Evaluation of phase retrieval approaches in magnified X-ray phase nano computerized tomography applied to bone tissue.

X-ray phase contrast imaging offers higher sensitivity compared to conventional X-ray attenuation imaging and can be simply implemented by propagation when using a partially coherent synchrotron beam. We address the phase retrieval in in-line phase nano-CT using multiple propagation distances. We derive a method which extends Paganin's single distance method and compare it to the contrast transfer function (CTF) approach in the case of a homogeneous object. The methods are applied to phase nano-CT data acquired at the voxel size of 30 nm (ID16A, ESRF, Grenoble, France). Our results show a gain in image quality in terms of the signal-to-noise ratio and spatial resolution when using four distances instead of one. The extended Paganin's method followed by an iterative refinement step provides the best reconstructions while the homogeneous CTF method delivers quasi comparable results for our data, even without refinement step.

Quantitative Assessment of Degenerative Cartilage and Subchondral Bony Lesions in a Preserved Cadaveric Knee: Propagation-Based Phase-Contrast CT Versus Conventional MRI and CT.

OBJECTIVE: The aim of this study was to quantitatively assess hyaline cartilage and subchondral bone conditions in a fully preserved cadaveric human knee joint using high-resolution x-ray propagation-based phase-contrast imaging (PBI) CT and to compare the performance of the new technique with conventional CT and MRI. MATERIALS AND METHODS: A cadaveric human knee was examined using an x-ray beam of 60 keV, a detector with a 90-mm2 FOV, and a pixel size of 46 × 46 µm2. PBI CT images were reconstructed with both the filtered back projection algorithm and the equally sloped tomography method. Conventional 3-T MRI and CT were also performed. Measurements of cartilage thickness, cartilage lesions, International Cartilage Repair Society scoring, and detection of subchondral bone changes were evaluated. Visual inspection of the specimen akin to arthroscopy was conducted and served as a standard of reference for lesion detection. RESULTS: Loss of cartilage height was visible on PBI CT and MRI. Quantification of cartilage thickness showed a strong correlation between the two modalities. Cartilage lesions appeared darker than the adjacent cartilage on PBI CT. PBI CT showed similar agreement to MRI for depicting cartilage substance defects or lesions compared with the visual inspection. The assessment of subchondral bone cysts showed moderate to strong agreement between PBI CT and CT. CONCLUSION: In contrast to the standard clinical methods of MRI and CT, PBI CT is able to simultaneously depict cartilage and bony changes at high resolution. Though still an experimental technique, PBI CT is a promising high-resolution imaging method to evaluate comprehensive changes of osteoarthritic disease in a clinical setting.

Dose fractionation in synchrotron radiation x-ray phase micro-tomography.

Phase sensitive x-ray imaging expands the applicability of standard attenuation based techniques by offering several orders of magnitude of increase in sensitivity. Due to the short wavelength, x-ray phase is not directly measurable, but has to be put in evidence by the use of phase contrast techniques. The phase can then be reconstructed from one or several phase contrast images. In this study, we consider synchrotron x-ray phase micro-computed tomography (µCT) based on free space propagation for heterogeneous and strongly absorbing objects. This technique generally relies on acquiring several scans of the sample at different detector distances. It is also generally believed that multi-distance phase µCT needs a higher dose input than single distance phase µCT. The purpose of this work is to study the impact of different means of dose fractionation on the reconstructed image quality. We define different acquistion schemes in multi-distance in-line phase µCT. Previously, the exposure time at each sample-to-detector distance was usually kept the same. Here, we let not only the number of distances vary but also the fraction of exposure time at each distance, the total exposure time being kept constant. Phase retrieval is performed with the mixed approach algorithm. The reconstructed µCT images are compared in terms of accuracy, precision and resolution. In addition, we also compare the result of dose fractionated multi distance phase µCT to single distance phase µCT using the same total radiation dose. In the multi-distance approach, we find that using different exposure times on each distance improves the image quality in the reconstructed image. Further, we show that, despite having the same total dose delivery, the multi distance imaging method gives better image quality than the single distance method, at the cost of an additional overhead from camera displacements and reference images. We show that by optimizing the acquistion parameters in terms of number of distances and exposure time at each distance, the resulting image quality can be improved. This means that for a desired image quality, a lower radiation dose can be used. This is important especially in high resolution imaging where the radiation dose used for imaging can be very large, potentially damaging the sample. Based on the acquired data, we define an optimal protocol for use in together with the heterogeneous object mixed approach.

Cartilage and soft tissue imaging using X-rays: propagation-based phase-contrast computed tomography of the human knee in comparison with clinical imaging techniques and histology.

OBJECTIVES: This study evaluates high-resolution tomographic x-ray phase-contrast imaging in whole human knee joints for the depiction of soft tissue with emphasis on hyaline cartilage. The method is compared with conventional computed tomography (CT), synchrotron radiation absorption-based CT, and magnetic resonance imaging (MRI). MATERIAL AND METHODS: After approval of the institutional review board, 2 cadaveric human knees were examined at an synchrotron institution using a monochromatic x-ray beam of 60 keV, a detector with a 90-mm field of view, and a pixel size of 46 × 46 µm. Images of phase-contrast imaging CT were reconstructed with the filtered back projection algorithm and the equally sloped tomography method. Image quality and tissue contrast were evaluated and compared in all modalities and with histology. RESULTS: Phase-contrast imaging provides visualization of altered cartilage regions invisible in absorption CT with simultaneous high detail of the underlying bony abnormalities. The delineation of surface changes is similar to 3-T MRI using cartilage-dedicated sequences. Phase-contrast imaging CT presents soft tissue contrast surpassing that of conventional CT with a clear discrimination of ligamentous, muscular, neural, and vascular structures. In addition, phase-contrast imaging images show cartilage and meniscal calcifications that are not perceptible on conventional CT or on MRI. CONCLUSIONS: Phase-contrast imaging CT may facilitate a more complete evaluation of the human knee joint by providing concurrent comprehensive information about cartilage, the underlying subchondral bone, and their changes in osteoarthritic conditions.