MRI Relaxation in the Presence of Fictitious Fields Correlates with Myelin Content in Normal Rat Brain

Purpose: Brain myelin plays an important role in normal brain function. Demyelination is involved in many degenerative brain diseases, thus quantitative imaging of myelin has been under active investigation. In previous work, we demonstrated the capability of the method known as Relaxation Along a Fictitious Field (RAFF) in the rotating frame of rank n (RAFFn) to provide image contrast between white and gray matter in human and rat brains. Here, we provide evidence pointing to myelin being the major source of this contrast. Methods: RAFFn relaxation time constant (T-RAFFn) was mapped in rat brain ex vivo. T-RAFFn was quantified in 12 different brain areas. T-RAFFn values were compared with multiple other MRI metrics (T-1, T-2, continuous wave T-1 rho, adiabatic T-1 rho and T-2 rho, magnetization transfer ratio), and with histologic measurements of cell density, myelin and iron content. Results: Highest contrast between white and grey matter was obtained with T-RAFFn in the rotating frames of ranks n = 4 and 5. T-RAFFn values correlated strongly with myelin content, whereas no associations between T-RAFFn and iron content or cell density were found. Conclusion: T-RAFFn with n = 4 or 5 provides a high sensitivity for selective myelin mapping in the rat brain. (C) 2015 Wiley Periodicals, Inc.