Based on magnetic resonance imaging (MRI), physicist Kenneth Kwong developed functional magnetic resonance imaging (fMRI) to visualize activity changes in different brain areas. In this method, changes in cerebral blood flow are measured, which are connected via the neurovascular coupling with activity changes in the corresponding brain areas.
This method exploits the different chemical environment of the measured hydrogen nuclei in the hemoglobin of oxygen-poor and oxygen-rich blood. Oxygenated hemoglobin (oxyhemoglobin) is diamagnetic while oxygen free hemoglobin (deoxyhemoglobin) has paramagnetic properties. The differences in the magnetic properties of the blood are also referred to as the BOLD (Blood-Oxygenation-Level Dependent Effect) effect. The functional processes in the brain are recorded in the form of sectional image series.
In this way, the activity changes in the individual brain areas can be examined by specific tasks on the subjects. This method initially serves as basic research for the comparison of activity patterns in healthy controls with the brain activities of persons with mental disorders. In a broader sense, however, the term functional magnetic resonance tomography also includes kinematic magnetic resonance tomography, which describes the moving representation of various organs.
Functional magnetic resonance imaging is a further development of magnetic resonance imaging (MRI). In classical MRI, static images of corresponding organs and tissues are displayed, while the fMRI reflects brain activity changes through three-dimensional images in the performance of certain activities.
Using this non-invasive procedure, the brain can be observed in a variety of situations. The physical basis of the measurement is based on nuclear magnetic resonance, as in classical MRI. By applying a static magnetic field, the spins of the protons of hemoglobin are longitudinally aligned. A high-frequency alternating field applied transversely to this magnetization direction provides for the transversal deflection of the magnetization to the static field up to the resonance (Lamor frequency). If the high-frequency field is switched off, it takes a certain amount of time, with the release of energy, until the magnetization again aligns itself along the static field.
This relaxation time is measured. The fMRI uses the fact that the different magnetization of deoxyhemoglobin and oxyhemoglobin is different. This results in different measurements for both forms, which are due to the influence of oxygen. However, because the physiological processes in the brain constantly change the ratio of oxyhemoglobin to deoxyhemoglobin, the fMRI makes continuous series recordings that register changes at all times. In this way, nerve cell activities can be displayed with millimeter precision in a time window of just a few seconds. Experimentally, the location of neuronal activity is determined by measurements of the magnetic resonance signal at two different times.
First, the measurement takes place at rest and then in an excited state. Then, the comparison of the images are performed in a statistical test procedure and spatially assigned the statistically significant differences. For experimental purposes, the stimulus can be presented to the subject several 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 resting phase are calculated and then depicted. In this procedure, it was possible to determine which brain areas are active in which activity. In addition, the differences of certain brain areas in psychological disorders were found in healthy brains.
In addition to basic research, which provides important insights into the diagnosis of psychological disorders, the procedure is also used directly in clinical practice. The main clinical application of the fMRI is the localization of language-relevant brain areas in the preparation of operations in brain tumors. This is to ensure that this area is largely spared during the operation. Further clinical applications of functional magnetic resonance imaging relate to the evaluation of patients with disorders of consciousness, such as coma, wake coma or MCS (Minimal State of Consciousness).
Despite the great successes of functional magnetic resonance imaging, this method should also be viewed critically. Significant connections between certain activities and the activation of corresponding brain areas could be found. The importance of certain brain areas for psychological disorders has also become clearer.
However, only the changes in the oxygen load of hemoglobin are measured here. Because these processes can be localized to specific areas of the brain, it is assumed that these brain areas are also activated due to neurovascular coupling. So the brain can not be observed directly in thinking. It should be noted that the change in blood flow occurs only after a latency of a few seconds after the neuronal activity. Therefore, direct assignment is sometimes difficult. In contrast to other non-invasive neurological examination methods, fMRI offers a much better spatial localization of activities.
However, the temporal resolution is much lower. Indirect determination of neuronal activity by blood flow measurements and hemoglobin oxygenation also creates some uncertainty. So a latency of over four seconds is assumed. Whether reliable neural activities can be assumed with shorter stimuli has yet to be investigated. However, there are also technical application limits of functional magnetic resonance imaging, which among other things are based on the fact that the BOLD effect is produced not only by the blood vessels, but also by the tissue adjacent to the vessels.