Blood-oxygen-level dependent

As BOLD contrast ( blood oxygenation level dependent of English, so " depends on the blood oxygen content " ) are referred to in the magnetic resonance imaging ( MRI), the dependence of the (image) signal from the oxygen content in the red blood cells. The main application of BOLD contrast is the functional MRI (fMRI ) to show the brain activity. Synonym is also used for blood oxygen level dependent or (more rarely) blood oxygen ( ation ) level dependence / dependency the acronym BOLD.

Historical Development

Already in 1935, introduced Linus Pauling stated that change the magnetic properties of the protein hemoglobin in red blood cells, depending on the degree of oxygenation. In 1982, Keith Thulborn and staff showed that hemoglobin in blood samples having different MRI signals as a function of oxygenation. The same effect is observed in 1990 Seiji Ogawa and employees in vivo in experimental animals; of them the name " blood oxygenation level dependent ( BOLD ) " contrast was coined. Ogawa also realized the potential that would have the BOLD contrast for functional MRI. The first results, which showed the brain activity of subjects after visual stimulation using the BOLD contrast, were published in 1992 by John W. Belliveau and employees. Nikos Logothetis and co-workers showed in 2001 that the so- measured BOLD response is directly related to neuronal activity.

Physical Basics

Deoxygenated hemoglobin ( desHb ) contains (due to the ionic bonding of the iron atom ) four unpaired electrons per heme group and is therefore paramagnetic. In oxygenated with oxygen hemoglobin ( oxyHb ), however, the iron binding covalently and there are no unpaired electrons before; oxygenated hemoglobin is therefore diamagnetic.

MRI scans in addition to the proton form ( density) distribution and the relaxation behavior of the hydrogen nuclei in the sample ( which is different for different fluids and tissues ) as the contrast. The strong magnetic dipole field of the paramagnetic deoxygenated hemoglobin leads to local magnetic field inhomogeneities and leads via the intermolecular dipole-dipole relaxation mechanism for dephasing of the initially coherently precessing nuclear spins. This dephasing is observed as truncated (transverse ) relaxation time in the vicinity of desHb and thus changes - depending on the desHb concentration - the contrast of the image. As shown in the adjacent figure, the relaxation rate varies linearly with the square of desHb concentration; with increasing desHB concentration increases the relaxation rate to and from the relaxation time. Even more pronounced than the change of the oxygenierungsabhängige change that underlies the contrast in gradient echo images. In these therefore the BOLD contrast change due to the change - time is particularly clear; a lesser extent, but it is also observed in spin -echo images due to the also oxygenierungsabhängigen time.

Mathematically describe the changes in the relaxation rates as

Wherein and the relaxation rates of ( diamagnetic ) are oxygenated hemoglobin and the amount of deoxygenated hemoglobin. varies from 0 ( exclusively oxygenated hemoglobin ) and 1 = 100 % (excluding deoxygenated hemoglobin). Often you will also find a description of the blood oxygenation dependent ( " saturation "), which is obtained if one sets.

Measured values ​​of the quantities and are in a magnetic field:

( In the measurements last listed the coefficients are set to 0, since the determination of and influence each other can and would thus lead to unreliable results. )

Applications of the BOLD effect

• The BOLD effect can spoke up for the measurement of neural activity using fMRI. Here one observes a signal increase in activated brain areas in - Weighted (or weighted ) MRI scans. This will be explained such that the neuronal activity resulting in an increased oxygen consumption and hence more first deoxygenated hemoglobin; This effect is more than offset by an increased cerebral blood flow with inflowing oxygenated hemoglobin ( " neurovascular coupling " ), so that eventually decreases the desHb concentration in activated brain areas and thus the transverse relaxation time ( and the observed signal ) increases.

• Due to the BOLD effect can be personalized with the suszeptibilitätsgewichteten imaging ( SWI) MR Venographien create. The SWI method was to start the name BOLD, this was then by the more general term " suszeptibilitätsgewichtet " replaced because BOLD -based Venographien are just one application purpose of this procedure.

• Another application is the BOLD imaging of the kidneys to measure the intrarenal oxygenation; in particular, the change of oxygenation by the administration of substances such as furosemide (Lasix ®) can be investigated. The subject of research is to use the BOLD contrast to study the oxygenation of tumors.