Multi-slice Segmented Spatiotemporally Encoded MRI With Simultaneous Echo Refocusing

Ultrafast magnetic resonance imaging(MRI)has been widely used in functional MRI(fMRI),diffusion tensor imaging(DTI),free-breath imaging heart imaging and other fields due to its good temporal resolution.Among ultrafast MRI,echo planar imaging(EPI)is most widely used owing to its excellent temporal and spatial resolution.With the help of a series of fast switching gradients,EPI provides all the data required for a two-dimensional magnetic resonance image under a single RF pulse excitation.However,EPI images are susceptible to distortion due to field inhomogeneity and chemical shift effects.To overcome the effects of field inhomogeneity and chemical shift,Frydman and coworkers propose a new single-scan method called spatiotemporally encoded(SPEN)MRI in 2006.Spatiotemporal encoding introduces a quadratic phase into nuclear spin evolution by a linear sweep pulse(chirp pulse).According to the stable phase theory,the intensity of SPEN MRI signals depends only on the local spin density in the corresponding spatial location at a certain sampling time under quadratic phase modulation,hence SPEN MRI has spatial selectivity.In addition,compared with EPI,the introduction of quadratic phase effectively broadens the bandwidth of phase?encoding dimension.Therefore,SPEN MRI has great robust to field inhomogeneity and chemical shift effects.By super-resolution reconstruction,SPEN images can reach spatial resolution comparable to EPI images.In multi-slice MRI,traditional SPEN method will lead to excessive specific absorption rate(SAR).Frydman and coworkers proposes a new method,which utlizes a hard pulse to store the encoded magnetization,to reduce the SAR values to a great extent.However,the drawback of this method is the signal loss due to T 1 relaxation.To reduce the signal loss caused by Ti relaxation,our group have proposed a multi-slice SPEN MRI approach based on segmenting slice-selective dimension(SeSPEN).In this thesis,the SeSPEN MRI is further accelerated with simultaneous echo refocusing.The main content of this thesis is as follows:1.The basic theory and method of SPEN MRI are described in detail,which includes the characteristics of quadratic phase,the spatial selectivity,and the robustness to the effects of field inhomogeneity and chemical shift.At the same time,the necessity of super-resolution reconstruction to SPEN images and the deconvolution super-resolved reconstruction algorithm are described.2.A multi-slice SeSPEN method with simultaneous double-echo refocusing method(ME-SeSPEN)is proposed.This method uses a series of dephase gradients to refocus signals from different slices at different moments of a same readout gradient period.The resulting images are reconstructed with the deconvolution super-resolution algorithm.Experimental results show that,compared with spin echo EPI,spin multi-echo EPI(SER)and SeSPEN MRI,ME-SeSPEN method can effectively shorten the sampling time and reduce eddy current effects.It can also maintain comparable robustness to field inhomogeneity,spatial resolution and signal-to-noise ratio to SeSPEN.3.Based on the double-echo ME-SeSPEN,a three-echo ME-SeSPEN method is provided to further shorten the sampling time while keeping the robustness of SPEN MRI to field inhomogeneity.Water phantom experimental results prove the feasibility of this method.