Functional magnetic resonance imaging (fMRI) is a magnetic resonance imaging technique used to image physiological changes in the body. It is based on the physical principles of nuclear magnetic resonance. In a narrower sense, the term is used in connection with the examination of activated brain areas.
What is functional magnetic resonance imaging?
Classical MRI displays static images of corresponding organs and tissues, while fMRI reproduces changes in activity in the brain through three-dimensional images during the performance of specific activities. Based on magnetic resonance imaging (MRI), physicist Kenneth Kwong developed functional magnetic resonance imaging (fMRI) for imaging changes in activity in different brain areas. This method measures changes in cerebral blood flow that are associated with activity changes in the corresponding brain areas via neurovascular coupling. This method takes advantage of the different chemical environment of the measured hydrogen nuclei in the hemoglobin of oxygen-depleted and oxygenated blood. Oxygenated hemoglobin (oxyhemoglobin) is diamagnetic, while oxygen-free hemoglobin (deoxyhemoglobin) has paramagnetic properties. The differences in the magnetic properties of blood are also known as the BOLD effect (Blood-Oxygenation-Level Dependent Effect). The functional processes in the brain are recorded in the form of cross-sectional image series. In this way, the changes in activity in the individual brain areas can be investigated by means of specific tasks performed on the test subjects. This method is initially used for basic research to compare activity patterns in healthy control subjects with the brain activities of individuals with mental disorders. However, in a broader sense, the term functional magnetic resonance imaging still includes kinematic magnetic resonance imaging, which describes the moving representation of various organs.
Function, effect, and goals
Functional magnetic resonance imaging is a further development of magnetic resonance imaging (MRI). Classical MRI displays static images of corresponding organs and tissues, while fMRI reflects changes in activity in the brain through three-dimensional images during the performance of specific activities. Thus, with the help of this non-invasive method, the brain can be observed under different situations. As with classical MRI, the physical basis of the measurement is initially based on nuclear magnetic resonance. Here, the spins of the protons of the hemoglobin are aligned longitudinally by applying a static magnetic field. A high-frequency alternating field applied transversely to this direction of magnetization ensures the transverse deflection of the magnetization to the static field until resonance (Lamor frequency) is reached. If the high-frequency field is switched off, it takes a certain time under energy dissipation until the magnetization aligns along the static field again. This relaxation time is measured. In fMRI, the circumstance of different magnetization of deoxyhemoglobin and oxyhemoglobin is exploited. This results in different readings for the two forms due to the influence of oxygen. However, since the ratio of oxyhemoglobin to deoxyhemoglobin is constantly changing during physiological processes in the brain, serial recordings are performed as part of fMRI, which registers the changes at each point in time. Thus, within a time window of a few seconds, neuronal activity can be visualized with millimeter precision. Experimentally, the location of neuronal activity is determined by measurements of the magnetic resonance signal at two different time points. First, the measurement is made in a resting state and then in an excited state. Then, the comparison of the recordings is performed in a statistical test procedure and the statistically significant differences are spatially assigned. For experimental purposes, the stimulus can be presented to the subject multiple times. This usually means that a task is repeated frequently. The differences from the comparison of the data from the stimulus phase with the measurement results from the rest phase are calculated and then displayed pictorially. With this procedure, it was possible to determine which areas of the brain are active during which activity.Furthermore, the differences of certain brain areas in psychological disorders to healthy brains could be determined. In addition to basic research, which provides important findings for the diagnosis of psychological disorders, the method is also used directly in clinical practice. The main clinical application of fMRI is the localization of language-relevant brain areas in the preparation of operations for brain tumors. The aim is to ensure that this area is largely spared during surgery. Other clinical applications of functional magnetic resonance imaging relate to the assessment of patients with disorders of consciousness, such as coma, waking coma, or MCS (minimally conscious state).
Risks, side effects, and hazards
Despite the great success of functional magnetic resonance imaging, this method should also be viewed critically in terms of its validity. Significant correlations between certain activities and the activation of corresponding brain areas could be established. The significance of certain brain areas for psychological disorders has also become clearer. However, only changes in the oxygen loading of hemoglobin are measured here. Because these processes can be localized to specific brain areas, it is assumed that these brain areas are also activated due to neurovascular coupling. So, the brain cannot be directly observed thinking. It must be noted that the change in blood flow occurs only after a latency period of several seconds after neuronal activity. Therefore, direct mapping is sometimes made difficult. However, an advantage for fMRI compared to other non-invasive neurological examination methods is the much better spatial localization of the activities. However, the temporal resolution is much lower. Indirect determination of neuronal activities by blood flow measurements and hemoglobin oxygenations also generates some uncertainty. Thus, a latency of more than four seconds is assumed. Whether reliable neuronal activities can be assumed for shorter stimuli remains to be investigated. However, there are also still technical limitations to the application of functional magnetic resonance imaging, based in part on the fact that the BOLD effect is produced not only by blood vessels but also by cellular tissue adjacent to the vessels.
