Nuclear magnetic resonance / magnetic resonance imaging

How NMR works: Under natural conditions, certain positively charged nuclei spin, creating a miniature magnetic field around themselves. Only nuclei with nonzero spin quantum numbers are "spin-active" or show resonance. Placed within a strong external magnetic force, nuclei align their spin with the overall magnetic field. NMR takes advantage of this phenomenon by using radio waves to stimulate a response from these nuclei. When these radio frequency (rf) waves hit the spinning nuclei, they stimulate them to tilt away from the applied magnetic field, sometimes causing them to flip spins out of alignment with the imposed magnetic field. When this flip relaxes (reverts to the original alignment), the change in direction is recorded as an electrical spike by an appropriately placed detector linked to a wire coil surrounding the sample (or patient in MRI). This change in orientation of individual spins with respect to the magnetic field is called resonance. The spike is caused by the fact from elementary physics that a moving magnetic field within a wire coil causes an electrical currentIn the context of NMR, nuclear stands for the nucleus of the atom. Recall, the nucleus is the positively charged "center" of an atom. Some atoms have nuclei which "spin" on an axis, examples are hydrogen, carbon, nitrogen, fluorene, and phosphorus. Why is this "spin" important to MRI? Consider hydrogen, with a nucleus of one proton. This nucleus spins around like a top. Since it has a positive charge, as it spins it generates a magnetic dipole along the spin axis, as shown in the figure to the right. In a normal, non-treated sampling of hydrogen atoms, the magnetic dipoles will orientate in random directions. However, if a magnetic field is applied, they will line up with the magnetic field:

I have shown the spinning nucleis' magnetic dipoles in two colors on purpose, because half of the dipoles will be of a higher energy "red" and half will be of a lower energy "blue". Once they are lined up by a magnet, they are zapped with energy in the form of low energy electromagnetic radiation (radiowaves), which causes the blue protons to become as energetic as the red protons:

The lower energy "blue" protons are able to accept one and only one size package (or quantum: quantum chemistry!) of energy -- energy in the form of light, or radiation -- and if that package is given to it, it jumps to a higher energy level, and becomes just like a red.But, there is only room in the higher energy level for half of the high energy dipoles, so half of them must soon give up their energy and drop back down. When they drop back down, they each give off a tiny amount of energy:

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