Functional Magnetic Resonance Imaging

Functional Magnetic Resonance Imaging is a relatively new addition to the brain imaging techniques which allows us to obtain information about brain function non invasively.

Generally speaking, there are other MR techniques that also provide functional information. MR angiography displays macroscopic flow in brain arteries and veins, perfusion and diffusion imaging depict semiquantitative tissue perfusion estimates and maps of water diffusion coefficients, respectively, while MR spectroscopy detects specific metabolites and other substances present in brain parenchima.

By fMR we mean a specific functional imaging approach targeting the correlate of neural activity associated with the most diverse mental processes. From simple motor, somatic sensory and parceptual function to complex patterns of motor learning, sensory discrimination, memory and attentional processes, this new way of looking at the human brain, still in its infancy, has already done immensurable contributions to our knowledge about it.

Differently from Positron Emission Tomography (PET) or Single Photon Emission Tomography (SPECT), no tracer is needed. The intrinsic contrast of the blood is used instead.

There exists a coupling between neuronal electrical activity and blood supply, which is mediated by some local vasodilating substances. Among them, nitric oxide (NO) figures as a high potency, fast acting one. Soon after neurons discharge, the local capillary bed receives a higher flow load, bathing the "activated" tissue. Contrary to one would expect, the oxigen contend of the local blood increases, due to a marked increase in delivered arterial blood and a hardly significant increase in oxigen extraction. The consequence is an increase in oxihemoglobin content as compared to deoxihemoglobin levels.

What does in mean for imaging purposes?

Deoxihemoglobin has paramagnetic properties, while oxihemoglobin is diamagnetic. In the MR context, this means that paramagnetic substances cause a substantial field inhomogeneity, causing loss of signal in the obtained images, what doesn't happen with diamagnet substances. In other words, regions of the brain containing higher oxigenated blood will appear slightly brighter. The problem is that this difference in signal intensity is extremely subtle, being undetectable by simply looking at the images.

In order to discern the activated areas, multiple dynamic scans are obtained during alternating task and non-task periods. A statistical analisys is necessary do detect which pixels of the image show significant increase in signal correlated to the task periods.

Finally, a coloured map of the activated regions is calculated and overlaid on high resolution images obtained in the same session from the same subject.

Shown here is the displaced sensorimotor cortex of a patient with a low grade astrocytoma, demonstrated by fMRI