Ionizing radiation from medical scans may be implicated in Alzheimer’s disease, according to a Danish study published in Oncotarget.

People receive about half of their annual average dose of 3.1 millisieverts from medical procedures and nuclear imaging, while the other half is natural background radiation from space, soil, water and materials like radon. The amount of medical imaging received by U.S. patients has been growing rapidly over the past 20 years, doubling or even tripling for some modalities.

The number of CT scans has nearly doubled, and approximately one-third of all diagnostic CT exams are scans of the head.

While most data on radiation damage is linked to accidental high-dose exposures, new findings from the University of Southern Denmark show that low doses of radiation may induce molecular brain changes resembling the pathology associated with Alzheimer’s.

“All these kinds of exposures are low dose and as long as we talk about one or a few exposures in a lifetime I do not see cause for concern,” said author Stefan J. Kempf. “What concerns me is that modern people may be exposed several times in their lifetime and that we don't know enough about the consequences of accumulated doses.”

The Danes exposed mice to chronic low-dose-rate radiation treatments, finding the dose rates were capable of targeting the neurons within synapses, eroding their ability to transmit messages. Synapse loss is a key early indicator of Alzheimer’s and has a strong correlation with the severity of dementia.

The mice showed particularly severe neurodegeneration in the hippocampus, a region that plays a role in formation of new memories. Shrinkage of the hippocampus is another strong signal of Alzheimer’s, as it is especially vulnerable to damage during early stages.

“It is crucial that we investigate the potential factors behind this disease,” says Kempf.