| Magnetic resonance imaging is a safe and effective tomography commonly used in clinical diagnosis.The nuclear magnetic resonance is the physical basis of magnetic resonance imaging.Radiofrequency transmit field B1+ play an important role in the process of magnetic resonance imaging.Acquiring the spatial distribution of B1+ is greatly benefit to many magnetic resonance application and researches,such as radiofrequency field shimming technology,T1 mapping,chemical exchange saturation transfer technique and magnetic resonance electric properties tomography.B1+ mapping technique uses special imaging sequence to acquire multiple images.By detecting the variety of signal magnitude or phase,the B1+-unrelated term can be eliminated from signal while keeping the B B1+ related term,which can be used to calculate the magnitude of Bt field.Bloch-Siegert shift method is one of the most commonly used methods among the existed Bt mapping methods.The precession frequency of the spin isochromats is changed by a strong off-resonance radiofrequency pulse,resulting in Bt dependent phase shifting.The magnitude of B1+ field can be deterrined by detecting the phase shift.In clinical magnetic resonance imaging scanner,multi-channel receive coils are commonly used as receiver.If Bi+mapping techniques are applied with multi-channel receive coils,each receive channel can acquire signal independently,obtaining multiple images.Theoretically,the measurement of B1+ magnitude is uncorrelated to receive coil so that all data from every receive channel can be used to calculate B1+ magnitude.However,there is noise in the signal.Signal-to-noise ratio is low in the regions where are far away from receive channel.Measuring |B1+| map from a single channel leads calculation errors.One solution considers channel combination in |B1+| calculation.In this dissertation,six B1+ mapping methods are theoretically investigated.Then,Bloch-Siegert Shift method is further studied and BSS sequence is designed.Finally,a channel combination method is proposed,which can improve the accuracy of B1+ mapping technique.In the study of BSS sequence design,the principle of designing off-resonance pulse and spatial encoding gradient is introduced.Fermi pulse is developed as off-resonance pulse and spatial encoding gradients are realized.BSS sequence is validated according to the following:(1)Using oscilloscope to measure the actual output sequence waveform of the spectrometer and observe whether the designed BSS sequence waveform conforms to it;(2)Make a comparison between nominal value and actual value of phase shift caused by off-resonance pulse;(3)Consider double angle method as reference and then compare the spatial distribution of Bloch-Siegert shift method and double angle method.The results show that BSS sequence is design successfully.In the study of channel combination method,the proposed method is applied with BSS method.According to the probability density distribution of signal phase noise,the probability density function of phase difference can be derived.Then,phase difference can be estimated through maximum likelihood method,achieving channel combination.The proposed method is compared with previous method and the results indicate that the proposed method show better robustness.The mean relative difference of the proposed method is 1.22%in the case of low signal-to-noise ratio,indicating a high accuracy in B1+ magnitude calculationIn this dissertation,several B1+ mapping methods are study theoretically and then BSS method are furtherly investigated.In the study of BSS sequence design,a method to design and verify BSS sequence is introduced.In the study of channel combination method,the accuracy of B1+ magnitude calculation is improved,which is greatly benefit to many researches such as T1 mapping.The proposed channel combination method provides theoretical guidance for data processing in actual imaging... |
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