Medical irradiation was the first area where x-rays gained practical application after their discovery and is currently the largest source of artificial ionizing radiation to which humans are exposed. Diagnostic x-rays, in particular, generate more than 95% of the total annual effective dose to the world’s population from artificial radiation sources. Excessive radiation exposure in humans can cause irreversible cellular damage and chromosomal aberrations, affecting the irradiated individuals and their offspring. It has been shown that the effective radiation dose for CT diagnostics is similar to that received by survivors of the Japanese atomic bomb blast several miles from the center of the blast (although the exposure method and type of radiation vary). In addition, studies have shown that people who have received x-rays are 0.6% more likely to develop cancer, mainly bladder, colon and leukemia, when their life expectancy exceeds 75 years. Children, especially younger children, are in the growth and development period, and their cell division and renewal rate is much higher than that of adults, so their sensitivity to radiation is also much higher than that of adults. The younger the child is exposed to radiation, the greater the risk of cancer, and the lifetime tumor mortality rate caused by radiation dose under the same scanning conditions is 10-15 times higher in 1-year-old children than in adults during CT examinations, and the radiation dose will continue to accumulate for the rest of the child’s life due to radiation examinations. Unfortunately, few researchers have focused on the radiation dose of diagnostic CT examinations and the risks they pose, and few physicians have the expertise to know how much radiation dose a patient is allowed to receive and how much radiation dose the patient will receive during this examination. In simple terms, the dose from a routine CT scan is approximately equivalent to taking 300 plain chest x-rays, with a risk equivalent to smoking 10 cigarettes per day for 1 year. It has been reported that if the radiation examination makes the patient every 10mSv radiation dose increase, the death rate will increase by 0.04%, which is equivalent to the risk of smoking 20 cigarettes per day or driving 10,000km in 6 months; moreover, every 10mAs increase in tube current is equivalent to the increase in radiation dose of 7 to 14 conventional x-ray chest films, so the risk of CT radiation cannot be ignored. CT image clarity, imaging speed, wide range of applications, high equipment penetration, there are still other examinations irreplaceable role, the contribution made in the prevention and treatment of disease is evident to all. Comprehensive view, the role of CT is more good than bad, the benefits far outweigh the risks, the current in clinical work still need to be widely used CT, but it causes the average annual effective dose of the population will therefore the possibility of a significant increase can not be ignored. Some information shows that in the United States, CT examinations account for 13% of all radiological examinations, but the radiation dose it causes patients to receive accounts for 70% of the total radiation dose received by patients. In China, there are more than 50,000 radiological treatment institutions, and about 250 million people receive treatment every year, with an installed capacity of about 5,000 units of CT equipment, ranking 3rd in the world. Multi-layer spiral CT (MSCT) can obtain more high-quality images in less time, and is entering the clinic in large numbers, gradually replacing single-layer CT. clear images and good display of lesions have intensified clinical reliance on radiological diagnosis, leading to a rising trend in the overuse of CT. Although the scan time of MSCT is shortened, the scan level is getting thinner and the radiation dose required to achieve a certain resolution is bound to increase. There are now more than 250 spiral CTs with 64 or more layers nationwide, and as MSCT becomes more and more widely used in clinical practice, CT radiation dose will inevitably become a constraint on its application if the dose to the subject is not controlled. Radiology staff have the obligation and responsibility to reduce the dose of CT radiation received by patients. In clinical radiology, the reduction of radiation dose should mainly obey the three principles of radiation protection for X-ray examinations proposed by the International Commission on Radiological Protection (ICRP) in 1997: (1) justification of practice, i.e., the practice of radiation exposure, unless the benefits to the exposed individual or (2) Optimization of radiation protection, i.e., radiation practices should be conducted in such a way as to ensure that the radiation dose is kept as low as reasonably achievable, taking into account economic and social factors; (3) Individual dose limits, i.e., the exposure resulting from the combination of all relevant practices. (3) individual dose limits, i.e., exposures from all relevant practices combined, to the selected individual dose limits. The purpose of specifying individual dose limits is to prevent deterministic effects from occurring and to limit stochastic effects to an acceptable level. In addition, the principle of “as low asreasonably achievable (ALARA)” should be followed in radiological examinations. That is, the principle of using the lowest dose to obtain diagnostic images to meet clinical needs. It is the principle of using radiation protection optimization methods to keep the dose to individuals, the number of people exposed, and the potential risk of exposure at as low a level as reasonably achievable in practices that have been judged to be appropriate and allowed to proceed. However, we have the following problems: (1) A survey shows that 16.8% of professional and technical personnel in China are unaware of dose limit standards; 70.0% of professional radiologists are unable to answer the question of what is meant by random and deterministic effects. (2) According to the literature, in CT examinations, without changing the scanning conditions, pediatric patients receive much higher effective radiation doses than adults, and CT examinations of the head and neck can be 2.5 times higher than those of adults. This indicates that the smaller the subject is when CT examinations are performed under the same conditions, the higher the radiation dose it receives. Therefore, in clinical work, we should be more alert to the problem of higher radiation doses during CT examinations in children. However, most hospitals are still using adult standards for CT examinations in children, and radiation dose has become one of the potential factors affecting children’s health. (3) In clinical work, most hospitals routinely use multi-phase scans. And Slovis’ study showed that multi-phase scans imply a multiplicative increase in radiation dose, but it does not increase the diagnostic accuracy exponentially as a result. (4) Crystals cannot be avoided in orbital, sinus, and middle ear scans; breast cannot be avoided in chest examinations; and the reproductive system cannot be avoided in sacroiliac, hip, or pelvic scans during conventional dose scans, while crystals and glands are extremely sensitive to radiation. To sum up, reducing the dose of x-ray radiation received by patients during CT examination is not only a major issue related to patients’ health, but also a major issue related to the future of CT, a powerful imaging tool and diagnostic radiology, and a major issue that every person engaged in the development, manufacture, use and medical radiation protection of CT should be concerned about and practice. The dose received by the examinee is 20% or more lower than the conventional dose to confirm the dose reduction. Reducing the radiation dose to patients from CT examinations can be achieved through various measures: (1) hardware, such as developing more effective filter plates, more precise collimators and more efficient detectors, etc.; (2) software, such as developing better algorithms, more flexible automatic dose modulation software, better noise reduction and artifact suppression software, etc., to improve image quality and provide space for dose reduction; (3) more reasonable optimization of scanning parameters, using individualized scan ranges, tube currents, tube voltages and pitches that are specific to the site and organ under examination and the purpose of the examination. The first two are mainly done by the R&D and manufacturing departments of CT, while the latter can and must be done by us medical imaging practitioners. According to CT imaging theory, the detection of lesions and the display of internal structures depend on the spatial resolution of CT, which is related to the density difference between the organ and tissue under test. For organs or sites with high contrast between tissue and gas, bone and soft tissue (e.g. temporal bone, sinus, nasopharynx, lung, bone), there is a good density difference between tissues due to high contrast between tissues and/or low absorption of x-rays by gas, and a certain degree of increase in noise does not cause a significant decrease in contrast between tissues, making it possible to envision low-dose CT scans of these sites, and the scans should be performed in a way that safeguards The scan should be performed with the lowest possible dose under the premise of ensuring diagnostic quality. The dose absorbed by the object is determined by the quality and quantity of the x-ray. The tube voltage of the x-ray tube determines the energy of the electrons emitted by the cathode filament, i.e., the quality of the x-ray (or the hardness of the x-ray, i.e., the ability to penetrate the substance); the tube current of the x-ray tube determines the number of electrons emitted by the cathode filament, i.e., the quantity of the x-ray. Under the premise of constant tube voltage, the x-ray radiation dose within a certain range determines the image quality. However, after the x-ray dose exceeds a certain range, too high a dose does not significantly help to improve the image quality. In children, due to their small size (thin thickness), more x-rays will reach the detector under the same conditions with a fixed x-ray tube voltage (120 kV), so the amount of x-rays required (tube current) is lower in children than in adults if they need to achieve the same image quality as adults. Cody et al. reported that at the same noise level, pediatric thoracoabdominopelvic scans can reduce the radiation dose by 60% to 90% compared to adults, so the principle of low-dose radiation optimization should be implemented for CT examinations in children. In abdominal examinations, the tube current value cannot be reduced too low because the image noise reduces the low contrast resolution and makes the contrast between soft tissues such as liver and between soft tissue organs and their lesions insufficient, so there is not much room to reduce the tube current in abdominal examinations, but at least it can be done without using too high a dose and without making the scan range more than what is actually needed. Reducing the radiation dose by reducing the tube current is easy to grasp and is a commonly used CT low-dose scanning scheme, but can also be achieved by reducing the tube voltage. x-ray irradiation of the human body, and human tissue photoelectric effect (PhotoeleCTric effeCT) and Compton scattering effect ((Compton effeCT), the relative strength of the photoelectric effect determines the material x The tube voltage is reduced. x-ray photon energy is lower and the photon energy (Key) is closer to the “K-edge” (Kev) of tissues or structures containing elements of high atomic number (such as bone, iodine-containing tissues or blood vessels), when the photoelectric effect is enhanced and the CT value is increased. The method of reducing tube voltage is more suitable for CT angiography (CTA), which can reduce both the dose and the amount of contrast agent. The prevailing problem is that the demand for CT image quality is higher than the actual diagnostic need. the reduction of CT scan dose may increase the noise of the image, but it should be accepted as long as it does not affect the diagnostic quality. The radiologist needs to learn to “tolerate” a certain amount of increased noise when performing low-dose scans to bring the benefit of reduced radiation to the patient, rather than just seeking a “pretty” image. In terms of reducing CT radiation dose, radiologists should practice actively and flexibly in their daily clinical work, and continuously adopt new technologies and methods to reduce CT radiation dose as much as possible. Radiology staff should have a clear medical purpose before conducting medical irradiation to patients and examinees, should analyze the pros and cons of different examination methods, and give priority to diagnostic techniques that have less impact on human health under the premise of ensuring diagnostic effects. Prior to the radiological examination, the examinee should be informed of the health effects of radiation. The principles of justification of medical exposure and optimization of radiation protection should be observed during the examination, with strict setting of scanning parameters, control of exposure dose, and control of unnecessary multi-phase scanning. The radiation dose received by the patient for each examination is also related to the scanning range, and the scanning range should be reduced as much as possible, which is conducive to reducing the exposure range. the apparent image quality may be affected by the adoption of low dose for CT scanning, and may be subject to clinical or social interrogation as a result. In this case, we should explain with reason and evidence, “reason” as mentioned above, and more can be found in the literature; “evidence” requires us to explore and study according to our own equipment conditions, and use our own data to explain that the dose reduction has not decreased the diagnostic level. The other party will understand and support after understanding the “reason” and seeing the “evidence”, because we all share the same belief that we are working for the health of mankind today and tomorrow. Since 2005, when we first called for attention to the dose of CT examinations in China and suggested that the dose should be reduced as much as possible under the premise of guaranteeing the diagnostic level, we have received responses from many colleagues in China, and more and more units have been involved in research on this issue, and many achievements have been made. In many aspects of CT low dose research is basically synchronized with the international, and the results achieved are similar. In reducing the tube current, more work has been done and more achievements have been made; in reducing the tube voltage, there are colleagues in China who are doing the work and have made certain achievements, which is very promising. Under the current medical level and medical environment, we cannot reduce the diagnostic level that CT can achieve, but we hope that people will have a more rational understanding of the relationship between the diagnostic ability of CT and dose in the future. Radiological diagnostic equipment varies widely from one medical institution to another, and it is impossible to set a uniform standard of scanning parameters. It is especially important for each hospital to figure out a personalized scanning plan suitable for its own equipment according to its own equipment conditions. As long as radiologists throughout the country and the world pay attention to follow the ALARA principle, it is bound to minimize the radiation dose load of x-rays to the examinees in CT examinations, so that CT can continue to play its advantages in the clinic and greatly benefit.