functional magnetic resonance imaging

functional magnetic resonance imaging
Name	functional magnetic resonance imaging
Caption	A typical BOLD fMRI scan showing brain activation patterns.
MeshID	D046149

functional magnetic resonance imaging is a neuroimaging technique that measures and maps brain activity by detecting changes in blood flow. This method relies on the coupling between neuronal activation and hemodynamic response, known as neurovascular coupling. It is a primary tool in cognitive neuroscience, psychology, and neurology for non-invasively studying the living human brain.

Principles and physics

The most common method relies on the blood-oxygen-level-dependent contrast, discovered by Seiji Ogawa. This BOLD signal arises because oxygenated hemoglobin and deoxygenated hemoglobin have different magnetic properties. When a brain region becomes active, local cerebral blood flow increases disproportionately, leading to a decrease in deoxygenated hemoglobin. This change is detected as a slight increase in the MRI signal. The physics involves the principles of NMR, where hydrogen nuclei in water align with a strong static magnetic field, typically generated by superconducting magnets like those from Siemens or GE. Radiofrequency pulses are applied to excite these nuclei, and the resulting signal, influenced by local magnetic field homogeneity, is measured.

Experimental design

Researchers design tasks to evoke specific cognitive processes, often using paradigms from experimental psychology. Common designs include block designs, where stimuli of the same type are presented together, and event-related designs, which allow for the analysis of responses to individual trials. Stimuli are presented using software like Presentation or E-Prime, often synchronized with the scanner via tools from Psychology Software Tools, Inc.. The IRB must approve all studies involving human participants to ensure ethical standards, as outlined in documents like the Belmont Report.

Data analysis

Raw data undergoes extensive preprocessing using software such as SPM, FSL, or AFNI. Steps include motion correction, often using algorithms like those from the MNI, spatial smoothing, and temporal filtering. Statistical analysis, frequently employing the general linear model, identifies voxels where the signal correlates with the experimental paradigm. Group-level analyses use techniques like random effects analysis to generalize findings across a population. Advanced methods include functional connectivity analyses, such as those developed for the Human Connectome Project.

Applications

It is widely used to map cognitive functions to brain regions, contributing to fields like neurolinguistics and social neuroscience. Clinical applications include pre-surgical mapping for patients with epilepsy or brain tumors to identify eloquent cortex near lesions. It aids in studying neurological disorders such as Alzheimer's, Parkinson's, and schizophrenia. The technique is also employed in neuromarketing by companies like NeuroFocus and in lie detection research, though the latter is controversial.

Limitations and criticisms

The BOLD signal is an indirect measure of neural activity, with a temporal resolution limited by the slow hemodynamic response. The signal is susceptible to artifacts from head motion, physiological noise from cardiac and respiratory cycles, and scanner drift. Statistical methods have faced criticism regarding multiple comparisons and low statistical power, leading to debates about reproducibility, as highlighted in papers by researchers like Edward Vul. The high cost of superconducting magnet systems and operational expenses limit accessibility. Furthermore, reverse inference—inferring mental states from activation patterns—is a significant logical challenge noted by scholars such as Russell Poldrack.

History and development

The foundational discovery of NMR was made by Felix Bloch and Edward Purcell. The development of MRI for medical imaging was advanced by Paul Lauterbur and Peter Mansfield, who shared the 2003 Nobel Prize. The key breakthrough for this specific technique was Seiji Ogawa's 1990 report on BOLD contrast in *Magnetic Resonance in Medicine*. Early human experiments were conducted at MGH and the University of Minnesota. The field expanded rapidly with the founding of the OHBM and initiatives like the BRAIN Initiative launched under the administration of Barack Obama.

Category:Neuroimaging Category:Magnetic resonance imaging Category:Medical physics