The Fukushima nuclear leak in Japan has made the public more aware of radiation, but it has also caused some degree of concern about radiation, as it is one of the major factors causing cancer. Radiation is everywhere and can only harm the human body after a certain dose and for a certain period of time. So, how much dose and how long does it take to receive radiation to cause damage to the organism? To figure out this problem, we must first understand two related terms: Sievert (Sievert, the unit symbol is Sv, abbreviated as Xi, Taiwan translated as Siever) and Gorey (Grey, the unit is consistent with the Gy, abbreviated as Gao). Sievert is the special name of the dose equivalent unit, the energy absorbed by 1 joule per kilogram of tissue is 1 Sievert, Sievert is a very large unit, commonly used milli-Sievert (mSv) to indicate the dose equivalent, 1Sv = 1000mSv. And Grey is the special term of the absorbed dose, the energy absorbed by 1 joule per kilogram of tissue per hour is 1 Grey, usually also expressed in milli-Gy, 1 Gy = 1000mGy. From its definition, it can be seen that the hivot indicates the dose of radiation, while the gorie indicates the absorbed dose. That is to say, 1Sv of radiation is completely absorbed by 1Kg of tissue is 1Gy, and different tissues and organs of the human body to the absorption of radiation is also different, with different absorption rates, such as the lungs for 0.12, that is, the lungs are exposed to 1Gy is equivalent to the whole body is exposed to 0.12Sv. The environment we live in itself has radiation, including air, soil and food among them all, each year receives about 2 ~ 4 mSv, China is 3.1mSv; daily life in the computer, refrigerator, TV, microwave ovens, etc. also have radiation, but in general the total amount will not exceed 5mSv / year. While the most radiation received by the public comes from medical diagnosis, including X-rays and CT, ultrasound and magnetic resonance imaging (MRI) is no radiation. An X-ray chest X-ray is about 0.02mSv, the damage to the human body is almost negligible; a head CT scan 2mSv, a chest CT scan 3mSv, the largest dose is the abdominal CT scan, once about 5mSv, which is the source of some people say that a CT scan is equal to taking hundreds of chest films, but some alarmist, because a long plane trip also has 0.2 mSv, according to this statement is equal to dozens of chest films, then smoking 20 per day up to 1mSV / year, is not equal to nearly a hundred chest films? People engaged in radiation work, the annual dose received does not exceed 50mSv, individual organs do not exceed 500mSv, eye crystals do not exceed 150mSv, meaning that the annual radiation dose below 50mSv are safe, it can be seen that X-ray radiography and CT are safe. From the medical point of view, to assess the radiation risk of X-ray and CT examinations, different groups of people should be distinguished. For the general public, unnecessary examinations should be avoided as much as possible; however, for high-risk and diseased people, the examinations should not be left undone, and should not be abandoned because of choking. According to the latest survey in Shanghai, lung cancer is the cancer with the highest incidence rate, and the most critical factor for effective treatment and prolonged survival is early detection. Most lung cancers detected by X-ray chest X-ray are already in the middle and late stages and have lost the best opportunity for surgery, while CT can detect early lung cancers of several millimeters in size, many of which are still in situ cancers, and can be fully resected by minimally invasive thoracoscopy without chemotherapy and radiotherapy after surgery, which can achieve This minimally invasive thoracoscopy can achieve clinical cure without chemotherapy and radiotherapy, minimizing patient pain and reducing medical burden. The U.S. Food and Drug Administration (FDA) has recommended low-dose chest CT as a medical screening program for high-risk groups to replace chest X-ray, because the radiation dose of a low-dose chest CT scan is only 30% to 50% of the conventional dose, but the diagnostic effect is comparable to that of conventional dose. There is a big gap between the current configuration of imaging equipment and the annual frequency of X-ray diagnosis of the population in China and that in developed countries, with 64 CTs per million people in the U.S., 88 in Japan, and only 8 in China, and the annual frequency of X-ray diagnosis per 1,000 people is only 1/5 of that in developed countries. However, for most of the less developed regions, long-term medical checkups are not conducted and it is common for minor diseases to go to hospitals, and the detection rate of advanced tumors is still very high, which brings great pain and heavy economic burden to patients and their families, and also greatly consumes the medical resources of the whole society. Therefore, it is urgent to vigorously promote the concept of medical checkups and make low-dose CT examination as a medical checkup for high-risk people, especially in big cities with medical conditions. But unfortunately, low-dose CT examinations are still not routine medical checkups, and chest radiographs still dominate medical examinations, and some places even still use chest X-rays! It is of concern that X-ray and CT examinations should be avoided as much as possible for children and women in their childbearing years, especially pregnant women, and in cases where the condition requires that the examination must be performed, it is important to be well protected. In most cases, ultrasound and MRI without radiation damage can be used, such as the heart, abdomen and pelvis; MRI has the best diagnostic efficacy for the head, spine, limbs and joints, and soft tissues, but it is more expensive. Happily, the current price of CT and MRI in Shanghai is the lowest in the world, only equivalent to 1/10~1/20 of the United States. In short, the radiation risk in medical diagnosis should be treated correctly, as long as the condition of the sick people need, X-ray and CT examination is a normal medical practice, do not need to dwell on the possible risks of radiation; low-dose CT examination should be promoted among high-risk groups. CT examination should be promoted among high-risk groups, especially in the case of many smokers, serious air pollution and high incidence of lung cancer in China, low-dose chest CT examination should be put on the agenda as a medical checkup. For the general population, unnecessary medical radiation should be avoided as much as possible, and attention should also be paid to avoid radiation factors in life, such as facing computers, TV and other bad habits for a long time. It is believed that with the progress of technology, the radiation dose in X-ray and CT examinations will be reduced, and the radiation dose of the latest 640-layer spiral CT can be reduced by up to 80%, and a low-dose CT examination of the chest is only equivalent to a few chest films. It also provides a “one-stop” solution that allows simultaneous 2D multi-planar reconstruction and 3D three-dimensional reconstruction, which greatly improves the diagnostic accuracy.