Fast Multislice Chemical Exchange Saturation Transfer(CEST) MRI With Dual-channel RF Irradiation

As one of the most advanced medical imaging technologies,MRI has profoundly changed the clinical diagnosis of anatomy and function of tissues and organs.However,the current MRI technology has very limited impact on the diagnosis at the molecular level.Chemical exchange saturation transfer(CEST)imaging is an emerging MRI technology that can provide novel contrast mechanism for visualizing human tissues at the molecular level.It is different from other contrast mechanisms in MRI.CEST technology mainly detects the macromolecular compounds with the exchangeable protons at the chemical shift of exchangeable protons,which makes it a unique molecular formation of MRI technology,and it can be used as a supplement to other conventional and functional MRI technologies.A typical scheme for a CEST pulse sequence includes a long RF irradiation module,followed by a fast image readout sequence.However,the duration of RF irradiation is time-consuming to generate steady-state CEST contrast.Therefore,fast chemical exchange saturation transfer imaging is still a challenging research field.In order to solve the above problem,we used dual-channel RF irradiation in the multislice chemical exchange saturation transfer imaging sequence.For the saturation sequence,channel 0 and channel 1 of the system are orthogonally distributed.Namely,when channel 0 is excited,channel 1 is not excited;when channel 0 needs to stop excitation to dissipate heat,channel 1 is excited,and vice versa,thus realizing the saturation pulse sequence with a duty cycle of 100%.During the whole excitation process of the saturation pulse sequence,the exchangeable protons exchange with the water protons continuously,and the saturation will transfer to the water,result in generating the steady-state CEST contrast.When the steady state is reached,the image is acquired by the balanced steady-state free precession sequence.For multislice sequence,because the RF pulse sequence is a selective saturation,it takes a period of time for the saturated signal to recover to static magnetization.Therefore,a long RF irradiation pulse train is used to generate the steady-state CEST contrast,and then a short RF irradiation pulse train is applied to maintain the steady-state contrast for multislice imaging acquisition or signal averaging.The proposed method has been verified by Bloch equation simulation,creatine phantom experiments and human experiments.Simulation results showed the feasibility of the proposed sequence.The results of creatine phantom experiments showed that the asymmetric magnetization transfer rate was not affected by the excitation angle of the balanced steady-state free precession sequence;the asymmetric magnetization transfer rate acquired with the proposed sequence based on the dual-channel RF irradiation was 31.54% higher than that obtained with the conventional method,and the former had better CEST effect near the exchangeable proton resonance frequency,and the CEST spectrum was higher.In multislice imaging,the RF saturation time could be reduced from 5seconds to about 2 seconds to maintain the steady-state CEST contrast,significantly reducing the acquisition time of CEST spectrum.In the human experiment,we acquired the MRI images of amide proton transfer in healthy volunteers,which proved that this method is suitable for clinical experiment.