Scattered radiation in cone beam computed tomography : analysis, quantification and compensation

Autor: Wiegert, Jens
Přispěvatelé: Aach, Til
Jazyk: angličtina
Rok vydání: 2007
Předmět:
Zdroj: Aachen : Publikationsserver der RWTH Aachen University XIII, 232 S. : Ill., graph. Darst. (2007). = Aachen, Techn. Hochsch., Diss., 2007
Popis: For imaging during minimally-invasive treatment in the so-called catheter laboratory conventional X-ray projection imaging is classically used. Particularly in cardio-vascular angiography and neuroradiology, so-called C-arm systems are used, enabling a flexible positioning of X-ray tube and detector. In the last years, these systems experienced the most important technical innovation with the introduction of 3D imaging functionality by means of cone-beam computed tomography (CBCT). With this technique a large number of projections is acquired during a rotation of the C-arm around the patient. Afterwards, these projections are reconstructed to volumetric images using algorithms similar to those used in classical computed tomography. The objective of current research is to improve the 3D image quality in order to extend the imaging capability to high quality low contrast imaging with C-arm X-ray systems. In this context, this thesis addresses the problem of scattered radiation. Because in CBCT with large area X-ray detectors the irradiated patient volume is substantially larger than in classical computed tomography, also the amount of scattered radiation reaching the detector is significantly larger and can even be superior to the amount of primary radiation. Therefore, scattered radiation is a major source of image degradation and nonlinearity in flat-detector based CBCT and is the most severe cause of inhomogeneity artifacts in reconstructed images. The primary objectives of this thesis are the detailed quantitative analysis of scattered radiation, the assessment of existing scatter compensation methods as well as the development of new effective methods for the reduction of scatter induced artifacts. After an introduction to the physical and algorithmic principles of CBCT in the first part of the thesis, at first a detailed quantitative analysis of the characteristics of scattered radiation in projections of CBCT is undertaken. This analysis is based on the advancements of a Monte-Carlo CBCT simulator allowing to study realistic and clinically relevant patient geometries obtained from real data sets of conventional computed tomography. With this method practically noise free reference data sets for typical measurement objects such as the head, thorax and pelvis region are generated that allow to exactly study the influence of scattered radiation and that are used in the course of the thesis for the assessment of the various methods for scatter compensation. Subsequently, the impact of scattered radiation on the reconstructed volume is quantitatively studied. For this purpose, and as one of the key contributions of this thesis, a mathematical description of the propagation of the most relevant image quality characteristics, signal, contrast, and noise from the projections into the reconstructed volume is derived. Based on this description and based on the well known Feldkamp-algorithm, new reconstruction algorithms are developed that – instead of the usual CT Hounsfield values – allow for reconstruction of the respective image quality feature, ie, voxel-wise inhomogeneities, voxel-wise decrease of object contrast, and voxel-wise standard deviations of the noise. Using the developed analysis method and based on the created reference data sets a comprehensive study of anti-scatter grids as the classical method of scatter suppression reveals that the quality of anti-scatter grids available for X-ray flat-detectors is not sufficient in order to effectively suppress scatter induced artifacts. Additionally, the investigation shows that usage of strongly scatter reducing anti-scatter grids has a negative impact on the signal-to-noise ratio. Therefore, in order to provide the desired image quality in low-contrast CBCT, it is essential to correct for scatter contained in the projections by means of software-based a-posteriori methods. In literature, however, so far no practical methods can be found. Therefore – as second important contribution – in this thesis a number of new scatter compensation methods have been developed. These can be grouped in four different classes: post-processing techniques performed in 3D reconstructed images, methods using model based Monte-Carlo simulations, methods based on single scatter estimation schemes, and iterative methods using artifact evaluation and feedback schemes. All correction methods are comparatively validated using the clinical reference data sets. It is shown that especially exploitation of both available data domains, the planar projection data and the 3D information, allows for combating the large scatter background present in this application and to meet the demanding accuracy requirements to achieve the expected image quality in CBCT.
Databáze: OpenAIRE
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