Pseudo-CT Generation for External Radiotherapy Planning from Magnetic Resonance Imaging Data on Brain Regions

Background: Nowadays, medical imaging has an important role in radiotherapy and treatment planning process. Despite the increasing usage of the magnetic resonance imaging (MRI) in the external radiotherapy (RT) design process, computed tomography (CT) remains a basic imaging modality in radiotherapy because of its relation with electron density value. In conventional external radiotherapy, MRI is used in functional tissue structures with registration on the CT image, which causes systematic errors during the registration of MRI and CT images. Objectives: The main purpose of this paper was to investigate the possibility of removing the CT simulator and replacing it with pseudo-CT for the electron density calculation in radiotherapy treatment planning. Materials and Methods: The pseudo-CT images were generated for 10 randomly chosen patients with brain disease. The data consisted of image voxels chosen within the segmented area of the brain in both MRI and CT images. The relationship between electron density information in CT images and MRI intensity value was derived from simple polynomial models. Results: It was found that the MRI intensity value is related to the Hounsfield unit (HU) value within different parts including the skull bone, sinus, and brain in CT images. The mean errors of the conversion model were 0.7479, 0.334 and -0.043 HU in the brain, skull, and air regions respectively, with the simple polynomial model in the upper slice. Conclusion: The proposed method generated pseudo-CT images from different segmented parts of the brain of MRI series. The application requires segmentation of the cortical bones in MR images. The result indicated that finding a complex model does not necessarily lead to achieving a better result.