The goal of this article is to provide a summary of the recent literature regarding the main various aspects between the children and adults in computed tomography imaging. This review is based on published articles, studies and guidelines in English and Romanian language related to the period 2000-2019, searched using electronic database with subjects focused on the potential danger of radiation exposure from CT in children or adults. Particularities on radiation protection in children and adults have been grouped in two aspects: techical – structural and practical. The techincal and structural features are referring to anatomy body structure, organ doses, kilovoltage technique, beam range doses, contrast media and its adverse reactions. The practical feature is concerning about the management of lowering the radiation dose, the global number of examinations, the effective dose and the equivalent exposure to natural radiation, the lifetime attributable risk of radiation-induced cancer, tissue sensitivity to radiation, prescan preparation, shieldering and sedation aspects.
Findings and procedure details
Tehnical and structural features
Between this goups of patients we do have anatomy differences especially in children like different red blood marrow distribution (children have highly radiosensitive red bone marrow in long bones, and adults have red bone marrow mainly in the axial skeleton) , non-ossified bones, imature organs, a higher cell turnover and mitosis rate, different body composition with more water, less fat and fibrous tissue. Children have low weights, immaturity of renal function, lower eGFR, fragile vascular access and lack of communicability.  In adults, we can say that the whole structural body has reached complete development. This differences comes with the concept of adapting the doses and the settings accordingly.
Due to differentions in tissue thickness, is required some changes in radiographic parameters when patients are smaller or larger than average for their age group. The best practice is to use the highest kVp and the lowest amount of mAs needed to provide an adequate exposure. The kVp is increased in each ascending age due to growth in tissue thickness requiring more energy penetration. KVp values varies between 40 kVp for a baby hand of 1 cm thickness, up to 66 kVp for an adult size knee of 12 cm thickness. 
If we use the same techniques factors for both adults and children on a CT with a typical 120 kVp x-ray beam, doses to the adult body range from 15–35 mGy while doses to the pediatric body are higher at 29–68 mGy.  Different authors have suggested that pediatric CT doses may be reduced to 30–50% to adult doses, by reducing the milliampere-seconds, with no loss of image quality. 
The organ doses were calculated on children in a study led by Lee C for different CT examinations. At the same technical setting of 100 mAs he concluded that the dose to the organs were higher by from 40 to 80% for newborn phantoms compared to those of 15-year phantoms. 
After intravenous injection the contrast medium travels throughout the body and its circulation is controlled by the cardiovascular system. First it runs to the right heart, the pulmonary circulation, and the left heart before reaching the central arterial system. 
Children have a larger relative circulating blood volume ( 120 mL/kg) on a small surface, heart rate (85-150 beats/minute) which distributes the blood faster and respiratory rate (20-40 breaths/minute). In adults we have smaller normal values of circulating blood volume ( 65-70 mL/kg) on a big surface, heart rate (60-100 beats/minute) with a slower distribution of blood and a lower respiratory rate (12-16 breaths/minute).   Considering this we need to adapt for each situation the contrast agent dose and the delay time to obtain a suitable image acquisition. The contrast dose is referred to as the maximum allowable contrast dose (5mL × body weight (kg)/serum creatinine (mg/dL) , children have an avarage of 2.0 mL/kg (concentration 300 mg I/mL)  and in adults is 1-2 mL/kg (concentration 300 mg I/mL). The injection flow rates is up to 5 mls/second in adults and 2 mls/second in children.  Although the dilution factor of oral contrast agents is usually higher than in adults (1–2% instead of 3% of iodinated contrast media (300–350 mg I/ml)), which provides better contrast with low tube voltage techniques. 
Iodine in a target organ causes a higher rate of absorption and a scattering of x-ray radiation which lead to an increase in attenuation and contrast medium enhancement on the CT image. The degree of CT contrast enhancement is directly related to the amount of iodine within the system and the level of x-ray energy therefore a higher contrast dose will create a greater attenuation with a high level of residual radiation.  Techniques such bolus tracking and the test bolus have the disadvantage of additional-monitoring scans increasing the radiation dose. An empirically determined fixed delay time usually suffices for most routine indications. 
Adverse reactions of contrast media in children have a lower incidence than adults, but have a significant impact on the development.  A study of 554 children subjected to a power injector revealed the incidence rate of extravasation to be 0.3%.  The reported incidence of intravenous contrast media extravasation related to power injection for CT ranges from 0.1 to 0.9% in adults. 
Reducing the radiation dose and its associated risks starts with performing only when properly indicated this is a justification for scan. To achieve this goal, it is need to exist a communication between the referring physician and radiologist, so only with a complete clinical context, the radiologist will be able to make a well-considered decision of which imaging modality will be the best choice to answer the clinical question. Moreover this information will help the radiologist to optimise the CT technique, reducing the risk of failed and repeated examinations. 
There are a few adjustable CT techniques in terms of lowering the patient radiation dose. We can decreased kVp and mA, have a faster tube rotation speed, a shorther scanning distance, a proper position in scanner and a low number of scan sequences. 
From 1991 to 1996 the total number of diagnostic medical examinations worldwide was about 2.4 billion and approximately 250 million in children below 15 years of age. In the period 1997–2007 the total number of examinations increased to more than 3.6 billion, with about 350 million examinations performed in children below 15 years of age. The most performed worldwide procedure is chest radiography witch represents 40%. On a global average frequency in diagnostic X-ray procedures for all ages and both sexes the chest examination have a 40% frequency, fallowed by limb and joint (8.4%), skull (3.2 %), abdomen, pelvin and hip (5.2%), and spine (7.4%). The most examinated region in children (0-15 years) is head/skull radiography (19%) and CT head (8%).
The effective dose on a chest X-ray for an adult and a 5 year old is the same (0.02 mSV) with an equivalent period of exposure to natural radiation of 3 days. For a CT head an adult is exposed to a 2 mSV to an equivalent period of exposure to natural radiation of 10 months where in a newborn it has a 2.5 years of equivalent exposure to natural radiation and a 6 mSV. For CT chest it is been observed that the doses are 7 mSV for an adult and 1.7 mSV for a newborn, with an equivalent of 3 years vs 8.6 months. For an CT abdomen it is a 7 mSV for an adult vs 5.3 mSV in newborn with an equivalent of 3 years vs 2.2 years. 
A study made by Stodickson et al, about the radiation exposure and lifetime attributable risk of radiation-induced cancer from computed tomographic scanning of adult patients shown that CT exposures were estimated to produce 0.7% of total expected baseline cancer incidence and 1% of total cancer mortality.  Another study made by David J.Brenner et al, demonstrated that cancer mortality risks attributable to the radiation exposure from a CT in a 1-year-old are 0.18% (abdominal) and 0.07% (head) a higher risk than for adults. In the United States, approximately 600,000 of head and abdominal CT examinations are annually performed in children and it is estimate that 500 of these individuals might die from cancer attributable to the CT radiation. 
In a review made by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) committee, speculated that children are clearly more likely to develop malignant conditions like leukemia, brain, breast, skin, and thyroid cancer. However, for other types of cancers, children are no more sensitive than adults to develop colon cancer or lung cancer.
To acquire an optimal CT examination it is important that the prescan preparation to be adequate. For both categories of patiences the preparation includes informations about de CT examination and the risks related to contrast media. In children category we have to talk with the parents, to let the children to adapt with the scanner environment and to influence them positively in order to reduce the anxiety. Adults understand in a different way the role of the examination. 
During the examination we may use a range of shields to protect the sensitive parts of the human body. A study made by M. Dobbs shown that the implementation of bismuth shielding in children especially bismuth breast and thyroid shields had succesfully reduced the dose to the tissues underneath the shield without artifacts the image quality. 
Children often need a sedation or a general anesthesia when performig CT scan mostly because they can not stay still and artifacts the image. Both categories of patients may need sedation or anesthesia due to the medical conditions. There have been different studies in children, that have investigated the association between exposure to anesthesia in childhood and neurodevelopmental outcomes. A review made by Wang et al found an association between neurodevelopmental impairment and anesthesia, and found that the number of exposures to anesthesia before 4 years of age was a more significant risk factor of worse outcomes than time of exposure alone.  For children and also for adults, sedation can have associated problems like insufficient level of sedation or oversedation. 
In order to proceed a sedation or a general anesthesia to perform a CT scan, especially in children we can use some helpful devices, like a vacumm pillow which can be wrapped around the baby or the part of the body to be investigated.The vacuum is ideal for the fixation of babies or certain parts of the body in infants and preventing the need for sedation or anaesthesia. A study made by Joseph J. Khan et al, included a program dedicated to reducing the frequency of pediatric sedation using a child life specialist, video googles, installing a moving color light show device in CT and adding a DVD player with a flat-screen monitor on a multijointed arm in the CT room. It was pointed up that there was a 44.9% decrease in the frequency of sedation from 27.1% prior to the program to 14.9% during the program.