Today we shall breifly learn as to how we can study the brain function using imaging techniques.
Positron Emission Tomography (PET)made debut in 1980s; two more imaging techniques came in the 1990s: functional magnetic resonance imaging (fMRI) and magnetoencepalography (MEG).
1) Positron Emission Tomography:
This technique makes it possible to see in an image which part of the brain is active during a particular task.
As we also know that although brain as a whole does not consume significantally more energy when it is active than when it is idle, metabolic activity does increase in circumscribed regions of the brain when these regions are functionally active.
This increased metabolic activity in the brain is the basis of PET.
In this technique a positron-emitting isotope is tagged to a molecule of biological interest such as glucose or a neurotransmitter.
For example, the positron-emitting isotope of fluorine (18F) is tagged to deoxyglucose and it is injected intravenously.
Deoxyglucose is taken up by neurons in the same way as glucose, but it can neither be fully metabolised nor can it come out of the neurons.
Since functionally active neurons take up more glucose, active regions of the brain accumulate more deoxyglucose.
So , following visual stimulation, 18F-deoxyglucose accumulation can be seen in the visual cortex. This signifies increased glucose metabolism in the visual cortex. Thus we have evidence for involvement of specific regions of the brain in specific functions
Positron emission is detected by appropriate detectors which construct a series of computerised images of the brain similar to those seen in computerised tomography (CT).
2) Functional MRI :
It is based on the principle that increased neuronal activity leads to a local increase in blood flow through the active part of the brain.
The increase in blood flow is somewhat greater than is warranted by the increase in oxygen consumption.
Therefore, blood flowing through the active, hyperemic region of the brain has more oxygenated haemoglobin than the blood flowing through less active regions of the brain.
The magnetic properties of oxygenated and deoxygenated haemoglobin are different, the magnetic resonance signals from the active region of the brain increase.
Functional MRI systems currently in common use give a spatial resolution of about 1 mm, but a resolution of 0.5 mm has been achieved in experimental settings. This is an important breakthrough because cortical columns also have a width of about 0.5 mm.
It can complement the information obtained from the conventional electroencephalography (EEG). MEG is based on the principle that neuronal activity in the cerebral cortex generates not only fluctuations in electrical potential (detected by EEG) but also magnetic fields. Unlike EEG signals, MEG signals are not distorted by the intervening tissues. These technical advances have given hope for rapid progress in localization of functions in the human brain.
So I hope ,this helps you guys to have a better picture on Brain Imaging.
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