Functional magnetic resonance imaging

Functional magnetic resonance imaging, fMRI abbreviated or fMRI ( functional magnetic resonance imaging for English ), is an imaging technique used to represent physiological functions inside the body with the methods of magnetic resonance imaging. fMRI in the narrow sense refers to procedures that (mostly based on the blood oxygenation ) can be represented with high spatial resolution activated brain areas; in a broader sense, other functional imaging techniques, such as dynamic cardiac MRI, the time-resolved MRI of joint movement or perfusion MRI are called functional MRI.

Introduction

Through fMRI recordings it is possible to make blood flow changes in brain regions visible which are attributed to metabolic processes, which in turn are associated with neuronal activity. This makes you look at the different magnetic properties of oxygenated and deoxygenated blood to use ( BOLD contrast). Upon activation of cortical areas, there is an increase of metabolism, causing the activated area with a disproportionate increase in blood flow responding ( so-called neurovascular coupling). Thus, the concentration of oxygenated ( diamagnetic ) increases relative to deoxygenated ( paramagnetic ) hemoglobin. About the intermolecular Elektronendipol - nuclear dipole relaxation mechanism, this concentration change causes a change in the effective transverse relaxation time of the observed hydrogen nuclear spins and thus leads to a signal change in MRI. This, to draw conclusions about the location of a neuronal activity, the magnetic resonance signal of the tissue is compared at two time points - for example, in the stimulated or experimental condition on the one hand and at rest or control condition on the other. The recordings can be compared by statistical methods with each other and the statistically significant differences ( corresponding to the stimulated areas ) are spatially associated and displayed.

An fMRI investigation usually runs in three phases:

Can for experimental purposes in an investigation of the brain of the subjects in the third partial scan such as a repeated stimulus are presented. Often the stimulus with a task for the subject is linked, such as the prompt to press at each illustrated object X is a key. The most common tests is the frequent repetition of the task. So can then take place with those from the rest by statistical methods, a comparison of recorded data from the stimulus phase. The calculated from this difference is then projected in false color to the previously conducted anatomical MR scan.

Especially the neurology and neuropsychology benefit from the opportunities of fMRI. How could, for example, by comparative studies with fMRI between people who suffer from mental disorders such as depression, anxiety and obsessive-compulsive disorder, and healthy controls significant and in some cases of persistent chronic differences in cerebral metabolism are detected.

Historical Development

In 1935 Linus Pauling had found that change the magnetic properties of hemoglobin depending on the degree of oxygenation. This effect forms the basis for the measurement of brain activity with functional MRI, which was developed in the 1980s and 1990s. In 1982, Keith Thulborn and staff showed that hemoglobin in blood samples as a function of oxygenation in his MRI signal is different. The same observation was made in 1990 by Seiji Ogawa and employees in vivo in healthy volunteers; the property of hemoglobin to cause different MRI signals, " blood oxygenation level dependent ( BOLD ) effect" was called. First fMRI results, which showed brain activity after visual stimulation were published by John W. Belliveau and colleagues in 1991.

Confines

Compared to other well-established non-invasive neurophysiological investigation methods, such as EEG, although the ( relatively young ) fMRI shows significantly more powerful options in the spatially - localized investigation, but an inherently much lower temporal resolution. An additional uncertainty arises from the indirect nature of the method - the neural activity is not directly measured but inferred from changes in blood flow and oxygenation. Here, a roughly linear relationship between stimuli that are longer than four seconds, and BOLD effect is assumed. Whether the BOLD effect reproduces reliable neuronal activity with shorter stimuli is controversial and still the subject of current research.

Other technical limitations of fMRI measurement are:

• In intact tissues of the BOLD effect is caused not only by the blood in the vessels, but also by the cellular tissue around the vessels around.
• , A minimum size of the measuring - voxel below when measuring the BOLD effect can vessels, which have a cross section which is larger than the defined voxel size, are incorrectly interpreted as neural activity.

In addition, it specifies the basic assumptions and possible findings from fMRI studies criticism based on the fact that the visualization of the measured data of the fMRI has a constructive component, thus creating more of the model concepts of the research could be presented as actual events. Furthermore, absent in many studies statistical correction calculations to exclude random results.