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1、MRI在人脑检查中的应用原理和实践操作John C. GoreConventional MRI is used extensively for radiological diagnosis and produces spatial maps of the properties of mobile hydrogen nuclei (single protons) that are contained mainly in water molecules. Conventional magnetic resonance images portray anatomic details with exq
2、uisite resolution (on the order of 1 mm or better), in three dimensions, and differentiate soft tissues very well. The contrast within images results from variations mainly in the density of water within tissues and in the manner in which water interacts with macromolecules Functional MRI (fMRI) is
3、now also well established in many centers and uses similar imaging techniques and the same equipment as conventional MRI. fMRI relies on detecting small changes in the signals used to produce magnetic resonance images that are associated with neuronal activity in the brain, and it is producing uniqu
4、e and valuable information for applications in both basic and clinical neuroscience. fMRI is safe, noninvasive, and repeatable in adults and children and thus has widespread potential uses. This short overview will attempt to illustrate the physical basis of fMRI, how it is performed in practice, so
5、me of the limitations of the technique, and some of the types of application that have been exploited to date.fMRI detects the blood oxygen level-dependent (BOLD) changes in the MRI signal that arise when changes in neuronal activity occur following a change in brain state, such as may be produced,
6、for example, by a stimulus or task One of the underlying premises of many current uses of functional imaging is that various behaviors and brain functions rely on the recruitment and coordinated interaction of components of large-scale” brain systems that are spatially distinct, distributed, and yet
7、 connected in functional networks. Thus, although the practice of phrenology is dead, identification of the neurobiological substrates associated with various specific functions of the brain is likely to shed light on how the brain determines behavior. In addition, geographic maps identifying the lo
8、cations of particularly critical areas, such as those involved in producing and understanding language, are of direct importance in clinical assessments and the planning of interventions.The physical origins of BOLD signals are reasonably well understood, though their precise connections to the unde
9、rlying metabolic and electrophysiological activity need to be clarified further. It is well established that an increase in neural activity in a region of cortex stimulates an increase in the local blood flow in order to meet the larger demand for oxygen and other substrates. The change in blood flo
10、w actually exceeds that which is needed so that, at the capillary level, there is a net increase in the balance of oxygenated arterial blood to deoxygenated venous blood Essentially, the change in tissue perfusion exceeds the additional metabolic demand, so the concentration of deoxyhemoglobin withi
11、n tissues decreases This decrease has a direct effect on the signals used to produce magnetic resonance images. While blood that contains oxyhemoglobin is not very different, in terms of its magnetic susceptibility, from other tissues or water, deoxyhemoglobin is significantly paramagnetic (like the
12、 agents used for MRI contrast materials, such as gadolinium), and thus deoxygenated blood differs substantially in its magnetic properties from surrounding tissues (1). When oxygen is not bound to hemoglobin, the difference between the magnetic field applied by the MRI machine and that experienced c
13、lose to a molecule of the blood protein is much greater than when the oxygen is bound On a microscopic scale, replacement of deoxygenated blood by oxygenated blood makes the local magnetic environment more uniform. The longevity of the signals used to produce magnetic resonance images is directly de
14、pendent on the uniformity of the magnetic field experienced by water molecules: the less uniform the field, the greater the mixture of different signal frequencies that arise from the sample, and therefore the faster the decay of the overall signal.The result of having lower levels of deoxyhemoglobi
15、n present in blood in a region of brain tissue is therefore that the MRI signal from that region decays less rapidly and so is stronger when it is recorded in a typical magnetic resonance image acquisition. This small signal increase is the BOLD signal recorded in fMRI (Figure 1) (2). It is typicall
16、y around 1% or less, though it varies depending on the strength of the applied field; this variability is one reason why higher-field MRI systems are being developed As can be predicted from the above explanation, the magnitude of the signal depends on the changes in blood flow and volume within tissue, as well as the change in local oxygen tension, so there is no simple relation between the signal change and any single physiological paramete匚 Thus fMRI does not report absolute
