Correction of streaking artifacts in CT images and its influence on Monte Carlo dose calculations

Introduction: Currently, treatment planning is done on the basis of CT (computed tomography) images from which a patient density matrix is determined. In conventional treatment planning the dose is calculated assuming patients are composed of water, at best considering electron densities. In comparison to that, in Monte Carlo Treatment Planning (MCTP), the CT image is segmented into a few materials (e.g. air, bone, tissue), and the dose is calculated taking into account these media. If there is an inaccuracy in the conversion of HU (Hounsfield units) to material type and density in the CT image, there might appear a significant dose miscalculation in both treatment planning methods. Streaking artifacts are one type of CT artifacts which are caused by very high attenuation of X-rays in some materials, such as metals. For example, between two metallic objects in a patient’s body there is practically no information about the patient’s geometry which can be detrimental for treatment planning. Such artifacts occur in prostate patients with two hip implants or for head and neck patients with tooth fillings. The latest approach of CT artifact correction deals with raw data sinograms and filling of missing projections corresponding to rays that passed through the highly attenuating media. In this work, a method for streaking artifact correction in sinogram space has been developed and the method was validated by dose calculation for MV photon beams. Materials and Methods: Three phantoms were used for the purpose of our study, as depicted in Fig. 1. All phantoms were filled with water and the pelvic phantom contained several materials of different contrasts. In CT images of all three phantoms, there is a significant image distortion due to streaking artifacts caused by the presence of the steel cylinders (Fig. 1).