fMRI: Isolating many brain events

fMRI produces images of brain regions by detecting indirect effects of neural activity on local blood volume, flow, and oxygen saturation.

THE CHALLENGE of science is to provide a biological basis of consciousness and the mental processes by which we perceive, act, learn, and remember. Are these processes localised to specific regions of the brain, or do they represent a collective property of the whole brain? The brain sends and receives millions of signals every second in the form of hormones, nerve impulses, and chemicals. The brain is made of nerve cells (neurons) with their processes (axons and dendrites) and the supporting glial cells.

A message generated or transmitted by a neuron travels along its axon as an action potential till it reaches a synapse (junction), where through a cascade of chemical events it is passed on to the other neurons. Tumour, injury or any other disease including psychiatric illness can affect the normal brain activity like reasoning, motor control and consciousness. Being fragile and difficult to access, except through surgery, imaging techniques are used as a non-invasive method of visualising the brain's structure, activity and function.

Magnetic Resonance Imaging (MRI) is the state-of-the art non-invasive imaging modality in clinical medicine and is an indispensable tool for clinicians for diagnosis of various diseases such as tumours, cancers, infractions, haemorrhage, spine abnormalities, heart diseases etc. MRI produces a series of cross-sectional images of human organs in any desired plane at any angle. It uses no harmful radiations such as X-rays or radioactive nuclear isotopes. Functional MRI (fMRI) is a variant of the regular MRI for determining which parts of the brain are activated by different types of physical activity, such as visual perception, sound or finger movements.

fMRI investigation differs from regular MRI scan in that it localises brain activity rather than the anatomy. It produces images of activated brain regions by detecting the indirect effects of the neural activity on local blood volume, flow, and oxygen saturation. It is assumed that an area is relatively more active when it has more oxygenated blood compared to another point in time. When the brain is active, arterial oxygenated blood will redistribute and increase to this area and is called blood oxygen level dependent (BOLD) phenomenon. These changes in tissue blood volume have been directly correlated with evoked brain activity. As fMRI images are acquired rapidly, one can get enough images to measure the relative differences between two mental states that differ in only one aspect in that everything is controlled except the behaviour in question. fMRI experiments are performed with several periods of stimulation (active) alternating with several periods of non-stimulation (rest). Images are then compared over the entire activation to the rest periods. Active regions glow more brightly than inactive regions.

fMRI is capable of isolating many simultaneous and coordinated brain events. It is useful to identify regions of the brain uniquely involved in visual perception, language generation, movement, memory, taste, mental status during yoga, etc.

Recently, fMRI has been used to find which part of the brain manages our perception of time. Such studies would lead to new drugs for patients with Parkinson's disease, who often experiences problems with time perception. fMRI has an important role in pain management and holds great promise for unravelling the neuroanatomic basis of many psychiatric diseases.

Work on using fMRI in clinical applications is under way to prove that consistent results could be got in those with pathological conditions. A number of patients with various brain disorders are to be investigated before it can be used as a routine investigation in clinical setting.

N. R. Jagannathan

Head, Dept of NMR
AIIMS, New Delhi

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