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The DTI parameters showed age-related changes in healthy
volunteers. FA increased and RD and MD decreased in all VOIs
( P<.001). AD in CST and ML did not show significant changes
( P>.126) but decreased in other regions (P<.006). FA of the
patient group negatively deviated from the normal age-related
change (ie, a2 < 0) and was statistically significant in the entire
pons, dTPF, vTPF, and MCP (P<.004). AD and RD also showed
negative deviations for all VOIs (P<.034) except for dTPF and
vTPF, where RD deviated positively (a2 > 0, P<.023). The
deviation from the normal pattern was not strongly dependent on
individual differences of dose (ie, a3 dose was not significant)
for most VOIs. Only CST showed a significant relation between
AD reduction and dose (a3 < 0, PZ.03).
The pairwise comparison between the pons and midbrain
showed that the decrease of FA in the pons was more pronounced
than that in the midbrain ). The ratio of the normalized FA
between the pons and midbrain showed a negative trend (b 3 Z
0.065, P<.001). The ratios of the normalized AD and MD also
showed significantly negative trends ( P<.001).
Further pairwise comparisons on substructures revealed that
the temporal changes of DTI parameters were not uniform within
the pons. shows the FA maps of the pons for a patient at
baseline and the 2 follow-up times. The decrease of FA in TPF
was manifested 18 and 45 months from the baseline. By contrast,
FA in the CST and ML showed smaller reductions at 18 months
and recovered to the baseline level at 45 months. shows
comparisons of the temporal changes of the normalized FA of the
CST and dTPF for all 42 patients. In most cases, dTPF showed
a greater drop than CST and remained at the lower value, whereas
CST either showed a smaller reduction or eventually recovered to
the baseline level.
The statistical analysis confirmed that the ratio of normal-
ized FA between the dTPF and CST showed a significantly
negative trend (b3 Z 0.135, P<.001) ). The analysis
with other DTI parameters was consistent with this result.
Taken together, these results suggest that compromised struc-
tural integrity is more pronounced in the dTPF than in CST:
dTPF showed more significant decrease in AD, increase in RD,
and increase in MD than did the CST (P<.001 for all
comparisons) b). Similar results were found when the
dTPF was compared with the ML or the MCP , d). On
the other hand, no significant differences were observed for the
pairs dTPF/vTPF, ML/CST, and MCP/CST in any of the DTI
parameters -g). In summary, the TPF showed more
changes reflecting white matter injury than did the CST, ML,
and MCP at the level of pons, regardless of dorsal or ventral
compartments, whereas there were no significant differences
among the other 3 VOIs.
This regional variation of the temporal changes could not
be explained unequivocally by the dose distribution. Although
Fig. 2. Average doses in the volumes of interest over the 42
patients. Error bars indicate standard deviation. CST Z cortico-
spinal tract; dTPF Z dorsal transverse pontine fiber; MCP Z
middle cerebral peduncle; ML Z medial lemniscus; vTPF Z
ventral transverse pontine fiber.
Fig. 3. Pairwise statistical comparison of the temporal changes of diffusion tensor imaging parameters between pairs of volumes of
interest. Comparisons between (a) Pons and Midbrain, (b) dTPF and CST, (c) dTPF and ML, (d) dTPF and MCP, (e) dTPF and vTPF, (f) ML
and CST, and (g) MCP and CST are presented. The number in the ordinate is b3 in the statistical model equation (see text) that indicates the
temporal change of the volume of interest in numerator with respect to the one in denominator. Error bars indicate standard error. AD Z
axial diffusivity; CST Z corticospinal tract; dTPF Z dorsal transverse pontine fiber; FA Z fractional anisotropy; MCP Z middle cerebral
peduncle; MD Z mean diffusivity; ML Z medial lemniscus; RD Z radial diffusivity; vTPF Z ventral transverse pontine fiber.
Volume 86 Number 2 2013 Differences in brainstem fiber tract response 295