ABSTRACT Purpose To improve the signal‐to‐noise ratio (SNR) and mitigate T 2 ‐decay‐related blurring in whole‐brain chemical exchange saturation transfer (CEST) imaging by optimizing the refocusing flip angles of a turbo spin echo (TSE) readout, also known as the SPACE‐CEST sequence. Methods The SNR of the SPACE‐CEST sequence was modeled as a function of flip angles, and a resolution penalty term was added to control blurring. Flip angles were optimized under two modes: Resolution‐only and SNR+Resolution mode. The optimized flip angles were compared to conventional variable/constant flip angles regarding transverse magnetization usage efficiency, longitudinal magnetization storage, and specific absorption rate (SAR). The SNR and resolution benefits of the proposed method were validated in a phantom, healthy volunteers, and brain tumor patients. Results The optimized flip angles in the Resolution‐only mode generated a constant signal, validating the framework. The optimized flip angles in SNR+Resolution mode exhibited higher transverse magnetization usage efficiency and greater longitudinal storage. The SNR+Resolution mode produced signals comparable to those from conventional constant flip angles but with significantly lower flip angles. Notably, the SNR+Resolution mode demonstrated high transverse magnetization usage efficiency even with 20% B 1 inhomogeneity and across a wide range T 1 and T 2 values, along with a 32.9% decrease in SAR. On average, the SNR+Resolution mode showed a 14.2% increase in SNR in humans and a 3.6% improvement in spatial resolution. Conclusion Higher transverse magnetization usage efficiency can be achieved by optimizing refocusing flip angles of the SPACE‐CEST sequence, leading to increased SNR and decreased SAR.
Yong et al. (Sat,) studied this question.