Functional DWI in Brain Mapping

The role of fMRI has become increasingly important in neuroimaging, for example in the presurgical mapping of gray matter (Hendler et al., 2003). Contrast mechanisms based on the blood oxygenation level, volume, and flow changes have been used to noninvasively detect brain activation secondary to the neuronal activity. Recently, several efforts have focused on alternative contrast mechanisms that may offer shorter temporal delays and more direct spatial localization in brain mapping. These include the detection of Lorentzian forces from neural activation from phase-sensitive images, the use of diffusion-weighting to sensitize the image to changes in incoherent displacements, and the mapping of small changes in the ADC from neural activity.

Phantom studies now suggest the possibility of detecting the destructive phase addition from the spatially incoherent, yet temporally synchronized, displacements caused by the Lorentz force experienced during electrical conduction within a strong magnetic field (Bellgowan et al., 2003).

Another approach has employed heavy diffusion weighting to remove the vascular signal and sensitize the minute and incoherent displacement in order to detect fast dynamic signal changes synchronized to a task (Li and Song, 2003). These authors observed fast functional signal changes consequent to the task activation by using a heavy diffusion-weighting protocol which possessed a different time course from the corresponding BOLD response and suggested an alternative origin different from the common BOLD signal sources. Li and Song concluded that the characteristics of the signal change suggested that it may be more directly linked to the neuronal activity temporally and spatially than the BOLD response. Song has recently expanded this approach to include the mapping of the diffusion tensor to detect synchronized fMRI signal changes during brain activation (Song et al., 2003).

Using fast-DWI, Le Bihan and colleagues observed changes in the apparent diffusion coefficient of water in the human brain visual cortex during activation by a flickering checkerboard task activation paradigm (Darquie et al., 2001). The ADC decrease was less than 1% but significant and reproducible, and closely followed the time course of the activation paradigm. The observed ADC findings were as cribed to a transient swelling of cortical cells that occurred with neural activation (Darquie et al., 2001).

These preliminary results from a variety of new imaging techniques suggest a new approach to the production of images of brain activation, with MRI from signals directly associated with neuronal activation rather than through changes in local blood flow. More importantly, the methods may in the near future provide noninvasive maps of the neural activation pathways in the central nervous system.

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