Creatine and phosphocreatine mapping of mouse skeletal muscle by a polynomial and Lorentzian line-shape fitting CEST method

Purpose: To obtain high-resolution Cr and PCr maps of mouse skeletal muscle using a polynomial and Lorentzian line-shape fitting (PLOF) CEST method. Methods: Wild-type mice and guanidinoacetate N-methyltransferase–deficient (GAMT-/-) mice that have low Cr and PCr concentrations in muscle were used to assign the Cr and PCr peaks in the Z-spectrum at 11.7 T. A PLOF method was proposed to simultaneously extract and quantify the Cr and PCr by assuming a polynomial function for the background and 2 Lorentzian functions for the CEST peaks at 1.95 ppm and 2.5 ppm. Results: The Z-spectra of phantoms revealed that PCr has 2 CEST peaks (2 ppm and 2.5 ppm), whereas Cr only showed 1 peak at 2 ppm. Comparison of the Z-spectra of wild-type and GAMT-/- mice indicated that, contrary to brain, there was no visible protein guanidinium peak in the skeletal-muscle Z-spectrum, which allowed us to extract clean PCr and Cr CEST signals. High-resolution PCr and Cr concentration maps of mouse skeletal muscle were obtained by the PLOF CEST method after calibration with in vivo MRS. Conclusions: The PLOF method provides an efficient way to map Cr and PCr concentrations simultaneously in the skeletal muscle at high MRI field.