Abstract
Diffusion functional magnetic resonance imaging (DfMRI) has been proposed as a method for functional neuroimaging studies, as an alternative to blood oxygenation level dependent (BOLD)-fMRI. DfMRI is thought to more directly reflect neural activation, but its exact mechanism remains unclear. It has been hypothesized that the water apparent diffusion coefficient (ADC) decrease observed upon neural activation results from swelling of neurons or neuron parts. To elucidate the origin of the DfMRI response at cellular level we performed diffusion MR microscopy at 17.2 T in Aplysia californica buccal ganglia and compared the water ADCs at cellular and ganglia levels before and after neuronal activation induced by perfusion with a solution containing dopamine. Neural cell swelling, evidenced from optical microscopy imaging, resulted in an intracellular ADC increase and an ADC decrease at ganglia level. Furthermore, the intracellular ADC increase was found to have a significant positive correlation with the increase in cell size. Our results strongly support the hypothesis that the ADC decrease observed with DfMRI upon neuronal activation at tissue level reflects activation-induced neural cell swelling.
Highlights
Diffusion functional magnetic resonance imaging (DfMRI)[1] has been proposed as an alternative for blood oxygenation level dependent (BOLD)-fMRI2
While the DfMRI signal acquired with a low degree of sensitization to diffusion in the mammalian brains contains a residual BOLD signal[23], the absence of blood in the excised Aplysia ganglia guarantees the absence of any BOLD effect in the DfMRI signal acquired in this study
Electrophysiological[24] and manganese enhanced MRI (MEMRI) studies[22] have previously shown that dopamine promotes neural activation in the Aplysia buccal ganglia, we investigated water apparent diffusion coefficient (ADC) changes induced by dopamine application using high resolution DfMRI
Summary
Diffusion functional magnetic resonance imaging (DfMRI)[1] has been proposed as an alternative for blood oxygenation level dependent (BOLD)-fMRI2. Based on earlier reports that the water apparent diffusion coefficient (ADC) decreases in relation to cell swelling[1, 9,10,11,12] and that neural swelling is one of the responses associated with neural activation[13,14,15,16,17,18,19], it has been hypothesized that the decrease in the water ADC observed during neural evoked responses would originate from the dynamic swelling of neurons or neuron parts, in line with the so-called neuromechanical coupling hypothesis[6] To investigate this hypothesis at cellular level, we performed diffusion magnetic resonance (MR) microscopy studies on the Aplysia californica buccal ganglia using an ultra-high magnetic field MRI (17.2 tesla).
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