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Valentina Carlile Osteopata
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How does fMRI work?


How does fMRI work?

Functional MRI records the so-called "full signal". This signal depends on the blood oxygenation of the subject under study. This is because small metal particles, when placed in the beam of the fMRI scanner, distort the signal such that at certain times the MRI signal measured from the areas where such metal is found is substantially lower. When a sufficiently large group of neurons is activated in a certain area of the brain, the area consumes more oxygen and an increase in the concentration of hemoglobin molecules occurs. There are molecules that have given up their oxygen to the neurons so that the neurons can function more effectively. In these early stages, for example, when the subject has just been shown a stimulus or has just started to perform a task, the proportion of oxidized and non-oxidized hemoglobin causes the signal being recorded to attenuate. However, there is an increase in blood flow to the area where neuronal activity has begun, so that the hemoglobin that has lost its oxygen is washed away, giving way to oxidized hemoglobin, i.e. hemoglobin that carries oxygen. This, inversely, leads to an increase in signal strength. In this way, by recording the full signal, we can indirectly verify a local increase in neuronal activation in this area of the brain. Because we know exactly what is happening and when during the scan, this technique allows us to tell exactly which areas of the brain are starting to function or become active at any given moment. The solid signal is known to have certain properties. At the beginning of neuronal activation, there is a slight decrease in the full signal, then the signal increases again before stabilizing. Usually, the peak is around 6 seconds. These properties of the full signal are used in the statistical analysis of individual MRI data, which helps us understand how the signal will appear in certain areas when the subject is presented with stimuli that are processed by those areas.

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