by Martin Buechert
from BioMedNet
In the late 18th century, the Austrian physician Franz Joseph Gall announced that different mental abilities resided in different parts of the brain. Scientists immediately began searching for ways to see which part did what. Some scientists mapped the bumps on the outside of the skull, thinking they must reflect the underlying structure. A century later, in the late 1800s, scientists shocked areas of the brain with electricity and found that zapping different areas stimulated different results, such as flexing one leg rather than the other. Today, scientists can peer at what goes on inside our heads by making images in a variety of ways. Some of the most exciting images arise from a technique called functional magnetic resonance imaging (fMRI), a further development of the older magnetic resonance imaging (MRI). In fMRI, images of the brain are created while it is stimulated in a certain way.
Several Web sites provide introductions to MRI and fMRI. Beginners might turn to the MRI Tutor, created by Ray Ballinger, a radiologist at the Veteran's Administration Medical Center in Gainesville, Florida. He also has a Ph.D. in medical physics. In the MRI Tutor's less technical summary, Ballinger writes, MRI is a way of getting pictures of various parts of your body without the use of X-rays. . . . A MRI scanner consists of a large and very strong magnet in which the patient lies. A radio wave antenna is used to send signals to the body and then receive signals back. These returning signals are converted into pictures by a computer attached to the scanner. Pictures of almost any part of your body can be obtained at almost any particular angle.
The MRI Tutor also offers visitors more sophisticated information - everything from an overview of the physics of magnetism to a specific explanation of fMRI. According to this description, fMRI is a technique that has recently been introduced to obtain functional information from the central nervous system. FMRI detects subtle increases in blood flow associated with activation of parts of the brain. FMRI may be useful for preoperative neurosurgical planning, epilepsy evaluation, and 'mapping' of the brain.
The Web site of the Laboratory of Functional MRI at the Memorial Sloan-Kettering Cancer Center also provides an introduction, About Functional MRI, that covers the basics of fMRI. The site also includes a historical view of the technique's roots and development and an overview of its current role and future prospects. A few illustrations give this introduction extra punch - one, for instance, shows how touching a subject's hand triggered activity in the back of the brain. This page also provides a tour of other sites about fMRI on the Web.
Many applications of fMRI depend on the so-called BOLD (blood-oxygen-level-dependent) effect. Although the details of the effect, first described in 1990, are still debated, researchers agree that it comes from magnetic differences in hemoglobin when it is and is not carrying oxygen. During activation of a certain area in the brain, a complex physiological reaction increases the demand for energy and, therefore, for oxygen. Hemoglobin in the blood transports oxygen to the active site in the brain, and the oxygenated blood can be seen in an MRI scan because of its distinctive magnetic properties. One place to learn more about such details is from an article called Functional Magnetic Resonance Imaging for the Psychiatrist, which was prepared by a group of physicians, largely from the Medical University of South Carolina, to help clarify the technique's capabilities and limitations for the benefit of psychiatric researchers and clinical practitioners.
To see how an area of the brain functions, investigators apply fMRI under different conditions - for instance, in resting and active states. Then they subtract the resting-state image from the active-state one to see where changes developed. Nevertheless, the BOLD effect creates very small differences, so an investigator must acquire lots of images, sometimes thousands of them. Then, statistical data analysis techniques - much more complicated than just subtracting one image from the other - are used to reveal more detail in the data.
To get a feel for the actual mechanics of the fMRI process, take a tour of the Neurovisualization Laboratory site at the University of Virginia. A section called How does fMRI work? describes the entire process. In addition, this site shows a device that looks a lot like a hockey goalie's mask and is used at the laboratory to keep subjects still during the procedure.
The Web site of the Center for Magnetic Resonance Research at the University of Minnesota includes another brief introduction to fMRI. The site also shows visitors some fMRI images in the form of three-dimensional overlays, with detailed descriptions of the experiment and of how the data were processed. Unfortunately, a promise of an animation of an fMRI study returned only several still images and failed to load anything animated when I tried it.
Researchers in the field can turn to the Center for Magnetic Resonance Research's page to find fMRI software. Stimulate, for example, is a GUI (graphical user interface)-based software package, developed at the center, that analyzes fMRI images. From the Stimulate pages, users can download the software and a complete HTML-based user's guide containing images and examples. The center's site also offers PhysioFix, a program that can be used to bring out finer details in an fMRI image. To learn more about such statistical approaches in imaging, take a look at the Statistical Parametric Mapping page at the Web site of the Wellcome Department of Cognitive Neurology of University College London, which describes one technique that can be used to test hypotheses about imaging data.
To see a series of images from an experiment that used real-time fMRI, link to the Web site of the Biophysics Research Institute at the Medical College of Wisconsin. This site also presents a project called fMRI of the Brain, which has a long-term objective of ... [creating] a foundation for the application of functional magnetic resonance imaging (fMRI) to medicine. In their pursuit of that goal, the investigators behind this site hope to define the physiological basis of the fMRI response, . . . use fMRI to investigate the cerebral organization of vision . . . , motor control . . . , and audition and language.
The most interesting part of the Biophysics Research Institute's site lies on the pages devoted to a software package called AFNI (Analysis of Functional NeuroImages), which was developed at the medical college. Investigators can use this free software package to analyze and display data sets created with fMRI. Visitors to the site can see sample images, download the software and movies made from it, and even get a laugh from a cartoon that pokes fun at the technique.
Some of the best examples of the use of fMRI in basic research appear on a site hosted by the Vision Research Group at the Royal Holloway College of the University of London. In addition to general information about vision research, this site displays some very interesting examples of state-of-the-art fMRI. For instance, clicking the link to fMRI from the main page leads to a set of images of a brain with the skull removed and parts of the tissue digitally dissected to reveal activity.
Other and even more spectacular examples of fMRI lie somewhat buried at the Massachusetts General Hospital NMR Research Center's site. To find the excitement there, follow a link called Cortical Surface Analysis Projects at the NMR Center and then another called Cortical Surface Reconstruction. This reveals algorithms for stripping off the skull and then reconstructing the complete folded cortical surface of each cerebral hemisphere from structural MRI images. Next, the algorithm can unfold the cortical surface - laying it out flat - to get a better view, for instance to see down into fissures. Several movies show the transition from three-dimensional MRI data to a so-called flat map of a cortical area. Of course, seeing this requires downloading an enormous amount of data, which may take a while; but it's well worth it.
With the ongoing propagation of powerful magnetic resonance scanners, fMRI experiments flow out of more and more labs. No one needs to feel a person's head, trying to read the bumps, in an effort to understand the activity of the brain inside. Instead, scientists can look inside, as if tuning a television to the cognitive station of the brain, and watch one thought-provoking program after another.
Martin Buechert is a postdoctoral researcher in the Magnetic Resonance Group at the University of Freiburg, Freiburg, Germany.
Endlinks
Laboratory for Clinical Cognitive Neuroscience - site of a Carnegie Mellon University group that is working to apply fMRI and other techniques to research in a variety of intriguing areas, including the basis of schizophrenia.
New Neuroimaging Links - a list of neuroimaging links ranging from introductory material to research results. Sponsored by the Department of Psychology at the State University of New York at Stony Brook.
Organization for Human Brain Mapping - site of a multidisciplinary organization designed to connect investigators working on all kinds of functional imaging. Includes links to the pages for the organization's past and future annual meetings.
Section for Medical Image Analysis and Pattern Recognition - a site at the University of Bergen in Norway that includes a section on research projects in fMRI, with several images and descriptions of techniques.
VA MRI Facility - an unusual and interesting page at the Gainesville, Florida, VA Medical Center that provides a series of teaching exercises with accompanying MRI scans.
Visualization of Cortical Activity Using fMRI - a slide show sponsored by Hewlett-Packard Laboratories and Stanford University, depicting many steps in fMRI, from techniques for stimulation to the results of different presentation approaches.
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