Diffusion tensor imaging: the normal evolution of ADC, RA, FA, and eigenvalues studied in multiple anatomical regions of the brain.

INTRODUCTION: The aim of our work was to investigate the process of myelination in healthy patients using the diffusion parameters apparent diffusion coefficient (ADC), relative anisotropy (RA), fractional anisotropy (FA), and eigenvalues. Age-dependent changes were assessed using the slope m of the fit functions that best described the data.
MATERIALS AND METHODS: Seventy-two patients (3 weeks-19 years) without pathological magnetic resonance imaging findings were selected from all pediatric patients scanned with diffusion tensor imaging over a 5-year period at our institution. ADC, RA, FA, and eigenvalue maps were calculated and regions of interest were selected in anterior/posterior pons, genu/splenium of corpus callosum (CC), anterior/posterior limb of internal capsule (IC), and white matter (WM) regions (frontal, temporal, parietal, occipital WM). Statistical analysis was performed using Spearman correlation coefficient and regression analysis.
RESULTS: Mean values ranged 71.6 x 10(-5) to 90.3 x 10(-5) mm(2)/s (pons/parietal WM) for ADC, 0.32-0.94 (frontal WM/CC) for RA, and 0.36-0.81 (frontal WM/splenium) for FA. Logarithmic fit functions best described the data. Strong age influences were observed for CC, pons, and parietal/frontal WM and changes were significant for all three eigenvalues, most pronounced for perpendicular eigenvalues. Changes in RA and FA differed depending on the structure anisotropy.
CONCLUSIONS: Changes observed for ADC, RA, FA, and eigenvalues with age were consistent with previous findings. Changes detected for RA and FA varied due to the different scaling of both parameters. We found that the use of the largely linear scaled RA adds more valuable information for the assessment of age-dependent structural changes as compared to FA. Additionally, we report normative values for the diffusion parameters studied.