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 When the FDA¹ issued a draft guidance document for imaging-equipment manufacturers in May 2012, Keith Strauss, MSc—a member of the steering committee of the Alliance for Radiation Safety in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physicist at Cincinnati Children’s Hospital Medical Center in Ohio, says that the biggest concerns raised by the FDA draft guidance document involve balancing those recommendations designed to add patient protection with vendors’ concerns about limiting access to imaging equipment and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Strauss says that a significant concern in the draft guidance document was whether it mandates that vendors’ applications specialists command enough specificity about the operation of their companies’ imaging devices in a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.”
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.
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