Functional Magnetic Resonance Imaging of Neuronal Activity

Functional magnetic resonance imaging (fMRI) has revolutionised our ability to observe brain function non-invasively by exploiting changes in magnetic resonance signals that accompany neuronal activity. The predominant contrast mechanism, known as blood oxygenation level-dependent (BOLD) fMRI, infers local neural activation from the haemodynamic response linked to neurovascular coupling. While BOLD fMRI offers excellent spatial coverage and clinical utility, its reliance on vascular dynamics imposes limits on temporal resolution and specificity. Recent advances seek to overcome these constraints by targeting more direct signatures of neural events. Diffusion functional MRI (DfMRI) probes activity-induced water movement, offering contrasts more closely tied to cellular swelling and microstructural changes. Parallel efforts aim to detect the weak magnetic fields or phase shifts generated by neuronal currents, through specialised pulse sequences and high-field instrumentation. Together, these approaches are refining our capacity to map neural processes with greater fidelity and may ultimately expand the scope of functional neuroimaging in both research and clinical settings.

Research from Nature Portfolio

Recent studies have provided new mechanistic insight into diffusion-based functional contrasts. In a high-field diffusion MR microscopy experiment, activation-induced swelling of individual neurons in Aplysia buccal ganglia led to measurable changes in intracellular and tissue-level apparent diffusion coefficients (ADC), directly linking the DfMRI signal to cell volume dynamics. This work supports the hypothesis that local ADC decreases during neural activation primarily reflect transient neuronal swelling rather than solely vascular contributions. In a complementary phantom study, a novel partial spin-lock sequence was introduced to capture phase variations arising from oscillatory magnetic fields akin to those produced by neural currents. Simulations and phantom measurements demonstrated that the sequence can resolve minute phase differences at frequencies corresponding to neuronal activity, indicating a promising route towards direct mapping of functional connectivity without haemodynamic intermediaries.

Functional Magnetic Resonance Imaging of Neuronal Activity publication trend

The graph below shows the total number of articles in functional magnetic resonance imaging of neuronal activity across all publications each year (not limited to Nature Index journals).

Technical terms

Blood oxygenation level-dependent (BOLD) fMRI: Functional contrast based on changes in deoxyhaemoglobin concentration following neural activation.
Diffusion functional MRI (DfMRI): Technique measuring activity-related alterations in water diffusion to infer cellular processes.
Apparent diffusion coefficient (ADC): Quantitative index of water molecule mobility within tissue, sensitive to microstructural changes.
Neurovascular coupling: Physiological mechanism linking neuronal activity to local changes in cerebral blood flow and volume.
Spin-lock preparation: MRI sequence module that applies a continuous radiofrequency field to probe relaxation processes sensitive to oscillatory magnetic fields.

References

1. Modulation of water diffusion by activation-induced neural cell swelling in Aplysia Californica. Scientific Reports (2017).
2. Neural magnetic field dependent fMRI toward direct functional connectivity measurements: A phantom study. Scientific Reports (2020).
3. Initial experiences with Direct Imaging of Neuronal Activity (DIANA) in humans. Imaging Neuroscience (2023).
4. Can MRI Be Used as a Sensor to Record Neural Activity?. Sensors (2023).
5. Water diffusion closely reveals neural activity status in rat brain loci affected by anesthesia. PLOS Biology (2017).