One of the main goals in medical imaging is to develop a technique and the technology to view changes in cell activity the brain in real time. While there are several non-invasive medical imaging techniques that can be used to take a snapshot of what is happening in the brain, they all have their limitations. Even the best option currently available, functional magnetic resonance imaging (fMRI), does not detect activity in real time. fMRI measures changes in oxygen levels in the brain and uses that as an indicator of brain activity. While quick, there is typically a 6 second delay between the event that triggered the change and a corresponding change in blood oxygen levels. During those 6 seconds, a lot of brain activity will be missed.
Now a team of researchers have developed a new method of imaging that can track brain activity 60 times faster on a time scale of 100 milliseconds. Rather than measure oxygen levels, the new technique measures changes in tissue stiffness.
Tissue stiffness can be determined using a technique called magnetic resonance elastography (MRE) on an MRI scanner. This technique is most commonly used to measure hardening of the liver but is equally useful for measuring the stiffness of tissues in the brain. It is not possible to measure tissue stiffness directly, but it is possible to measure the speed at which vibrations pass through tissue. The faster the vibrations travel, the stiffer the tissue.
The research started with a plan to study scar tissue in the lung by combining MRE with another MRI scanning method, but there were complications. The researchers decided to use the technique on mouse brains instead.
The researchers noticed the auditory cortex, which is used to hear, was stiffer than other parts of the cortex. The researchers hypothesized that this was possibly due to increased blood flow in response to the noise from the MRI scanner. Gel was put in one ear to dampen the sound and the results were dramatic. The researchers hypothesized that the reduction in the stimulus resulted in a reduction in blood flow, which made the tissue less stiff.
Further investigation revealed the change in tissue stiffness was not due to changes in blood flow. The researchers determined this by switching a stimulus on and off faster than the blood system can respond. They were still able to measure the changes in tissue stiffness. The researchers increased the speed of the stimuli and found it was possible to detect a 10% change in stiffness with changes in stimulus states every 100 milliseconds.
The researchers are now preparing a similar study in healthy human brains. Initial findings suggest that it is possible to measure differences in tissue stiffness in a time frame of around 24 milliseconds.
The researchers hope that with further development their new imaging technique will be invaluable for studying brain diseases such as Alzheimer’s disease, dementia, and multiple sclerosis. Since the technique does not rely on blood flow measurements, it could also be used when fMRI scans are not possible, such as in patients with large brain tumors that interfere with blood flow.
Details of the new imaging technique are detailed in the paper – Imaging localized neuronal activity at fast time scales through biomechanics – which was recently published in the journal Science Advances DOI: 10.1126/sciadv.aav3816