Radioactive sprinkles keep machines true

Radioactivity-detecting medical scanners, such as positron emission tomography (PET) imagers, are becoming so sharp-eyed that it’s more challenging than ever to find the limits of the machines’ vision. Doing this requires uniform radiation sources smaller than the minimum-size object the scanners are supposed to discern. Without such verification, uncertainty about the resolution of the machines’ detectors could lead to misinterpretations of scans.