Basic Science

How do brain scans work?

As far as we know, there’s only one thing in our solar system sophisticated enough to study itself: the human brain. But this self-investigation is incredibly challenging; a living brain is shielded by a thick skull, swaddled in layers of protective tissue, and made up of billions of tiny, connected cells.

That’s why it’s so difficult to isolate, observe, and understand diseases like Alzheimer’s. So how do we study living brains without harming their owners? We can use a trio of techniques called EEG, fMRI, and PET. Each measures something different and has its own strengths and weaknesses, and we’ll look at each in turn. First is EEG, or electroencephalography, which measures electrical activity in your brain.

As brain cells communicate, they produce waves of electricity. Electrodes placed on the skull pick up these waves, and differences in the signals detected between electrodes provide information about what’s happening. This technique was invented almost 100 years ago, and it’s still used to diagnose conditions like epilepsy and sleep disorders. It’s also used to investigate what areas of the brain are active during learning or paying attention.

EEG is non-invasive, relatively inexpensive, and fast: it can measure changes that occur in just milliseconds. Unfortunately, it’s hard to determine exactly where any particular pattern originates. Electrical signals are generated constantly all over the brain and they interact with each other to produce complex patterns. Using more electrodes or sophisticated data-processing algorithms can help.

But in the end, while EEG can tell you precisely when certain activity occurs, it can’t tell you precisely where. To do that, you’d need another technique, such as functional magnetic resonance imaging, or fMRI. fMRI measures how quickly oxygen is consumed by brain cells. Active areas of the brain use oxygen more quickly.

So watching an fMRI scan while a person completes cognitive or behavioral tasks can provide information about which regions of the brain might be involved. That allows us to study everything from how we see faces to how we understand what we’re feeling. fMRI can pinpoint differences in brain activity to within a few millimeters, but it’s thousands of times slower than EEG. Using the two techniques together can help show when, and where, neural activity is occurring.

The third, even more precise, technique is called positron emission tomography and it measures radioactive elements introduced into the brain. That sounds much scarier than it actually is; PET scans, like fMRI and EEG, are completely safe. During a PET scan, a small amount of radioactive material called a tracer is injected into the bloodstream, and doctors monitor its circulation through the brain.

By modifying the tracer to bind to specific molecules, researchers can use PET to study the complex chemistry in our brains. It’s useful for studying how drugs affect the brain and detecting diseases like Alzheimer’s. But this technique has the lowest time resolution of all because it takes minutes for the tracer to circulate and changes to show up. These techniques collectively help doctors and scientists connect what happens in the brain with our behavior.

But they’re also limited by how much we still don’t know. For example, let’s say researchers are interested in studying how memory works. After asking 50 participants to memorize a series of images while in MRI scanners, the researchers might analyze the results and discover a number of active brain regions.

Making a link between memory and specific parts of the brain is an important step forward. But future research would be necessary to better understand what’s happening in each region, how they work together, and whether the activity is because of their involvement in memory or another process occurring simultaneously.

More advanced imaging and analysis technology might one day provide more accurate results and even distinguish the activity of individual neurons. Until then, our brains will keep measuring, analyzing, and innovating in pursuit of that quest to understand one of the most remarkable things we’ve ever encountered.

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