Three-pool model of white matter

Purpose: To investigate the use of a three-pool relaxation model to measure myelin, myelinated axon, and mixed water-pool fractions in white matter (WM) during myelination. Materials and Methods: MRI at 1.9 Tesla, and conventional spin-echo imaging were used to acquire T1 and T2 relaxation data in 15 normal children ranging in age from 3 months to 13 years 4 months. Three equations with three unknowns were solved to calculate three water-pool fractions for each child in a frontal association fiber area and a frontal parietal projection-fiber area. The temporal trend of the fractions was compared with a theoretical three-pool myelination model. Results: The myelin level in the projection-fiber area rose earlier than in the association fiber area following the standard caudal-to-rostral trend. The temporal trend of the three-pool fractions followed that predicted by the theoretical myelination model in both brain areas. The myelinated-axon and mixed pool sizes were significantly different in the two WM areas following early myelination, although their myelin pools were similar. T1 values correlated more highly with the myelinated-axon and mixed pool fractions than with the myelin pool fraction. Conclusion: the three-pool relaxation model provides measurements of water pool fraction in WM that follow values predicted during myelination.