Fractional anisotropy | EPFL Graph Search

Fractional anisotropy (FA) is a scalar value between zero and one that describes the degree of anisotropy of a diffusion process. A value of zero means that diffusion is isotropic, i.e. it is unrestricted (or equally restricted) in all directions. A value of one means that diffusion occurs only along one axis and is fully restricted along all other directions. FA is a measure often used in diffusion imaging where it is thought to reflect fiber density, axonal diameter, and myelination in white matter. The FA is an extension of the concept of eccentricity of conic sections in 3 dimensions, normalized to the unit range. A Diffusion Ellipsoid is completely represented by the Diffusion Tensor, D. FA is calculated from the eigenvalues () of the diffusion tensor. The eigenvectors give the directions in which the ellipsoid has major axes, and the corresponding eigenvalues give the magnitude of the peak in that direction. with being the mean value of the eigenvalues. An equivalent formula for FA is which is further equivalent to: where R is the "normalized" diffusion tensor: Note that if all the eigenvalues are equal, which happens for isotropic (spherical) diffusion, as in free water, the FA is 0. The FA can reach a maximum value of 1 (this rarely happens in real data), in which case D has only one nonzero eigenvalue and the ellipsoid reduces to a line in the direction of that eigenvector. This means that the diffusion is confined to that direction alone. This can be visualized with an ellipsoid, which is defined by the eigenvectors and eigenvalues of D. The FA of a sphere is 0 since the diffusion is isotropic, and there is equal probability of diffusion in all directions. The eigenvectors and eigenvalues of the Diffusion Tensor give a complete representation of the diffusion process. FA quantifies the pointedness of the ellipsoid, but does not give information about which direction it is pointing to. Note that the FA of most liquids, including water, is 0 unless the diffusion process is being constrained by structures such as network of fibers.

Using medical images acquired during routine clinical care for research purposes: a comprehensive medical informatics approach applied to the study of brain development from birth through 2 years of age as indexed by diffusion MRI

Vincent Alexis Roch

Biologic variability and dramatic changes of brain development in children aged 0 to 2 years make it challenging to accurately detect subtle abnormalities in single Magnetic Resonance Imaging (MRI) scans. Diffusion MRI (dMRI) indices such as Apparent Diffusion Coefficient (ADC) are reliable measures of water content in the brain and thus an excellent surrogate marker for brain development. Developing robust age-specific diffusion biomarkers for quantitative measurement of normative brain evolution would enhance our ability to detect subtle alterations due to tissue injuries or neuropathological disorders. Obtaining significant numbers of normative MRI scans for this age group means redirecting clinical data from hospital databases for research purposes. Therefore, this pilot project demonstrates the feasibility of identifying, retrieving and analyzing pediatric clinical dMRI data to investigate normal brain development from birth to 2 years. Research Patient Data Registry (RPDR) at Massachusetts General Hospital (MGH) was used to collect patient medical information and identify healthy children according to radiology reports. Corresponding MRI data were retrieved from MGH Picture Archiving and Communication System (PACS) using the prototype of Medical Imaging Informatics Bench to Bedside (mi2b2) software. A specific pipeline was created to handle the volume of studies and extract technical scan information used to identify comparable diffusion series; 193 studies were used for analysis. Two markers, whole brain average of ADC and Fractional Anisotropy (FA) values (WBAADC and WBAFA), were computed for each patient, their age-evolution across patients was investigated with different models. WBAADC and WBAFA seem to exhibit biexponential decay and increase respectively and might be gender-specific. These results have clinical implications for potentially determining the health status of an unknown individual, and research utility for continued development of these tools.

2011

Vincent Alexis Roch

2011