New MRI procedure measures brain activity in real time
Functional magnetic resonance imaging (fMRI) can be used to measure neuronal activity via blood flow changes in the brain. However, this so-called neurovascular effect only sets in after a delay of a few seconds. Rapid changes in the range of 10 to 100 milliseconds are not recorded.
An international research team led by Samuel Patz from Brigham and Women's Hospital in Boston has now developed an MRI technique that can also image brain processes in the millisecond range. It is not the blood flow that is registered, but another mechanism that has been known for a long time: the biomechanical properties of neurons change depending on their activity. The axons swell, the dendrites contract, and this changes their stiffness.
The new procedure is based on classic magnetic resonance elastography (MRE), which doctors have been using for years to detect disease-related tissue stiffness. The central principle behind this: When sound waves hit tissue, the latter shifts depending on its elasticity. A standard MR scanner can measure this process.
To find out whether MRE can also be used to image rapid changes in stiffness in brain tissue, the researchers anaesthetized laboratory mice, fixed them in a custom-made apparatus and stimulated one of the hind legs with electrical impulses of varying frequency. In this way, Patz and his team wanted to activate brain areas that process pain stimuli. At the same time, the researchers conducted sound waves through the mouse brain. A small MR coil, two centimeters in diameter, picked up the resulting shift. At the highest frequency level, the current pulses alternated between on and off within 100 milliseconds. And indeed, the researchers were able to record the biomechanical changes in brain tissue even under such rapid impulse changes.
Patz and his colleagues believe that the method can also be used in humans. In this way, brain functions could be measured more directly in the scanner than ever before.