Abstract
PurposeThe apparent propagator anisotropy (APA) is a new diffusion MRI metric that, while drawing on the benefits of the ensemble averaged propagator anisotropy (PA) compared to the fractional anisotropy (FA), can be estimated from single‐shell data.Theory and MethodsComputation of the full PA requires acquisition of large datasets with many diffusion directions and different b‐values, and results in extremely long processing times. This has hindered adoption of the PA by the community, despite evidence that it provides meaningful information beyond the FA. Calculation of the complete propagator can be avoided under the hypothesis that a similar sensitivity/specificity may be achieved from apparent measurements at a given shell. Assuming that diffusion anisotropy (DiA) is nondependent on the b‐value, a closed‐form expression using information from one single shell (ie, b‐value) is reported.ResultsPublicly available databases with healthy and diseased subjects are used to compare the APA against other anisotropy measures. The structural information provided by the APA correlates with that provided by the PA for healthy subjects, while it also reveals statistically relevant differences in white matter regions for two pathologies, with a higher reliability than the FA. Additionally, APA has a computational complexity similar to the FA, with processing‐times several orders of magnitude below the PA.ConclusionsThe APA can extract more relevant white matter information than the FA, without any additional demands on data acquisition. This makes APA an attractive option for adoption into existing diffusion MRI analysis pipelines.
Highlights
The term diffusion magnetic resonance imaging refers to a set of diverse imaging techniques that, when applied to brain studies, provide useful information about the microscopic organization and connectivity of the white matter
Diffusion tensor MRI (DTMRI) brought to light one of the common issues of diffusion magnetic resonance imaging (dMRI) techniques: in order to carry out clinical studies, the information given by the selected diffusion analysis method must be translated into some scalar measures that describe different features of diffusion within every voxel
The apparent propagator anisotropy (APA) is able to obtain similar results as the fractional anisotropy (FA) with a smaller sample size. This feature makes the APA a robust alternative to the FA even with datasets collected for diffusion tensor (DT)-MRI-based analysis, that is, single-shell data with b ≈ 1000 s ∕ mm2
Summary
The term diffusion magnetic resonance imaging (dMRI) refers to a set of diverse imaging techniques that, when applied to brain studies, provide useful information about the microscopic organization and connectivity of the white matter. One relevant feature of dMRI is its ability to measure orientational variance in the different tissues, that is, anisotropy. The most common way to estimate the anisotropy is via the diffusion tensor (DT).. Diffusion tensor MRI (DTMRI) brought to light one of the common issues of dMRI techniques: in order to carry out clinical studies, the information given by the selected diffusion analysis method must be translated into some scalar measures that describe different features of diffusion within every voxel. Despite the strong limitations that the underlying Gaussian assumption imposes, the FA is still widely used in clinical studies involving dMRI Metrics like the fractional anisotropy (FA) were defined with the DT as a starting point. Despite the strong limitations that the underlying Gaussian assumption imposes, the FA is still widely used in clinical studies involving dMRI
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