Diffusion tensor imaging (DTI)
- DTI is a noninvasive technique for mapping information on the coherence and direction of white matter fibers in the brain.
- There is increasing interest in combining fMRI and DTI to map connections between regions of activation by using the functional activation as a seeding point for DTI fiber reconstructions.
- Conceptually, this raises several issues: (1) fMRI activations are limited to gray matter; DTI is limited to white matter; (2) seeding points are highly sensitive to arbitrary fMRI thresholding; and (3) the angle of intersection between fMRI and DTI eigenvectors can vary such that caution is needed to avoid propagating connections in erroneous directions.
- Potential solutions include developing probabilistic DTI models and over-sampling.
- There remains a fundamental lack of understanding of the relationship between what we see at the MRI level, at the diffusional MRI level contrast, and the microscopic structure of the tissue.
- Comparison of the amount of a marker for intracellular space, e.g., N-acetyl aspartate (NAA), may help distinguish intra- and extra-cellular DTI signals.
- Decreased fractional anisotropy, as an indication of the structural integrity of fibers, may reflect fewer axons or less myelin; a finding of less NAA might help constrain the interpretation of the data.
- Similar ambiguity characterizes other imaging measures, such as cortical thickness.
- To overcome the correlational nature of MR, longitudinal causational validation studies involving animals are needed.
