Multicontrast x-ray computed tomography imaging using Talbot-Lau interferometry without phase stepping

Abstract

Purpose: The purpose of this work is to demonstrate that multicontrast computed tomography (CT) imaging can be performed using a Talbot-Lau interferometer without phase stepping, thus allowing for an acquisition scheme like that used for standard absorption CT.

Methods: Rather than using phase stepping to extract refraction, small-angle scattering (SAS), and absorption signals, the two gratings of a Talbot-Lau interferometer were rotated slightly to generate a moiré pattern on the detector. A Fourier analysis of the moiré pattern was performed to obtain separate projection images of each of the three contrast signals, all from the same single-shot of x-ray exposure. After the signals were extracted from the detector data for all view angles, image reconstruction was performed to obtain absorption, refraction, and SAS CT images. A physical phantom was scanned to validate the proposed data acquisition method. The results were compared with a phantom scan using the standard phase stepping approach.

Results: The reconstruction of each contrast mechanism produced the expected results. Signal levels and contrasts match those obtained using the phase stepping technique.

Conclusions: Absorption, refraction, and SAS CT imaging can be achieved using the Talbot-Lau interferometer without the additional overhead of long scan time and phase stepping.

Figure 1

Schematic of the image extraction process. A two-dimensional Fourier transform (a) is applied to a single projection image. The resulting transform shows distinct peaks representing the DC and first harmonic terms. The position of the first harmonics is determined by the period of the moiré pattern and will be located at 1/p_m. The DC and first harmonic peaks are used to generate the refraction (b), small-angle scattering (c), and attenuation (d) projection images. Note that the refraction image (b) is differential in nature, as expected from Eq. 6.

Figure 2

CT reconstructions of the phase (a), small-angle scattering (b), and attenuation (c) contrast mechanisms. The top row shows axial slices from the center of the image volume, while the bottom row shows volume renderings in the plane of the wooden dowel and PTFE rod. The display range is [1.0, 5.5] × 10⁻⁷ for the phase image, [0.0, 0.015] mm⁻¹ for the absorption, and [0.01, 0.065] for the SAS. Note that the wooden dowel, which gives very little signal in the phase and attenuation images, is highly scattering and is easily seen in the SAS image. Also note that the PMMA rod, at the top of the volume renderings, is visible in the phase contrast image and not in the attenuation image.

Figure 3

CT reconstructions of the phase (a),(d); small-angle scattering (b),(e); and attenuation (c),(f) contrast mechanisms. The top row shows axial 100-slice averages from the moiré reconstructions. The bottom row shows axial 100-slice averages from the phase stepping reconstructions. Note the similarity between the two rows, indicating the ability of the moiré technique to faithfully extract each contrast mechanism. The display range is [1.0, 5.5] × 10⁻⁷ for the phase image, [0.0, 0.015] mm⁻¹ for the absorption, and [0.01, 0.065] for the SAS. Image (a) shows the lines over which the plots in Fig. 4 are shown.

Figure 4

Line plots of the lines drawn in Fig. 3a. In each plot, the dashed line is from the moiré dataset, while the solid line is from the phase stepping dataset. Plot (a) shows the level of the PMMA and POM cylinders. Plot (b) shows the PTFE and air cylinders. As expected from the images in Fig. 3, both image acquisition techniques show the same contrast levels for each material. Additionally, one can see from these plots that the in-plane spatial resolution is maintained for the moiré technique.