Structural vs. Functional MRI, and Why it Matters
Structural MRI shows how brain tissue looks; functional MRI shows what it does
Have you ever had an MRI scan? MRI, short for magnetic resonance imaging, uses a huge magnet to “take pictures” of the inside of your body. MRI is less risky than other imaging methods, such as CAT scans (which use X-rays) and PET scans (which use a radioactive chemical).
If you’ve gotten an MRI, you probably had to remove any metal on your body and lie still in a scanner. You heard a series of sounds as the magnet sent brief pulses of force to your body. The image shows how fast the atoms in your body return to normal after disruption by the magnetic force. Different types of tissues — white matter, gray matter, skin, bones, etc. — do so at different speeds, which creates differences in brightness.
Each magnetic pulse produces an image of a “slice” of your brain. It takes many slices to show your entire brain. MRI creates 3 differently shaped kinds of slices, as shown below.
This sort of imaging is called structural MRI, because it shows the physical structures of your brain. Doctors use structural MRI (MRI for short) to see the size and location of strokes, tumors, traumatic injury, bleeding, and atrophy.
However, structural MRI doesn’t tell you what the brain is doing.
It doesn’t show how neurons are firing or what neurotransmitters are released. It can’t detect conditions where there is no damage, yet the brain functions differently — such as migraines, chronic pain, chronic fatigue, or mental illness. To see such things, you need functional MRI (fMRI).
Like structural MRI, functional MRI uses magnetic pulses to produce images of brain slices. Unlike structural MRI, it shows where the brain is especially active.
The brain images in news articles typically come from fMRI.
fMRI measures changes in brain activity indirectly, based on blood flow. Brain areas that are more active use more glucose, so more blood flows there to deliver it. Because blood flow is much slower than neural firing, changes in brain activity show up in fMRI after a several-minute delay.
Structural and functional MRI differ in other important ways.
Structural MRI doesn’t require much interpretation or data analysis; functional MRI does. Structural MRI results are visible on the scan itself, while fMRI results are created later by humans.
Functional MRI is like a layer added on top of a structural MRI scan. For each point in the structural MRI scan, fMRI gives a number representing amount of blood flow there. Using computer programs, researchers superimpose these numbers onto the structural MRI slices, creating new maps of the fMRI data. They create one map for each condition in an experiment. If the experimental conditions are “reading” versus “doing nothing,” researchers create a “reading” map and a “doing nothing” map.
Next, researchers use statistics to compare blood flow in different conditions. They ask questions like these: where is there a statistically significant difference between activity while reading versus while doing nothing? Which areas are significantly active at the same time and probably communicating with each other?
Finally, researchers create a new visual to represent the statistical results for each statistical comparison. The resulting image looks like the one below. A color spectrum represents the amount of difference between conditions (for example, the difference between “reading” and “doing nothing”). In other words: where structural MRI shows brain tissue, fMRI represents numbers. And, unlike structural MRI, fMRI is a comparison.
The most important thing to remember: the brightly colored blobs shown in news articles aren’t photos of brain activity. They are several times removed, created to display the results of a statistical analysis.
Why does the difference matter?
You’ve probably never experienced an fMRI scan. Structural MRI is used in medical practice, while functional MRI is used only in research. (There’s one exception: some surgeons use functional MRI to make sure that they’re not damaging parts of the brain used for language).
Functional MRI rarely sees clinical use because technical limitations make it impractical. Until recently, to get meaningful information about an individual patient, you had to collect as much as 16 hours of data.
People have wanted this situation to change for over 20 years, predicting fMRI will “bring psychiatry into the 21st century”.
That might actually happen soon. At a neuroscience conference on October 7, the eminent neuroscientist Damien Fair claimed that functional MRI can be used clinically now, and doctors will start adopting it soon.
We might soon have an objective measure of invisible brain conditions — such as migraines, chronic pain, or mental illness. That could make it easier to diagnose them, treat them, and take them seriously.
Structural vs. Functional MRI
To summarize:
Structural MRI shows the physical structures of your brain; functional MRI shows their amount of activity.
Structural MRI results are visible on the images the MRI machine produces; fMRI results are created later by humans.
Structural MRI shows a physical thing (brain tissue), fMRI represents numbers.
Structural MRI shows one thing (brain tissue), while fMRI represents a comparison between conditions.
We need both to understand the brain.
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Writing this post was a nostalgic experience. I used to enjoy doing fMRI analyses.
What brought you here? Was anything unclear? Did this post raise new questions? Do you have an experience with MRI to share? Leave a comment below.
A future post will compare the 2 main types of fMRI — task based and resting state. Stay tuned!