(
BOLD) In
MRI the changes in blood oxygenation level are visible. Oxyhaemoglobin (the principal
haemoglobin in arterial blood) has no substantial magnetic properties, but deoxyhaemoglobin (present in the draining veins after the oxygen has been unloaded in the tissues) is strongly paramagnetic. It can thus serve as an intrinsic paramagnetic
contrast agent in appropriately performed
brain MRI. The concentration and relaxation properties of deoxyhaemoglobin make it a
susceptibility , e.g.
T2 relaxation effective
contrast agent with little effect on
T1 relaxation.
During activation of the
brain, the oxygen consumption of the local tissue increase by approximately 5% with that the oxygen tension will decrease. As a consequence, after a short period of time vasodilatation occurs, resulting in a local increase of blood volume and
flow by 20 - 40%. The incommensurate change in local blood
flow and oxygen extraction increases the local oxygen level.
By using
T2 weighted gradient echo EPI
sequences, which are highly
susceptibility sensitive and fast enough to capture the three-dimensional nature of activated
brain areas will show an increase in
signal intensity as oxyhaemoglobin is diamagnetic and deoxyhaemoglobin is paramagnetic. Other MR pulse
sequences, such as spoiled
gradient echo pulse
sequences are also used.
As the effects are subtle and of the order of 2% in 1.5 T
MR imaging, sophisticated methodology, paradigms and data analysis techniques have to be used to consistently demonstrate the effect.
As the
BOLD effect is due to the deoxygenated blood in the draining veins, the spatial localization of the region where there is increased blood
flow resulting in decreased oxygen extraction is not as precisely defined as the morphological features in
MRI. Rather there is a physiological
blurring, and is estimated that the linear dimensions of the physiological
spatial resolution of the
BOLD phenomenon are around 3 mm at best.