Research On Early Radiation-induced Brain Injury In Patients With Nasopharyngeal Carcinoma Based On Diffusion MRI

Nasopharyngeal carcinoma(NPC)is one of the common malignant tumors in south China,which has obvious radiosensitivity,and radiotherapy is the most effective treatment.However,radiotherapy can inevitably damage the brain tissue at the base of the skull while killing tumor cells,and the NPC patients will suffer a series of cognitive dysfunction in the early stage after radiotherapy,including memory,vision,attention,learning,executive function and motor function.In recent years,relevant studies have found that the cognitive dysfunction of patients after radiotherapy is closely related to the changes of their brain structure.However,the neurological damage mechanism of early radiation-induced brain injury in NPC patients has still not been fully clarified,and there is a lack of objective index to reflect the degree of brain injury in clinic.Diffusion magnetic resonance imaging(dMRI)is an imaging technology that uses the diffusion mode of water molecules to non-invasively detect the integrity of nerve cells and the microstructure information of brain tissues.In this thesis,three post-processing analysis methods based on dMRI were used to explore early radiation-induced brain injury with longitudinal studies in NPC patients.The first research part is based on the metrics of diffusion kurtosis imaging(DKI)and neurite orientation and density imaging(NODDI).The radiation-induced gray matter structural changes were analyzed by GM-based spatial Statistics(GBSS)analysis method.In this experiment,22 NPC patients were selected as experimental subjects.We collected dMRI data at the following stages for each patient:before radiotherapy,0～3 months after radiotherapy,6 months after radiotherapy,12 months after radiotherapy.After the standard preprocessing steps of MRtrix3 and FSL software,the gray matter skeleton was obtained from the average population gray matter fraction map,and the significant difference at the whole brain level of the diffusion metrics between groups was statistically analyzed on the gray matter skeleton.The experimental results showed that the brain injury of NPC patients after radiotherapy was mainly concentrated in the temporal lobe,frontal lobe and other brain regions close to the radiotherapy target.The radiation-induced grey matter injury is most severe at the early-delayed period,and then slowly recovers at the late-delayed period.The second research part is based on Fixel-based analysis(FBA)to study the density and morphological changes of white matter structure after radiotherapy.The Fibre Orientation Distribution(FOD)model was used to calculate the metrics FD,FC and FDC to reflect the local information of fiber populations in each voxel.The FBA can explore the abnormal changes of fiber bundles density and cross-sectional size in different directions in each voxel which can overcome the defects of the traditional Voxel-based analysis(VBA)method.The results showed that the reduction of the cross-sectional size of the fiber bundles caused by atrophy was the main cause of the early radiation brain injury.The third research part is based on the brain network analysis to study the changes of the brain topological property of NPC patients after radiotherapy.In this research,diffusion tensor imaging(DTI)model and probabilistic tracking algorithm were used to construct a structured brain network,and graph theory was used to analyze changing pattern of brain network topology.The results showed that the local efficiency,node property and within-module connectivity of relevant module in the brain network changed significantly after radiotherapy,and the changes of some nodal parameters in the temporal lobe were dose-dependent.Our study found that white matter and gray matter structures of NPC patients showed significant changes in related brain regions,mainly in temporal lobe,frontal lobe and occipital lobe,and the radiation-induced brain injury were most severe at 6 months after radiotherapy.The results of our study provide imaging basis for further study on the mechanism of nerve damage in NPC patients with early radiation-induced brain injury.