The Influence of Patient Positioning and Immobilization Equipment on MR Image Quality and The Accuracy of MR-CT Image Registration

Introduction: CT scan as gold standard image of RT is taken by fixation equipment that is an essential requirement in radiotherapy (RT) for the reproducibility of patient positioning in RT sessions. MRI is used in combination with CT image in RT due to the variety of imaging protocols and high soft tissue contrast. However, used special coils in MRI do not allow to fix the patient like CT scan in conventional diagnostic setup. The purpose of this study was to evaluate the performance of the fixation base designed for MR-simulation brain scanning. We evaluated the image quality and the accuracy of CT-MRI image registration of the RT setup in comparison with a diagnostic setup.Materials and methods: CT and MR images of ۱۱ patients with brain tumor that have thermoplastic mask and head rest as fixation equipment have been used in this study. T۱w and T۲w MR scanning protocol were done with the MR head coil (radiology setup (RL)) and with ۶ channels body matrix coil (RT setup). For MR image quality assessment, signal to noise ratio (SNR) was calculated based on dedicated regions in MR images. The influence of fixation devices on CT-MRI rigid registration has also been evaluated based on line profile and external contour methods.Results: SNR reduction in T۱w MR images is more than T۲w images. The max of SNR reduction is acquired in posterior tissues with amount of ۱۸.۳۳% and ۱۵.۵۹% for T۱w and T۲w MR images, respectively. Mean error in rigid registration of CT and RL MR images was obtained ۴ mm that reduce to ۱.۰۲ mm with fixation equipment in RT MRI imaging. DSC of external contour was obtained ۸۳.۸۵% and ۹۲.۲۰% for the registration of CT-RL MRI and CT-RT MRI, respectively, that shows the increase in the accuracy of images registration using fixation equipment in RT position.Conclusion: Image quality of MRI decreased depending on the anatomical regions in RT position. The maximum decrease in SNR was observed in the posterior region of the MR images. Also, the error of CT-MRI image rigid registration decreased to ۱ mm in RT position.

کلیدواژه ها:

MRI ، CT ، Brain ، Thermoplastic mask ، Radiotherapy ، Image quality ، Image Registration