While the brain shrinks with age, brain cell density remains constant in cognitively normal adults, according to images from the world's most powerful medical magnetic resonance imaging (MRI) machine.
The images provide the first evidence that in normal ageing, cell density is preserved throughout the brain, not just in specific regions, as previous studies on human brain tissue have shown, researchers said.
The findings also suggest that the maintenance of brain cell density may protect against cognitive impairment as the brain gradually shrinks in normal ageing.
The images were made at University of Illinois at Chicago (UIC) with a powerful 9.4 Tesla MRI, the first of its kind used in human imaging.
The 9.4 Tesla magnetic field is more than three times stronger than that of a typical MRI machine in a doctor's office and is currently approved only for research.
"The information provided by these 9.4-Tesla scans may be very useful in helping us to detect tiny losses of brain cells and the reduction in cell density that characterises the early stages of neurodegenerative diseases that can take decades to develop before symptoms appear, like Alzheimer's disease," said Dr Keith Thulborn, director of MRI research in the UIC College of Medicine and lead author of the study.
"If we can identify when Alzheimer's pathology starts, the efficacy of new drugs or other interventions to slow or prevent Alzheimer's disease can be tested and monitored when the disease starts, instead of after it's developed for 20 or 30 years and becomes clinically apparent," said Thulborn.
Although neuroscientists have known for long that the brain shrinks with age, it was believed that the loss in volume was due to the loss of brain cells.
This was disproven by studies that showed the neurons themselves shrink, while the number of cells remains the same in normal older adults.
Thulborn and colleagues scanned the brains of 49 cognitively normal adults ranging in age from 21 to 80.
According to the researchers, the 9.4 Tesla MRI measures sodium ions, which are less concentrated by several orders of magnitude than the fat and water molecules detected with standard MRI.
Sodium ions are present throughout the body and pumped in and out of neurons to generate the electric potentials needed to spark nerve impulses.
Sodium concentrations in the brain reflect neuron density. Areas of low sodium concentration indicate lots of neurons packed tightly together, while higher sodium concentration can indicate more space between cells - or the loss of cells, as in the case of neurodegenerative diseases.
"We can use the 9.4 Tesla to look at brain cell loss in real time in patients experiencing stroke, or to see whether chemotherapy for brain tumours is working in higher resolution that is just not available using the current 3 Tesla clinical scanners," Thulborn said.
The study was published in the journal NMR in Biomedicine.
