BACKGROUND AND PURPOSE:In the chronic phase after traumatic brain injury, D
TI findings reflect WM integrity. D
TI interpretation in the subacute phase is less straightforward. Microbleed evaluation with SWI is straightforward in both phases. We evaluated whether the microbleed concentration in the subacute phase is associated with the integrity of normal-appearing WM in the chronic phase.
MATERIALS AND METHODS:Sixty of 211 consecutive patients 18 years of age or older admitted to our emergency department ≤24 hours after moderate to severe traumatic brain injury matched the selection criteria. Standardized 3
T SWI, D
TI, and
T1WI were obtained 3 and 26 weeks after traumatic brain injury in 31 patients and 24 healthy volunteers. At baseline, microbleed concentrations were calculated. At follow-up, mean diffusivity (MD) was calculated in the normal-appearing WM in reference to the healthy volunteers (MD
z).
Through linear regression, we evaluated the relation between microbleed concentration and MD
z in predefined structures.
RESULTS:In the cerebral hemispheres, MD
z at follow-up was independently associated with the microbleed concentration at baseline (left: B = 38.4 [95% CI 7.5–69.3],
P = .017; right: B = 26.3 [95% CI 5.7–47.0],
P = .014). No such relation was demonstrated in the central brain. MD
z in the corpus callosum was independently associated with the microbleed concentration in the structures connected by WM tracts running through the corpus callosum (B = 20.0 [95% CI 24.8–75.2],
P < .000). MD
z in the central brain was independently associated with the microbleed concentration in the cerebral hemispheres (B = 25.7 [95% CI 3.9–47.5],
P = .023).
CONCLUSIONS:SWI-assessed microbleeds in the subacute phase are associated with D
TI-based WM integrity in the chronic phase.
These associations are found both within regions and between functionally connected regions.
The yearly incidence of traumatic brain injury (
TBI) is around 300 per 100,000 persons.
1,2 Almost three-quarters of patients with moderate to severe
TBI have traumatic axonal injury (
TAI).
3 TAI is a major predictor of functional outcome,
4,5 but it is mostly invisible on C
T and conventional MR imaging.
6,7D
TI provides direct information on WM integrity and axonal injury.
5,8 However, D
TI abnormalities are neither specific for
TAI nor stable over time. Possibly because of the release of mass effect and edema and resorption of blood products, the effects of concomitant (non-
TAI) injury on D
TI are larger in the subacute than in the chronic phase (>3 months).
4,9,10 Therefore, D
TI findings are expected to reflect
TAI more specifically in the chronic than in the subacute phase (1 week–3 months).
4 Even in regions without concomitant injury, the effects of
TAI on D
TI are dynamic, possibly caused by degeneration and neuroplastic changes.
6,11,12 These ongoing pathophysiological processes possibly contribute to the emerging evidence that D
TI findings in the chronic phase are most closely associated with the eventual functional outcome.
12,13Although D
TI provides valuable information, its acquisition, postprocessing, and interpretation in individual patients are demanding. SWI, with which microbleeds can be assessed with high sensitivity, is easier to interpret and implement in clinical practice. In contrast to D
TI, SWI-detected traumatic microbleeds are more stable
1 except in the hyperacute
14,15 and the late chronic phases.
16 Traumatic cerebral microbleeds are commonly interpreted as signs of
TAI. However, the relation is not straightforward. On the one hand, nontraumatic microbleeds may be pre-existing. On the other hand, even if traumatic in origin, microbleeds represent traumatic vascular rather than axonal injury.
17 Indeed,
TAI is not invariably hemorrhagic.
18 Additionally, microbleeds may secondarily develop after trauma through mechanisms unrelated to axonal injury, such as secondary ischemia.
18D
TI is not only affected by pathophysiological changes but also by susceptibility.
19 The important susceptibility-effect generated by microbleeds renders the interpretation of D
TI findings at the location of microbleeds complex. In the chronic phase, mean diffusivity (MD) is the most robust marker of WM integrity.
4,6 For these reasons, we evaluated MD in the normal-appearing WM.Much
TAI research focuses on the corpus callosum because it is commonly involved in
TAI
5,18,20 and it can reliably be evaluated with D
TI,
5,21 and
TAI in the corpus callosum is related to clinical prognosis.
6,20 The corpus callosum consists of densely packed WM tracts that structurally and functionally connect left- and right-sided brain structures.
22 The integrity of the corpus callosum is associated with the integrity of the brain structures it connects.
23 Therefore, microbleeds in brain structures that are connected through the corpus callosum may affect callosal D
TI findings. Analogous to this, microbleeds in the cerebral hemispheres, which exert their function through WM tracts traveling through the deep brain structures and brain stem,
24,25 may affect D
TI findings in the WM of the latter.Our purpose was to evaluate whether the microbleed concentration in the subacute phase is associated with the integrity of normal-appearing WM in the chronic phase. We investigated this relation within the cerebral hemispheres and the central brain and between regions that are functionally connected by WM tracts.
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