A mainstay of monitoring progress in Perthes is with x-rays. This often causes parents to wonder if there is a significant threat of harm from the radiation exposure of the x-rays. The Image Gently campaign was created by the Society for Pediatric Radiology, American Society for Radiologic Technologists, and others to address just that!
The goal in medical imaging is to weigh the costs vs the gains, and to minimize the risks where possible. In the case of Perthes and traditional x-rays this means using the appropriate dose for the size of patient and body part and achieving the best images with the least amount of repeat images. This may mean careful explanation of the procedure and even immobilizing a child to decrease movement.
Ultimately, x-rays are accessible and reliable when it comes to standardized interpretation. When you have a small x-ray study, such as the AP and Lat used to evaluate Perthes, you receive about 0.01 mSv. We all receive background radiation from the atmosphere daily. In fact you absorb 4 times the radiation when you go on a flight than when you have an x-ray!