The nuclear leakage in Fukushima, Japan, has made the public more aware of radiation, but it has also aroused to a certain extent the public’s concern about radiation, which is one of the major factors leading to cancer. Radiation is ubiquitous and only harms the human body after it reaches a certain dose and lasts for a certain period of time. So, how much dose and for how long can radiation be received before it causes damage to the organism? To figure this out, it is first necessary to understand two related terms: Sievert (unit symbol is Sv, abbreviated as Xi, which is translated as Siever in Taiwan) and Grey (unit conformity is Gy, abbreviated as Go). Sievert is the special name of the unit of dose equivalent, every kilogram of tissue absorbs 1 joule of energy that is 1 Sievert, Sievert is a very large unit, commonly used millisievert (mSv) to express the dose equivalent, 1Sv = 1,000mSv. And Grey is the special name of the absorbed dose, every kilogram of tissue absorbs 1 joule of energy per hour that is 1Gy, usually also expressed in milliGy, 1Gy = 1,000mGy. From its definition, it can be seen that Sv denotes the dose of radiation, while gorey denotes the absorbed dose. That is to say, 1Sv of radiation is 1Kg of tissue completely absorbed is 1Gy, and different tissues and organs of the human body on the absorption of radiation is also different, with different absorption rates, such as the lungs for 0.12, that is, the lungs are irradiated by 1Gy is equivalent to the whole body is irradiated by 0.12Sv. We live in the environment itself there is radiation, including the air, soil, and food in the, each year, about to receive 2 ~ 4mSv, our country is 3.5mSv, the radiation dose of 1Gy. 4mSv, our country is 3.1mSv; daily life of computers, refrigerators, television, microwave ovens and other radiation, but generally the total amount will not exceed 5mSv / year. The radiation that the public receives the most comes from medical diagnosis, including X-rays and CT, while ultrasound and magnetic resonance imaging (MRI) have no rays. An X-ray chest scan is about 0.02mSv, which is almost negligible damage to the human body; a CT scan of the head is 2mSv, a CT scan of the chest is 3mSv, and the largest dose is a CT scan of the abdomen, which is about 5mSv, which is the source of the claim that a CT scan is equivalent to hundreds of chest X-rays, but it is a bit of an alarmist statement because a long airplane trip is also 0.2mSv, and according to this claim it would be 0.2mSv. mSv, according to this statement is equal to dozens of chest radiographs, then smoking 20 per day can be up to 1 mSV / year, is not equal to nearly a hundred chest radiographs? People engaged in radiological work receive an annual dose of no more than 50mSv, no more than 500mSv to a single organ, and no more than 150mSv to the crystalline lens of the eye, implying that an annual radiation dose of less than 50mSv is safe, and it can be seen from this that X-ray radiography and CT are both safe. From a medical point of view, to assess the radiation risk of X-ray and CT examinations, we should distinguish between different groups of people. For the general public, unnecessary examinations should be avoided as much as possible; however, for high-risk groups and sick people, the examination should not be done without choking. According to the latest survey in Shanghai, lung cancer is the cancer with the highest incidence rate, and the most crucial factor for effective treatment and prolonging survival is early detection. Most of the lung cancers detected by X-ray chest radiographs are in the middle to late stage, losing the best chance of surgery, while CT can detect early lung cancers of several millimeters in size, many of them are still carcinomas in situ, and they can be completely cut by minimally invasive thoracoscopy. It can achieve clinical cured effect, minimize the pain of patients and reduce the medical burden. The U.S. Food and Drug Administration (FDA) has recommended low-dose chest CT as a physical examination 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 not comparable to that of the conventional dose. China’s current configuration of imaging equipment and population X-ray diagnostic annual frequency compared with developed countries there is a big gap between the United States per million people have a CT is 64 units, Japan is 88 units, while China has only 8 units, the annual frequency of X-ray diagnosis per 1,000 population is only 1/5 of the developed countries. in the north of Guangzhou and other developed areas, the concept of physical examination has been popularized, the rate of early detection of cancer has been significantly increased, and life expectancy is close to or even exceeding the level of developed countries, and the average life expectancy is about 1,000 years. In developed areas such as North, Shanghai and Guangzhou, the concept of physical examination has been popularized, the early detection rate of cancer has been significantly increased, the per capita life expectancy is close to or even exceeds the level of developed countries, and advanced tumors are rare; however, for most of the underdeveloped areas, it is common that people don’t have physical examination for a long time and don’t go to the hospitals for minor illnesses, and the discovery rate of advanced tumors is still very high, which has brought great pain and heavy economic burden to the patients and their family members, and has also greatly consumed the medical resources of the society. Therefore, it is urgent to vigorously promote the concept of physical examination and make low-dose CT examination as the physical examination program for high-risk groups, especially in big cities with medical conditions. Unfortunately, low-dose CT examination is still not a routine medical check-up program, and chest X-ray still occupies a major position in medical check-ups, and chest X-ray is even still used in some places! It is a matter 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 an examination must be performed, it must be well protected. In most cases, ultrasound and magnetic resonance (MRI) examinations without radiation damage can be used, such as the heart, abdomen and pelvis; MRI has the best diagnostic performance for the skull, spine, joints of the limbs and soft tissues, etc., but it is more expensive. Happily, Shanghai’s current CT and MRI prices are the lowest in the world, only equivalent to the United States 1/10 ~ 1/20. In short, for medical diagnosis of radiation risk should be treated correctly, as long as the sick people are in need of medical conditions, X-ray and CT examination is a normal medical behavior, do not have to be entangled in the risk of radiation; high-risk groups should be promoted in the low-dose CT examination, especially in our country. CT examination, especially in China’s many smokers, serious air pollution, lung cancer incidence rate remains high, low-dose chest CT examination as a medical examination should be put on the agenda; and for the general population, should try to avoid unnecessary medical radiation, but also pay attention to avoid the life of radiation factors, such as prolonged exposure to computers, television and other bad habits. It is believed that with the advancement of technology, the radiation dose in X-ray and CT examination will be reduced, and the radiation dose of the latest 640-slice spiral CT can be reduced by up to 80%, and a low-dose CT examination of the chest is only equivalent to taking a few chest X-rays, and it provides a “one-stop” solution, which allows simultaneous two-dimensional multi-planar reconstruction and three-dimensional stereo reconstruction, greatly improving the quality of the medical examination and the quality of the medical treatment. It also provides a “one-stop” solution for 2D multi-planar reconstruction and 3D stereo reconstruction at the same time, which greatly improves the diagnostic accuracy.