Image Gently Responds to the FDA
One of the leaders driving low-dose, high-quality pediatric imaging is the multisociety advocacy group Image Gently®. When the FDA¹ issued a draft guidance document for imaging-equipment manufacturers in May 2012, Keith Strauss, MSc—a member of the steering committee of the Alliance for Radiation Safety in Pediatric Imaging (the Image Gently Alliance)—responded to governmental questions about emerging best practices with some common-sense answers honed by years in the field: He participated, with a delegation from Image Gently, in Device Improvements for Pediatric X-ray Imaging (a public workshop held by the FDA on July 16 in Silver Spring, Maryland). Strauss, a clinical pediatric-imaging physicist at Cincinnati Children’s Hospital Medical Center in Ohio, says that the biggest concerns raised by the FDA draft guidance document involve balancing those recommendations designed to add patient protection with vendors’ concerns about limiting access to imaging equipment and altering the course of dose-sensitive manufacturing processes already underway. In essence, Strauss says, these boil down to three chief concerns: user training, hardware configuration, and proposed warning labels for equipment used to image pediatric patients. Strauss says that a significant concern in the draft guidance document was whether it mandates that vendors’ applications specialists command enough specificity about the operation of their companies’ imaging devices in a pediatric setting. “What all trainers know how to do is tell you how the controls on the machine change its performance when imaging adults,” Strauss explains. “The challenge is making sure that the operator not only knows what the controls do, but when to apply that change to obtain diagnostic images at properly managed patient doses for adults and children.” Almost every piece of imaging equipment sold will, at some point, be used on children. As dedicated pediatric hospitals account for only about 20% of pediatric imaging performed in the United States, Strauss says, the other 80% is necessarily conducted at facilities (and using equipment) serving patients of all ages. For that reason, he says, applications specialists must teach basic knowledge about the imaging device needed for the proper imaging of both children and adults. “Kids are small; because their bodies are not as thick, they don’t need as much radiation to penetrate the body,” he explains. “A different set of operational choices, compared with the set used in adult imaging, is necessary to allow proper visualization of small details.” He continues, “For example, a smaller focal-spot size delivers a sharper image, lower kilovoltages compensate for the limited contrast of pediatric organs, and shorter exposure times overcome more patient motion in the uncooperative patient. When imaging children, removal of the grid designed to remove scatter radiation reduces patient dose with limited loss of image quality.” Hardware Configuration Some of the options now provided by manufacturers might make their equipment better suited for pediatric imaging, Strauss says. The challenge is that a manufacturer might not understand the pediatric application of an option because the option was developed for a different adult application. “Standard equipment is capable of doing good work in pediatric imaging if it’s configured correctly,” he says. “If it’s not configured correctly for pediatric imaging, the image quality may be better than necessary, with an associated patient dose that is larger than needed.” That’s why, when the FDA’s draft guidance document asks questions about the appropriateness of choosing one body region to categorize pediatric subgroups, Strauss reminds the FDA that children’s body parts increase in thickness at different rates as the children age. “Growth of the abdomen is linear with age, while the head is 90% of adult size by age five,” Strauss says. Manufacturers should determine pediatric patient sizes ranging from 500 g to 120 kg, or premature neonate to adult, as outlined in the draft guidance document. The recommendations also stipulate that “equipment features or settings that are expected to vary depending on patient size should be evaluated for acceptable outcomes,”1 and Strauss agrees. He adds that the FDA has solicited help in identifying size-dependent tests, including automatic brightness control, focal-spot size, filter thickness, tube current, and pulse width. What is unclear from the draft guidance document, he says, is whether equipment users are free to ignore—without FDA approval—vendor-developed (and FDA-approved) pediatric protocols in favor of those developed at their own sites. “What happens if someone like me does a research project, collects data, and shows that doing something in a different way is an improvement?” Strauss asks. Even when researchers publish their findings, he says, “There’s no guarantee that the vendor whose equipment you used is going to pick up on that, spread that message, or take it to heart.” A Universal Phantom The draft guidance document asks whether a large adult-sized phantom and a small pediatric-sized phantom would be sufficient to demonstrate coverage of the entire range of patient sizes. They would not, Strauss says, for step-function algorithms typically used in automatic exposure-control devices. The essential features of a universal attenuation phantom would be a cost-effective polymethyl methacrylate construction with a continuously adjustable thickness. Anthropomorphic phantoms are an additional expense, he says, and are still not capable of demonstrating safe and effective use when image-processing changes are to be evaluated. Strauss says, “I can check the image-acquisition features of imaging equipment with tissue-equivalent phantoms; I cannot check the image-quality algorithms of the equipment with the same phantoms because they do not model the subtle characteristics of human tissue that influence image quality.” He continues, “The draft guidance document recommended putting warning labels on the equipment; this was roundly discouraged by pretty much everybody. The warning label would state that the equipment should not be used for patents of less than a specified age if the manufacturer had not submitted performance data for patients below that age.” Equipment sold off the shelf can do adequate imaging of children if configured properly, Strauss says. The issue is not that this equipment is incapable of doing good pediatric imaging, but it does not typically happen automatically. “You’ve got to know what you’re doing, and it’s best if you have the options that enhance pediatric imaging, which may or may not have been purchased with the unit,” he says. Marilyn J. Goske, MD, is chair of the Alliance for Radiation Safety in Pediatric Imaging and is a professor of radiology and pediatrics at Cincinnati Children’s Hospital Medical Center. She also gave input to the FDA on behalf of Image Gently. She believes that a final guidance document that is too prescriptive could limit access to pediatric imaging, adversely affecting pediatric patients.