Frequency-stabilized chemical exchange saturation transfer imaging with real-time free-induction-decay readout

Purpose To correct the temporal B(0)drift in chemical exchange saturation transfer (CEST) imaging in real-time with extra free-induction-decay (FID) readout. Theory and Methods The frequency stabilization module of the recently proposed frequency-stabilized CEST (FS-CEST) sequence was further simplified by replacing the original three k-space lines of gradient-echo (GRE) readout with a single k-space line of FID readout. The B(0)drift was quantified using the phase difference between the odd and even parts of the FID signal in the frequency stabilization module and then used to update the B(0)frequency in the succeeding modules. The proposed FS-CEST sequence with FID readout (FID FS-CEST) was validated in phantoms and 16 human subjects on cross-vendor scanners. Results In the Siemens experiments, the FID FS-CEST sequence successfully corrected the user-induced B(0)drift, generating consistent amide proton transfer-weighted (APTw) images and magnetization transfer ratio asymmetry (MTRasym) spectra with those from the non-frequency-stabilized CEST (NFS-CEST) sequence without B(0)drift. In the Philips experiments, the FID FS-CEST sequence produced more stable APTw images and MTR(asym)spectra than the NFS-CEST sequence in the presence of practical B(0)drift. Quantitatively, the SD of the APTw signal values in the deep gray matter from 15 subjects was 0.26% for the FID FS-CEST sequence compared to 1.03% for the NFS-CEST sequences, with the fluctuations reduced by nearly three-quarters. Conclusions The proposed FS-CEST sequence with FID readout can effectively correct the temporal B(0)drift on cross-vendor scanners.