Rapid measurement of brain macromolecular proton fraction with transient saturation transfer MRI

Purpose To develop an efficient MRI approach to estimate the nonwater proton fraction ( f ) in human brain. Methods We implement a brief, efficient magnetization transfer (MT) pulse that selectively saturates the magnetization of the (semi‐) solid protons, and monitor the transfer of this saturation to the water protons as a function of delay after saturation. Results Analysis of the transient MT effect with two‐pool model allowed robust extraction of f at both 3 and 7 T. This required estimating the longitudinal relaxation rate constant (R 1,MP and R 1,WP ) for both proton pools, which was achieved with the assumption of uniform R 1,MP and R 1,WP across brain tissues. Resulting values of f were approximately 50% higher than reported previously, which is partly attributed to MT‐pulse efficiency and R 1,MP being higher than assumed previously. Conclusion Experiments performed on human brain in vivo at 3 and 7 T demonstrate the ability of the method to robustly determine f in a scan time of approximately 5 min. Magn Reson Med 77:2174–2185, 2017. © 2016 International Society for Magnetic Resonance in Medicine