Functional mri4/12/2023 Meanwhile the MRI scanner tracked the signal throughout the brain. While lying in the MRI scanner the subject watched a screen which alternated between showing a visual stimulus and being dark every 30 second. This image is the result of the simplest kind of FMRI experiment. The blood flow peaks after around 6 seconds and then falls back to baseline, often accompanied by a "post-stimulus undershoot". This means the blood oxygenation actually increases following neural activation. This is followed by a period where the blood flow increases, not just to a level where oxygen demand is met, but overcompensating for the increased demand. There is a momentary decrease in blood oxygenation immediately after neural activity increases, known as the “initial dip” in the haemodynamic response. You might expect blood oxygenation to decrease with activation, but the reality is a little more complex. One point to note is the direction of oxygenation change with increased activity. This form of MRI is known as blood oxygenation level dependent (BOLD) imaging.ĭiagram of the BOLD effect - Courtesy of Stuart Clare, FMRIB Since blood oxygenation varies according to the levels of neural activity these differences can be used to detect brain activity. This difference in magnetic properties leads to small differences in the MR signal of blood depending on the degree of oxygenation. Haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated. When neuronal activity increases there is an increased demand for oxygen and the local response is an increase in blood flow to regions of increased neural activity. Oxygen is delivered to neurons by haemoglobin in capillary red blood cells. This provides a means of discriminating between grey matter, white matter and cerebral spinal fluid in structural images of the brain. The key to MRI is that the signal from hydrogen nuclei varies in strength depending on the surroundings. In FMRI it is the magnetic signal from hydrogen nuclei in water (H 2O) that is detected. When pointing in the same direction, the tiny magnetic signals from individual nuclei add up coherently resulting in a signal that is large enough to measure. The stronger the field the greater the degree of alignment. Normally atomic nuclei are randomly oriented but under the influence of a magnetic field the nuclei become aligned with the direction of the field. The magnetic field inside the scanner affects the magnetic nuclei of atoms. A typical research scanner (such as the FMRIB Centre scanner) has a field strength of 3 teslas (T), about 50,000 times greater than the Earth’s field. The cylindrical tube of an MRI scanner houses a very powerful electro-magnet. Sensory regions of the brain - Courtesy of Peter Hobden, FMRIB How does MRI work? FMRI is also being applied in clinical and commercial settings. Over the last decade it has provided new insight to the investigation of how memories are formed, language, pain, learning and emotion to name but a few areas of research. The attractions of FMRI have made it a popular tool for imaging normal brain function – especially for psychologists. It is easy for the experimenter to use.It has excellent spatial and good temporal resolution.It is non-invasive and doesn’t involve radiation, making it safe for the subject.As a brain imaging technique FMRI has several significant advantages: The development of FMRI in the 1990s, generally credited to Seiji Ogawa and Ken Kwong, is the latest in long line of innovations, including positron emission tomography (PET) and near infrared spectroscopy (NIRS), which use blood flow and oxygen metabolism to infer brain activity. FMRI can be used to produce activation maps showing which parts of the brain are involved in a particular mental process. This content is associated with The Open University's Health Science courses and qualifications.įunctional 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.
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