The majority of fMRI studies use T2 or T2* weighted scans. Studies have shown that diffusion MRI scans can detect activation. However, the exact biophysical mechanism remains unclear. We will explore the physiology of neuronal activation, the BOLD response, fMRI and diffusion MRI, and how to disentangle the BOLD response and microstructure changes.
The vast majority of fMRI [1] experiments detect neuronal activation indirectly via the BOLD (blood oxygen level dependent) response. T2 and T2* weighted scans are used but there has been strong interest in using diffusion MRI (dMRI), which is sensitive to tissue microstructure. Studies have shown that dMRI is sensitive to neuronal activation in the CNS [2] but the exact biophysical contrast mechanism is not fully understood [3-8].
We will explore the electrophysiological changes in the axon and CNS due to activation, and subsequent changes in microstructure and vasculature. We will discuss dMRI studies that seek detect activation more directly and further dMRI studies that seek to disentangle the changes in vasculature and microstructure.
A particular study of interest involves an MRI viable isolated tissue (VIT) system that keeps extracted nervous tissue physiologically and electrically viable for extended periods of time [9,10]. Furthermore, tissue in the VIT chamber can be simulated chemically and electrically. Use of this chamber excludes the effects of the bold response since there is no blood. Preliminary results indicate that dMRI can detect activation of chemically and electrically stimulated nerves in the absence of the BOLD response and that the signal changes are small compared to signal changes seen in the presence of the BOLD response [11].
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