On some mornings Rose Chuderewicz, 80, can’t remember how to get dressed. She writes notes to remind herself to do daily tasks, then forgets to read them.

Her memory loss could have resulted from a stroke, mental illness, Parkinson’s, or Alzheimer’s, a disease the World Health Organization says affects about 18 million people globally and is likely to double by 2025. Doctors at the University of Pittsburgh, using a novel brain imaging procedure, confirmed Alzheimer’s, the relentless destroyer of brain tissue causing about two-thirds of all dementias.

The researchers are pioneering a way to peer inside the living brain to prove the mind-robbing ravages of the illness. A radioactive dye they invented attaches to proteins called amyloid that define the disease and highlights them on an imaging device. The dye, backed by General Electric Co., holds promise of diagnosing the disease early for better treatment and showing drugmakers if a medicine is slowing or reversing brain damage.

“Alzheimer’s disease will bankrupt the health-care system as we know it right now without a therapy,” a neurologist at the Mayo Clinic in Rochester, Minn., who is also on an advisory council to the Chicago-based Alzheimer’s Association, Ronald Petersen, said. “And the earlier you can get in to treat, the better. But how do you know before there are symptoms?”

The dye, called Pittsburgh Compound B, or PiB, may be one answer. Though it isn’t yet approved by the U.S. Food and Drug Administration for widespread use, it’s being tested on Ms. Chuderewicz and others who have the forgetfulness, disorientation, and poor judgment that leads doctors to suspect Alzheimer’s.