How can MRI pictures show slices through an object? And how do you get an image from using a magnet?
MRI images show where hydrogen nuclei (protons) are located in a person’s body. Protons are magnetic particles that have only two possible states in a magnetic field: aligned with the field or aligned against the field (also called “anti-aligned”). This limited range of alignments is the result of quantum physics. Normally, the protons in a person’s body are equally divided between aligned one way and aligned in the opposite way. But when a person is placed in a strong magnetic field, the protons in their body tend to align with the magnetic field and the distribution of aligned and anti-aligned protons shifts. There are then somewhat more aligned protons than anti-aligned protons.
Once there are more aligned protons than anti-aligned protons, it becomes possible to flip them about. Flipping these protons from aligned to anti-aligned takes energy and this energy can be provided by a radio wave. But not just any radio wave will do: its frequency must be just right in order to provide the proper amount of energy or the proton won’t flip. When the right radio wave is provided, some of the aligned protons will flip to become anti-aligned. This flipping of protons can be detected by a sensitive radio receiver.
By placing the person in a non-uniform magnetic field and by adjusting the frequencies and timings of the radio waves, an MRI device can determine where protons are located in the person’s body to with a few millimeters. A computer records where the protons are and then displays information about them as cross sectional images. For example, the computer can display a dense concentration of protons as white and a region with few protons as dark. MRI is particularly good at imaging tissue because tissue contains lots of hydrogen atoms and their protons.