Functional Magnetic Resonance Imaging (fMRI)

Studies of how the brain functions in healthy people use functional magnetic resonance imaging (fMRI) as a tool to measure which areas of the brain are active while the person is doing a task, such as a memory test, or looking at images, or perhaps pressing buttons in a particular sequence. The measurements with fMRI gives an indication of which regions of the brain are associated with the task the person is doing, and hence gives us information about the functional structure of the brain.  Performing similar experiments in patient groups provides information on which regions may be functioning abnormally thus increasing our understanding of the disease and potentially over the long term help us in developing preventative and therapeutic methods.

Brain activation is a general term used to refer to activation of neurons in the brain. The basic unit of information processing in the brain is a neuron, and ensembles of neurons are activated when a particular task is done, for example learning the name of a person one may just have met for the first time. Neurons require energy, such as oxygen and glucose, which is supplied by the local blood vessels and when neurons are more active, more blood is supplied to the neurons. Functional neuroimaging methods, such as fMRI, are able to measure the changes in blood flow due to the increased energy demands of neurons. As such they do not directly measure neuronal function and the changes in neuronal activity but measure the physiological changes due to increased neuronal activity. The physiological changes are the increased blood flow to supply oxygen and glucose to neurons. Glucose and oxygen are bound to haemoglobin molecules and the differing magnetic properties of oxygenated haemoglobin and deoxygenated haemoglobin alter locally the magnetic field.

Functional magnetic resonance imaging is a technique that uses a magnetic field to measure the physiological changes due to neuronal activation. As oxygenated and deoxygenated haemoglobin increases (or decreases) in a brain region, so is the local magnetic field in that part of the brain region altered. The magnetic resonance imaging scanner allows for measurements of these tiny changes in the magnetic field and hence from these tiny changes we can construct images that indicate which areas have increased blood supply due to neuronal activation.

How are the small magnetic field changes measured? Magnetic resonance imaging relies on the nucleus of hydrogen, a component of water and the most common molecule in the body. The main magnet of the magnetic resonance imaging (MRI) scanner provides a relatively homogeneous magnetic field inside the scanner and smaller magnets within the MRI scanner are used to increase the energy of the hydrogen protons. The response of the hydrogen protons to the increased energy will be dependent on the strength of the magnetic field where the proton is located, thus for example, if the magnetic field on the brain is identical in all regions the dynamics of the hydrogen protons will be identical in all brain regions. But if a region of the brain is activated, for example, someone is learning a new name, the increased blood supply to that specific brain region will lead to a change in the ratio of oxygenated and deoxygenated haemoglobin. The change in the ratio of oxygenated and deoxygenated haemoglobin will alter locally the magnetic field (a change due to the magnetic properties of haemoglobin) and the altered magnetic field will also alter the dynamics of the hydrogen protons. The MRI scanner allows us to measure these changes in hydrogen protons and hence produce images of brain activity.