X-ray attenuation models to account for beam hardening in computed tomography.

We introduce a beam-hardening correction method for lab-based X-ray computed tomography (CT) by modifying existing iterative tomographic reconstruction algorithms. Our method simplifies the standard Alvarez-Macovski X-ray attenuation model [Phys. Med. Biol.21, 733 (1976)] and is compatible with conventional (i.e., single-spectrum) CT scans. The sole modification involves a polychromatic projection operation, which is equivalent to applying a weighting that more closely matches the attenuation of polychromatic X-rays. Practicality is a priority, so we only require information about the X-ray spectrum and some constants relating to material properties. No other changes to the experimental setup or the iterative algorithms are necessary. Using reconstructions of simulations and several large experimental datasets, we show that this method is able to remove or reduce cupping, streaking, and other artefacts from X-ray beam hardening and improve the self-consistency of projected attenuation in CT. When the assumptions made in the simplifications are valid, the reconstructed tomogram can even be quantitative.

Nonparaxiality stabilizes three-dimensional soliton beams in Kerr media.

On the basis of simple physical arguments involving energy flows and power invariants, we show that nonparaxiality stabilizes (1+2D) soliton beams in Kerr media.

Polarization-domain solitary waves of circular symmetry in Kerr media.

We present a new class of dark circularly symmetric solitary wave in bulk, self-defocusing Kerr media. The waves comprise two orthogonally polarized fields mutually guiding each other and forming separate polarization domains. The stability of these new solutions and the dynamics of related structures are briefly investigated.

Collisions, steering, and guidance with spatial solitons.

By colliding two self-guided beams (solitons), we generate one, two, three, four, or possibly more stable solitons for a broad class of nonlinear material that spans threshold to power-law nonlinearities. This reproduction is shown to depend critically on beam stability, on the standard waveguide parameter V, and on a scaled angular beam separation theta/theta(c). The beams can also annihilate one another or create a stable beam from two unstable beams. Beam steering is possible by differentially changing soliton power or phase. Also, the colliding solitons induce versatile optical devices for the switching and steering of small-signal beams.