Three-Dimensional Single-Shot CEST Imaging at 3T Based on True FISP Readout.

To simultaneously fit multiple-pool effects, spectrally selective 3D chemical exchange saturation transfer (CEST) imaging typically requires single-shot readouts to save time. However, to date, fast low angle shot (FLASH) and echo planar imaging (EPI) have been the primary pulse sequences used for this purpose. They suffer from low signal-to-noise ratio (SNR) or image distortion related to B0 field inhomogeneity. In this work, we developed a 3D single-shot CEST sequence using true fast imaging with steady-state precession (True FISP) readout, also known as balanced steady state free precession (bSSFP), and optimized the scanning parameters through simulations. The performance of the CEST sequence was validated using an egg white phantom, 10 healthy volunteers, and two patients with brain tumors on a 3T human scanner. Subsequently, the proposed CEST sequence using True FISP was compared with the commonly used FLASH-based CEST sequence, focusing on SNR and image contrast, while maintaining identical pre-saturation modes, repetition time, echo time, and scan time. In the simulation experiments, the maximum CEST signal obtained from the True FISP was significantly greater than that obtained from the FLASH sequence. In the egg white phantom, the SNRs of amide proton transfer-weighted (APTw) and nuclear Overhauser enhancement (NOE) effect images obtained from the True FISP were 68.3% and 57.0% higher than those obtained from the FLASH sequence, respectively. In healthy volunteers, saturated images collected with the True FISP sequence at 3.5 ppm showed an approximately 84% increase in mean temporal SNR compared to those collected with the FLASH sequence. Compared to the FLASH sequence, the CEST images obtained from the True FISP sequence could display more detailed brain tissue structures of both normal individuals and patients with brain tumors. Therefore, due to the high SNR inherent in the sequence, True FISP has the potential to be used for fast and high-quality 3D image readout of CEST contrasts in clinical applications.