A. Quantitative phase-contrast micro-CT

Three methods have been developed for quantitative in-line phase-contrast micro-CT: (1) phase retrieval with a single distance propagation-based phase-contrast computed tomography (PPCT) data, (2) phase retrieval with two distances PPCT data, and (3) phase retrieval based on phase-attenuation duality (PAD), respectively [4-6]. Performance of different single sample-to-detector distance (SDD) phase retrieval algorithms was studied [7].

An approach was developed to determine the absorption correction factor (ACF) accurately for the Modified Bronnikov algorithm (MBA) to direct reconstruct 3D-refractive index from single SDD X-ray PPCT data [1]. It takes into account the wavelength, SDD and phase-attenuation duality of PPCT. The method was evaluated with a sample of nylon, polystyrene and PMMA fibers. Fig. 3(a) shows the reconstructed result, Fig. 3(b) shows histogram of Fig. 3(a), and the results indicate that three materials are well distinguished.

Fig. 3.Full-screen View

Reconstructed result of the wire sample (a) and its histogram (b).

Compared with phase retrieval with a single distance, phase retrieval with two SDD images can improve the precision apparently in quantitative X-ray in-line phase contrast imaging. However, a critical point that determines the retrieval efficiency of this method is optimization of SDD z₁ and z₂. We investigated the problem systematically based on computer simulation and related experiments. The results show that the highest retrieving precision could be obtained while z₂=3z₁, as shown in Fig. 4 [5].

Fig. 4.Full-screen View

Retrieving error vs. Z₂, with Z₁ being set to (a) 50 cm, (b) 100 cm, (c) 120 cm and (d) 150 cm.

Extending validity of the phase-attenuation duality (PAD) method, the PPCT at 60 keV of synchrotron radiation for a sample with both light and dense components was investigated. The results show that the PAD-based PPCT is effective in imaging simultaneously the weak and strong absorption components. Compared with the direct PPCT technique, the PAD-based PPCT technique demonstrates its excellent capability in material discrimination and characterization (Fig. 5) [6].

Fig. 5.Full-screen View

(Color online) Reconstructions and histograms of electron density distribution of samples by direct (a) and PAD-based (b) PPCT at 60 keV.

B. Spiral micro-CT

The maximum beam size of BL13W1 is 45 mm×5 mm. For a long sample, it is difficult to obtain 3D images in one single CT scan. SR-based spiral micro-CT (SRS micro-CT) is able to obtain volumetric projection data of a long sample in one continuous scan, which is advantageous in its short acquisition time, high spatial resolution and less moving artifacts. An SRS micro-CT system has been built on BL13W1 [8]. SRS micro-CT of bamboo was performed. The 3D image and slices were of high spatial resolution, as shown in Fig. 6. The length of scanned bamboo was 10.54 mm, which is 2.73 times of vertical beam size. The inner fiber structures can be seen clearly.

Fig. 6.Full-screen View

(Color online) SRS micro-CT reconstructed images of bamboo.

C. X-ray Fluorescence CT

X-ray fluorescence computed tomography (XFCT) allows non-destructive reconstruction of elemental distribution in a sample. The XFCT system was established and experiment results were obtained [9, 10].

In this system, horizontal scanning geometry is used, with an energy-sensitive detector coupled to a multichannel analyzer to register the fluorescence signals simultaneously. A water-cooled X-ray CCD camera is used for direct imaging and alignment purposes. The XFCT system has been opened to public service. XFCT was applied to the reconstruction of a naturally dried cirrhotic liver of the Sprague-Dawley (SD) rat to investigate the distributions of Fe and Zn [11] (Fig. 7). Recently, ordered-subsets expectation maximization (OSEM) algorithm has been introduced into XFCT to speed up the data acquisition process. We are now studying accelerating X-ray fluorescence CT based fast scanning and OSEM (Fig. 8).

Fig. 7.Full-screen View

(Color online) Fe and Zn distributions in the liver of an SD rat.

Fig. 8.Full-screen View

(Color online) Cd distribution in test sample with different algorithm Left, filtered back projection; right, OSEM.

D. Local micro-CT

An improved algorithm, pseudo-global tomography, is proposed to solve the local CT reconstructing problem. In order to compensate the lost part of projections in the local CT, an effective strategy is applied to reduce DC-shift and low-frequency artifacts caused by projection truncation. With the new algorithm, one is able to reconstruct the correct inner structure of the sample among all the region of interest (ROI) areas regardless of the location of the ROI in the sample without prior knowledge of it, as shown in Fig. 9 [12].

Fig. 9.Full-screen View

(Color online) Experimental results of a phantom. (a) the global CT slice for the whole sample; (b) & (d), the ROIs taken from the global reconstruction with 258×258 pixels; (c) & (e), ROIs reconstructed by Pseudo-global tomography; (f) & (g) the profiles (yellow lines).

E. Grating-based differential phase-contrast imaging

For low-Z materials, X-ray grating-based differential phase contrast imaging is of higher sensitivity than conventional X-ray imaging [13]. A set of X-ray grating-based imaging system, with one phase grating and one transmission grating, was installed on this beamline. Using the double-grating system, the complex coherence factor was measured and the transverse coherence length and source size were analyzed [14]. The reconstructed slices of absorption coefficient, decrement of the real part of refractive index and scattering coefficient of a mouse brain were obtained (Fig. 10) [15]. Without any staining, various tissues, such as caudate putamen (CPu), lateral ventricle (LV) and corpus callosum (cc), were distinguished in the reconstructed slices.

Fig. 10.Full-screen View

Reconstructed slices of absorption coefficient, real part decrement of refractive index and scattering coefficient of a mouse brain.

F. Fast micro-CT based on EST

In order to reduce the absorbed dose and speed up the data collection time for micro-CT, we developed a low dose and fast imaging system based on equally sloped tomography (EST). In traditional filtered back projection (FBP)-based micro-CT, a large number of projections are needed to ensure high spatial resolution, which results in long exposure time and high absorbed dose.

EST-based micro-CT can be applied to solve these problems. In method, the projections are equally sloped angled, not equally spatial angled. The reconstruction results of a cement material by FBP from 1000 projections and EST from 250 projections are shown in Fig. 11. It can be seen that a comparable image quality is achieved by EST with only 1/4 of the projections by FBP. Based on EST, In vivo micro-CT with 75% dose reduction and high pressure micro-CT with incomplete angle range of projections (samples in the diamond-anvil cell) have been realized on BL13W1. An In vivo micro-CT experiment for a living insect was realized in 4 s. The radiation dose on the sample was 1.69 Gy and the dose absorbed by the insect was 0.67 Gy. If we used equal angle tomographic method such as the FBP algorithm, at least 720 projections would be needed for this case, and the radiation dose would be 3.6 times higher. The EST-based micro-CT imaging system will open to users soon.

Fig. 11.Full-screen View

(Color online) A comparison between the reconstruction results of FBP and EST.

G. Multi-energy micro-CT combined with DCM

A data-constrained modelling (DCM) method with multi-energy CT data sets was developed for characterizing sub-resolution compositional microstructures, enabling one to obtain quantitative compositional fractions on each image voxel. The approach has been successfully applied to characterize and predict the microstructure of a range of different materials, including distributions of zinc corrosion product and mineral phases in hydrocarbon reservoir limestone [16, 17]. Microscopic 3D chemical distributions of a corroded zinc wire are shown in Fig. 12. This result is important for understanding the cathodic and anodic processes that occur during corrosion, and is vital when attempting to control the overall rate of corrosion. Also, 3D distributions of dolomite and calcite in a limestone sample were obtained.

Fig. 12.Full-screen View

(Color online) DCM-predicted microstructures: a cross section of a corroded zinc wire. (a) Zn, (b) ZnO, (c) the compositional volume fractions. (d) 3D compositional image.

H. Local and remote fast micro-CT reconstruction

Fast CT reconstruction system was established at BL13W1. The CT reconstruction can be completed in 5 min, and remote data processing and transfer is realized. The system includes two parts: GPU parallel computing server and CT reconstruction software. The configuration of Nvidia Tesla C2050 with 448 GPU cores is 112 times faster than a 4-core CPU. The fast CT reconstruction software X-TRACT SSRF was developed by international cooperation of SSRF with CSIRO of Australia. Now it opens to users with about 100 registered users. These users can visit the website https://ct.ssrf.ac.cn, and download the client software X-TRACT SSRF.

I. Ring artifacts correction for CT reconstruction

Caused by non-uniform response of detector elements and instabilities of light source, ring artifacts in CT slices greatly influence three-dimensional reconstruction and quantitative analysis. A correction algorithm based on polar-coordinate transform and Fourier transform is proposed for elimination of the ring artifacts [18]. Experimental results indicate that this algorithm not only eliminates the ring artifacts effectively, but also keeps the image edges and increases the signal-to-noise ratio (Fig. 13). The ring artifacts correction software for micro-CT reconstruction is based on the correction algorithm, it has been developed and opened to the users.

Fig. 13.Full-screen View

Fossil slices before (a) and after (b) ring artifacts correction.

A. Biomedicine

BL13W1, of large beam size, high energy level and high coherence, achieves high resolution imaging for inner microstructure of soft tissues and low Z materials, and dynamic imaging for living animals in biomedicine. Users have carried out researches in different areas, including tumour diagnosis and treatment, nervous system, imaging methodology of phase contrast, safety of synchrotron radiation-based medical imaging, biomedical materials, scar of dermal tissue, acupuncture points, pharmaceutics etc [19-27].

Synchrotron radiation angiography provides a novel solution to directly monitor the success of middle cerebral artery occlusion. Twenty adult SD rats for middle cerebral artery occlusion models with different suture head silicone coating were prepared randomly, and the In vivo imaging experiment was performed on BL13W1 [28]. Silicone-coated suture is superior to uncoated suture for producing consistent brain infarction. Additionally, silicone coating length is an important variable controlling the extent of ischemic lesion: infarcts affected predominantly the caudate-putamen with large variability (<2 mm), both the cortex and caudate-putamen (2–3.3 mm), and most of the hemisphere, including the hypothalamus (>3.3 mm), as shown in Fig. 14.

Fig. 14.Full-screen View

Synchrotron radiation angiography images of middle cerebral artery (MCA) occlusion using silicone coated sutures with different coating lengths. The MCA was only partially occluded when the suture head was not coated (A) or the coating length was <2 mm (B). The MCA was successfully occluded when the coating length was 2.0 mm to 3.3 mm without affecting the hypothalamic artery (HTA), posterior cerebral artery (PCA), and anterior choroidal artery (AChA; C). The MCA was occluded when the coating length >2 mm together with AChA/PCA, and HTA (D).

TIEG1 (TGFβ inducible early gene 1) can induce apoptosis of cancer cells, but its role in inhibiting invasion and metastasis has not been reported and is unclear. The decreased TIEG1 expression is associated with increased EGFR (epidermal growth factor receptor) expression in breast cancer tissues and cell lines [29]. TIEG1 significantly inhibits breast cancer cell invasion, suppresses mammary tumorigenesis in xenografts in mice, and decreases lung metastasis by inhibition of EGFR gene transcription and the EGFR signaling pathway. The microangiography experiment for blood vessels was performed at BL13W1. The results show that TIEG1 binds the EGFR promoter to inhibitor its transcription, hence the reduction of VEGF and the neovascularization (Fig. 15).

Fig. 15.Full-screen View

Synchrotron radiation angiography images of blood vessels and quantification of microvessel density of tumor sections in the mice bearing MDA-MB-231HM/TIEG1, MDA-MB-231HM/Vector, and MDA-MB-231 cells.

The regenerative treatment of large osseous defects remains a formidable challenge in orthopedic surgery today. Particularly, strategies to control the appropriate degradation rate adapting to the tissue reconstruction are essential for tissue regeneration. The osteoinductive growth factor, recombinant human bone morphogenetic protein-2 (rhBMP-2) was incorporated into calcium/magnesium-doped silica-based scaffolds [30], disulfide-containing PEG-based scaffolds [31], calcium silicate/calcium phosphate cement scaffolds [32], gelatin sponges based implant loaded with nanoparticle [33] for bioactivity and biodegradation promotion. The in vivo effects of scaffolds in rabbit radius or femur critical defect model were investigated by synchrotron radiation-based micro-computed tomography (SR micro-CT) imaging. The results indicate that the scaffolds underwent gradual resorption and replacement by new bone and induced reunion of bone marrow cavity in 12 weeks, much better than the effect of self-repairing group (Fig. 16). Both osteoinduction and appropriate degradation played a crucial role in accelerating and promoting bone augmentation and effective proangiogenesis.

Fig. 16.Full-screen View

3D reconstruction of cross-section images by SR-μCT at week 4 and 12 after scaffolds implant in the rabbit femur.

B. Archaeology and Paleontology

In archaeology, to protect important culture heritage, noninvasive characterization of 3D CT can acquire high-resolution inner structure of a sample. Eye bead, an article of western Asia style, is an evidence of East-West culture communication during the Warring State Period (475BC-221BC), but its production technology needs exploration. SR-μCT was used for the first time to obtain 3D images of an eye bead samples (Fig. 17). The ablate air bubble was found that will not appear in the normal glass phase, and the inlay production technology of eye bead was testified according to micro analysis [34]. By using scanning electron microscope and SR-μCT, crustacean fossils mainly focusing on a few phosphatocopines with nearly complete preservation of stalked eyes, antennula and limbs were examined successfully [35].

Fig. 17.Full-screen View

(Color online) (a) View of sample from the perforation, (b) one transverse section with outlined white layers, (c) 3D reconstruction of a spherical air bubble in the glass phase, (d) 3D reconstruction of an oblate air bubble located at the boundary of a horned eye and glaze.

C. Materials science

BL13W1 provides hard X-rays of 8–72.5 keV to reveal inner microstructure of the samples nondestructively. And its high flux ensures observation of dynamic process of materials structure. Users performed studies on dendrite growth behaviour in the solidification process of metallic alloy [36, 37], hydrogen evolution in the electrochemistry reaction [38], intermetallic compound (IMC) growth behaviour and evolution rule of the bubbles in soldering process at the brazing interface [39, 40], etc. SR micro-CT can be used for characterizing the 3D structure of materials, such as 3D dendrite structure of alloy [41], composites and their distribution of cement in the different hydratation stages, the influence of foaming technology on the porous structure of polymer [42], the change of volume and density for materials in extreme condition of high pressure. Fast micro-CT based on EST or CS reconstruction algorithms can realize the evolution researches of 3D structures for materials.

Combining synchrotron-based computed tomography with scanning electron microscopy-ultramicrotomy, the distribution of micrometer-sized inorganic particles within a polymer matrix was studied [43]. It was found that the inorganic particles distributed in a polymeric matrix can form independently, but interpenetrate clusters of varying size and fractal dimension with the largest fractal dimension of 2.36 (Fig. 18).

Fig. 18.Full-screen View

(Color online) Embedded Fractal Clusters: (a) section through composite containing SrCrO₄ (red) embedded in an epoxy matrix with two densities nominated as low density or LDE (green) and normal (blue); (b) 3D reconstruction of LDE (partially cut away) and all the SrCrO₄ particles; (c) 3D reconstruction of LDE (partially cut away) and the five largest SrCrO₄ clusters; (d) View of all clusters (rendered in different colours) with the largest cluster in blue. A, B, C indicate three different regions of the largest cluster; (e) entanglement of clusters: if tension is applied to the materials then the clusters provide a mechanical resistance improving the shear properties.

Microstructure evolution of metal-ceramic materials Al-SiC was observed (Fig. 19) and related sintering kinetics parameters were obtained, different mechanisms of ceramic and metallic materials with microwave-dielectric loss and eddy current losses were analyzed. Further analysis of the mechanism of interfacial polarization and micro-focus were carried out. The experimental results [44] help to reveal the mixed-interaction mechanisms of microwave sintering of metal-ceramic materials and offer some significant references for actual production.

Fig. 19.Full-screen View

(Color online) 3D structure images of SiC-Al in microwave sintering.

Using a special tensile testing device, high-resolution in situ observations showed the fracture process for randomly oriented short carbon fiber/epoxy (SCF/EP) composites. Based on this method, the response to mechanical load of two samples containing untreated SCFs or oxidation-treated SCFs were compared (Fig. 20). An increase of about 6% in the proportion of broken fibers to other types of damaged fiber in the sample with oxidation-treated fibers, was observed. Also, the oxidation treatment reduced the ineffective lengths of the composite fibers by about 20%, hence the improved mechanical properties of the composites [45].

Fig. 20.Full-screen View

(Color online) SR-CT 3D images of SCF/EP samples. Sample 1, 15% SCFs (untreated); Sample 2, 15% SCFs treated by oxidation.

D. Pharmaceuticals

Structure of a pharmaceutical plays an important role in its function and effectiveness. Osmotic drug delivery systems (ODDS), delivering active agents by constant inner osmotic pressure, provide desired drug concentrations at the absorption site, maintaining the plasma drug level within the therapeutic range for a period of time, reducing the dosing frequency. The dynamic internal structural change in felodipine release process was investigated, and the remaining tablet core was correlated well with the percentages of felodipine (Fig. 21) [46, 47]. This provides the new insight to the structure-based design of ODDS. SR micro-CT was also used to monitor blend homogeneity of binary mixtures composed of microcrystalline cellulose and starch [48].

Fig. 21.Full-screen View

(Color online) The 3D structure and quantitative analysis of controlled release kinetics of monolithic osmotic pump tablets.

E. Phytology

Identification of traditional Chinese medicines (TCMs) is an important way for quality control of TCMs. Compared with conventional microscopic identification methods, synchrotron radiation X-ray phase contrast micro-tomography (SR-XPCMT) provides clearer and finer microstructures of TCMs with better spatial and temporal resolutions nondestructively. Many investigations on different kinds of TCMs have been done since 2009 [49-51], including root and rhizome, fruit, flower, leaf, and so on. Fig. 22 shows 3D structure of a part of main root of a wild ginseng and volume distribution of the calcium oxalate cluster crystals. The distribution, quantity and volume of the calcium oxalate cluster crystals were obtained. The results show that SR-XPCMT is promising methods for identifying TCMs, especially for valuable and difficult-slicing samples. Due to its nondestructive naturer, SR-XPCMT provides the feasibility of quantitative study on microstructures of TCMs. In addition, the complex structure of bamboo nodes was revealed by the synchrotron X-ray microtomography [52] and the cavitation and water-refilling processes in plants were observed with X-ray phase contrast microscopy [53].

Fig. 22.Full-screen View

(Color online) (a) 3D structure of a part of the main root of the wild ginseng; (b) 3D distribution of calcium oxalate cluster crystals; (c) Volume distribution of calcium oxalate cluster crystals of the wild ginseng main root, where Vi is the distribution volume of calcium oxalate crystals with different size of Ai and V_sum is the total volume of all calcium oxalate crystals.

F. Other fields

In space science, ground calibration with high energy X-rays of BL13W1 was performed for the Gamma-Ray Burst polarimeter-POLAR, which will be the first space instrument for precise polarization measurement of Gamma-Ray Burst. Moreover, the X-ray radiation dose effects of typical optoelectronic and large-scaled processor for 20–70 keV aerospace X-rays was studied to provide reference data of ionization radiation effect for typical large-scaled processor for aerospace.

Researches in environmental science, physics, energy resources, and industrial applications were carried out on this beamline. The 3D structural characteristics and aggregation processes of farmland soil aggregates were studied, and the results revealed that different ways of fertilization in a long period could change morphology of the soil aggregate [54]. The compaction process of rods under tapping using CT method was researched to testify that the corresponding relaxation time versus tapping intensity follows an Arrhenius behaviour, which is reminiscent of the slow dynamics in thermal glassy systems [55]. The physical structure of Yangquan coal sample was characterized and the 3D visualization of compositions and volume fractions was realized. The results are relevant in quantitative modelling of coal-bed gas transportation and coal processing [56]. In the industrial applications, with 55 keV X-ray of BL13W1, 3D internal geometry of stainless steel diesel nozzle was obtained. This research has significant effect on improving the design of nozzle size and structure, and increase of the combustion efficiency [57].