Research And Implementation Of Multinuclear Simultaneous MRI Pulse Sequences

Nuclear magnetic resonance(NMR)techniques manipulate nuclear spin with flexible pulse sequences to obtain microscopic chemical environment,kinetic information and weighted images of different parameters.This technology has been widely used in biochemical analysis,material physics and life sciences.Traditional MRI is the imaging of proton(1H)in organism,which can provide morphological and indirect functional information.It has the advantages of imaging in any slice direction,high contrast of soft tissue,rich image information and no ionizing radiation.It has become one of the important tools in clinical diagnosis and basic research of life science.With the development of high field MRI system,the effective imaging and spectroscopy detection of endogenous non-proton nuclei(23Na,31p,35Cl,17O,etc.)and exogenous probes(19F,129Xe,3He,13C,etc.)are also gradually developed.Non-proton can provide additional information that can not be obtained from proton detection alone,such as cell function,ion disorder,energy metabolism,tissue pH and drug-targeting sites at the subcellular or molecular level.It is helpful to further study the process of pathological changes.Commercial MRI systems are optimized for proton imaging,and provide a highlevel programming language for pulse sequence development,hindering researchers from developing and transforming new methods.The development of multinuclear imaging pulse sequences and the integration of new techniques are more difficult.Therefore,the research work of multinuclear and multichannel pulse sequence development platform is carried out,and then,the research of multinuclear simultaneous imaging method is carried out.The details are as follows:(1)According to the control requirements of multinuclear,multichannel and the flexible design requirements of pulse sequence,a text representation method of pulse sequence is proposed,which is parallel expandable,easy to analyze,compact and easy to master.Firstly,the functional modules of magnetic resonance spectrometer are abstracted into four kinds of expandable logical channels.Secondly,the events executed by each module are cut into segmented sequence elements and described as one or more functions.Thirdly,the synchronization design of pulse sequence is simplified by combining time slices and channel docking functions.Among them,the design of left and right gradient function can realize all kinds of bridged gradient and shape gradient,facilitate the design of a variety of k space trajectory,and support the design of acoustic noise reduction sequences.The method has been integrated into the self-developed software,and a variety of pulse sequences have been designed for testing and verification.The delayed excitation and sampling technology is applied to the design of pulse sequence to increase the robustness of the sequence and alleviate the influence of gradient delay and short-time constant eddy.The single nuclear and multinuclear simultaneous imaging results that meet the expectations are obtained,which lays a foundation for the development and research of multinuclear imaging methods.(2)To mitigate the risk of image registration and positioning error caused by large difference in image resolution of different nuclides in multinuclear simultaneous imaging,a dual-nuclei simultaneous imaging method with adjustable resolution is proposed.In a TR,proton and X-nuclei are excited successively,and the X-nuclei and the proton signals are acquired successively.The parameter λs which adjust the slice selection gradient intensity of X-nuclei is introduced to adjust the slice thickness,and the slice dephasing gradient is applied in advance to ensure the recovery of dispersed phase before signal acquisition.The parameters λr and λp which adjust the frequency and phase-encoding gradient intensity of X-nuclei are introduced to adjust the corresponding FOV.Firstly,X-nuclei and proton are space encoded in the plane,and the encoded X-nuclei signal is acquired;Then frequency and phase-encoding compensation gradients are applied to make the proton k coordinate return to the target point,and then the proton signal is encoded and acquired.This method has the function of flow compensation in frequency-encoding and slice selection direction for proton.The proposed method can be applied to the simultaneous imaging of any two kinds of nuclides,and the echo time and flip angle of each nuclear can be controlled separately,which is beneficial to obtain multi-parameter weighted images.(3)The imperfection of the gradient system leads to the deviation of the k space sampling trajectory from the trajectory designed by the pulse sequence,and artifacts are introduced to the image,especially in the non-cartesian sampling trajectory imaging method,so it is necessary to measure or estimate the k space trajectory.The slice selection method is widely used in trajectory measurement,which uses the phase of the FID to characterize the evolution trajectory of the gradient.However,the FID signal is modulated by the Fourier transform of the excited slice profile.During high resolution imaging,the signal amplitude will be close to zero,and the measured trajectory is extremely inaccurate.By modifying the slice selection method,a k space trajectory measurement method with high signal to noise ratio(SNR)is proposed.The slicerefocusing gradient intensity is adjusted to shift the low signal area,the phase of the high SNR signal is extracted to constract the k space trajectory.This method was applied to ultrashort echo time(UTE)imaging and high quality images were obtained.This method can be used to measure the evolution trajectory of arbitrary shape gradient.It is suitable for high resolution imaging and the parameter setting is simple.It has potential application value in multinuclear simultaneous imaging with pure frequency encoding.