heated attention in recent years. In this review, we first analyze the magnitudes of scatter in CBCT and its resultant errors in the reconstructed images. The existing CBCT scatter correction methods are then summarized in several categories: pre-processing methods, and post-processing methods including measurement-based, software-based, hardware-based decomposition and hybrid methods. An important issue related to the post-processing methods, the noise increase in the scatter corrected images, is also discussed. Although numerous scatter correction methods have been proposed in the literature, each approach has its own strengths and drawbacks and an optimal and standard method is still elusive. This review provides a comprehensive summary of the current research on scatter correction, and suggests future directions from the authors' perspective.