Dose reduction measures benefit pediatric patients

December 1, 2005

Balancing the benefits of a procedure involving ionizing radiation against the possibility of unwanted damage is often difficult. Regulations on exposure must consider medical, economic, and ethical aspects of radiation as well as the individual and collective dose of the population.

Balancing the benefits of a procedure involving ionizing radiation against the possibility of unwanted damage is often difficult. Regulations on exposure must consider medical, economic, and ethical aspects of radiation as well as the individual and collective dose of the population.

A European Council directive on the safe use of medical x-rays (97/43/Euratom) states that children and pregnant women should receive special protection. Due to the highly mitotic state of a child's cells, radiation risk depends on the child's age at the time of exposure. There is no evidence to suggest the presence of a threshold dosage, below which the risks associated with radiation disappear. Thus, any radiation dose, no matter how small, can cause harm. The probability, though not the severity, of harm increases with added exposure.1

The first years of life appear to be the most vulnerable period for radiation-induced cancer.2 The risk of contracting one of the cancers that are most readily induced by radiation, such as leukemia, thyroid cancer, and breast cancer, increases markedly with decreased age at exposure. The risk of contracting childhood leukemia and cancer varies from 2 x 10-3 to 3 x 10-3 per Gy. Prenatal radiation exposure of just 10 mGy has been reported to increase this risk by 0.06% to 40%.3-5 Children up to the age of four who are exposed to ionizing radiation are very susceptible to thyroid cancer.

Results from a study published in 2004 on the effects of radiation dose on infants' brains are alarming.6 Exposure of the neonatal brain to ionizing radiation at doses equivalent to those of a head CT scan can impair intellectual development. Premature infants and those who are ill at birth may require a number of x-ray examinations during their early weeks, resulting in a large accumulated radiation dose. A study of 43 newborns (23 premature) who underwent x-ray examinations in their early years (Figure 1) found that chest and chest/abdomen scans were the most common studies.7 Only a few studies of the skull, bone age, and hip joints were performed. The majority of radiographs were acquired during the babies' first month.

Exposure to medical x-rays should be justified in advance for all patients. This involves determining the objectives of the exposure and the patient's specific circumstances. The referring physician requesting the examination should assess the feasibility of an alternative modality. Previous x-rays and teleradiology should be used for consultations if possible.

Practitioners can protect their patients from excessive ionizing radiation by continually comparing the doses they deliver with standard dose reference levels.8 Experts recommend recording dose measurements and data specific to the procedure (kV, mAs, grid use, image receptor system, and generator) in order to estimate the dose delivered to that patient for future reference.

Effective doses vary widely, even for the same examination performed with similar imaging receptors.7,9 These variations are particularly important in imaging children, especially newborns (Figure 2).7 A positional shift of just 1 or 2 cm can cause a huge change in the dose delivered to different organs. The size of children of the same age can also vary widely. A newborn may weigh as little as 0.3 kg and a 16-year-old more than 120 kg.

Children who are ill from birth, such as those with respiratory distress syndrome, vesicoureteral reflux, or heart problems, are likely to need many radiological follow-up examinations.10 Even healthy children may undergo x-ray examinations of their teeth or chest.

Studies of pediatric imaging examinations in Europe have found wide variations in techniques, equipment performance, and dosage. One analysis of pediatric chest radiography, for example, revealed that some children received 40 times more radiation than others who had the same examination.11

Evidence suggests that users of conventional and digital x-ray systems could make substantial dose reductions without loss of image quality. In children's examinations, the EU directives involve the use of a speed class of 400 to 800 with a screen-film system.11

Digital radiography users should avoid excessively high kV settings and tube currents. This applies across the range of competing digital technologies.

Radiography staff should be able to optimize dose levels so that increasing dosage "just to make sure" is unnecessary. Flat panels have been shown to provide good resolution with no significant difference in diagnostic quality at reduced radiation doses.12,13 Researchers have reported lowering doses by 50% to 75% with no impact on image quality.14,15 Another group reported a 50% decrease in radiation dose compared with the screen-film system speed class 400.16 They attributed the reduction to use of a flat-panel system.

The European Commission's DIMOND III project addressed the issues of dose constraints with the objective of safer, more cost-effective healthcare. One of its main requirements is for "quality as good as necessary, not as good as possible, and dose value as low as possible." Speed class 800 results in image quality good enough for almost all x-ray examinations, according to the DIMOND III recommendations.17 Speed class 1600 is recommended for flat-panel detectors. Changing the nominal speed class from 400 to 800 reduces the patient dose by 50%.

SCREEN-FILM STRATEGIES

Beam energy, filtration, collimation, grid use, the anode heel effect, patient size, screen-film combination, and film processing conditions all affect the patient dose in conventional radiography, and most affect CR examinations as well. The higher the kV, the farther radiation penetrates and the lower the required dose. Exposure times are also shorter with higher kV values, reducing the blurring from patient motion that can be a problem with newborns and children.

Use of an antiscatter grid is unnecessary with infants and small children. Additional tube filtration of 0.1 to 0.2 mm of copper (or equivalent) is recommended for examinations performed on equipment with an existing total filtration of about 2.5 mm of aluminum. The additional filtration should absorb the soft part of the radiation spectrum, which would otherwise be absorbed by the patient's skin.

Practitioners should avoid routine use of automatic exposure controls when imaging small children, but if they are used, precise adjustment is essential. A change in film density from 1.2 to 1.7, for example, means a 40% higher dose. Beam collimation is difficult with children because the size of the exposed area in newborns can vary widely, even in infants of the same height (Figure 3).7 The effective dose will be 15% higher if the primary radiation beam is increased from 20 x 20 cm to 22 x 22 cm. A dose area product meter should be routinely employed on x-ray tubes used in pediatric x-ray examinations.

The smallest patients can require the most powerful machines. Use of a 12-pulse or high-frequency multipulse generator may reduce dose by cutting exposure time. It takes 20 times longer to achieve the same level of blackening on a 10-month-old baby's chest x-ray, and the entrance surface dose is 2.15 times higher, with a one-pulse generator than with a converter generator.11

Use of low-attenuation materials such as carbon in table tops and film cassettes can also reduce dose. The radiation dose is greater in the cathode side of the radiation beam, due to the anode's shadow-to-focus distances. Because there is less radiation on the anode side, the right positioning of the output beam reduces dose on the thinner part.

Keeping dosage low in fluoroscopy requires use of a pulsed mode, a short overall imaging time, high kV, and low mA. The "last image hold" function is essential. A dose area product meter is recommended. The grid should be removed and additional filtration used to optimize image quality.18 CT examinations of children should adhere to tight indications and dedicated protocols.

Lead and rubber shielding must be used in pediatric x-ray imaging to protect nearby parts of the body from primary and scattered radiation. Radiosensitive cell-forming bone marrow is present in most bones at birth. The developing breast, thyroid, and gonads are also sensitive to ionizing radiation and should be protected. Selecting a projection (anterior/posterior, or vice versa) that results in the child's body absorbing the soft beam can minimize the dose delivered to these organs.19

Contact vinyl rubber placed on the infant's body provides the best protection in neonatal radiography. Personal shields should be used in incubators; they are hygienic and help minimize any drop in the baby's body temperature. If personal shields are not available, alternative strategies include hanging lead from the collimator to cast a shadow on the primary beam and placing a lead shield on the top of the incubator lid. The radiation beam should be collimated carefully, regardless of lead shielding, so that only the area of interest is exposed.

PERSPECTIVES

Many factors must be optimized to obtain acceptable diagnostic images of newborns and young patients, but the lowest possible radiation dose must be employed. Exposure factors (kV, mAs) should be selected on the basis of a patient's size rather than age. Training programs, dissemination of dose information, and collaboration among physicists, radiographers, and radiologists can substantially reduce the amount of radiation children receive.

Only trained staff should take x-ray examinations of children. Use of patient care sheets, on which exposures for previous examinations are recorded, help tailor examination parameters to an individual's dose history.20 Medical imaging management with PACS permits better monitoring of radiology practice and enables receipt of a second, specialized opinion via teleradiology.

Diagnostic dose reference levels are set nationally for all radiological examinations. The dose reference level corresponds to the third quartile. About 75% of people receive a dose less than this value; i.e., these levels should not be exceeded for standard procedures when diagnostic and technical performance follows good practice. Evaluating image quality against patient dose and meeting the criteria for good radiographic technique are key to achieving a significant dose reduction.

The varying weight of newborns (from 0.4 kg to 6 kg) requires several diagnostic dose reference levels for x-ray examinations. Exposure factors (kV, mAs) are not reliable dose descriptors. Measures of entrance surface dose and dose area product are more valuable. The term "effective dose" is suitable when comparing modalities.

The radiation dose a child receives in a single x-ray examination will be small; effective dose is usually under 1 mSv. Doses from CT scanning and interventional radiography procedures are higher, however. Because some children are exposed regularly to medical x-rays and receive substantial doses, imaging techniques should be recorded on film or digitally. Future PACS and electronic patient record systems may include features to record total dose.

Dose should be optimized every time a child is exposed to ionizing radiation. Radiography staff who specialize in pediatric imaging should learn to produce tightly collimated high-quality images, discuss the procedure with patients and parents, and perform x-ray examinations safely and confidently.

DR. KETTUNEN is a radiographer and principal lecturer at Oulu Polytechnic, School of Health and Social Care, in Oulu, Finland.

References

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13. Strotzer M, Volk M, Frund R, et al. Routine chest radiography using a flat-panel detector: image quality at standard detector dose and 33% dose reduction. AJR 2002;178(1):169-171.

14. Volk M, Hamer OW, Feuerbach S, et al. Dose reduction in skeletal and chest radiography using a large-area flat-panel detector based on amorphous silicon and thallium-doped cesium iodide: technical background, basic image quality parameters, and review of the literature. Europ Radiol 2004;14(5):827-834.

15. Hamers S, Freyschmidt J, Neitzel U. Digital radiography with a large-scale electronic flat-panel detector vs. screen-film radiography: observer preference in clinical skeletal diagnostics. Europ Radiol 2001;11(9):1753-1759.

16. Ludwig K, Lenzen H, Kamm KF, et al. Performance of a flat-panel detector in detecting artificial bone lesions: comparison with conventional screen-film and storage-phosphor radiography. Radiology 2002;222(2):453-459.

17. DIMOND III Final Report. Image quality and dose management in digital projection radiography. 13 July 2005, http://www.dimond3.org

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19. Cook J, Kyriou J, Pettet A, et al. Key factors in the optimization of paediatric x-ray practice. Br J Radiol 2001; 74(887):1032-1040.

20. Duggan L, Warren-Forward H, Smith T, Kron T. Investigation of dose reduction in neonatal radiography using specially designed phantoms and LiF:Mg,Cu,P TLDs. Br J Radiol 2003;76(904):232-237.