【正文】 e 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 in regional blood flow and metabolism within the brain. The principle is simple. Neurons that are active demand more glucose and oxygen. The brain vasculature (脈管系統(tǒng) ) responds to neural activity by directing more blood to the active regions. Thus by detecting changes in blood flow, PET and fMRI reveal the regions of brain that are most active under various circumstance. Until recently,”mind reading” has been beyond the reach of science. With the introduction of PET and f MRI, it is possible to observe and measure changes in brain activity associated with the planning and execute of specific tasks. PET imaging was developed in the 1970s, by two groups of physicists, one at at Washington University, led by M. Terpogossian, and the second at UCLA, led by Z. H. Cho. The basic procedure is simple. A radioactive solution containing atoms that emit positrons (positively charged electron) is introduced into the bloodstream. Positrons, emitted wherever the blood goes, interact with the electrons produce photons (光子 ) of electromagic radiation. The location of the positronemitting atoms are found by detector that pick up the photons. One powerful application of PET is the measurement of metabolic activity in the brain. In a technique developed by Louis Sokoloff and his colleagues at the National Institute of Mental Health, a positronemitting isotope of fluorine or oxygen is attached to 2deoxyglucose (2DG). This radioactive 2DG is injected into the blood stream, and it travels to the brain. Metabolically active neurons, which normally use glucose, also take up the 2DG. The 2DG is phosphorylated by enzymes inside the neuron, and this modification prevents the 2DG from leaving. Thus, the amount of radioactive 2DG accumulated in a neuron and the number of positron emissions indicated the level of neuronal metabolic activity. In a typical PET application, a person’s head is placed in an apparatus surrounded by detectors. By use of puter algorithms, the photons resultin