Fractional anisotropy

Fitting Attempt Reference Relaxation Time (s) Reference Volume (cm)3 Standard Deviation Volume Fraction (%) Anisotropy Constant (erg/cm3)... 

Figure 50.10. Diffusion tensor imaging results from a mouse brain imaging study at 7 T. (A) mean diffusisivity (D ) (B) Fractional Anisotropy (FA), (C) Color coded FA orientation map.

Brain lesion analysis is also an active area of research. DTI metrics have been found to help differentiate between tumor types and promises to provide a sensitive method for abnormal tissue characterization. Fractional anisotropy has already been found to correlate with the cell density of brain tumors (Beppu et al., 2003 Beppu et al., 2005). The ability of FA to correlate with cell density likely accounts for the sensitivity of FA to tissue typing in brain lesions. Fractional anisotropy has been found to differentiate between low grade and anaplastic astrocytomas, to help differentiate between brain metastasis and high grade gliomas, and there is evidence that FA may help to detect tumor infiltration not visible by conventional MRI (Holmes et al., 2004 Tsuchiya et al., 2005 Goebell et al., 2006). Other DTI metrics have been utilized in brain lesion analysis and the early findings point to the breadth of future possibilities. 

Beppu T, Inoue T, Shibata Y, Kurose A, Arai H, Ogasawara K, Ogawa A, Nakamura S, Kabasawa H (2003) Measurement of fractional anisotropy using diffusion tensor MRI in supratentorial astrocytic tumors. J Neurooncol 63 109-116. 

For a scalar measurement of the amount of anisotropy in tissue, rotationally invariant metrics from the diffusion tensor have been proposed by Basser et al. [2]. The fractional anisotropy (FA) map is defined as the proportion of the diffusion tensor D that can be assigned to anisotropic diffusion. It is given by ... 

Indices of diffusion anisotropy - fractional anisotropy (EA) and lattice index (LI), calculate the degree of differences in diffusion in different directions... 

Fig. 7.15 Wallerian degeneration in the right corticospinal tract 3 months after an infarction in the right MCA territory. Fractional anisotropy images demonstrate hypointensity secondary to reduced FA in the tight corticospinal tract...

Ozsunar, Y, et al.,Evolution of water diffusion and anisotropy in hyperacute stroke significant correlation between fractional anisotropy and T2. AJNR Am J Neuroradiol, 2004. 25(5) p. 699-705. 

DTI identifies the motion of water that is quantified by voxel and region-based methods measuring diffusivity and fractional anisotropy. Diffusivity, a measurement of amplitude of diffusional motion, is increased with neuronal loss and gliosis. 

Fractional anisotropy reflects directional motion of water. Huid motion in the brain is restricted to the same axis as the axon or myeUn sheath. With neuronal damage, fractional anisotropy decreases secondary to less restricted motion in various axes. 

DTI provides a measure (fractional anisotropy) of the coherence and integrity of myelinated pathways. Children with ADHD displayed a reduced fractional anisotropy in the right neostriatum and premotor cortex, and in the left cerebellum and parieto-occipital cortex compared to matched controls (Ashtari et al., 2005). The lower the cerebellar fractional anisotropy, the more severe were the ratings of symptoms of inattention (Ashtari et al., 2005). These findings point to a link between white matter abnormalities and the symptomatology of ADHD. 

Subset of young adults, Cincinnati Lead Study (N = 91) Diffusion tensor imaging to relate PbB to changes in white matter structure Study subjects prior PbBs Decreased fractional anisotropy with increased PbB through white matter amount Brubaker etal. (2009)... 

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