P2–270: Diffusion–tensor imaging tractography: Correlation with processing speed in aging

Prior studies using diffusion–tensor imaging (DTI) demonstrated an age–related decline in the integrity of white matter, mostly in anterior regions. Processing speed has been shown to be correlated with white matter integrity. We developed a DTI tractography–based metric for assessing the relationship between the structural integrity of specific white matter pathways and cognitive functioning. The normalized total weighted length (NTWL) metric is the summed length of all computer–generated fibers (streamtubes) of a tract–of–interest (TOI) weighted for average linear anisotropy and then normalized for estimated intracranial volume. TOIs with lower integrity will have lower linear anisotropy which, in turn, should cause the streamtube generation algorithm to terminate prematurely resulting in shorter streamtubes and lower values of NTWL. To determine the relative contribution of interhemispheric vs. cingulum bundle fibers to psychomotor processing speed ability in the elderly using quantitative DTI tractography. DTI was obtained on 12 cognitively normal individuals 49 to 83 years old NTWL was calculated on three TOIs: interhemispheric fibers (IHF) passing through the corpus callosum and right and left cingulum bundles. Trail Making Test part A (TMT–A) was used to measure psychomotor processing speed. Multiple linear regression was used to test the association between TMT–A and NTWL in the three TOIs after accounting for age. Age was entered at step 1 and the TOI variables were entered at step 2. Age accounted for 28% of the variance in TMT–A performance (p =.079, trend). The NTWL metrics for the three TOIs accounted for an additional 41% of the variance at step 2 (p = .101, trend for F change). Examination of standardized beta weights for the individual TOIs showed that only NTWL in the right cingulum bundle was a significant predictor (beta = –.585, p = .044). After accounting for age, performance on TMT–A appears to be more closely related to the ultrastructural integrity of the right cingulum bundle than to the left cingulum bundle or to interhemispheric fibers. Although the results are limited by the small sample, they demonstrate the potential of quantitative DTI tractography for correlating cognitive functions with specific white matter pathways.