Computed Thomography is a relatively new field in the area of non destructive imaging.It allows to reconstruct the internal structure of opaque objects without destroy them. This is a great advantage compared to conventional microscopy techniques, any optical or electronic microscope, in fact, provides information on the internal structure of samples only if samples are properly processed and sectioned. Information about three-dimensional structure could be obtained by the image of a surface or a combination of several thin slices, but in both cases information cannot be certain since methods of cutting and preparation can dramatically change the structure of the sample. Microcomputed tomography, commonly referred to as µCT, like conventional computed tomography is based on the collection of projections of X rays through a specimen and the application of tomographic principles to reconstruct the 3-D structure of the specimen. Itis based on the interaction of X-rays with matter. The attenuation ofX-rays, passingthrough an object, is dependent on thedensity and atomic number of the object under investigation. This radiation is converted in a radiographic image of the object. Images obtained from different angles are analyzed by analgorithm called Filter back projection in order to reconstruct a virtual slice through the object. When different consecutive slices are reconstructed, a 3D visualizationcan be obtained.
The term "micro" denotes a scanning system much higher in resolution than conventional clinical scanners. Clinical tomographic scanners may have resolutions on the order of a millimeter or less. However, high-resolution µCT scanners may have resolutions below five microns.
The high resolution of this system makes it useful in the analysis of small objects such as trabecular bone samples. Trabecularbone consists of a complicated three-dimensional network of plates and rods, arranged ina lattice-like network.The architectural parameters of trabecular bone could be strongly influenced by aging or bone diseases such as osteoarthritis or osteoporosis.
Until recently, information about thesestructural parameters of trabecular bone were only available by histomorphometry, adestructive procedure limited to two-dimensional analysis. Nowadays Micro-CT, because of its capability to allow three-dimensional and non destructive analysis, found largeapplications in pre-clinical bone research.The increasing incidence and prevalence of bone pathologies on the population, increases the interest of improve an accurate bone characterization by Micro-CT. Micro-CT system, object of this study is the Skyscan 1072, located at the Technology and Health Department of the Italian National Institute of Health.One of the goal of this research is set at optimizing the system for the analysis of bone samples.
The first part is dedicated on determining the resolution of the system. The performance of an imaging system is usually described by the measurement of its Modulation Transfer Function or MTF whichgives a description of how much contrast at a specific spatial frequency is maintained by the imaging process.The second part of this study is focused on the process of images reconstruction, fundamental in a Micro-CT analysis. Micro-CT images are affected by several artifacts which will be widely discussed in the following chapters. One of the most difficult artifact is beam hardening. It depends on the polychromatic X-ray tube used in these systems. The X-rays beam investing the sample is composed of X-rays with a spectrum of different energies. The attenuation of an X-ray depends on its energy, the lowestX-ray energies are preferentially absorbed. Assuming that the grey level of CT images corresponds to the linear coefficientof attenuation, which is constants depending on the material, because of the beam hardening, the attenuationof a given material is not strictly proportional to its thickness. This implies visual distortions on the images and the consequent origin of quantitative problems. In order to better understand the effect of beam hardening on Micro-CT images, the filtered back projection algorithm will be implemented in LabVIEW (Version 8.2). The Skyscan 1072 allows to correct the effect of beam hardening during the process of images reconstruction by the definition of a proper parameter. In order to define the correct value of this parameter for a bone sample analysis, a comparison between the results of both the algorithm implemented and the Skyscan reconstruction software will be evaluated.
After the optimization of the system for bone analysis, nineteen trabecular bone samples, extracted from femoral heads of eight patients subject to a hip arthroplasty surgery, will be analyzed. The main problem of bone analysis by micro-CT is the processing of the reconstructed cross-sections images for the sample morphometric analysis. The post-processing of the images for the morphometric characterization usually requires a process named binarization of the images which consists on the definition of a threshold value of grey-level, necessary to distinguish bone from background. The choice of this value is a crucial task since a standard method doesn’t exist. Moreover, the inhomogeneity of bone causes another problem during the binarization process. Binarization associates each pixel of the image to bone or air, not considering that each pixel can be composed by both of them. This effect is called Partial Volume Effect and it affects especially pixels at the edges of the analyzed sample. In order to avoidproblems related to the binarization, the main goal of this study is the evaluation of a new method for the histomorphometric analysis of bone sample from the direct processing of the greylevel histogram of the images.
Finally, the last part of this research will be dedicated on the remodeling process of bone. The remodeling of bone is an important research topic because of its importance in the study of bone pathologies such as osteoporosis. Osteoporosis is a bone disorder characterized by an inadequate amount and faulty structure of bone, resulting in fractures from relatively minor trauma. It leads to a bone mineral density (BMD) reduction, a bone microarchitecture deterioration and an alteration of the amount and variety of proteins in bone. Aging is the main factor of osteoporosis incidence but in the last years, another factor related to long-duration spaceflight, has been considered. Because of the difficult in reproducing in-vivo space conditions, the development of numerical models is a good alternative for the remodeling process study
