Robust And Sensitive Magnetic Resonance Chemical Exchange Saturation Transfer Imaging

Chemical exchange saturation transfer（CEST）imaging,capable of indirectly detecting a variety of low-concentration metabolites possessing water-exchangeable protons,has been widely used for the diagnosis of many diseases.However,CEST imaging still faces several major technical challenges,including B₀ frequency drift,inadequate saturation duration,inhomogeneous radiofrequency（RF）transmission,and the optimization of saturation schemes.In this thesis,we aim to address these technical challenges to improve the robustness and sensitivity of CEST imaging.Firstly,we proposed a frequency-stabilized CEST imaging sequence with the gradient echo readout（GRE-FS-CEST）to correct the B₀ drift in real time.Specifically,a frequency stabilization module was inserted in front of the conventional non-frequency-stabilized CEST（NFS-CEST）sequence.In this frequency stabilization module,three non-phase-encoded k-space lines were acquired to quantify the B₀ drift value.Then,the center frequency of all the succeeding pulses was adjusted to realize the real-time correction of B₀ drift and ensure the efficacy of fat suppression.The phantom and human experiments results showed that the GRE-FS-CEST sequence can successfully correct the artifacts of the CEST images attributed to the B₀ drift.Secondly,we proposed the frequency-stabilized CEST imaging sequence with the free induction decay（FID）readout（FID-FS-CEST）by further simplifying the stabilization module of the GRE-FS-CEST sequence.The FID-FS-CEST sequence replaced the original three k-space lines of GRE readout with a single k-space line of FID readout,which reduced the complexity of the sequence design,shortened the minimal readout time,and allowed a greater phase-wrap-free frequency to be corrected.A cross-vendor validation of the simplified FID-FS-CEST sequence was performed on a Siemens scanner and a Philips scanner.The proposed FID-FS-CEST sequence can effectively correct the B₀ drift on cross-vendor scanners.Thirdly,we proposed a parallel-transmission based CEST（p Tx-CEST）sequence to prolong the saturation duration and improve the RF transmission inhomogeneity.With the p Tx functionality,the p Tx-CEST sequence enhanced the sensitivity of CEST imaging by achieving arbitrarily long saturation duration with a 100%duty cycle,and the longer saturation duration achieved by the pTx-CEST sequence resulted in higher tumor-normal CEST contrast.Meanwhile,the p Tx-CEST sequence was run with the dual RF channels controlled independently via amplitude and phase modulations,which helped reduce the RF saturation inhomogeneity due to the dielectric shading effects in abdominal CEST imaging.Fourthly,we optimized the saturation scheme taking the consideration of the B₀inhomogeneity to improve the accuracy of the B₀ inhomogeneity correction.Results of Bloch-Mc Connell simulations showed that a larger degree of oscillation in the z-spectrum could lead to a larger error that occurred in the B₀ inhomogeneity correction.The oscillation of the z-spectrum can be reduced by adopting saturation schemes with non-rectangular pulses,longer pulse width,and interpulse spoilers.The phantom and in vivo results demonstrated that the optimized saturation scheme obtained from Bloch-Mc Connell simulations can eliminate the artifacts caused by the error of B₀ inhomogeneity correction,improving the robustness of CEST imaging.In summary,we achieved real-time B₀ drift correction,prolonged the CEST saturation duration,improved the RF transmission inhomogeneity of saturation pulses,and optimized the CEST saturation scheme considering the B₀ spatial inhomogeneity.The combination of these aforementioned four pieces of work improves the robustness and sensitivity of CEST imaging,which further promotes the clinical application of CEST imaging.