fMRI

Functional magnetic resonance imaging (fMRI) is a neuroimaging technique that measures brain activity by detecting changes in blood oxygenation. It relies on the blood-oxygen-level-dependent (BOLD) contrast, an indirect indicator of neural activity derived from T2*-weighted MRI signals. The method does not measure neural activity directly but infers it from vascular responses to neuronal activation.

During fMRI, subjects perform tasks or rest while the scanner repeatedly acquires whole-brain images, often using echo-planar imaging. Spatial resolution is typically 2-3 millimeters isotropic, and temporal resolution is on the order of 1-2 seconds per volume. The BOLD signal reflects a hemodynamic response that lags neural activity by several seconds and typically peaks about 4-6 seconds after onset.

Data are processed through preprocessing steps such as slice timing, motion correction, normalization to a common brain space, and spatial smoothing. Statistical analysis commonly uses a general linear model to relate the experimental design to the observed BOLD signals, producing activation maps while controlling for multiple comparisons. Resting-state fMRI analyzes spontaneous low-frequency fluctuations to infer functional networks without a task.

Applications include localizing brain regions involved in cognitive functions in task-based studies, and mapping intrinsic networks in resting-state studies. fMRI is used in cognitive neuroscience, clinical research, and presurgical planning. Limitations include the indirect nature of the BOLD signal, relatively poor temporal resolution, sensitivity to motion and artifacts, and high cost. Safety relies on MRI compatibility, with contraindications for certain implants and devices. The technique emerged in the early 1990s and has since become widely adopted.