Chemical exchange saturation transfer imaging of phosphocreatine in the muscle

Purpose: To determine the exchange parameters for the CEST of phosphocreatine (PCrCEST) in phantoms and to characterize PCrCEST in vivo in the muscle at different saturation powers and magnetic fields. Methods: Exchange parameters were measured in PCr solutions using varying saturation power at 15.2 T. Z-spectra were analyzed using multipool Lorentzian fitting in the hindlimb using various powers at 2 different fields: 9.4 T and 15.2 T. Modulation of PCr signal in PCrCEST and phosphorus MRS was observed in the mouse hindlimb before and after euthanasia. Results: The exchange rate of PCr at physiological pH in phantoms was confirmed to be in a much slower exchange regime compared with Cr: k(ex) at pH 7.3 and below was less than 400 s(-1). There was insufficient signal for detection of PCrCEST in the brain, but PCrCEST in the hindlimb was measured to be 2.98% +/- 0.43 at a B-1 of 0.47 mu T at 15.2 T, which is 29% higher than 9.4T values. The value of PCrCEST at a B-1 of 0.71 mu T was not significantly different than that measured at a B1 of 0.47 mu T. After euthanasia, PCrCEST signal dropped by 82.3% compared with an 85% decrease in PCr in phosphorus MRS, whereas CrCEST signal increased by 90.6%. Conclusion: The PCrCEST technique has viable sensitivity in the muscle at high fields and shows promise for the study of metabolic dysfunction and cardiac systems.