A Primer on MRI and Functional MRI

Functional brain mapping with magnetic resonance imaging (MRI) is a rapidly growing field that has emerged in only the past several years. Functional MRI (fMRI) is the use of MRI equipment to detect regional changes cerebral metabolism or in blood flow, volume or oxygenation in response to task activation. The most popular technique utilizes blood oxygenation level dependent (BOLD) contrast, which is based on the differing magnetic properties of oxygenated (diamagnetic) and deoxygenated (paramagnetic) blood. These magnetic susceptibility differences lead to small, but detectable changes in susceptibility-weighted MR image intensity. Relatively low image signal-to-noise ratio (SNR) of the BOLD effect, head movement, and undesired physiological sources of variability (cardiac, pulmonary) make detection of the activation-related signal changes difficult. Fortunately, rapid image acquisition techniques can be used to generate data sets with hundreds of images for each slice location which can be statistically analyzed to determine foci of brain activity. Finally, the basic properties of the human activation response have implications for experimental design and statistical processing techniques used for extraction of functional information from the four-dimensional data sets. An Introduction to NMR Magnetic resonance imaging (MRI) has been used for over 15 years to generate exquisite images of the soft tissue anatomy of the human body. Only recently has MRI been employed to image the functioning human brain in a completely non-invasive fashion contrast [1-4]. MRI is based on the physics of Nuclear Magnetic Resonance or NMR, a property of atomic nuclei that was discovered in the 1940's and has a long history of use in chemical analyses. The NMR phenomenon is based on the quantum mechanical property of nuclear spin. Nuclei having an odd number of neutrons, odd number of protons, or both will have a net magnetic moment and will therefore be NMR active. Among the most commonly used nuclei are 1H, 13C, 19F, 23Na, and 31P. For imaging in biological systems, 1H or “proton” NMR is the most common, primarily due to its high concentration in the human body and high sensitivity (it gives rise to very large NMR signals). Consider an object that contains many hydrogen nuclei, each with the characteristic nuclear spin. In an NMR experiment, the object to be imaged is placed in a large, static magnetic field. The magnetic field is commonly supplied by a superconducting magnet that remains “on” continuously, often for several years at a time. The magnetic moments associated with nuclear spin tend to align themselves parallel or anti-parallel to the static magnetic field, B0 (this is