Functional magnetic resonance imaging (fMRI) measures the small changes in blood flow that occur with brain activity. It may be used to examine the brain’s functional anatomy, (determine which parts of the brain are handling critical functions), evaluate the effects of stroke or other disease, or to guide brain treatment. FMRI may detect abnormalities within the brain that cannot be found with other imaging techniques. FMRI is becoming the diagnostic method of choice for learning how a normal, diseased or injured brain is working, as well as for assessing the potential risks of surgery or other invasive treatments of the brain. Physicians perform fMRI to:
- Examine the functional anatomy of the brain.
- Determine which part of the brain is handling critical functions such as thought, speech, movement and sensation, which is called brain imaging.
- Help assess the effects of stroke, trauma, or degenerative disease (such as Alzheimer’s) on brain function.
- Monitor the growth and function of brain tumors.
- Guide the planning of surgery, radiation therapy, or other invasive treatments for the brain.
Functional MRI is still evolving and improving. While it appears to be as accurate in finding the location of brain activity as any other method, overall there is less experience with fMRI than with many other MRI techniques. Your physician may recommend additional tests to confirm the results of fMRI if there are critical decisions to make (such as in planning brain surgery).
The popularity of fMRI derives from its widespread availability (can be performed on a clinical 1.5T scanner), non-invasive nature (does not require injection of a radioisotope or other pharmacologic agent), relatively low cost, and good spatial resolution. Increasingly, fMRI is being used as a biomarker for disease, to monitor therapy, or for studying pharmacological efficacy. fMRI is of course based on MRI, which in turn uses Nuclear Magnetic Resonance coupled with gradients in magnetic field to create images that can incorporate many different types of contrast such as T1 weighting, T2 weighting, susceptibility, flow, etc. In order to understand the particular contrast mechanism predominantly used in fMRI it is necessary to first discuss brain metabolism.
Task activation fMRI studies seek to induce different neural states in the brain as the visual, auditory or other stimulus is manipulated during the scan, and activation maps are obtained by comparing the signals recorded during the different states. The typical fMRI task activation experiment utilizes visual, auditory or other stimuli to alternately induce two or more different cognitive states in the subject, while collecting MRI volumes continuously.
Many MRI scanner manufacturers now supply add-on features that allow standard fMRI procedures to be performed easily. These include suitable pulse sequences, peripheral devices for presentation of stimuli to the subjects in the scanner, devices for recording responses from the subject, and even statistical analysis and display packages that allow assessment of the data while the subject remains in the magnet. In an fMRI experiment a large series of images is acquired rapidly while the subject performs a task that shifts brain activity between two or more well defined states. Several hundred such volumes may be collected in a single session while the subject does different tasks.
Surgery offers the possibility of improved seizure control or cure, especially for patients with temporal lobe epilepsy, but demands an understanding of language lateralisation for surgical planning. However, current clinical methods for language lateralisation (for example, the Wada test) are highly invasive. fMRI offers a promising alternative approach. While there is good agreement between conventional invasive Wada testing and fMRI results, fMRI is more sensitive to involvement of the non-dominant hemisphere. fMRI also provides more specific anatomical information. The reproducibility of distinct patterns of activation in individual subjects is good, potentially allowing clinical decisions to be made on the basis of results.
fMRI studies are showing that functional reorganisation is a general response to brain injury. A wide range of regions within the spatially distributed cortical motor network may contribute to this. fMRI can be sensitive to early (and even preclinical) stages of brain pathology. A pioneering illustration of this approach was an fMRI based memory study of a group of apparently healthy subjects at risk of developing earlier onset Alzheimer’s disease .One year after fMRI scanning, those who were beginning to develop memory problems in early clinical expression of presumed Alzheimer’s disease were identified. A significant difference in the pattern and volume of activated cortex with the memory task was found in these subjects relative to those who did not develop memory impairment. Using fMRI to guide therapeutic development is clearly one of the most exciting prospects for the technique. Initial work has not just been for drug development and response monitoring. The greatest impact may be on areas in which sensitive and objective end points were previously difficult to define. fMRI studies of brain stem changes associated with pain suggest that the periaquaductal grey matter is an important locus for such action.
It may be used to examine the brain’s functional anatomy, (determine which parts of the brain are handling critical functions), evaluate the effects of stroke or other disease, or to guide brain treatment. fMRI may detect abnormalities within the brain that cannot be found with other imaging techniques.
FMRIB Centre Functional magnetic resonance imaging, or FMRI, works by detecting the changes in blood oxygenation and flow that occur in response to neural activity – when a brain area is more active it consumes more oxygen and to meet this increased demand blood flow increases to the active area.
FMRI scans use the same basic principles of atomic physics as MRI scans, but MRI scans image anatomical structure whereas FMRI image metabolic function. The images generated by FMRI scans are images of metabolic activity within these anatomic structures.