*The sample (we will consider hydrogen atoms because that is what a majority of the human body is composed of) has many protons, each of which have a natural alignment within the body.
*The MRI machine scans section by section and in each section there are a certain number of voxels (volume units) and in each voxel there are a certain number of protons. The computer studies the protons in each voxel and compiles the results.
This shows a section with a thickness, Thk, and one of its MANY voxels, each with a volume of about 3 cubic millimeters.
*When the sample is placed within the magnetic field of the MRI scanning machine, approximately all of the protons will line up parallel or anti parallel to the direction of the magnetic field and effectively "cancel" each other out. However, in each voxel there is going to be one or two protons that are "left over" and these protons are the ones from which the computer compiles its data.
In this voxel, there is one "left over" proton. Since there are so many protons in the human body, the data from the one or two "left over" protons per voxel is enough for the MRI computer to produce an image.
*The MRI machine's magnetic field will cause these protons to have magnetic moments and they will precess about the axis of the magnetic field. The rate at which they precess is called the Lamour frequency.
This shows the proton of the nucleus precessing about the axis of the magnetic field.
*The Lamour frequency of a proton is dependent on the type of atom from which it comes and on the strength of the external magnetic field acting on it (which would be the combined magnetic field of the constant and changing magnetic fields of the MRI machine).