農(nóng)學(xué)]第二章人體基本結(jié)構(gòu)概述(參考版)
2025-01-22 02:00本頁面
【正文】 Magic Resonance Imaging (MRI) While still used widely, CT is gradually being replaced by a newer imaging method, magic resonance imaging, or MRI. The advantages of MRI are that it yield a much more detailed map of the brain than does CT, it does not require Xirradiation, and images of brain slices can be made in any plane desired. MRI uses information about how hydrogen atoms in the brain respond to perturbation of a strong magic field. The electromagic signals emitted by the atoms are detected by an array of sensors around the head and fed to a powerful puter that constructs a map of the brain. The information from an MRI scan can be used to build a strikingly detailed images of the whole brain. In the most mon form of MRI, the hydrogen atoms are quantified—for instance, those in water or in fat in the brain. An important fact of physics is that when a hydrogen atoms is put into a magic field, its nucleus can exit in either two states: a high energy state or a low energy state. Because hydrogen atoms are abundant in the brain, there many protons in each state. The key to MRI is making the protons jump from one state to the other. Energy is added to the protons by passing an electromagic wave (i. e. a radio signal) through the head while it is positioned between the poles of a large mag. When the radio signal is set at just the right frequency, the protons in the lowenergy state absorb energy from the signal and hop to the highenergy state. The frequency at which the protons absorb energy is called the resonant frequency (hence the name magic resonance). When the radio signal is turned off, some of the protons fall back down to the lowenergy state, emitting a radio signal of their own at a particular frequency. This signal can be picked up by a radio receiver. The stronger the signal, the more hydrogen atoms between the pole of the mag. If we used this procedure, we would simply get a measurement of the total amount of hydrogen in the head. However, it is possible to measure hydrogen amount at a fine spatial scale by taking advantage of the fact that the frequency at which protons emit energy is proportional to the size of the magic field. In the MRI machine used in hospital, the magic fields vary from one side of the mag to the other. This gives a spatial code to the radio waves emitted by the protons: Highfrequency signals e from hydrogen atoms near the strong side of the mag, and lowfrequency signals e from weak side of the mag. The last step in the MRI process is to orient the gradient of mag at many different angles relative to the head and measure the amount of hydrogen. It takes 15 min to make all of the measurement for a typical brain scan. A sophisticated puter program is used to make a single image from the measurement, resulting in a picture of the distribution of hydrogen atoms in the head. It is possible to see the effect of demyelinating (脫髓鞘 ) diseases on white matter in the brain. MRI images also reveal lesions(損害 ) in the brain, because tumors and inflammation generally increase the amount of extracellular water . PET and fMRI (Functional Brain Imaging) CT and MRI are extremely valuable for detecting structural changes in the living brain, such as brain tumors and brain swelling (腫脹 ) after a head injury. Noheless, much of what goes on in the brain—healthy or diseased—is chemical and electrical in nature and not observable by simple inspection of the brains anatomy. Amazingly, however, even these secrets are beginning to yield to the newest imaging techniques. The two “functional imaging” techniques now in widespread use are positron emission tomography(正電子發(fā)射計(jì)算機(jī)斷層掃描 ), or PET, and functional magic resonance imaging, or fMRI. While the technical details differ, both methods detect changes
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