Simultaneous Neuromelanin Sensitive Imaging And Quantitative Susceptibility Mapping In The Whole Brain

Neuromelanin sensitive magnetic resonance imaging(NM-MRI)and quantitative magnetic susceptibility mapping(QSM)have shown great potential applications in neurodegenerative diseases and mental diseases.The combination of NM-MRI and QSM technology is beneficial for improving the performance of disease diagnosis and comprehensively understanding the pathological and physiological changes in diseases,which is recently receiving increasing attention.However,there is no consensus on the NM-MRI imaging parameters and the contrast to noise ratio of NM-MRI images is relatively low.In addition,blood flow of arteries in the brain creates artifacts.3D magnetization transfer(MT)gradient recalled echo(GRE)sequence can image neuromelanin and magnetic susceptibility simultaneously.However,the sequence requires long MT saturation pulse duration and the effect of MT pulse on susceptibility values remains unclear.This thesis focuses on the above issues and mainly carries out the following three parts of work:1.Study on signal simulation of NM-MRI imageThe contrast to noise ratio of NM-MRI images largely depends on the parameter settings of the MT pulse module and imaging module.This study aims to optimize the setting of the parameter for improving visualization of neuromelanin using numerical simulation method.The results showed that appropriately shortening the duration of MT Gaussian RF pulses only slightly reduced the contrast of substantia nigra and surrounding tissues,but can greatly improve the scanning speed.Choosing the shortest echo time achieved higher tissue contrast in NM-MRI images.Choosing the shortest repetition time saved total scanning time.The optimal flip angle was determined by the echo time and repetition time,usually less than 25°.In summary,the contrast to noise ratio of target nuclei on NM-MRI images can be improved and the total scanning time can be saved by optimizing scanning parameters.2.Study on artifact suppression technology of artery flow in NM-MRIThe arterial blood flow around the substantia nigra of the brain is easy to produce artifacts,which affects the accuracy of image analysis and clinical diagnosis.This study analyzed the effects of three artifact suppression techniques,namely flow compensation technology,spatial saturation,and phase encoding direction adjustment,on the quality of NM-MRI images.The results indicated that the application of flow compensation technology reduced flow artifacts in the phase direction.The placement of saturation bands effectively reduced the signal of blood,thereby suppressing artifacts caused by blood flow.Changing the phase encoding direction avoided the impact of flow artifacts on the substantia nigra region.In conclusion,the reasonable settings of three artifact suppression techniques can basically eliminate the effects of blood flow artifacts around the substantia nigra on NM-MRI images.3.Study on simultaneous NM-MRI and QSM imaging of the whole brain based on optimized MT radio frequency pulsesAt present,the long duration of MT radio frequency pulses limits its widespread clinical application.This study realized simultaneous NM-MRI and QSM imaging of the whole brain on 3D GRE sequence by developing MT Gauss radio frequency pulse with adjustable duration and MT binomial radio frequency pulse with shorter duration.Results showed that 3D GRE sequence with 10ms or 5ms of MT Gauss pulse and 2.1ms of MT binomial pulse provided the same saturation effects on highlighting neuromelanin,and yielded similar susceptibility values in the deep gray matter nuclei to that of sequence without MT pulse.In conclusion,short MT pulse provides a practicality mean to simultaneously image the neuromelanin and magnetic susceptibility within the specific absorption ratio limit.