Functional magnetic resonance imaging Guide, Meaning , Facts, Information and Description
Functional Magnetic Resonance Imaging (or fMRI) is the use of MRI to learn which regions of the brain are active in a specific cognitive task, as in speech or in the conjugation of a verb. It is one of the most recently developed forms of brain imaging
As nerve cells "fire" impulses, they metabolize oxygen from the surrounding blood. Approximately 6 seconds after a burst of neural activity, a haemodynamic response occurs and that region of the brain is infused with oxygen-rich blood.
Because oxygenated haemoglobin is diamagnetic, while deoxygenated blood is paramagnetic, MRI is able to detect a small difference (a signal of the order of 3%) between the two. This is called a blood-oxygen level dependent, or "BOLD" signal. The precise nature of the relationship between neural activity and the BOLD signal is a subject of current research.
scan of the brain. to view an animation of the scan from top to bottom.]]BOLD effects are measured using a T2 imaging process, which is different from the T1 scan taken in ordinary structural MRI images (the former measures the rate of change of spin phases, while the later detects the half-life of inverted spins). T2 images can be acquired with moderately good spatial and temporal resolution; scans are usually repeated every 2-5 seconds, and the voxels in the resulting image tend to represent cubes of tissue approximately 2.5 millimeters on each side. Other non-invasive functional medical imaging techniques can improve on one of these figures, but not both.
The science of applying fMRI is quite complicated and multi-disciplinary. It involves:
- A good understanding of the physics of MRI scanners.
- Statistical analysis of results. Because the signals are very subtle, correct application of statistics is essential to both "tease out" observations and avoid false-positive results.
- Psychological study design. When conducting fMRI on humans, for example, it is essential to employ carefully designed experiments which allow the precise neural effect under consideration to be separated.
- For a non-invasive scan, MRI has moderately good spatial resolution, but relatively poor temporal resolution. Increasingly, it is being combined with other data collection techniques such as EEG or MEG, which have much higher recording frequencies.
- Integration with other areas of neuroscience in order to better understand the location (and role) of the signals which fMRI is able to detect. This includes a great deal of neuroanatomy but also other sub-fields such as neurochemistry and neuropathology.
- By using a injected contrast agent, e.g., MION, causing a local disturbance in the magnetic field that is measurable by the MRI scanner. The signal associated with these kind of contrast agents are proportional to the cerebral blood volume. Other methods of investigating blood volume which do not require an injection are a subject of current research.
- By using what is called arterial spin labelling ASL. The associated signal is proportional to the cerebral blood flow, or perfusion.
Another recently developed functional MRI technique is diffusion tensor imaging (DTI). As protons are directed along certain axes in the brain (for example, as water flowing down a neuronal axon within a bundle of nerve fibers in cerebral white matter), this directionality can be measured. Connectivity between brain regions may be inferable from diffusion images, and illnesses that disrupt the normal organization or integrity of cerebral white matter (such as multiple sclerosis) have a quantitative impact on DTI measures.
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2 Is fMRI worthwhile? 3 See also |
There are also questions about the reproducibility of fMRI studies, which some contend are not being addressed properly.Scanning in Practice
Subjects in a fMRI are asked to lie very still, and are often restrained with padded headbraces to prevent small motions from disturbing measurements. It is possible to correct for some amount of motion with postprocessing of the data, but significant motion can easily render these attempts futile. For purposes of actual scanning, slices tend to use a standard horizontal orientation, Talairach Space, in which the anterior and posterior commissures form a line (Known as the AC-PC line). This line can easily be located in scan results, and with proper scaling, can be used to map analogous brain regions between subjects with high precision.Is fMRI worthwhile?
Ever since its inception, fMRI has been critised for only asking "where" brain activity occurs. Some authors, such as Uttal, go so far as to suggest that fMRI is just a modern-day phrenology and is therefore destined to fail and fundamentally uninformative. Not surprisingly, there are plenty of functional imagers who offer up counter-arguments (e.g. Donaldson 2004). This debate is sure to continue for the foreseeable future. In the mean time, fMRI is likely to go from strength to strength as it continues to pursue questions relating to the functional organisation of the human brain.