CT is mainly diagnosed by the different contrast between bone, muscle and fat, while the resolution is very high, is the most commonly used diagnostic modality, can be seen as a three-dimensional version of the chest X-ray, but the disadvantage is that the dose of radiation is very high.MRI's imaging principle is completely different from the CT, the contrast of MRI to the soft tissues is quite good, so it is often used to do the imaging of brain tissue as well as the liver. However, its resolution is much lower than CT and the imaging time is long. But the absolute advantage of MRI is that there is no radiation to the human body, which makes it a very safe means of diagnosis compared to CT. Currently, there are two types of MRI: continuous wave (CN) and pulsed Fourier transform (PFT). Continuous wave NMR***Vibrators are composed of magnets, RF transmitters, detectors and amplifiers, and recorders (see Figure 8-5). Magnets are used to generate a magnetic field, there are three main types: permanent magnets, magnetic field strength of 14,000G, frequency of 60MHz; electromagnets, magnetic field strength of 23,500G, frequency of 100MHz; superconducting magnets, the frequency can be up to more than 200MHz, the highest up to 500 ~ 600MHz. frequency of the instrument, the resolution of the large, high sensitivity, the map is simple and easy to analyze. The magnet is equipped with a scanning coil, which is used to ensure that the magnetic field generated by the magnet is uniform and can be varied continuously and accurately within a narrow range. An RF transmitter is used to generate a fixed frequency wave of electromagnetic radiation. Detectors and amplifiers are used to detect and amplify *** vibration signals. The recorder plots the *** vibration signals into a *** vibration map. An equal number of carbon and hydrogen atoms are placed in the same nuclear magnetic **** vibration instrument with the same external magnetic field strength and temperature, and the nuclear magnetic **** vibration signal of carbon is only 1/6000 of that of hydrogen, which indicates that there is a large difference in the sensitivity of different nuclei to be detected in the same magnetic field.The natural abundance of 13C is only 1.108% of that of 12C. Because of the small detected sensitivity and low abundance, detecting 13C has more technical difficulties than detecting 1H.