Nikola Stikov1, Lee M. Perry2, Aviv Mezer2, John M. Pauly3, Brian A. Wandell2, Robert F. Dougherty2
1Montreal Neurological Institute, McGill University, Montreal, QC, Canada; 2Psychology, Stanford University, Stanford, CA, USA; 3Electrical Engineering, Stanford University, Stanford, CA, USA
In myelinated axons, the ratio between the axon caliber (diameter) and the total caliber of the axon plus its myelin sheath (i.e., the fiber caliber) is relatively constant and is observed to be near the theoretically optimal value of 0.6. Recently, variations in this axon-to-fiber ratio (the "g-ratio") have been proposed to be associated with differences in brain development. Here we describe a method to estimate the g-ratio in-vivo by combining diffusion imaging and quantitative magnetization transfer. The methods described here form a novel MR contrast mechanism that may be useful for quantifying the development of white matter and the deterioration of white matter in demyelinating diseases.