Magnetic resonance imaging (MRI) refers to a method for taking a picture by using magnets and radio waves. In the realm of human memory, MRI can be used to look at the anatomy of the brain while someone is still alive, without any invasive procedures and without any radiation (like is used with X-ray scans or computed tomography [CT] scans). When brain damage occurs, the damaged tissue can be observed on an MRI scan. Based on which part of the brain is damaged and on the pattern of memory impairment, one can infer the function of the damaged or missing brain tissue.
Understanding how you can get a picture of the brain with MRI using magnets and radio waves involves some basic physics. The first thing to understand is that all brain tissues can be thought of as liquids with varying amounts of other molecules mixed in. For example, cerebrospinal fluid is almost entirely water, grey matter is composed largely of water and protein, and white matter is composed mostly of fat and contains little water. As you may remember from chemistry class, the symbol for water is H2O (that means there are two hydrogen atoms and one oxygen atom). Because water is present in almost every part of the body, there are many hydrogen atoms in the body. Each hydrogen atom has one positively charged proton and one negatively charged electron. This proton forms the basis of all routine MRI imaging.
Here is how it works. A proton is like a small planet that rotates on its own axis. This spinning electrical charge creates a tiny magnetic field in the direction of the axis of the spinning proton. The axes of all hydrogen atoms point in all possible directions, so people do not experience themselves as having any magnetic charge. However, the MRI machine itself is a very strong magnet (it is actually 30,000 to 60,000 times stronger than the earth's magnetic field). When someone is in an MRI machine, all of his or her protons begin to point in the same direction as the big MRI magnet. Now the person becomes a magnet.
To measure the tiny magnetic fields in every part of the brain, you have to send in a radio wave to tip the magnetic fields of the protons on their side (90 degrees from the direction of the MRI's magnetic field). Once the protons are on their side, it becomes possible to measure the magnetic signal with sensors in the MRI machine. What is measured is the time it takes the protons to reorient back to the direction of the MRI's magnetic field. Certain types of tissue with a lot of water (e.g., cerebrospinal fluid or grey matter) take a long time to reorient themselves to the MRI's magnetic field. Other types of tissue (e.g., white matter) reorient themselves faster. Before scanning, the brain is divided up into little virtual cubes called voxels of about 1 millimeter by 1 millimeter by 1 millimeter each. The sensors measure how long the reorientation process takes for every voxel in the brain. For these types of scans, cerebrospinal fluid shows up black, grey matter shows up grey, and white matter shows up white. Pathological or diseased tissue frequently has a higher water content than the surrounding tissue. More water means more hydrogen protons, so the MRI signal is different for healthy tissue than for unhealthy tissue.
MRI scans can be acquired in one of three planes. An axial plane is horizontal, as if you were looking at the brain from the top. A coronal plane is vertical, as if you were looking at the brain from the front. A sagittal plane is another vertical plane, but this time as if you were looking at the brain from the side. MRI scans are particularly useful for memory researchers who use the anatomical mapping as a basis for locating areas of activity in functional magnetic resonance imaging (fMRI) scans. These scans allow researchers to make inferences about areas that are important for both memory encoding and memory retrieval.
GLOSSARY Magnetic resonance imaging (MRI) is a procedure that utilizes the magnetic resonance signal produced by the protons of tissue...
We’ve been able to harness the power of magnetism in so many aspects of our everyday lives, right down to the ability to look inside the human body
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