Revising the lower statistical limit of x-ray grating-based phase-contrast computed tomography.

Mathias Marschner,Franz Pfeiffer,Julia Herzen,Peter B Noël,Marian Willner,Lorenz Birnbacher,Michael Chabior,Martin Bech

doi:10.1371/journal.pone.0184217

Abstract

Phase-contrast x-ray computed tomography (PCCT) is currently investigated as an interesting extension of conventional CT, providing high soft-tissue contrast even if examining weakly absorbing specimen. Until now, the potential for dose reduction was thought to be limited compared to attenuation CT, since meaningful phase retrieval fails for scans with very low photon counts when using the conventional phase retrieval method via phase stepping. In this work, we examine the statistical behaviour of the reverse projection method, an alternative phase retrieval approach and compare the results to the conventional phase retrieval technique. We investigate the noise levels in the projections as well as the image quality and quantitative accuracy of the reconstructed tomographic volumes. The results of our study show that this method performs better in a low-dose scenario than the conventional phase retrieval approach, resulting in lower noise levels, enhanced image quality and more accurate quantitative values. Overall, we demonstrate that the lower statistical limit of the phase stepping procedure as proposed by recent literature does not apply to this alternative phase retrieval technique. However, further development is necessary to overcome experimental challenges posed by this method which would enable mainstream or even clinical application of PCCT.

Highlights

Limited soft-tissue contrast is a major limitation of x-ray computed tomography (CT), a widely used clinical imaging modality
We evaluate a scan with long exposure, i.e. high photon counts, to test the accuracy of the reverse projection (RP) method, thereby investigating its applicability to tomographic scans of biological soft-tissue
It is clear that the error that is introduced due to the linear approximation of the sinusoidal phase stepping curve depends on the value of the differential phase-contrast signal

Summary

Introduction

Limited soft-tissue contrast is a major limitation of x-ray computed tomography (CT), a widely used clinical imaging modality. One way to address this shortcoming is to employ phase-sensitive imaging methods, which access the real part of the refractive index of the measured object [1,2,3,4,5,6,7]. The refractive index decrement leads to a phase shift of x-rays when passing through an object. The cross-section of this interaction is orders of magnitude higher in comparison to the cross-section of the attenuation interaction [8]. When this information can be used for imaging, increased contrast is reached.

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Discussion

Conclusion