TSFA (Tract-Specific Fractional Anisotropy) at crossing-fiber regions with clinical diffusion MRI
To accommodate complex “wiring” in the human brain with a volume of around 1.2 liters, a significant percentage of white matter (WM) fibers need to cross with each other. Diffusion magnetic resonance imaging (dMRI) in routine clinical research is usually acquired within a limited time of less than 5 minutes and with a relatively low b-value such as 1000s/mm2. The resultant voxel size is usually from 2x2x2mm3 to 3x3x3mm3 and crossing fibers exist in a large proportion of WM voxels. It is believed that almost half of the brain voxels contain crossing fibers. Fractional anisotropy (FA) derived from the single tensor model and ranging from 0 to 1, has been widely used to infer the WM microstructural changes associated with many neurological or psychiatric disorders. However, single tensor FA is significantly underestimated at the crossing-fiber regions. In addition, tract-specific analyses including tract statistics tract morphometry and tract metric analysis have become important due to great clinical significance of the tracts. With the extensive applications of FA in neurological and psychiatric studies such as phenotype characterization, drug testing and therapy monitoring, biased conclusions from the underestimated single tensor FA in tract analysis will have significant negative impacts on these studies. It is, therefore, critical to correct FA at the crossing-fiber regions for the effects of free water and crossing-fiber geometry and obtain the tract-specific FA (TSFA) for each crossed tract with routine clinical dMRI, namely single shell dMRI with 25-30 diffusion orientations.
For details, please see our Publication (Mishra et al., Magnetic Resonance in Medicine 2014).