title: Three-pool model of white matter.
abstract: OBJECTIVE
  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.
  
  
  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.
  
  
  CONCLUSIONS
  The three-pool relaxation model provides measurements of water-pool fractions in WM that follow values predicted during myelination.
authors.affiliation: University of Texas Health Science Center at San Antonio, Research Imaging Center, San Antonio, Texas 78284, USA. jlancaster@uthscsa.edu
authors.forename: Jack L
authors.initials: JL
authors.lastname: Lancaster
authors.forename: Trevor
authors.initials: T
authors.lastname: Andrews
authors.forename: L Jean
authors.initials: LJ
authors.lastname: Hardies
authors.forename: Stephen
authors.initials: S
authors.lastname: Dodd
authors.forename: Peter T
authors.initials: PT
authors.lastname: Fox
mesh_terms: D000293:Adolescent; D001921:Brain / Q000033:anatomy & histology*; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D008279:Magnetic Resonance Imaging; D008954:Models, Biological; D009186:Myelin Sheath / Q000502:physiology; D012016:Reference Values