Performance evaluation of x-ray differential phase contrast computed tomography (PCT) with respect to medical imaging.

PURPOSE: To evaluate the maximum performance gain that can theoretically be achieved with differential phase contrast computed tomography (PCT) compared with absorption-based CT, applied to in vivo medical imaging.
METHODS: We develop a mathematical framework for analyzing the performance of PCT relative to CT under ideal conditions. We validate our model by interpreting the results of published PCT experiments. Finally, we utilize our framework to evaluate the relative performance of PCT versus CT for in vivo medical imaging of the human body, investigating several clinically relevant material contrasts.
RESULTS: We show that the performance of PCT relative to CT depends on the ratio of phase contrast and absorption contrast of the examined materials and increases with increasing effective coherence length and increasing spatial resolution. The introduced effective coherence length characterizes an experimental PCT setup; it comprises coherence of the beam as well as properties of the x-ray interferometer. Whole body medical CT will not benefit from phase-contrast imaging, because the higher phase contrast is overcompensated by the low coherence lengths of PCT setups with low-brilliance sources, and by limited spatial resolution. The relative performance of PCT, which is inferior to CT for all examined material contrasts at the resolution level of today's medical CT, can be improved by increasing spatial resolution at the expense of increased patient dose. At the break-even point of equal performance for PCT and CT, a radiation dose at least 1 order of magnitude higher than today is required. Mammographic CT already operates at higher spatial resolution and may benefit from PCT for some applications in terms of reduced patient dose at equal image quality.
CONCLUSIONS: Phase-contrast imaging utilizing low-brilliance x-ray sources has limited potential for an application in routine whole body CT. Breast CT, however, may benefit from phase-contrast imaging. These conclusions are due to fundamental arguments and independent of whether technical issues (quality of gratings, etc.) can be solved. PCT will only be suitable for in vivo medical imaging if x-ray sources with much better spatial coherence are routinely available.