Pediatr Radiol (2015) 45 (Suppl 3):S370–S374 DOI 10.1007/s00247-015-3379-8

ADVANCES IN PEDIATRIC NEURORADIOLOGY

Update on radiation safety and dose reduction in pediatric neuroradiology Mahadevappa Mahesh 1

Received: 23 December 2014 / Revised: 25 February 2015 / Accepted: 22 April 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract The number of medical X-ray imaging procedures is growing exponentially across the globe. Even though the overall benefit from medical X-ray imaging procedures far outweighs any associated risks, it is crucial to take all necessary steps to minimize radiation risks to children without jeopardizing image quality. Among the X-ray imaging studies, except for interventional fluoroscopy procedures, CT studies constitute higher dose and therefore draw considerable scrutiny. A number of technological advances have provided ways for better and safer CT imaging. This article provides an update on the radiation safety of patients and staff and discusses dose optimization in medical X-ray imaging within pediatric neuroradiology. Keywords Children . Computed tomography . Radiation dose . Radiation protection . Radiation safety

Introduction Radiation safety in pediatric radiology can be examined from two angles. First, the radiation safety of children can be examined within the premise of appropriateness, justification and dose optimization; in addition, communication plays a key role in advancing radiation safety in children. Second, the radiation safety of staff can be discussed under the princi-

* Mahadevappa Mahesh [email protected] 1

The Russell H. Morgan Department of Radiology and Radiological Science, Johns Hopkins University School of Medicine, 601 N. Caroline St., Baltimore, MD 21287-0856, USA

ples of radiation protection. The main purpose of this article is to provide an update on the radiation safety of patients and staff and to discuss dose optimization in medical X-ray imaging with a focus on imaging studies performed within pediatric neuroradiology. Before embarking on the discussion of radiation safety in pediatric radiology, it is crucial to examine the types of imaging procedures performed in a typical pediatric radiology division or department. In the absence of published data, it is reasonable to explore information regarding imaging procedures that is published on various pediatric radiology Web sites and surveys. Accordingly, the typical medical imaging procedures performed in pediatric radiology are radiography and fluoroscopy (>60%), ultrasound (~15%), CT (~10%) and MRI (5–10%) (personal communication, Morrison B., pediatric radiology imaging billing data for Department of Radiology, The Johns Hopkins Hospital, December 19, 2014). These are followed by procedures such as nuclear medicine, cardiovascular services and others, which account for 1– 5% of total procedures. However, many pediatric clinics are striving to perform proportionately more MRI studies on pediatric patients. This trend is growing but confined to academic and large pediatric hospitals. In many practices, the cost and the non-availability of MR scanners result in more X-ray imaging studies than MR studies. Pediatric CT studies, according to national surveys, account for about 10% of all CT procedures [1]. The radiation dose associated with X-ray imaging procedures, especially CT studies, has drawn considerable scrutiny in the last few years. This is largely in response to the 2009 National Council on Radiation Protection and Measurements report titled “Ionizing Radiation Exposure of the Population of the United States” [1]. The report indicates that the radiation exposure to the U.S. population from medical sources alone accounts for nearly 50% of all the radiation exposure of the

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U.S. population and that dose from CT studies contributes to nearly half of all medical radiation exposures [1]. According to the BEIR VII report [2], the average risk for radiation-induced cancer (stochastic effects) in the general population is 5% per Sievert. However, with children the long-term radiation risks can be 2–3 times higher than for average adults. This is because younger patients are more sensitive to radiation and have a longer lifetime in which to realize the radiation effects (stochastic risks). And for the same imaging techniques, children absorb more radiation than adults. It is important to note that a Sievert (Sv) is a large quantity of radiation dose, and typical imaging doses are in the ranges of 0.01–30 milli-Sieverts (mSv) [3]. The estimation of cancer risks from medical X-ray imaging including CT is often mired in conflicting arguments and models utilized in estimating long-term risks. Most risk estimations are derived from the studies on survivors of Hiroshima and Nagasaki radiation exposure [2], wherein biological risks are substantiated at radiation dose levels far higher (>250 mSv) than those observed in medical X-ray imaging (typically 0.01–30 mSv per study) [3]. Irrespective of the associated controversies regarding radiation exposure and associated cancer risks, one needs to recognize topics that all groups can agree on and work toward optimizing imaging protocols. The four tenets of radiation safety for patients are justification, optimization, dose limitation and communications [4].

Justification All X-ray imaging studies, especially for pediatric patients, require strong justification. Studies that have strong clinical indications are to be completed after exhausting all other nonX-ray imaging options. All X-ray imaging studies are to be performed when the medical benefit is appropriately high. Adopting American College of Radiology appropriateness criteria and or other established criteria at the point of ordering of X-ray procedures can to a large extent eliminate unwanted X-ray imaging studies and reduce overall radiation risk.

Optimization For studies that require X-ray imaging, such as CT, radiography and fluoroscopy, optimization of the imaging protocols is crucial. For example, optimization of CT protocols requires thorough understanding of scan parameters [5], available radiation dose reduction methods and technological advances. In addition, it is important to tailor the CT protocols according to child size, body regions and, most important, the clinical questions [6, 7]. With all the efforts to reduce radiation dose in

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CT or other X-ray imaging studies, considerable attention is required in terms of image quality. Radiation dose and image quality are like two sides of the same coin: if any one side of the coin is defaced, the value of overall X-ray imaging study will be compromised.

Dose limitation Even though limiting radiation doses should be the broader goal of imaging protocol optimization, it should not be the primacy goal of any X-ray imaging study, because limiting dose without balancing with the image quality can jeopardize the overall result of the study. For example, if radiation dose is lowered too much, it can yield images of poor quality (images with high image noise) that may impact the diagnosis. And in some instances the patient is required to repeat the imaging study because of poor image quality, so in these cases the overall goal of dose limitation is futile because repeat studies result in additional radiation dose to patients [4].

Communication Communication plays a key role in radiation safety in patients. Good communication between the ordering physicians and image providers can often eliminate unnecessary imaging studies. A good rapport between pediatric radiologists and ordering physicians often allows pediatric radiologists to make a decision on appropriate imaging studies rather than routine studies. Also, it allows pediatric radiologists to suggest alternative and effective imaging studies that are appropriate to a particular clinical condition.

Steps for minimizing dose There are several steps to minimize radiation dose in pediatric patients: (1) Perform X-ray exams only when medical benefits are appropriately high. (2) Consider alternative modalities such as US and MRI. (3) Adjust scan techniques to the size of the patient in order to keep the radiation dose at a minimum. (4) Image only the indicated area. This is especially important with digital radiographs, wherein sometimes in order to ensure complete anatomical coverage technologists take radiographs exceeding the anatomical coverage required and provide digitally collimated images for radiologists to review [7]. For example, skull radiographs of children should be collimated to show only the head and not include the neck and upper chest. Routine quality

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control on images can eliminate this extraneous exposure in children. (5) Avoid repeat and multiphasic exams. More important but often difficult is to avoid repeated exams. With the increasing availability of electronic health records, one can access previous studies or studies performed at a different clinic and hence avoid repeat studies [8]. Especially in children, head CT is routinely performed to evaluate trauma, and often when patients are transported to a different hospital or clinic, head CT is repeated for lack of availability of prior studies. Patient condition permitting, a reasonable attempt should be made to obtain the outside study. This does not mean that all repeat studies should be refused. If clinically justified, particularly with a critically injured patient or changing clinical exam, a repeat study may be necessary. No exam should be refused just because there is a prior examination, but rather for lack of justification. Because radiation doses from CT studies are usually higher than those from radiography and fluoroscopy studies [3], considerable attention has been given to reducing CT doses. In the last few years, technological advances have been designed to reduce CT dose without jeopardizing image quality. These advances include automatic tube current modulation, highefficiency CT detectors, volume CT scanners (256–320-row multi-detector CT), dual-source CT scanners, dynamic collimation, iterative reconstruction and use of lower tube voltages.

Strategies to reduce CT dose Automatic tube current modulation techniques Among technological advances, the most radiation dose reduction is possible with the automatic tube current modulation or dose modulation strategy. Prior to this innovation, tube current remained constant for the entire CT gantry rotation, irrespective of patient size. The opportunity to reduce dose is achieved by varying the tube current based on patient thickness. Even though this was not so beneficial in children in earlier generations, with recent maturation of the technology it is now possible to achieve significant dose reduction by simply turning on the dose modulation option. However, it is important to understand how such dose modulation strategies operate, because each CT manufacturer implements dose modulation differently [9, 10]. For example, dose modulation in CT scanners operates with the user selecting noise index levels (Noise Index Auto mA from GE Healthcare, Waukesha, WI; Standard Deviation Sure Exposure from Toshiba Medical Systems, Otawara-shi, Tochigi-ken, Japan), which determine the range of dose modulation based on the desired noise level

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in the acquired image. On the other hand, Siemens uses the reference-effective mAs concept, which basically requires users to select a reference-effective mAs for dose modulation. By doing so, the scanner has built-in range of tube current to operate for the selected reference-effective mAs (Reference mAs, CARE Dose 4D from Siemens Healthcare, Erlangen, Germany; mAs/slice, Z-DOM from Philips Healthcare, Best, the Netherlands). Singh et al. [11] reported a 50–75% dose reduction with dose modulation techniques stratified for different clinical indications in chest and abdomen CT in children. One caveat to a successful implementation of dose modulation strategy is the requirement to position the patient at the center of the CT gantry (iso-center) and make sure the user-defined parameters are well understood and kept at optimal settings. Tube voltage selection Choosing appropriate tube voltage for CT studies has the double advantage of lowering radiation dose to the child and also improving image contrast. This is especially true with regard to small- and medium-size patients [12]. Current CT scanners are calibrated to select tube voltage as low as 70 kV. Because dose varies as a power of tube voltage, a large portion of dose savings can be realized with use of lower tube voltage techniques. Volume CT scanners Multi-detector CT (MDCT) scanners are often classified based on the number of detector rows in the z-direction, which indicates the volume covered per CT gantry rotation. With the goal of covering a larger anatomical area per gantry rotation, there has been a race to innovate large-volume CT scanners. Among them, the 320-detector row MDCT scanners cover 160 mm of anatomy, enabling large-volume scanning in shorter time [13]. These scanners have a particular advantage in scanning children because a large scan area can be covered in a relatively short time and overlap between scans can be avoided. When utilizing a volume MDCT scanner, certain pediatric CT protocols can cover the entire scan region (e.g., a chest CT in an infant) in a single scan, which has the advantage of minimal scan overlap and lessens the need for sedation. Volume CT scanners have the potential to reduce overall exam time and minimize patient motion, both of which are critical in quality pediatric imaging. Dual-source CT scanners Dual-source CT scanners have two X-ray tubes mounted at a 90° angle, with corresponding CT detectors at opposite ends. These scanners have the capability to cover larger areas in relatively shorter periods of time because of the fast table

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speed [14]. Dual-source CT scanners provide a greater opportunity for optimal pediatric imaging because rapid table speed (43 mm/gantry rotation) accompanied by high pitch values can cover larger anatomical areas (up to 120 mm) in less than 1 s. Shorter scan times are critical in pediatric populations because shorter times help to minimize patient motion, reduce the need for patient sedation, and create opportunities to reduce the amount of contrast material used for certain CT protocols. Dual-source CT scanners also enable users to acquire dualenergy data by setting each X-ray tube at a different tube voltage. The applications of dual-energy CT are slowly proliferating in the clinical arena. In addition, new-generation dual-source CT scanners are equipped with very low noise detectors that allow the acquisition of CT data with low-scan techniques, resulting in significant reduction in pediatric dose. Iterative reconstruction Conventionally, CT images are reconstructed using the filtered back-projection (FBP) method. The image quality is directly impacted by the scan settings, especially the tube current. The tube current (mA) and the exposure time product (mAs) have a linear relationship with radiation dose. Therefore if the tube current is reduced to lower radiation dose, it results in increased noise and artifacts that degrade image quality and render the CT image suboptimal. To overcome these limitations and to improve image quality at low-dose settings, CT manufacturers have introduced new reconstruction algorithms. Iterative reconstruction methods acquire CT data at a much lower tube current and process raw data to lower image noise by performing multiple iterations with the goal of preserving image quality [15]. Iterative reconstruction techniques have been shown to reduce radiation by as much as 40–80% while maintaining diagnostic quality. A caveat of using iterative reconstruction techniques is that they require a period of adjustment by readers to adapt to the image appearance. Manufacturers provide different strengths of iterative reconstruction (IR) levels that each user has to become familiar with prior to adopting IR images routinely. One option is to reconstruct CT images using both the conventional reconstruction method (FBP) and the IR method and compare the image quality between the sets before choosing the most appropriate IR strengths. With increasing computing power, the reconstruction of CT data into various image datasets is no longer a major issue. Multiple CT scans within a CT exam Multiples scan series within a CT exam is of greater concern because each series adds to the total radiation dose per study. For example, performing non-contrast and contrast-enhanced

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head CT results in double the radiation exposure to pediatric patients. The need for dual-phase or multiple-phase studies is to be evaluated extra-carefully, and these studies should be performed only when the clinical need is well justified. In order to lower total dose from such studies, many strategies are under consideration. One such is the use of dual-energy CT data, which has been shown to lower radiation dose during multiple scan series by reconstructing virtual non-enhanced images at image quality similar to that of true non-enhanced images, with the potential to avoid additional scans [16]. Limited sequence versus entire scan sequence Certain populations of children undergo frequent CT scans for evaluation of specific clinical conditions. One such group is children with shunted hydrocephalus, who undergo frequent CT scans for the evaluation of potential shunt malfunction. In these children, the use of limited-sequence scans rather than entire scan sequences can reduce radiation dose significantly. One study of limited-sequence scanning has shown that there are no significant differences between the entire head CT scan and a limited CT scan [17]. MR imaging is another option that is increasingly replacing CT scans for evaluation of shunt malfunction [18]. However, use of MR scans depends on the availability of MR scanners, use of specially designed rapid MR imaging protocols and proper selection of patients. Radiation dose reports MDCT scanners display CT dose for each exam. The basic radiation dose descriptors in CT are the CT dose index volume (CTDIvol) and dose-length product. With these dose descriptors, one can estimate effective dose based on published conversion factors for standard CT scans such as head, neck, abdomen and pelvic CT scans [19]. For assessing effective dose estimations for children one needs to correct for the pediatric size, because the displayed dose descriptors are based on standard adult phantom sizes (16-cm-diameter acrylic head phantom and 32-cm-diameter abdomen phantom) [19]. Also, because the dose received is dependent on both patient size and scanner radiation output (CTDI vol ), the American Association of Physicists in Medicine (AAPM) has recommended the size-specific dose estimation method, which takes into account patient size [20]. This is particularly important for children because the displayed dose descriptors are based on only two phantom sizes. In addition, the radiation dose displays are often saved as an image file. Also available are the Digital Imaging and Communications in Medicine structured dose reports, which provide detailed information on various scan parameters for each CT series. Structured dose reports are a valuable source of information for auditing and internal quality-control purposes.

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CT protocol reviews CT protocols are to be reviewed routinely, with the emphasis on lowering dose without jeopardizing image quality. In addition, protocols can be compared with the specific CT protocols developed by vendors, campaign Web sites such as Image Gently and Image Wisely, and the AAPM Web sites [21].

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Radiation safety of staff Radiation safety for staff requires following the principles of radiation protection. The three pillars of radiation protection are time, distance and shielding. Except for fluoroscopic procedures, in CT and radiographic studies, no one other than the patient should be inside the X-ray room during imaging. Therefore, most of the discussion about radiation safety for staff applies to fluoroscopy safety. A number of excellent publications detail the various steps one can take to minimize radiation risks during fluoroscopy procedures [22, 23].

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Conclusion 14.

Recent advances in CT technology and X-ray detector technologies accompanied by concerns about radiation dose and risks are now enabling radiologists to perform pediatric imaging including pediatric CT scans at remarkably low doses without compromising the quality of the scans. By employing many of the strategies discussed in this article, one can achieve radiation dose reduction while maintaining diagnostic examination quality.

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18. Conflicts of interest The author has no financial interests, investigational or off-label uses to disclose. 19.

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