Functional MRI is a specialized approach to magnetic imaging that capitalizes on the oxygen level in haemoglobin to determine the level of brain activity. Haemoglobin's magnetic properties change depending on the amount of oxygen being carried. Functional MRI therefore measures this magnetism and correlates this to brain activity as a function of oxygen level. Another more sensitive contrast mechanism that depends on blood oxygenation level is known as blood oxygen level dependent (BOLD) contrast. The mechanism is still not clearly understood, but this provides a very sensitive method for measuring brain activity based on blood oxygen levels.
Functional MRI is expected to provide detailed information of the brain areas involved in various human mental activities. The use of fMRI will enable doctors to see how the brain is functioning while a person is performing a specific task such as reading or exercising. This provides a distinct benefit over the anatomical information currently available from conventional MRI images.
Functional MRI is a specialized approach to magnetic imaging that capitalizes on the oxygen level in haemoglobin to determine the level of brain activity. Haemoglobin's magnetic properties change depending on the amount of oxygen being carried. Functional MRI therefore measures this magnetism and correlates this to brain activity as a function of oxygen level. Another more sensitive contrast mechanism that depends on blood oxygenation level is known as blood oxygen level dependent (BOLD) contrast. The mechanism is still not clearly understood, but this provides a very sensitive method for measuring brain activity based on blood oxygen levels.
Functional MRI is expected to provide detailed information of the brain areas involved in various human mental activities. The use of fMRI will enable doctors to see how the brain is functioning while a person is performing a specific task such as reading or exercising. This provides a distinct benefit over the anatomical information currently available from conventional MRI images.
fMRI exploits magnetic differences between oxyhaemoglobin and deoxyhaemoglobin .Deoxyhaemoglobin in the blood is paramagnetic whereas oxyhaemoglobin is diamagnetic. As the level of deoxyhaemoglobin in the blood vessel increases, a susceptibility difference is generated between the blood vessel and surrounding tissue. A dephasing of the magnetic resonance proton signal results and the image is darker than normal. Oxyhaemoglobin however is diamagnetic and does not cause dephasing. Therefore changes in the oxygenation level of the blood can be observed as signal changes in the T2 weighted images.
The point should be made that it might be expected that upon increased neural activity, the oxygen consumption is increased and the level of deoxyhaemoglobin might also increase. This however is not the case as the flow of blood to the brain also increases, bringing more oxyhaemoglobin, decreasing the paramagnetism and thus increasing the signal. The prognosis for fMRI and/or BOLD MRI remains to be seen as these techniques are optimized for use in humans.