Determining the optimal choice of attenuation filters and propagation distance for polychromatic phase-contrast micro-computed tomography of a multi-material electromotor using synchrotron radiation.

Optimizing phase-contrast micro-computed tomography (µCT) for a given object is not trivial if the radiation is polychromatic and the object multi-material. This study demonstrates how an optimal combination of propagation distance and mean energy (set by attenuation filters) may be derived for such an object (an electromotor scanned on beamline BM18 at ESRF in Grenoble, France). In addition to appropriate image quality metrics, it is mandatory to define a task. In that respect, raising E_mean from 100 keV to 164 keV mitigates beam hardening by metal parts, yet raising E_mean further to 230 keV deteriorates CNR² (where CNR is contrast-to-noise ratio) due to higher image noise. Propagation distances between d = 2 m and 25.3 m are evaluated crosswise with energy. While longer propagation distances generally yield higher CNR², shorter distances appear favorable when discerning plastic near metal parts. SNR² (where SNR is signal-to-noise ratio) power spectra and modulation transfer (MTF) are evaluated independently from two-dimensional projections supporting volume image analysis for which image sharpness depends strongly on the digital filters (Paganin and Wiener) which are applied along with filtered back-projection. In summary, optimizing synchrotron µCT scans remains a very complex task which differs from object to object. A physically accurate model of the complete imaging process may not only allow for optimization by simulation but also ideally improve CT image reconstruction in the near future.