英语翻译Functional MRI (fMRI) is the use of MRI to detect locali

英语翻译
Functional MRI (fMRI) is the use of MRI to detect localized changes in brain activity,
usually in the form of changes in cerebral metabolism,blood flow,volume,or
oxygenation in response to task activation.These changes are interrelated and
may have opposite effects.For example,an increase in blood flow increases blood
oxygenation,whereas an increase in metabolism decreases it.The most common
means of detection is measuring the changes in the magnetic susceptibility of
hemoglobin.Oxygenated blood is dimagnetic and deoxygenated blood is para-
magnetic.These differences lead to a detection method called blood oxygen level
dependent contrast (BOLD).Changes in blood oxygenation can be seen as differences
in the T2 decay constant,so T2 weighted images are used.Because these changes
are very small,images before and after task initiation are subtracted from each
other and the resultant difference image is overlaid on a standard image.
Special care must be taken when obtaining these images because the effect is
small and can be corrupted by several sources of noise.Typically,hundreds of images
are taken for each slice plane position and statistical analysis is used to produce
the final image.Sources of noise errors are thermal noise,head movement,
and respiratory and cardiac cycles.An example of an fMRI image is shown in
Figure 16.52.