95 Microstructures | Syndromic Craniosynostosis A ROI approach was used for white matter tract analysis, with the MRI Atlas of Human White Matter as a guideline.17 “OR/SEED” and “AND” operators were used when tracts were allowed to pass through, and “NOT” operators were used when tracts were not allowed to pass through. Occasionally, “NOT” operators were used to avoid aberrant or crossing fibers from other bundles. To secure measuring identical parts of the different white matter tracts, 2 AND operators at both ends of a bundle to extract always the same segment of the particular white matter tract were used. We measured the tracts as reported previously.14 DTI Metrics The white matter metrics from DTI, voxel-by-voxel, are mathematically based on 3 mutually perpendicular eigenvectors, whose magnitude is given by 3 corresponding eigenvalues sorted in order of decreasing magnitude as ʎ1, ʎ2 and ʎ3. An ellipsoid is created by the long axis of ʎ1, and the small axes ʎ2 and ʎ3, from where the measured length of the three axes are the eigen values. These eigenvalues are used to generate quantitative maps of fractional anisotropy (FA), the derivation of MD, RD and AD. FA represents the amount of diffusional asymmetry in a voxel, which is presented from 0 (infinite isotropy) to 1 (infinite anisotropy). AD stands for the diffusivity along the neural tract: ʎ1. The diffusivity of the minor axes, ʎ2 and ʎ3, is called the perpendicular or radial diffusivity. The mean of these diffusivity ʎ1, ʎ2 and ʎ3 is known as MD. FA, MD, AD and RD are used as indirect markers of white matter microstructure of these young patients.18 However, the mathematical coupling in the FA, MD, RD and AD equations means that our statistical approach will first need to assess for differences in the eigenvalues before analysing the impact of summary measures of diffusivity. The following equations were used: unit of measure FA scalar value ranging between 0-1 MD mm2/sec RD mm2/sec AD mm2/sec 6
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