Low-dose, simple, and fast grating-based X-ray phase-contrast imaging

Abstract

Phase sensitive X-ray imaging methods can provide substantially increased contrast over conventional absorption-based imaging and therefore new and otherwise inaccessible information. The use of gratings as optical elements in hard X-ray phase imaging overcomes some of the problems that have impaired the wider use of phase contrast in X-ray radiography and tomography. So far, to separate the phase information from other contributions detected with a grating interferometer, a phase-stepping approach has been considered, which implies the acquisition of multiple radiographic projections. Here we present an innovative, highly sensitive X-ray tomographic phase-contrast imaging approach based on grating interferometry, which extracts the phase-contrast signal without the need of phase stepping. Compared to the existing phase-stepping approach, the main advantages of this new method dubbed "reverse projection" are not only the significantly reduced delivered dose, without the degradation of the image quality, but also the much higher efficiency. The new technique sets the prerequisites for future fast and low-dose phase-contrast imaging methods, fundamental for imaging biological specimens and in vivo studies.

Fig. 1.

Working principle of the grating interferometer. (A) Through the Talbot effect, a periodic interference pattern is formed behind the phase grating (G1), in the plane of the analyzer grating (G2) (17). (B) Plot of the intensity oscillation (shifting curve) as a function of the grating position x_g for a detector pixel over one period of the analyzer grating. The dots correspond to the measured values (normalized to unit) while the gray line shows a sinusoidal fit.

Fig. 2.

Tomographic reconstructions of a demineralized mouse joint, acquired at a voxel size of 3.5 × 3.5 × 3.5 μm³. A1 to A3 show the data obtained with the phase-stepping (PS) protocol, while B1 to B3 the reconstruction using the reverse-projection (RP) method. A1 and B1 shows an axial slice: B1 is sharper than A1, and there are no ring artifacts. A2 and B2 depict a coronal slice through the joint, clearly showing that the RP protocol is less sensitive to typical horizontal stripes artifacts observed with the PS method (see enlarged inset). A3 and B3 show a sagittal view through the joint. The dotted lines mark the locations where the axial views (A1 and B1) have been taken. Scale bar, 500 microns.

Fig. 3.

Coronal slice of a rat brain, obtained after tomographic reconstruction using PS (A) and the RP (B) protocol. Qualitatively, both reconstructions are very similar. In B the effects of the grating imperfection (ring artifacts), as expected, are more evident. The plot in C shows a quantitative comparison between two line profiles extracted at the position marked by the color bars (hippocampus region). Scale bar, 1 mm.

Fig. 4.

Imaging of a rat paw. (A) Differential phase-contrast radiography (7 stacks, RP protocol), (B1) axial and (B2) coronal slices through the paw acquired with the PS protocol, (C1) axial and (C2) coronal slices through the same sample obtained with the RP protocol. Structural details of both soft tissue (muscles, fat) and hard tissue (bone) are well visible. Scale bars, 2 mm (A), 1 mm (B1–2 and C1–2).