|MLA Citation:||Bloomfield, Louis A. "Question 644"|
How Everything Works 22 Jan 2018. 22 Jan 2018 <http://howeverythingworks.org/print1.php?QNum=644>.
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.