Optimisation of image quality and radiation dose in computed tomography using iterative image reconstruction

Background: Computed tomography (CT) is one of the most important diagnostic tools for medical imaging. CT is known as a high-dose modality and for producing images that allow for high diagnostic confidence. The challenge is to establish an optimal image quality for the specific diagnostic task with regard to the lowest radiation dose to the patient. Various technological advances have been introduced to reduce radiation dose in CT, such as iterative reconstruction (IR) of the images. Aims: The overall aim of this thesis is to investigate and evaluate IR algorithms in terms of image quality and radiation dose, and in clinical practise to evaluate the potential of a new model-based IR (MBIR) method for reconstructing paediatric abdominal CT examinations at a reduced radiation dose. Material and methods: The image quality for six IR algorithms in four CT systems of brain CT was evaluated with an image quality phantom using different radiation dose levels and iterative image optimisation levels (Paper I). Philips IR and MBIR were futher investigated for abdominal CT (Paper II) with regard to image quality and radiation dose. An image quality phantom and an anthropomorphic torso phantom were used to investigate how image quality depends on IR and MBIR variations with different radiation dose levels and tube voltages. Philips MBIR was investigated with thinner slice thickness and compared with standard slice thickness with IR for abdominal CT (Paper III). This study included investigation of image quality in both the image quality phantom and anthropomorphic phantom and one clinical case. An investigation of Philips MBIR was performed with paediatric abdominal CT involving a 32% lower radiation dose compared to the standard examination (Paper IV). Various approaches for subjective and objective evaluation of image quality were used and assessed. Results: In this thesis, the effects on image quality and radiation dose from the use of advanced reconstruction methods for CT was investigated in both phantoms and patients. The IR algorithms have different strengths and weaknesses, and in conclusion all IR algorithms improved the main part, or all of the image quality parameters compared to filtered back projection. For all IR algorithms, subjective image quality was improved. The objective image quality parameters; noise, signal-to-noise ratio, and contrast-to-noise ratio were all improved although to a variable degree. Spatial resolution was improved only for MBIR algorithms and GE IR (adaptive statistical iterative reconstruction). The IR algorithms improved image quality regardless of radiation dose. The MBIR algorithms improved image quality progressively with decreasing radiation dose but affected the image appearance, which can be attributed to a change in the noise power spectrum. For Philips MBIR (IMR), it is possible to use thinner slice thickness with maintained or even improved image quality. Furthermore, with Philips MBIR (IMR), it is possible to reduce the radiation dose and improve low-contrast resolution for low-contrast scanning in both brain and abdomen. A dose reduction of 32% with IMR in paediatric abdominal CT was possible without degradation of image quality. There is great potential to lower radiation dose by using IR and MBIR. Conclusion: The radiation dose can be reduced with new iterative reconstruction methods while maintaining or in some cases improving image quality compared to older reconstruction methods.