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Purpose: To confirm that CrCEST in muscle exhibits a slow‐exchanging process, and to obtain high‐resolution amide, creatine (Cr), and phosphocreatine (PCr) maps of skeletal muscle using a POlynomial and Lorentzian Line‐shape Fitting (PLOF) CEST at 3T. Methods: We used dynamic changes in PCr/CrCEST of mouse hindlimb before and after euthanasia to assign the Cr and PCr CEST peaks in the Z‐spectrum at 3T and to obtain the optimum saturation parameters. Segmented 3D EPI was employed to obtain multi‐slice amide, PCr, and Cr CEST maps of human skeletal muscle. Subsequently, the PCrCEST maps were calibrated using the PCr concentrations determined by 31P MRS. Results: A comparison of the Z‐spectra in mouse hindlimb before and after euthanasia indicated that CrCEST is a slow‐exchanging process in muscle (<150.7 s−1). This allowed us to simultaneously extract PCr/CrCEST signals at 3T using the PLOF method. We determined optimal B1 values ranging from 0.3 to 0.6 μT for CrCEST in muscle and 0.3–1.2 μT for PCrCEST. For the study on human calf muscle, we determined an optimum saturation time of 2 s for both PCr/CrCEST (B1 = 0.6 μT). The PCr/CrCEST using 3D EPI were found to be comparable to those obtained using turbo spin echo (TSE). (3D EPI/TSE PCr: (2.6 ± 0.3) %/(2.3 ± 0.1) %; Cr: (1.3 ± 0.1) %/(1.4 ± 0.07) %). Conclusions: Our study showed that in vivo CrCEST is a slow‐exchanging process. Hence, amide, Cr, and PCr CEST in the skeletal muscle can be mapped simultaneously at 3T by PLOF CEST. [ABSTRACT FROM AUTHOR]

Purpose: To assess the feasibility of CEST‐based creatine (Cr) mapping in brain at 3T using the guanidino (Guan) proton resonance. Methods: Wild type and knockout mice with guanidinoacetate N‐methyltransferase deficiency and low Cr and phosphocreatine (PCr) concentrations in the brain were used to assign the Cr and protein‐based arginine contributions to the GuanCEST signal at 2.0 ppm. To quantify the Cr proton exchange rate, two‐step Bloch–McConnell fitting was used to fit the extracted CrCEST line‐shape and multi‐B1 Z‐spectral data. The pH response of GuanCEST was simulated to demonstrate its potential for pH mapping. Results: Brain Z‐spectra of wild type and guanidinoacetate N‐methyltransferase deficiency mice show a clear Guan proton peak at 2.0 ppm at 3T. The CrCEST signal contributes ∼23% to the GuanCEST signal at B1 = 0.8 μT, where a maximum CrCEST effect of 0.007 was detected. An exchange rate range of 200–300 s−1 was estimated for the Cr Guan protons. As revealed by the simulation, an elevated GuanCEST in the brain is observed when B1 is less than 0.4 μT at 3T, when intracellular pH reduces by 0.2. Conversely, the GuanCEST decreases when B1 is greater than 0.4 μT with the same pH drop. Conclusions: CrCEST mapping is possible at 3T, which has potential for detecting intracellular pH and Cr concentration in brain. [ABSTRACT FROM AUTHOR]