3 - Functional magnetic resonance imaging

Summary

Introduction

Investigating the neural basis of cognitive development necessarily requires sensitive measures of brain activity that may be used to obtain repeated observations of subject populations over an extended period of time. MRI methods allow rapid and noninvasive determination of both brain structure and brain function, characteristics that are of particular importance in studies involving children. These imaging techniques employ a combination of static and modulated magnetic fields to obtain local estimates of chemical concentrations in different brain regions. Most commonly, both structural and functional images are derived from proton signals reflecting the local environment of water molecules in various tissue types. The variable environment in different tissue types results in corresponding intensity variations in reconstructed brain images, referred to as tissue contrast. In structural images, the variable tissue contrast provides a means to visualize the spatial distribution of gray matter, white matter, and cerebrospinal fluid throughout the brain. In functional imaging, additional small modulations of signal intensity occur because of changes in tissue blood flow and oxygenation.

Signal intensity changes can be recorded as a function of time and their relation to behavior examined with a variety of signal processing techniques that enhance the behavioral task-related signal change while suppressing undesirable physiologic noise arising from head motion, respiratory artifact, or global changes in cerebral blood flow. In the simplest case, signal intensity in a control condition is subtracted from signal intensity recorded during task performance in order to compute an estimate of taskrelated brain activity.