Simultaneous measurement of saturation and relaxation in human brain by repetitive magnetization transfer pulses

Magnetization transfer (MT) by equidistant pulse trains can be described as being analogous to progressive partial saturation, where 'direct' saturation of water is amplified by MT contributions that are dependent on macromolecular content and differential saturation. This concept was applied to study the transition to steady state in the human brain using similar MT-pulses as in imaging. Up to 41 bell-shaped MT-pulses of 12 ms duration were applied at frequency offsets between 0.5 and 15 kHz with flip angles between 1080 and 1440 degrees . Central white and parietal gray matter was studied in human subjects using STEAM for localized read-out (TE = 30 ms, TM = 13.7 ms). The apparent degree of saturation, delta(app), and the longitudinal relaxation of the water pool during the pulse repetition period (PR) were fitted to the transient behavior after signal correction for cerebro-spinal fluid. PR was varied between 15 and 100 ms to assess the PR-dependence of the fitted parameters. The MT-term in delta(app) exceeded the direct saturation and attained its maximum at PR > or = 100 ms. The macromolecular pool was only partially saturated by a single MT-pulse. The offset may be increased to 2.5 kHz to reduce direct saturation without sacrificing MT in white matter. The estimated relaxation rates (1.04 +/- 0.11 s(-1) in WM; 0.76 +/- 0.13 s(-1) in GM) were faster than are commonly observed at 1.5 T. The apparent saturation is a measure for MT that is not confounded by relaxation. To maximize MT in brain tissue, MT-pulses should be applied at PR = 100 ms or longer. At shorter PR, a larger steady state saturation is obtained at the cost of increased contributions from direct saturation. Since this accelerates the convergence, PR should be decreased to reach the steady state within a specified time. A faster transition can always be achieved at a reduced frequency offset via increased direct saturation.