AUGMENTING SOFT TISSUE CONTRAST USING PHASE-CONTRAST TECHNIQUES IN MICRO-COMPUTED TOMOGRAPHY IMAGING

X-ray imaging and computed tomography is a widely used tool in clinics as well as in research applications. Micro-computed tomography (μCT) is used in research for visualization of cross-sectional slices of a sample as well as for reconstruction of data for 3D visualization. Conventional X-ray imaging is based on the attenuation of X-rays in the sample and the technique is referred to as absorption imaging. This technique provides useful information when the differences in attenuation coefficients is sufficient resulting in good contrast. On the other hand, low density samples such as soft tissues consist of various structures with similar densities, resulting in poor contrast in absorption images. This necessitates the use of a stain or phase imaging to enhance contrast in the sample during imaging. Staining has some limitation as not all samples can be stained, stains can be toxic, and staining can be time consuming. Therefore, new imaging techniques are required to image low density samples without the use of a stain. A promising imaging possibility on a μCT is the use of phase information of an X-ray beam to generate an image of the sample. This technique is known as phase-contrast imaging and it is based on the refraction of X-rays due to differences in refractive indices. The Zeiss Xradia MicroXCT-400 provides the possibility of using propagation-based phase-imaging. In this Master’s Thesis, the phase-contrast imaging protocol on the Zeiss Xradia MicroXCT400 was optimized using thin polylactic acid fibers in order to enhance the visibility of low density samples. The optimization consisted of source and detector distance, power, and voltage variation measurements. The projection images were analyzed for fiber visibility and contrast-to-noise ratios (CNR) were calculated. The results of the optimization were applied to two types of collagen samples embedded in air, ethanol, and water. The results of the analysis showed enhanced contrast for phase-contrast images compared to absorption images. Not all of the results were in alignment with theory, which is most likely due to the specifications and non-ideal operation of the X-ray source. The most important results indicated that the source does not need to placed at the negative limit to obtain useful phase information and the visibility increases with increasing sample-to-detector distance. For a magnification of 4x, the optimal detector distances, with sufficient CNRs and reasonable exposure time, for source distances of 160mm, 230mm, and 300mm are 80mm, 90mm, and 90mm, respectively. Finally, significantly enhanced contrast was obtained for the collagen sample embedded in water using phase-imaging techniques compared to absorption imaging. Even though enhanced contrast was obtained using phase-imaging techniques, the technique is limited due to the focal spot size and voltage of the X-ray source. As phase and absorption data cannot be separated, the final images are a combination of both. As a result, the optimal imaging settings are one which provide absorption information without a significant increase in noise as well as one which provides the edge enhancement effect typical for propagation-based phase-imaging. Further improvement of contrast and the use of phase information would require the application of phase retrieval on the raw projection data or the use of gratings to obtain phase contrast information.