In order to describe the magnetic properties of a hydrogen nucleus, we refer to its magnetic moment, which is an indication of how strong a magnet it is. Every hydrogen nucleus in the universe (as far as we know) has the same magnetic moment and spins with the same speed and direction. Typically in water, or lipids, or in any form, the hydrogen nuclei are randomly oriented and their individual magnetic moments cancel each other out. However, when placed inside the strong magnetic Ÿeld of an MRI system (which we will call B0), the nuclei tend to align with the magnetic Ÿeld. Now, it is important to keep in mind that this is only the hydrogen nuclei aligning; the orientation of chemical bonds and entire molecules are not a˜ected, nor is the motion of the molecules. A good analogy for this situation is a compass needle, which tends to point north (Figure 3.2).  e compass itself is not pulled north; just the needle orientation is a˜ected.