In children, as in adults, CT is an invaluable imaging modality. Like any tool, however, it offers the greatest relative benefit when used appropriately. One byproduct of inappropriate use is an increased radiation dose. This has unique implications for children, including a closer relationship between CT radiation doses and the risk of cancer.¹ It is therefore imperative that radiologists and all radiology personnel become familiar with strategies to protect the welfare of children.

We will use the term “radiation dose” in a simplified fashion, unless otherwise specified, as a general descriptor representing any and all radiation a child receives from CT. There will be no attempt at distinction between dose measurements such as the weighted CT dose index, surface dose, organ dose, or effective dose.

The benefits of CT far outweigh any potential risks. It can be used for virtually any region of the body or organ system. While traditional applications have included oncologic, traumatic, and inflammatory categories, such technological developments as helical and multidetector CT have resulted in its use in more common disorders, including appendicitis and renal calculi. As a consequence, the number of annual CT examinations is estimated to have increased by nearly 600% between 1980 and 1995.¹

Increased use in children is likely due in part to faster scanning and a concomitant decreased need for sedation.² Additional explanations for the increased use of CT as compared with MR imaging include a shorter wait for CT appointments, lower cost, and a relatively short examination time. Furthermore, multidetector CT (MDCT) technology and improved workstation capabilities offer new applications for CT angiography. CT arteriograms can be obtained even in newborns, who may not be as easily or quickly evaluated by MR angiography (Figure 1).³

Radiation Problems

A number of issues are related to CT radiation in children, and they have become more prominent as use has increased. They include the following:

• contribution of CT to a relatively large radiation dose to the population;
• increased organ radiosensitivity of children relative to adults;
• longer lifetime risk for radiation-induced cancer; and
• lack of adjustments of pediatric CT parameters based on size or region of scanning.

Moreover, the increased complexity of MDCT technology poses new challenges in performing body CT examinations in children and presents even greater potential for inappropriate radiation dose. Guidelines for pediatric body MDCT have only recently become available.⁴

While CT examinations account for only about 5% of medical x-ray imaging modalities, CT contributes at least 40% of the medical radiation dose.^5,6 It is thus the single largest contributor of medical radiation exposure to the population. Estimates, probably conservative, of the number of pediatric CT examinations begin at about 600,000 annually for children 15 or younger.¹ Given the increase in CT use in general,^1,6 CT radiation dose can be considered a public health issue.

This is particularly important for children because their organs are more radiosensitive than those of adults,⁷ and they have a recognized longer lifetime in which radiation-induced cancer can be manifested.¹ Knowing this, it would seem reasonable that adjustments would be made in CT technique to reduce the amount of radiation in children, but this is not always the case. Paterson et al reviewed 58 pediatric CT examinations and found little to no adjustment based on age as an estimate of size for tube current, pitch, and slice thickness, three parameters that influence radiation dose (Table 1).⁸ Moreover, although dose can be decreased when scanning the chest compared with the abdomen, there was little difference in parameters for examinations of these two regions. The authors point out that many of the techniques used even in small children were similar to those routinely used in adults.

One explanation for this lack of adjustment may come from the difference in image quality between CT and radiography. With radiography, excessive exposure factors adversely affect diagnostic usefulness, while in CT there is no diagnostic penalty for such excess.⁹ A chest CT using a tube current of 320 mAs in an infant has less noise than one obtained at 40 mAs, although no proven gain in diagnostic ability accompanies the 700% increase in dose.

These unique risks in children are greater with newer multidetector technology, given the multitude and complexity of options from which radiologists and technologists have to choose (Table 2).¹⁰ The paucity of guidelines for single-detector and MDCT scanning has important implications for CT radiation dose. While it may be acceptable to use the higher dose examination in certain situations, radiologists must understand the wide range of doses that can result from apparently small changes in CT parameters and should work to minimize the dose by selecting the lowest settings appropriate for the individual scan.

Much of the current focus on CT radiation in children results from a series of articles published in the February 2001 issue of the American Journal Roentgenology^1,8,11,12 and the subsequent, and often incorrect, distillation of this information in the popular press.¹³ The first of the AJR articles emphasized the lack of adjustments in CT parameters for children, as mentioned above.⁸ The second article investigated the CT radiation dose and estimated cancer risks.¹ Using the approximate number of pediatric CT examinations performed each year, lifetime expectancy, and low-dose radiation exposure from atomic bomb survivors, investigators found an increased cancer risk of 0.35% above the background rate from a single (albeit high-dose) CT. This equates to about 500 deaths (lifetime), given an estimated 600,000 annual pediatric CT examinations.

Additional evidence links the amount of radiation associated with a CT examination to cancer development.⁷ Although certain debatable estimations are made in these investigations, it is recognized that the radiation doses to children from CT examinations and radiation-induced cancer are more closely related than was previously believed.

The third of the three articles provided guidelines for single-detector helical body CT using size-based adjustments in some of the parameters that determine radiation dose.¹¹

Solutions

The solutions to these problems, as well as strategies to minimize risks, can be divided into those that are more immediate and those requiring a long-term approach.

Certain solutions are the responsibility of radiology personnel, including radiologists and CT technologists. First, the CT examination should be performed only when appropriate. If a modality such as MR imaging or sonography can provide sufficient information, the risk of radiation should be avoided altogether. A panel of experts has noted that 25% to 40% of all CT examinations were questionable in terms of necessity.¹⁴ The best way to reduce dose for CT would be not to perform it at all. Sufficient justification should be made for each CT examination performed.

A second strategy the radiology team can apply is judicious use of CT examinations, including focusing the examination to the clinical questions. If pelvic scanning, for example, is not necessary, this part of an abdominal examination—which often seems to be performed out of habit—can be eliminated. If there is a specific question that can be answered by a few slices, this modification should be made (Figure 2).

Limiting multiphase IV contrast examinations is another example of judicious use. In the study by Paterson et al, about 30% of all pediatric abdominal examinations were multiphase IV contrast-enhanced examinations.⁸ No scientific data support the routine use of precontrast body CT examinations in children, and we believe that multiphase CT should be used infrequently. Precontrast CT needs to be justified in every case.

Once it has been determined that the CT examination is necessary, and the examination is focused as much as possible to the region in question, the examination parameters that are adjustable need to be determined. An easy way to do this is to use size- or weight-based tables.^4,11 In making these adjustments, however, it is important to have a basic understanding of the individual contribution of parameters to radiation dose (Table 3).

Milliamperes (mA) and gantry rotation cycle time (in seconds) are combined to provide a measure of radiation commonly referred to as tube current (mAs). A linear relationship exists between tube current and radiation dose such that reducing the tube current by 50% will also decrease the dose by half. This can be done by adjustments to gantry rotation cycle time or milliamperes.

The relationship of peak kilovoltage (kVp) and dose is more complex. Basically, lowering the kilovoltage will also lower the dose. With this increase in noise, there is a concomitant decrease in image contrast, but data suggest that kVp as low as 80 can provide acceptable image quality in neonates.⁹

Section thickness and table speed also affect dose. The fastest table speed and thickest slice should be selected based on the scan indication. In general, most pediatric body CT scanning can be performed at a pitch of 1.5.^11,15 Adjustments in slice thickness and pitch need to be balanced against the potential loss in spatial resolution from increased image noise.

Individual adjustments in CT parameters should be based on several considerations, one of which is scan indication. Large lesions, surveys, or follow-up examinations are considerations for lower dose CT. Parameters should also be adjusted based on the organ system or region scanned. Relatively high contrast areas such as the lung parenchyma and bone are more amenable to decreases in tube current or kilovoltage than areas within the abdomen. No investigations are under way to specifically address lower dose CT scanning of the abdomen in children; limited pediatric recommendations are available for pediatric body CT scanning of the chest^16-19 and pelvis.²⁰ There are, to the best of our knowledge, no data for intrinsically high-contrast skeletal imaging in children, but the same principles for tube current reduction in the chest apply. Finally, CT parameters should be selected based on size of the child (Table 4).^4,11

An additional area in which a pediatric dose can be minimized relates to the complexity of multidetector scanning. This complexity accounts for a large variation in scan parameters, some of which provide relatively high dose. Systems that simplify multidetector CT scanning and reduce the options to a reasonable range should be increasingly available. One such scanner is a color-coded, weight-based system⁴ that has shown a significant simplification of scanning, with reduction in scan variation (or error), including parameters affecting radiation dose. A large group of CT technologists found this system simpler to use and preferable to other formats for protocols.

In addition to these immediate modifications that can minimize radiation dose in children, longer term strategies are critical. Full discussion of these is beyond the extent of this article. Increased efforts to educate radiologists, radiologic technologists, and clinicians, including pediatricians and family practitioners, about the benefits and risks of CT radiation would be helpful. Manufacturers must also become more involved in providing pediatric protocols for single- and multidetector CT scanners.

Research support through a variety of funding options, including industry, federal agencies, and other organizations or societies, should be directed toward the issue of radiation reduction research. The Society for Pediatric Radiology supported an investigation that provided information about dose reductions in body CT scanning in children.²¹ In this study, simulated tube current reductions were performed by adding noise. The authors concluded that radiation dose reductions of 33% to 66% could be achieved, depending on whether a structure or lesion was of intrinsically high or low visibility. Efforts of this type can help establish guidelines for organ- or region-based CT examinations.

The Society for Pediatric Radiology held an educational forum in August that addressed many of these issues. The meeting, called “The Conference on the ALARA Concept in Pediatric CT—Intelligent Dose Reduction,” arrived at many conclusions that are reflected in this article and should be increasingly available in different forms over the next few months.

References

1. Brenner DJ, Elliston CD, Hall EJ, et al. Estimated risks of radiation-induced fatal cancer from pediatric CT. AJR 2001;176:289-296.
2. Pappas JN, Donnelly LF, Frush DP. Reduced frequency of sedation of young children using new multi-slice helical CT. Radiology 2000;215(3):897-899.
3. Cohen RA, Frush DP, Donnelly LF. Data acquisition for pediatric CT angiography: problems and solutions. Pediatr Radiol 2000;30:813-822.
4. Frush DP, Soden B, Karen KS, Lowry C. Improved pediatric multidetector body CT using a size-based, color-coded format. AJR 2001: In press.
5. UNSCEAR: United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation. NY: United Nations, 1993.
6. Mettler FA Jr, Wiest PW, Locken JA, Kelsey CA. CT scanning: patterns of use and dose. J Radiol Prot 2000;20(4):347–348.
7. Pierce DA, Preston DL. Radiation-related cancer risks at low doses among atomic bomb survivors. Radiat Res 2000;154:178-186.
8. Paterson A, Frush DP, Donnelly LF. Helical CT of the body: are settings adjusted for pediatric patients? AJR 2001;176:297-301.
9. Huda W, Scalzetti M, Levin G. Technique factors and image quality as functions of patient weight at abdominal CT. Radiology 2000;217(2):430–435.
10. Berland LL, Smith IK. Multidetector array CT: once again, technology creates new opportunities. Radiology 1998: 209;327–329.
11. Donnelly LF, Emery KH, Brody AS, et al. Minimizing radiation dose for pediatric body applications of single-detector helical CT. AJR 2001;176:303-306.
12. Rogers LF. Taking care of children: check out the parameters used for helical CT. AJR 2001;176:287.
13. Sternberg S. CT examinations in children linked to cancer later. USA Today January 22, 2001, 1A.
14. Slovis T, personal communication. Society for Pediatric Radiology conference. The ALARA concept in Pediatric CT-intelligent dose reduction. Chicago: August 2001.
15. Vade A, Demos TC, Olson MC, et al. Evaluation of image quality using 1:1 pitch and 1.5:1 pitch helical CT in children: a comparative study. Pediatr Radiol 1996;26:891-893.
16. Lucaya J, Piqueras J, Garcia-Pena P, et al. Low-dose high-resolution CT of the chest in children and young adults: dose, cooperation, artifact incidence, and image quality. AJR 2000;175:985–992.
17. Ambrosino MM, Roche KJ, Genieser NB, et al. Application of thin-section low-dose chest CT (TSCT) in the management of pediatric AIDS. Pediatr Radiol 1995;25:393–400.
18. Ambrosino MM, Genieser NB, Roche KJ, et al. Feasibility of high-resolution, low-dose chest CT in evaluating the pediatric chest. Pediatr Radiol 1994;24:6–10.
19. Rogalla P, Stover B, Scheer I, et al. Low-dose spiral CT: applicability to paediatric chest imaging. Pediatr Radiol 1998;28:565-569.
20. Kamel IR, Hernandez RJ, Martin JE. Radiation dose reduction in CT of the pediatric pelvis. Radiology 1994;190:683-687.
21. Frush DP, Slack CC, Hollingsworth CL, et al. Computer-simulated radiation dose reduction for pediatric abdominal helical CT. 4th International Pediatric Radiology Meeting, Paris, France: May 28–June 1, 2001.

Table 1.

Unadjusted Tube Current (mA) for Pediatric Body CT

Source: Survey of 58 CT examinations of children referred for consultation from a variety of institutions and practices.⁸

Table 2.

Potential Parameters for Chest CT in a 10-kg Child*

Options

• Thickness (mm) 3.75, 5
• Table speed (mm per cycle) 7.5, 11.25, 15, 22.5, 30
• Gantry cycle (sec) 0.5, 0.6, 0.7, 0.8, 0.9, 1
• Tube current (mA) 60–120 (increments of 10)
• Kilovoltage (kVp) 100, 120, 140

Total > 750 combinations

*Light Speed QX/i, GE Medical Systems

Table 3.

Radiation Dose Range Based on Hypothetical Single-Slice Helical CT for Follow-Up of Abdominal Abscess in a 10-kg Child

• “High” dose: 140 mAs, 140 kVp, pitch 1–4.5 mSv*
• “Medium” dose: 100 mAs, 120 kVp, pitch 1.5–1.6 mSv
• “Low” dose: 60 mAs, 120 kVp, pitch 2–0.7 mSv

Total > 500% difference in dose

* mSv = millisieverts (effective dose)

Source: Personal communication, W. Huda, Syracuse, NY

Table 4.

Suggested Tube Current (mA) for Single-Detector Helical CT, by Weight of Pediatric Patients

Source: Reference 11, with permission