PEDIATRICS Vol. 114 No. 1 July 2004, pp. 205-209
Reduction of Cancer Risk Associated With Pediatric Computed Tomography by the Development of New Technologies
Mark G. Benz, ScD* and
Matthew W. Benz, MD, SM, FAAP
* Engineering Horizons International, Lincoln, Vermont
Southboro Medical Group, Southboro, Massachusetts
Abbreviations: CT, computed tomography • MRI, magnetic resonance imaging
Although helical and multidetector (multislice) computed tomography (CT) approaches are invaluable and rapidly growing imaging modalities for pediatric patients, the radiation exposure associated with these technologies has come under increasing scrutiny.1 Ionizing radiation has many documented harmful effects, the most serious being the induction of fatal cancers.2 Children are at 10-fold greater risk, compared with middle-aged adults, because dividing cells are most susceptible to radiation-induced neoplastic transformation and because there is more time for these genotoxic effects to be manifested during the child's remaining lifetime.3
The radiation doses of typical pediatric, diagnostic, CT studies overlap the doses received by some World War II Japanese atomic bomb survivors, among whom excess cancer mortality rates have been observed.4,5 On the basis of predictions from these survivor data, the radiation doses from typical pediatric CT studies may cause the eventual cancer-related death of 1 of 1000 children examined.3 This rate is considered by many to be unacceptable, which has spurred efforts to obtain high-quality, diagnostic, clinical imaging with reduced radiation exposure.6–9
Immediate measures to address this problem have focused on minimizing CT radiation exposure among children with recommendations for more judicious use of CT examinations6,7 and with adjustment of the exposure parameters of existing CT equipment, to deliver "as low as reasonably achievable" radiation doses while still yielding high-quality images.8–12 Although progress has been made on each of these fronts, wide variability in scanning techniques still exists, exposing many children to radiation doses that are higher than necessary.13 In addition, newer multidetector (multislice) CT scanners are now being used and are inherently more complex, presenting additional challenges with respect to dose reduction.14 These difficulties have stimulated the search for long-term, new-technology-based approaches to CT radiation dose reduction for children. In this search, 2 questions are asked, as follows. Can new "kid-size" technologies be developed to achieve the desired dose reduction? If developed, can these technologies be retrofitted readily to existing systems and used easily throughout the entire health care system? Two especially promising, new-technology, design strategies are discussed in this article, ie, 1) development of an enhanced x-ray detector-based system for pediatric CT, allowing greatly reduced radiation doses, and 2) development of a moving-target version of pediatric magnetic resonance imaging (MRI), leading to greater utilization, when appropriate, of this radiation-free competitive modality by reducing the need for physician-supervised sedation during the procedure.
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ENHANCED X-RAY DETECTOR-BASED SYSTEM FOR PEDIATRIC CT
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All x-ray-based imaging systems contain 2 major functional units, namely, an x-ray detector and an x-ray source (x-ray tube or e-beam x-ray generator). In conventional radiography, film serves as the x-ray detector. Adjustments of the x-ray source (mA and/or kV), referred to as technique by radiologists, are made to achieve the appropriate image exposure. The technique is decreased if the image is overpenetrated (too dark) and increased if it is underpenetrated.
For non—film-based systems, such as digital radiography and CT, the x-ray detector usually consists of an array of transparent or translucent, ceramic, solid-state devices that absorb x-rays and convert some of the energy into light. This light is transmitted to and excites attached photodiodes, which in turn provide information to the computer for image construction. With this type of detector, there is no visual clue indicating excess radiation exposure. Images of children scanned at adult settings (with radiation doses many times higher than needed) are still of high quality.
In most current CT systems, the x-ray detector and the x-ray source are mounted in opposing positions on a gantry that spirals axially around the body. The x-ray source is a high-voltage, rotating-anode, vacuum tube that generates x-rays by the deceleration of high-energy electrons (for example, 120 kV). Less than 1% of the kinetic energy from this deceleration is converted into x-rays, however. Most of the energy is converted into heat; thus, thermal management of the system becomes the major design challenge for tube manufacturers. Because of the enormous amount of heat generated, the face or track of the x-ray source is fabricated from a tungsten-rhenium alloy, selected in part because of its very high melting temperature. The x-ray wavelengths generated with tungsten-rhenium consist of a wide-band spectrum (Fig 1). Although the very longest wavelengths are filtered out before they reach the body (because they would be totally absorbed in the body), x-ray detectors must respond to the remaining wide-band spectrum of wavelengths. Detectors with this wide-band wavelength capability are inherently inefficient.
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Fig 1. Intensity versus wavelength for x-rays produced by deceleration of 120-kV electrons in tungsten.15 The spectrum of wavelengths for an alloy of tungsten plus 5% to 10% rhenium would be similar.
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Current measures to reduce the radiation exposure for pediatric CT have focused on the adjustment of system parameters determining exposure to source x-rays, namely, tube current (mA), peak kilovoltage (kVp), pitch, slice thickness, and table speed. Little attention has been directed toward detector sensitivity. Enhancement of the sensitivity of the x-ray detectors in a CT system would allow reduction of the intensity of the x-ray source while maintaining an equivalent data stream to the computer for high-quality image construction. This reduction in intensity would reduce the patient's radiation exposure.
In an analog example with optical wavelengths, this is the approach used for grocery store bar code readers. Enhanced-sensitivity detectors allow reduction of the intensity of the red laser light source to safe levels, while maintaining an equivalent data stream to the computer for bar code interpretation.
For the proposed enhanced x-ray detector-based system for pediatric CT, the active materials used for the x-ray detector and for the face or track of the x-ray source (and associated source filters) would be changed. These materials would be chosen to absorb and generate x-rays in a narrow band of wavelengths at or just below the absorption edge for the active atomic species in the detector. X-ray absorption is maximized for wavelengths at or just below an absorption edge. This is shown for the rare earth atoms gadolinium and lanthanum in Fig 2. Mixtures of atomic species with overlapping absorption peaks could be used for the detector, should it be necessary to adjust the wavelength range between peaks. The complementary, narrow-band, x-ray source for this system would be a conventional, rotating-anode, vacuum tube, with modification of the surface composition of the face or track to an atomic species that generates high-intensity x-rays at the wavelengths most effective for the detector. Appropriate source filters would be used to maximize the performance of the detector and minimize the radiation dose to patients. It should be possible to retrofit these changes (ie, enhanced detector and modified narrow-band x-ray source) to current CT systems easily.
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Fig 2. Calculated relative energy absorption versus wavelength for gadolinium and lanthanum.16 The absorption edges for these atoms are 0.2468 and 0.3184 angstroms, respectively (1 angstrom = 0.1 nm).17
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MOVING-TARGET VERSION OF PEDIATRIC MRI
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The functional components of a typical MRI scanning system include magnetic field devices (steady and gradient) and radiofrequency components (coils, power supply, and receiver). No ionizing radiation is involved. Although each scan requires only a fraction of a second, repetitive scans are obtained for several minutes, to provide sufficient information from the replicate observations for identification and removal of random noise from the data.
Existing MRI systems are considered to be of a stationary-target design. The MRI image-generation algorithm assumes that the target imaging volume remains stationary and that no spatial movement of the patient occurs between the repetitive scans. This requirement creates a complication for pediatric observations, in which such motion is difficult to control. To comply with this no-movement assumption, sedation is often used. This sedation requirement limits the ease with which observations can be scheduled and carries the risk of sedation-related complications.
Various fast-scan MRI techniques under development have reduced the stationary-target requirement somewhat, but these methods are primarily directed toward minimizing motion artifacts resulting from physiologic processes (ie, cardiac and respiratory movements) and have not allowed for significant gross movement (ie, the "wiggle factor") among pediatric patients.
For the proposed moving-target version of pediatric MRI, the design paradigm would be changed to one that allows some motion of the patient between the replicate scans used for noise reduction. A motion-sensitive, target volume element sensor would be added to the system to detect and record patient motion (Fig 3). Existing MRI frequency-based technology would be used to identify the location of each magnet volume element within the magnet. A modified image-generation algorithm would be developed to combine the target volume element motion sensor data with the stationary magnet volume element data for generation of the desired image. These changes would reduce the need to control patient movement with physician-supervised sedation during the procedure, making MRI a more attractive, radiation-free alternative to CT in many situations. It should be possible to retrofit these changes (ie, motion sensor and modified image-generation algorithm) to current MRI systems easily.
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Fig 3. Schematic view of proposed pediatric MRI system to solve the moving-target problem. A motion-sensitive target volume element (TVE) sensor is added to detect motion of the patient between replicate observations used for noise reduction.
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CONCLUSIONS
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The development of new pediatric CT systems and alternative MRI technologies is being pursued in earnest. Two especially promising new-technology design strategies are discussed here, ie, 1) development of an enhanced x-ray detector-based system for pediatric CT, allowing greatly reduced radiation doses, and 2) development of a moving-target version of pediatric MRI, leading to greater utilization, when appropriate, of this radiation-free competitive modality by reducing the need for physician-supervised sedation during the procedure. These design strategies are now ready for increased university- and industry-based research and development.
The challenge to reduce the radiation-associated risks for pediatric CT is similar to that encountered for mammography in the early 1970s. For mammography, reductions in radiation doses of almost 90% have been achieved since 1974 (Fig 4). During the same period, significant improvements in detection sensitivity and image quality (phantom scores) have also been achieved. The introduction of new technologies has accounted for most of these improvements. This progress and the efforts that created it are inspiring and provide a useful model for the approach to the current challenge.
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Fig 4. Radiation dose and phantom score associated with a typical mammogram according to year.18,19 The dose has been reduced dramatically, and the phantom score (a measure of detection sensitivity and image quality) has been increased. Ref. 18 data were updated with permission of the authors.
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Strong public advocacy helped create the environment for the improvements in mammography. This advocacy focused public funding, university- and industry-based research and development, and ultimately acts of Congress (the Mammography Quality Standards Act of 1992, reauthorized in 200220,21) on the challenge. The new technology developed and the progress achieved continue to be ensured by the Mammography Quality Standards Act, which requires annual inspections of all mammography facilities in the United States. Equipment and procedures for screening mammography must meet stringent Food and Drug Administration accreditation and quality standards.
Public advocacy for children is needed to create a similar environment for improvements in pediatric CT. Significant progress could be made if public funding was focused on the discovery and development of new technologies. The pediatrics community is encouraged to become public advocates in this case. While awaiting new lower-dose technologies, pediatricians, family practitioners, emergency department physicians, pediatric subspecialists, residents, and medical students are urged to assist in the implementation of strategies being introduced for the reduction of ionizing radiation dose risks among children undergoing CT evaluations with current technologies22 (see below for a summary).
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STRATEGIES FOR REDUCTION OF RADIATION DOSE RISKS FOR PEDIATRIC CT
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For pediatricians considering the risks and benefits of performing a CT examination of a child, the following radiation dose-reduction strategies are recommended. These strategies could form the basis for increased dialogue with radiologists and CT technicians at imaging centers. The last recommendation addresses long-term follow-up care for patients who underwent CT examinations as children and have grown to adulthood. - Avoid unnecessary examinations. It has been estimated that approximately one-third of all pediatric CT examinations may be unwarranted.23 Therefore, greater adherence to accepted practice guidelines24 and development of newer, definitive, clinical decision-making pathways are advised.
- Consider utilizing alternative imaging modalities, such as MRI and sonography, when the clinical situation permits.
- Confirm that pediatric CT radiation-minimizing protocols that conform to the latest recommendations are implemented and routinely updated at local referral centers. Confirm that CT exposure parameters (ie, mA, kVp, slice thickness, pitch, and table speed) are adjusted for pediatric patients. It is especially important to confirm that institutions using multidetector CT systems adhere to the latest pediatric protocols, because it is more difficult to reduce radiation doses with such scanners.
- Perform focused CT examinations by limiting the field of view to the specific area of the body in question. For example, abdominal CT scans do not always require inclusion of the pelvis.
- Limit multiphase IV contrast studies by obtaining postcontrast examinations only, because there is rarely a need for a comparative precontrast study. The resultant reduction in radiation dose is 50%.
- Insist that all specialists and/or consultants, especially in emergency department settings, evaluate patients before ordering CT scans.
- Encourage the development of a system for quantifying and documenting cumulative medical ionizing radiation exposure (especially from CT examinations) in a child’s medical record, to be available to both pediatric and future adult primary care providers for ongoing surveillance during annual physical examinations and health screening. Until such a documentation system is available and implemented, it is recommended that, for children who underwent CT examinations as children, this ionizing radiation exposure history be underscored in the medical record and passed along to adult primary care providers for review.
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ACKNOWLEDGMENTS
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We gratefully acknowledge the helpful discussions with Charles D. Greskovich, retired from General Electric Global Research Center, on the nature of x-ray absorption in solid-state detectors for CT, and with Gary H. Glover, director, Radiological Sciences Laboratory, Lucas Center for Magnetic Resonance and Imaging, Stanford University, on the challenge of solving the moving-target problem for pediatric MRI.
We also acknowledge the efforts of Dr David C. Spelic, of the FDA Division of Mammography Quality and Radiation Programs, and his collaborators for providing very current and helpful data regarding recent trends in the reduction of radiation dose associated with screening mammography.
In addition, we acknowledge articles in the media on this subject written by Steve Sternberg. They have helped to raise public awareness of this issue, intensify the dialogue, and speed remedial actions.
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FOOTNOTES
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Received for publication May 30, 2003; Accepted Oct 20, 2003.
Address correspondence to Mark G. Benz, ScD, Engineering Horizons International, 1909 York Hill Rd, Lincoln, VT 05443. E-mail: benz{at}engineeringhorizons.com
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PEDIATRICS (ISSN 1098-4275). ©2004 by the American Academy of Pediatrics
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