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MRI stands for Magnetic Resonance Imaging. It is a popular imaging technique that can measure signals coming from water inside your body. However, these signals can only be measured when you are inside a magnetic field. Basically, an MRI scanner is a big magnet. That sounds scarier than it is, since you may know that we are always surrounded by the magnetic field of the earth, although the earth’s field is much less strong than a MRI scanner. For comparison, a typical MRI scanner that you find in hospitals has a magnetic field strength that is about 300 times stronger than a typical fridge magnet.
The way in which signals from water in MRI are measured can be compared with how a radio works. Different radio stations will transmit at different frequencies. When you change the settings of the radio, it will change the radio station that you can listen to. In the same way a clinician can change the settings of the MRI scanner so that it can measure the different frequencies of water signals. These different water signals may come from different regions of the body, so that you can measure signals from different types of tissues such as muscle, blood and fat. By using all these properties of an MRI scanner, detailed images of the body can be obtained in a harmless way.
MRI does not require harmful radiation to be disposed in your body. MRI is completely safe, the only thing to watch out for is that magnets attract metals. Just in the same way paperclips will attach themselves to fridge magnets. Therefore it is important to not have anything magnetic on your body before going inside an MRI scanner.