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 (T₁, T₂, continuous wave T_1ρ, adiabatic T_1ρ and T_2ρ, 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.