BrainTools: Magnetic Resonance Imaging (MRI)


Scientists have a lot of different tools for studying the brain. BrainTools is a series that aims to give you some background information on the different gadgets and methods that are being used by scientists today.

Breaking news: our heads are not transparent!

Because of this inconvenient little fact, it isn’t very easy to see what our brains look like. For a long time, the only way to get a peek at the brain was to remove the top of the head with a sharp knife. Needless to say, most of the earliest images of brains came from the heads of dead people. Fortunately, today we have a number of noninvasive options for looking inside our heads.

One of the most popular ways to take pictures of the brain is with magnetic resonance imaging, or MRI. As the name implies, MRI works using a powerful magnet that’s about 200 times more powerful than the magnets stuck on your refrigerator. The MRI magnet is shaped like a donut, and whatever you’re trying to take a picture of needs to fit inside the hole in the middle.

How does a giant magnet create an image of the brain? It only works because people are made out of (mostly) water. A lot of water means a lot of hydrogen atoms, and a lot of hydrogen atoms means a lot of positively charged protons (each hydrogen atom has one proton in its nucleus). The magnetic field inside the MRI scanner is so powerful, that it causes a lot of the protons to spin in sync with each other, in alignment with the magnetic field. Once all the protons are spinning together, the scanner sends radio pulses at a specific frequency, causing the protons to flip their spins, creating a varying magnetic field. The physics and equations behind all of this are way above my head, but the scanner measures the changes in the magnetic field, and a computer can transform that data from the scanner into pictures.

(If all that talk about magnetic fields and radio pulses doesn’t make any sense to you, that’s OK - you can still understand MRI studies!)

MRI can be used to study the brain in a number of different ways. Since MRI creates really high-resolution images, scientists can look at the structural differences between people’s brains and see how those differences relate to differences in behavior or personality. For example, a recent study used MRI to look at the brains of criminals diagnosed with antisocial personality disorder (ASPD), and found that certain parts of the brain involved with empathy and social behavior were smaller in the criminals than in non-offenders.

Beyond size and shape, MRI can also be used to measure the thickness of different parts of the brain. At the Cognitive Neurology and Alzheimer Disease Center (CNADC) here at Northwestern, the Super Agers project uses MRI to study the brains of older adults who seem to be more resilient to the cognitive declines associated with healthy aging than the average person. All of the folds in our brain normally get a bit thinner over the course of our lives, as a consequence of healthy aging. Researchers at the CNADC found that the Super Agers have significantly less thinning than average for people of the same age, and had the same amount of thinning as people several decades younger than them.

To give you a little perspective, before MRI, the only way to answer the kinds of questions asked by the previously mentioned studies was to physically remove a brain from a person’s head, meticulously dissect that brain, and manually measure the sizes and masses of the different pieces. The process can take countless hours, and the consequences of mistakes were often irreversible. One misplaced cut, and you were out of luck!

Understanding how MRI can be used to further our understanding of the brain is very important, but if you only take one thing away from this post, make sure it is this: big magnets are very dangerous! MRI magnets can turn any metal object, even something as small as a paperclip or hairpin, into a dangerous projectile. This is why the people who do MRI research make sure to screen people very carefully before letting them near the magnet. For people who pass the screening, MRI is safe and painless (Scientists have raised animals for their entire lives inside a magnetic field as strong as an MRI, and these animals don’t turn out any differently than animals raised under “normal” conditions).

The advent of MRI literally opened up a window into our heads (or whatever part of the body you want to see). While MRI isn’t perfect - just like any camera, it has a hard time capturing an image of something that is moving - it is a significant improvement over older techniques (like X-ray), and far more tolerable for people than surgery. And the principles of MRI can be extended to measure more than just size and shape, but what parts of the brain are actually involved in specific behaviors.

Questions? Thoughts? Leave a comment, or get in touch with me on Twitter, @jimkloet.



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