Ok, so full disclosure. The graphic at the bottom of the home page shows the MRI as a static field of 0 Hz. That is the least interesting part of the story. There is SO MUCH MORE going on here!
Basically, the body is composed of roughly 60% water. Water is composed of 2 hydrogen and 1 oxygen atom. Lets concentrate on the proton of the hydrogen atom. It is spinning on an axis. These axis are randomly oriented. Now, lets put them in a very high, very uniform magnetic field. These axis now align parallel to the field. To differentiate, some align "Parallel" and some align the opposite direction, or "Antiparallel". So, in 2 million atoms, 1 million +1 orient one way, and 1 million -1 orient the other. Net result, in 2 million atoms, 2 have to be different! These two that choose to be different are what makes the difference, and therefore the picture.
So, all these hydrogen protons are happily spinning Parallel or Antiparallel in the magnetic field. We put an antenna around the inside of the bore of the magnet and hit them with a 10 to 15 Kw RF field. (You are the subject of interest inside the bore). The frequency is dependent on the magnetic field intensity. A 1 Tesla field, (10,000 Gauss, or 40,000 times the normal field of the earth), results in a resonant frequency of 42 Mhz.
The subject in a 1T perfectly uniform magnetic field, is hit with 10 KW of RF at 42 Mhz. All of the aligned spinning protons are knocked 90 degrees off axis. When the RF is extinguished, the protons "Precede" back to Parallel or Antiparallel. Upon doing so, they emit a signal at the same frequency they were excited at. (1.5T would be 63Mhz, 2T would be 84Mhz) Those 2 in 2 million that were different are pertinent now.
Hitting a gazillion atoms with RF and getting a gazillion echos back is good if you want to see that there is a body in the magnet, but not much else. Since the resonant frequency is directly related to the magnetic field intensity, we can modify it. If we put a coil of wire inside the magnet's bore, we can put a DC current on it and warp the field. Create a Magnetic Gradient. So, the magnetic field at your head is 1.1T and 0.9T at your feet. That means that the resonant frequency at your head is 46.2 Mhz and the resonant frequency at your feet is 37.8 Mhz. If you excite the body with a spectrum of 37.8 to 46.2 Mhz, the echos will respond likewise, and be mapable to a specific location in the body. A spectrum analysis will reveal this info. Mathematically, this is accomplished by a fourier transform. The first transform gets you from the time / voltage domain to the time / frequency domain.
So, we put gradient coils from head to foot, left and right, and top and bottom. We energize one set of coils to warp the magnetic field and transmit an RF pulse. We energize an other set of coils and listen for echos. We do one fourier transform to translate from time / voltage to time / frequency domain. A second fourier transform translates us from time / frequency to the time / spatial domain. From there, we can generate an image. The technology used here is the same as used in top secret military satellites. So, if you are familiar with this technology, you best keep it secret.
What's all the banging about? Well, when you hit a gradient coil with 100 to 150 amps of DC current, and it is in a 1T magnetic field, it generates a substantial magnetic field opposing the 1T field and tries to move. Violently! Thereby the noise.
So, modern day computers do a lot more tricks. Repulsing at specific times to accentuate certain other materials. Analyzing other frequencies to look for other atomic signatures. The list goes on and ever increases as computer technology expands.
But it is NOT simply a static field!