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Magnetic Resonance Imaging
The reason functional imaging can be achieved by detecting the oxygenated blood
roots in the physiology of the biological tissues. For instance, neuronal function has
a metabolic activity that is directly related to changes of the oxygenation state of the
neurons as follows. The oxyhemoglobin carries the oxygen to the neurons, and, during the metabolic activity of the nerve cells, the oxygen uptake by the neurons causes
oxyhemoglobin to change into deoxyhemoglobin. In addition, neurons require glycogen, obtained from the red blood cells, for their metabolic processes during electric
activity. The glucose uptake can be measured with the chemical fluorodeoxyglucose
(FDG) that has an intrinsic magnetic moment and is incorporated in the cellular
metabolism as regular glucose. The oxygen and glucose requirement by the neurons during cellular depolarization activity has as a direct consequence an increased
blood perfusion in the capillaries surrounding the nerve cells.
The exact f MRI procedure is designed based on the functional characteristics to
be evaluated to ensure that there is a reasonable correlation between these characteristics and certain traceable metabolic aspects. For instance, if the motor-control
activities of the brain need to be evaluated, the patient will be asked to move a toe or
a finger while the system collects data.
One issue in the task design is the concern that the sequence of the tasks assigned
to the patient must be in a specific order to derive a correlation between the brain
activities and a particular cognitive or motor task. An additional requirement is that
any inappropriate and unrelated actions must be filtered out by this methodology.
Other typical examples of specific tasks involve audio or visual input stimuli to
evoke a cognitive response in the brain.
Two commonly used methods apply a fast data acquisition protocol to reduce
the errors from lapses in attention or motion artifacts. The first method is called
fast low angle shot (FLASH). During FLASH imaging, there is a short interval
between the f MRI RF pulses. This short interval reduces the flip angle and consequently reduces the realignment rate, T 1 . As a result, the acquisition sequence
can be accelerated. The second method is echo planar imaging (EPI). EPI applies
small amplitude RF pulses combined with a high gradient in the magnetic field.
The steep gradient provides better contrast and faster acquisition with typical
acquisition times under 100 ms. The low intensity and high gradient have the disadvantage of introducing distortions and a low signal amplitude. In particular,
in the brain, the magnetic inhomogeneities start playing a significant role in the
image formation. However, due to EPI’s speed, it is one of the most commonly
used techniques in f MRI.
15.5.1 BOLD MRI
The oxygen consumption increases only slightly during the metabolic activity of
neurons. This means that in such cellular activities the oxyhemoglobin-to-deoxyhemoglobin ratio changes only marginally. This has led to more specialized versions of
f MRI procedures for functional brain imaging. This type of imaging falls in a class
of its own that is named after the principles of the technique. Blood oxygenation
level dependent (BOLD) contrast imaging uses the disparity in magnetic properties
of oxygenated (diamagnetic) and deoxygenated (paramagnetic) blood.
Magnetic Resonance Imaging
The reason functional imaging can be achieved by detecting the oxygenated blood
roots in the physiology of the biological tissues. For instance, neuronal function has
a metabolic activity that is directly related to changes of the oxygenation state of the
neurons as follows. The oxyhemoglobin carries the oxygen to the neurons, and, during the metabolic activity of the nerve cells, the oxygen uptake by the neurons causes
oxyhemoglobin to change into deoxyhemoglobin. In addition, neurons require glycogen, obtained from the red blood cells, for their metabolic processes during electric
activity. The glucose uptake can be measured with the chemical fluorodeoxyglucose
(FDG) that has an intrinsic magnetic moment and is incorporated in the cellular
metabolism as regular glucose. The oxygen and glucose requirement by the neurons during cellular depolarization activity has as a direct consequence an increased
blood perfusion in the capillaries surrounding the nerve cells.
The exact f MRI procedure is designed based on the functional characteristics to
be evaluated to ensure that there is a reasonable correlation between these characteristics and certain traceable metabolic aspects. For instance, if the motor-control
activities of the brain need to be evaluated, the patient will be asked to move a toe or
a finger while the system collects data.
One issue in the task design is the concern that the sequence of the tasks assigned
to the patient must be in a specific order to derive a correlation between the brain
activities and a particular cognitive or motor task. An additional requirement is that
any inappropriate and unrelated actions must be filtered out by this methodology.
Other typical examples of specific tasks involve audio or visual input stimuli to
evoke a cognitive response in the brain.
Two commonly used methods apply a fast data acquisition protocol to reduce
the errors from lapses in attention or motion artifacts. The first method is called
fast low angle shot (FLASH). During FLASH imaging, there is a short interval
between the f MRI RF pulses. This short interval reduces the flip angle and consequently reduces the realignment rate, T 1 . As a result, the acquisition sequence
can be accelerated. The second method is echo planar imaging (EPI). EPI applies
small amplitude RF pulses combined with a high gradient in the magnetic field.
The steep gradient provides better contrast and faster acquisition with typical
acquisition times under 100 ms. The low intensity and high gradient have the disadvantage of introducing distortions and a low signal amplitude. In particular,
in the brain, the magnetic inhomogeneities start playing a significant role in the
image formation. However, due to EPI’s speed, it is one of the most commonly
used techniques in f MRI.
15.5.1 BOLD MRI
The oxygen consumption increases only slightly during the metabolic activity of
neurons. This means that in such cellular activities the oxyhemoglobin-to-deoxyhemoglobin ratio changes only marginally. This has led to more specialized versions of
f MRI procedures for functional brain imaging. This type of imaging falls in a class
of its own that is named after the principles of the technique. Blood oxygenation
level dependent (BOLD) contrast imaging uses the disparity in magnetic properties
of oxygenated (diamagnetic) and deoxygenated (paramagnetic) blood.