Improving contrast and spatial resolution in crystal analyzer-based x-ray dark-field imaging: Theoretical considerations and experimental demonstration.

PURPOSE: This paper describes and experimentally validates a methodology for improving contrast and spatial resolution of the x-ray dark-field imaging (XDFI) by cutting the monochromator-collimator asymmetrically and thinning the Laue angle analyzer. METHODS: We measure the spatial resolution of our XDFI setup using a test object consisting of wolfram tungsten meshes and compare it with the theoretical prediction. Using x-ray dark-field computed tomography of breast cancer specimens (lobular carcinoma in situ), we demonstrate that the resolution of XDFI is sufficient for histopathologic analysis. RESULTS: Our experimental results show that the overall spatial resolution of XDFI can be improved by approximately a factor of 2 when these modifications are implemented. The reconstructed images of breast cancer specimens provide sufficient details for radiologic histopathology. CONCLUSIONS: By cutting the monochromator-collimator and thinning the Laue angle analyzer, XDFI can achieve the resolution sufficient for radiologic histopathology.

Reconstruction of the acoustic impedance profile in a plate using an inverse spectral procedure.

An inverse spectral procedure was applied to reconstruct the acoustic impedance profile along the thickness direction of a plate using its thickness resonance frequencies, density and thickness. For a successful reconstruction, the material-property profile must be symmetric about the mid-plane of the plate. Several cases of numerical simulations, including plates with a few layers and with a high number of layers are described. The calculated resonance frequencies were used to reconstruct the acoustic impedance profile, a process that was successful for all cases. We assume that a plate with a high number of layers, each with a different but constant acoustic impedance, simulates a plate with a smoothly varying acoustic impedance profile. It can be concluded that such a plate, which generates small, virtually undetectable, internally reflected waves, can also be reconstructed. In the special case of a plate of unknown thickness and unknown but constant density, the method is still useful, because a relative variation of the material property can be reconstructed using only the resonance frequencies. An experiment using a resonance-mode electromagnetic acoustic transducer (resonance-mode EMAT) is also described. EMAT is a non-contact ultrasonic method that can measure thickness resonance frequencies, making it appropriate for this method. Some examples of applications are measurement of the temperature profile inside a rolled metal sheet, measurement of a clad metal plate, and monitoring of a metal casting.

Measurement of velocity variations along a wave path in the through-thickness direction in a plate.

In this paper there is given a method to predict ultrasonic wave velocity variations along a wave path in the through-thickness direction in a plate from thickness resonance spectra. Thickness resonance spectra are numerically calculated and two simple rules used to predict the entire ultrasonic wave velocity variation are derived. In the calculation, the wave path is assumed to be straight along the thickness direction and the velocity variation is assumed to be either as a parabolic curve dependence or a linear dependence with respect to the distance from the surface and to be symmetric with respect to the plate center. To see if the numerical calculation method is reliable, thickness resonance frequencies of a sample with three-layers were measured by EMAT (electromagnetic acoustic transducer) with a good agreement between the measured and the calculated frequencies. This method can be applied to the ultrasonic measurement of material characteristics, internal stress or various other properties of plate materials.