Increased Anisotropy in Acute Stroke

Abstract

Background and Purpose-The increase in fractional anisotropy (FA) in acute stroke has yet to be explained. Using an engineering methodology known as pq diagrams, we sought to explain the increase in FA by describing changes in the total magnitude of the diffusion tensor (L) as well as the isotropic (p) and anisotropic (q) components. Methods-Diffusion tensor imaging was performed in 10 patients with stroke Ͻ27 hours old. The diffusion tensor was decomposed into the p and q components and plotted to describe the diffusion trajectories. FA was also calculated and compared. Results-There was significant and consistent reduction in p, q, and L (p: mean, Ϫ50.0%; range, Ϫ36.6% to Ϫ64.5%; q: mean, Ϫ50.8%; range, Ϫ30.8% to Ϫ72.8%; L: mean, Ϫ50.3%; range, Ϫ37.0% to Ϫ65.1%). There were inconsistent changes in FA (mean, Ϫ0.5%; range, Ϫ44.9% to ϩ45.0%). Five patients had elevated FA due to proportionately higher loss of L than q. Conclusions-Changes in FA only occur when there is a change in the ratio of q/L. Acute elevation of FA occurred in the context of a larger reduction in L than q. The elevation in FA occurs in the context of a reduction in the anisotropic tensor and therefore is a consequence of ratio-metric measurement. This appears to clarify the reported increase in FA in terms of alterations in the shape of the apparent diffusion tensor. pq diagrams appear to offer improved resolution of acute diffusion changes in ischemia.

The Relationship between Diffusion Anisotropy and Time of Onset after Stroke

“…For major WM tracts, diffusion anisotropy was elevated in 4/8 patients r7 h after stroke onset (mean rFA = 1.2070.16 (P < 0.01) in these 4 patients) and 0/13 patients > 7 h. Subcortical WM in the gyri showed similar elevated rFA findings in 4/7 patients r7 h (mean rFA = 1.1770.21 (P < 0.01) in these 4 patients) and 0/12 patients > 7 h. Our findings are in agreement with five human DTI studies that have reported elevations of FA less than 6 h (13% to 21% rise) (Green et al, 2002;Ozsunar et al, 2004a, b;Schaefer et al, 2003;Yang et al, 1999). However, one study had 3 patients less than 6 h with only 1 showing elevated FA (Green et al, 2002), and another acquired only 3-direction diffusion-wt images in the majority of their patients (Yang et al, Figure 1 T 2 -weighted images (T2W; b = 0 sec/mm 2 ), isotropic diffusion-weighted images (DWI; b = 1000 sec/mm 2 ), Trace/3 apparent diffusion coefficient (ADC) maps, and fractional anisotropy (FA) maps from 2 patients depicting hyperacute (patient 1; 2 h postsymptom onset) and acute (patient 18; 14 h postsymptom onset) infarcts in the middle cerebral artery distribution involving major white matter (WM) tracts and subcortical WM regions (refer to Table 1). Over 19 slices in patient 1 (51-year-old male patient), 75% of the major WM regions-of-interest (ROIs) evaluated showed an increase in rFA, whereas 25% showed reductions in rFA.…”

“…However, studies reporting changes in FA have been variable with either reductions (Munoz Maniega et al, 2004;Sorensen et al, 1999), elevations (Schaefer et al, 2003), a mixture of changes (Green et al, 2002;Ozsunar et al, 2004a, b;Yang et al, 1999), or no change (Harris et al, 2004). Elevations of 8% to 45% in FA have been noted to occur from < 4 h up to 24 h after symptom onset (Green et al, 2002;Ozsunar et al, 2004a;Schaefer et al, 2003;Yang et al, 1999). Interestingly, similar studies have also reported FA reductions of 2% to 44% in individual patient data from 4 h up to 27 h after symptom onset (Green et al, 2002;Yang et al, 1999).…”

“…Elevations of 8% to 45% in FA have been noted to occur from < 4 h up to 24 h after symptom onset (Green et al, 2002;Ozsunar et al, 2004a;Schaefer et al, 2003;Yang et al, 1999). Interestingly, similar studies have also reported FA reductions of 2% to 44% in individual patient data from 4 h up to 27 h after symptom onset (Green et al, 2002;Yang et al, 1999). Although biologic variability is certainly plausible, the wide discrepancy in FA results after stroke onset may be analytical in nature, arising from the practice of sampling larger regionsof-interest (ROI) that encompass different types of WM tracts or both WM and GM areas within the ROI in the ischemic core (Green et al, 2002;Harris et al, 2004;Ozsunar et al, 2004a;Schaefer et al, 2003) and/or that patient data from r24 h was grouped for analysis, thereby obscuring any time-dependent trends (Munoz Maniega et al, 2004;Sorensen et al, 1999).…”

[P1–450]: Diffusion Tensor Image Segmentation of the Cerebrum Provides a Single Measure of Cerebral Small‐vessel Disease Severity Related to Cognitive Change

“…Due to the way in which FA is calculated, proportional differences in the Euclidian magnitude of a tensor and the magnitude of its anisotropic component will result in the same FA value (Green et al, 2002) and consequently a given FA value will not be unique. This introduces a certain level of ambiguity in the interpretation of FA values for O.A.…”

A Riemannian scalar measure for diffusion tensor images

“…While FA and MD are the most popular scalar measures in clinical research, in [17,18] plotting the FA in the region of interest (ROI) on a (|M I | F , |M A | F )-plane is seen to be useful. So far, all the scalars in this section have been pointwise measures, that is they contain information only on the diffusion tensor in a particular voxel.…”