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REVIEW ARTICLE |
* Department of Radiology, Duke University Medical Center, Durham, North Carolina
Department of Radiology, Cincinnati Children’s Hospital Medical Center, Cincinnati, Ohio
Department of Radiology, Memorial Sloan-Kettering Cancer Center, New York, New York
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ABSTRACT |
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 TOP ABSTRACT IMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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Key Words: CT, infants and children • helical CT • radiation dose
Abbreviations: CT, computed tomography • SPR, Society for Pediatric Radiology • mSv, milliSievert • CTDIw, weighted CT dose index • DLP, dose length product • mA, milliamperage • ALARA, as low as reasonably achievable • kVp, kilovoltage peak
It has been more than 3 decades since computed tomography (CT) first became available for diagnostic imaging. Technical developments have resulted in a number of distinct generations of scanners, including the helical CT, in the early 1990s and, most recently, multidetector (or multislice) scanners. Because of these technical advancements, CT has become an increasingly used tool.1–3 However, as with any tool, the greatest benefit is derived from a combination of sufficient technical understanding and appropriate application. Without this, the tool may not be as useful, and can be dangerous. One important potential danger of misuse of CT is an inappropriate amount of radiation. Recently, many health care providers as well as the general public became more aware of unnecessary CT radiation exposure, including potential cancer risks.4 This information was based on a series of 3 articles published in the American Journal of Roentgenology5–7 and the subsequent media attention (at times based on inaccurate reporting) of this series. The initial reaction of health care providers as well as the public was swift and intense, and there was a lack of readily accessible information to address these concerns for radiologists as well as pediatric health care providers.8
The role of pediatric care providers in CT is paramount for several reasons. First, health care providers are responsible for ordering and providing indications and justifications for CT examinations. In addition, the nature of the physician-patient relationship can make the pediatric care provider the principal (or only) source of information about imaging studies, including potential risks. For these reasons, the following material is intended to serve as a concise and contemporary review of CT and related radiation issues for the pediatrician, pediatric subspecialist, nurse practitioner, family practitioner, or other pediatric health care provider. Material emphasizes the unique aspects of pediatric CT and includes patterns of use and applications, radiation risks, and strategies that can be implemented to manage radiation: that is, to reduce or eliminate unnecessary radiation children get from CT examinations.
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IMPORTANCE OF CT FOR CHILDREN |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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The number of CT examinations performed in the United States has been estimated to be as high as nearly 60 million,14 and the use has been increasing. Notably, in a recent review, it was estimated that there was a 600% increase in all CT examinations for the decade spanning the mid-1980s to the mid-1990s,15 with a an increase in the pediatric population from 
4% to >11% of all CT examinations.9,15 Approximately 33% of all pediatric CT examinations are performed in children in the first decade of life, with 17% in children at or under the age of 5.16 These numbers are important because organs and tissues in younger children are more susceptible to radiation-induced cancer, as will be discussed later in greater detail. Screening examinations such as those for coronary artery disease, lung cancer, and potentially colon cancer obviously have much more application in adults.17,18 However, these procedures are conceivably used in higher risk populations in young adulthood at a time when care may still be primarily through pediatric providers. Therefore, the same principles of minimizing radiation exposure in children with a long life expectancy should also apply to young adults.18 Despite the increase in use and increased attention,19 there has not been a parallel increase in understanding of the risks or the use of techniques for reducing these risks. For example, it was recently (November 2002) that a symposium sponsored by the National Council on Radiation Protection and Measurements identified that CT radiation is still an important topic. In particular, recommendations from this symposium included educating the medical community.14
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HISTORICAL PERSPECTIVE: CT AND THE DISCOVERY OF X-RAYS |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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The Society for Pediatric Radiology (SPR), established in 1958, is an organization that is dedicated to medical imaging in children. One of the founders for this society, who most consider the father of pediatric radiology, was Dr John Caffey. Dr Caffey, whose formal training was in pediatrics, was long a proponent of minimizing radiation in children. He considered one of his most important and enduring contributions the debunking of the use of radiation therapy to treat the normal prominent thymus for respiratory distress in infants and children.21 The SPR, as well as other pediatric radiology societies, have continued to serve as resources for imaging research and imaging welfare in children, including CT and radiation.22 For example, at the 2001 International Pediatric Radiology meeting (which includes the SPR, the European Society of Pediatric Radiology, and a number of other pediatric radiology societies) the award-winning paper for a scientific investigation was based on dose reduction CT in pediatric abdominal scanning.23
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CT AND RADIATION: UNIQUE CONSIDERATIONS IN CHILDREN |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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5% of all x-ray imaging and yet the radiation from CT examinations is 40% to 67% of all medical radiation.9,24,25 For example, in a recent review of CT use and radiation dose, the effective dose (a measure of whole body dose based on individual organ doses and sensitivities of these organs) of a chest CT was 54 times that of a mammogram, and nearly 68 times the dose of a chest x-ray.9 Finally, when CT is not necessary, or when settings provide an unnecessarily high dose, there will be a greater population dose, a potential public health concern.22 Considerations unique to the pediatric population include increased radiosensitivity of certain tissues, particularly in infancy,26 a longer lifetime for radiation-related cancer to occur, and a lack of size-based adjustments in technique. The thyroid gland, breast tissue, and gonads are structures that have an increased sensitivity to radiation in growing children. This means that a similar radiation dose per gram weight of tissue has a greater potential for developing fatal cancer.15 Some of these regions are routinely involved in scanning in the chest, such as the thyroid and breast tissue. However, some of these regions are routinely included in examinations usually thought to exclude these areas. For example, the breast tissue may be included on the uppermost images obtained during an abdomen scan. The ovaries might be in the lowermost images of an abdomen scan that extends down through the upper pelvis, and the testicles could be included in an examination of the pelvis where the lower images extend down below the symphysis pubis or even above that point when the testicles are retracted. If close but not directly in the area of scanning, scatter, or internal deflection of the x-ray particle (photon) path, may also affect organs.
In addition to increased organ sensitivity, small children also receive a greater radiation dose than larger children or adults from the same CT settings. Although the energy imparted (a measure of radiation dose) is less for smaller patients than adults, the corresponding organs are even smaller and the actual dose to the organ is higher.27
One of the major current issues with radiation risk from CT in children is that children have a longer lifetime in which to manifest radiation-related cancer. Moreover, the cancer risk is cumulative over a lifetime. Each CT examination (including multiple series per examination) contributes to the lifetime exposure. Radiation for older adults and the elderly does not carry the same cancer risk because many radiation-induced cancers, particularly solid malignancies, will not be evident for decades.
The final point related to the unique consideration of CT radiation in children is that, despite most of the rest of pediatric dosing, such as with antibiotics, adjustments in CT technique have not been traditionally made based on variations in size in children.5 One of the articles in the American Journal of Roentgenology from February 2001 concluded that there were very few age-based adjustments in CT scanning for children. Importantly, tube current (units of mA) for helical scanning was as high for the youngest age group as it was for those who were 12 to 16 years of age. Although this investigation dealt with a relatively small sample size from regional institutions, the conclusions likely reflect routine practice.5 In particular, it has been our experience based on discussions with fellow pediatric radiologists, dialogue at national conferences, work in committees and organizations such as the SPR that these data apply to much of the body scanning in children. A recent survey of pediatric radiologists indicates that, although some adjustments are being made, there is still a substantial need for size-based adjustments in pediatric body CT scanning.28 One reason for this lack of adjustment is likely that CT is a digital technology and there is no penalty for high doses of radiation. That is, higher radiation doses improve image quality unlike radiography where higher doses can result in overexposed, or dark, examinations.
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RISKS OF LOW-LEVEL RADIATION AND CT |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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100 milliSivierts (mSv).29 The dose from a single CT examination (a CT examination can have from 1 to 4 different series, including pre- and postcontrast evaluation) can range from <1.0 mSv to >27.0 mSv.24 For perspective, background exposure, that is from natural sources, is 3.0 mSv per year. Support for and against increased cancer risk with low-level radiation can be found in the scientific literature. One such perspective is hormesis. Hormesis is the view that low-level radiation may actually be protective.30–32 In a recent article, Cohen30 reviewed investigations that supported hormesis. Investigations in support of hormesis included the following: prior exposure to low-dose radiation decreased chromosomal aberrations after high-dose exposure; immune response increased with low-level exposure; and epidemiologic studies that report a lower level of cancer than expected after low-level exposure. Data were also reviewed which indicate that harmful effects are not present until doses ranging from 200 to 500 mSv were given.30
The contrary, and more prevalent view, is that there is a statistically significant increased risk of fatal cancer from low-dose radiation—that is, in the range of 50 to 100 mSv, and possibly 10 to 50 mSv.6 These types of results are derived from 50-year follow-up of atomic bomb survivors, medical exposures (especially radiation oncology) and occupational exposures. In an article by Brenner et al6 in which data were based on a certain estimations including the total number of CT examinations performed during a year in infants and children and relatively high radiation dose technique, there was the potential for inducing an increase in the number of cancer fatalities from a single CT. Although these estimates were based on CT examinations that impart a relatively high radiation dose compared with contemporary practice, adjustments to these techniques still provides a substantial lifetime risk of cancer.15 Although estimates have put this risk as low as 1 fatal cancer per 1000 pediatric CT examinations,15 it is important to realize that this estimated increase is <0.5% over the baseline for lifetime cancer mortality.6,15 On the other hand, it is also important to realize that discussions about low-level exposure relate to fatal cancer and not the incidence of nonfatal cancer. The health and economic ramifications of nonfatal cancer from low-level radiation need to be clarified.
As noted above, CT examinations can result in doses that are in the range of low-level exposure.15,25,33 As noted above, the dose from each CT examination is cumulative over the life of an individual. This becomes an important factor in those situations in which multiple examinations may be performed. In fact, 30% of all individuals having a CT will have a total of at least 3 examinations.9 For example, a single CT of the abdomen could provide a dose of 11 mSv. If there are 3 phases in this examination, the actual dose is 33 mSv (3 x 11 mSv). If this child is 1 of the 30% who have 3 or more examinations, the lifetime dose is at least 100 mSv, clearly in the range of doses associated with induction of fatal cancer.
Part of the problem with radiation dose estimations in children (and assignment of risk) is the lack of consensus for the measurement (or estimation) of dose. Basically, risk is determined using either direct measures of dose, such as organ dose, or a weighted measure of radiation dose taking into account various organ doses and sensitivities (effective dose). Other measures that are increasingly familiar to radiologists and technologists are the weighted CT dose index (CTDIw) and dose length product (DLP). These measurements are not applicable for risk assessments. The usefulness of these measures is that modifications of scan parameters are reflected in the CTDIw or DLP displayed on the scan monitor. This serves as an aid in designing scan protocols or in real-time modification of the examination just before the child is scanned. For example, doubling one CT scan parameter, the tube current, will double the CTDIw displayed on the monitor. Without knowing the actual organ dose (and therefore risk), this measure tells the radiologist or technologist that this organ dose in the child will also be doubled. Because there is a call for documentation of some measure of CT radiation dose for CT, and because CTDIw and DLP are relatively easy measures of radiation that are increasingly available on scanners, these are becoming familiar terms in the everyday practice of radiology. The various methods for dose determination and implications in changes of various parameters in radiation dose for CT have recently been reviewed in greater detail.34
The major difficulty in defining specific organ (or effective) dose and cancer risk is that this is very complicated, with approximations based on mathematical models, computer simulations, or anthropomorphic pediatric phantoms. For example, size-based pediatric anthropomorphic phantoms are relatively rare and quite expensive, and dose determinations are cumbersome. Although at this time there is no consensus for the degree of detail or actual measure for describing and documenting radiation dose for the routine practice of CT, radiologists together with a variety of organizations, are working toward that end.14
Irrespective of one’s stance with regard to cancer and low-dose radiation, the medical community must follow the ALARA (as low as reasonably achievable) principle for CT in children.35 That is, unnecessarily high doses should be avoided. To this end, strategies should be developed that minimize or eliminate the amount of unnecessary radiation exposure36,37 (Table 1).
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STRATEGIES THAT SHOULD BE USED FOR MINIMIZING RADIATION EXPOSURE |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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The first step in minimizing radiation exposure in children is to decide that CT is in fact the most appropriate modality to answer the specific question. A comprehensive evaluation of the appropriateness criteria for CT examinations in children or review of outcomes based on comparison with other modalities is beyond the scope of this article. For example, the benefits and limitations of CT versus sonography in the setting of trauma and appendicitis continue to evolve and be debated.38,39 Computed tomography is recognized as a useful modality in a wide variety of settings including cancer surveillance and follow-up, trauma, and as a problem-solving tool in a number of clinical scenarios.1,2,10 Communication between pediatric health care providers and radiologists is nevertheless critical in deciding whether or not a CT is appropriate. With sufficient clinical information, the radiologists may be able to suggest an acceptable alternative that does not use ionizing radiation, such as sonography or magnetic resonance imaging. When a CT is indicated, the information supplied can help in designing the CT scan with the clinical questions in mind. When this type of communication occurs, all health care providers become more educated, clinical pathways can be developed to assist physicians in selection of appropriate imaging strategies, and better health care for that child, as well as for subsequent children, results.
Once deemed necessary, the CT examination should be designed to answer the specific question. One way to do this is for the radiologist to limit the examination to the region in question. For example, routine scanning of the pelvis as part of an abdomen CT is not always necessary; in this way, exposure to the gonads in boys and girls will be reduced or eliminated. There are many potential scenarios in which this type of limited CT is a consideration. For example, the region scanned could be limited for follow-up examinations for resolution of a localized process such as pseudocyst, lung or abdominal abscess, or suspicion of pyelonephritis with a plegmon. Lead shielding during radiography is standard for examinations such as an abdomen x-ray (gonad shielding) or scoliosis evaluation (breast shielding). With CT, shielding of areas outside of the immediate scan area provides little benefit. Radiation to these areas is minimal, and usually attributable to internal scattering of photons that is unaffected by surface shielding. However, shielding of the breast tissue and thyroid gland during chest CT is a strategy to reduce radiation dose. Investigations of chest CT in both adults40 and children41 have noted a reduction in breast tissue dose of 29% to 57% with no appreciable loss in diagnostic quality.
Once it has been determined that a CT is the appropriate examination, and the region of coverage is determined, selection of appropriate scan parameters is important. Parameters that affect radiation in helical CT scanning that are selected by the radiologist (or CT technologist) include the number of scans through the region in question (multiphase scanning), speed at which the patient travels through the gantry (table speed), gantry rotation cycle time, kilovoltage, and tube current.
Multiphase examinations are those examinations in which repeat examinations of an area are performed (usually with the same CT settings) at different times with respect to the administration of intravenous contrast material. Precontrast studies are perhaps the most familiar but arterial (early), venous (mid-) and late-phase CT scanning can also be performed. Essentially, every additional phase increases the radiation dose (and potential risk) by the multiple of the total number of phases. In body scanning, it has been reported that multiphase scanning occurs in 30% of children, many times with 3 phases.5 Radiologists usually determine when multiphase scanning should be performed. There is little justification for the routine use of multiphase examinations in infants and children. We very rarely (<3% of body CT) perform these types of examinations in our practices.
It is also important to keep in mind that CT parameters need to be adjusted based on the size of the child, region examined, and indication for the examination. Guidelines on weight-based scanning for helical CT in children have recently become available.1,7 However, these are just guidelines and individual adjustments need to be made based on the individual needs and preferences of the practice. Different CT parameters should be used for different regions or organ systems scanned. Chest CT and skeletal CT do not typically require as much radiation as abdomen or head CT examinations. Finally, CT parameters should also be adjusted based on scan indication. For example, detection of relatively conspicuous abnormalities such as a retroperitoneal hematoma, or conceivably renal calculi (renal stones are readily evident by CT because of calcification) is possible with lower radiation dose techniques.23 At this point and time, data are lacking for specific threshold for detection of diseases. It is perhaps partly because of this that helical CT in children has defaulted to adult techniques.5
Probably the most recognized contribution to radiation dose by radiology personnel is the CT parameter of tube current, essentially the number of photons generated by the x-ray tube. When this is multiplied by gantry rotation time (1 complete rotation, in seconds), the unit mAs result. Therefore, adjustments in either tube current or gantry speed can affect mAs. Basically, reducing the tube current by 50% and keeping all other factors identical also decreases the radiation dose by 50%. Decreasing the gantry rotation time from 1.0 second to 0.5 second also reduces the radiation dose by 50%. Lower tube currents are recommended for chest scanning,7,42,43 because there is less solid tissue that the x-ray photons are traversing, and for small-size children the same reasoning applies.5,7 Neither of these adjustments, however, has routinely been made in children.5 The cost for reducing tube current or rotation time is that the signal-to-noise ratio decreases because the number of image-forming photons decreases (the images are noisier, similar to static on a television screen). Because of this, spatial resolution decreases (Fig 1). This may be acceptable in a child in whom a large abnormality is suspected, such as a retroperitoneal hematoma, or an abscess. Therefore, as mentioned above, tube current should be adjusted for patient size, scan indication, and region scanned. Tube current could also be adjusted during a CT scan based on regional needs for more or less x-ray photons. For example, use of a single mA through the chest may mean that there is an unnecessarily high dose through the upper lungs, where there is little tissue to stop the x-ray photons, versus through the lower chest, including the heart, where the x-ray amount is appropriate. New CT scanners are equipped with this modulation, referred to as automatic exposure control or automatic tube current modulation.
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39% in a body phantom.34 Only recently has attention focused on adjustments in kilovoltage for pediatric CT, which are the same as those used in adult, in the range of 120 to 140 kVp. Recent investigations suggest that kilovoltage can be lowered in small children in certain applications,44 and ongoing research is directed at determining the relationship between kilovoltage in the range of 80 to 120, image quality, and radiation dose in children. For a CT examination, the child lies on a bed (also called a table). It is this bed that moves the child through the scanner as the gantry rotates. The gantry contains the radiation source, the tube, and the detectors opposite the tube. Table speed is another parameter that contributes to radiation dose. Table speed (numerator) and the width of the x-ray beam (denominator) can be combined. This combination is pitch. Faster table speeds without adjusting the beam width give higher pitches. In general, lower pitches (<1.0) give better image quality but higher radiation doses. Pitches from 1.0 to 2.0 are lower in terms of dose but have more artifact and can be lower in terms of detail. For a more detailed discussion of pitch, the reader is referred to a recent review.34 The faster the table speed, the shorter amount of time the area of scanning is exposed to the x-ray beam, and the lower the radiation dose. Adjustments in table speed have not been made based on indications in children. Slower speeds and highest quality have been the apparent defaults.4 Other parameters such as the width of each x-ray detector or the number of detector rows in the scanner (one detector row—or single-slice CT, vs 4-, 8- or 16-slice—or multidetector CT) also contribute to radiation dose but discussion of these parameters is beyond the scope of this article.34
Because of the variety of selectable options, a large number of different scan techniques could be used with both single and multidetector CT. With multidetector CT, the number of options has become much more complex and confusing.3,45 This complexity, as with any aspect of pediatric care, can lead to incorrect scanning technique. A method of trying to minimize inappropriate body CT for children was recently reported. The format was based on a color-coded system used in the emergency setting in children.46 Using this format, investigators found CT scanning was significantly simpler and preferred by technologists.45
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CONCLUSIONS |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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FOOTNOTES |
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Reprint requests to (D.P.F.) Division of Pediatric Radiology, 1905 McGovern-Davison Children’s Health Center, Department of Radiology, Duke University Medical Center, Erwin Rd, Durham, NC 27710. E-mail: frush943{at}mc.duke.edu
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REFERENCES |
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TOPABSTRACTIMPORTANCE OF CT FOR...HISTORICAL PERSPECTIVE: CT AND...CT AND RADIATION: UNIQUE...RISKS OF LOW-LEVEL RADIATION...STRATEGIES THAT SHOULD BE...CONCLUSIONSREFERENCES |
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