Mike Albright's NMR Tutorial Home Page
Part 1 - magnetic dipoles
Basic NMR: It has occured to me that some people reading this may not really have a strong NMR background, and since the original idea was to add insight to anyone interested in NMR/MRI/MRS (acronyms), this section is meant to give a quick, very basic overview of NMR physics.
SPIN quantum numbers of Nuclei, Gyroscopes, and North/South bar magnets (like a person scout compass..):
This is the reason that NMR or MRI are possible! Nuclei (the center of atoms) of certian isotopes can act as if they are spinning (i.e., have angular momentum). Isotopes that have an even atomic number and an even mass number have a spin quantum number of zero; I=0 (i.e., have NO angular momentum). Other isotopes have spin numbers which are multiples of 1/2 can be NMR visible! Although any spin larger than 1/2 (i.e. 1, 3/2, 2, 5/2, etc.) have a quadrupole moment, and I am going to have enough trouble with the simple North/South bar magnet analogy. Hence we will stick with 1/2 spins for the moment (no dipole pun intended).
MAGNET + GYROSCOPE = 1/2 SPIN NUCLEUS
(I did say that this was very basic.... didn't I?)
OK, now that we have the basics, let us do a few Gedanken experiments. Remember the science experiment with iron filings on top and a bar magnet under the table? The iron filings aligned with the magnetic lines of flux. Well, now imagine a whole lot of the very tiny compasses (like a Cracker Jack prize size..) all closely packed on a table. Now imagine a really big Bar Magnet under the table with the North pole fartherest from you. What happens to all the compass needles? Now turn the Bar Magnet 90 degress clockwise.
All spin 1/2 nuclei are like the tiny compass needles and will try to align with the North/South bar magnet when these nuclei are put in a very strong magnetic field. However, since the nuclei have angular momentum, they do not align like the compass needles would, but rather they align with a spin wobble like a toy gyroscope (when it is trying to right itself against the force of gravity). This phenomonen is called "precession". The frequency of this precession is related to the strength of the magnetic field, and is on the order of many Megahertz (millions of cycles per second) for the NMR experiment!
Now that we have lots of really tiny spins precessing at a Megahertz rate in a very large static magnetic field, we can do an experiment by flipping almost all the spins South/North in large North/South magnetic field. They should come back to be aligned with the large magnetic field again. But, how can we flip these spins so that they are lined up opposite to the large North/South magnet. It turns out that an oscillating electrical field (a radio-frequency one in this case) has an orthoganal oscillating magnetic field. Think of the RF sine wave as a three dimensional rotating field (look at a slinky from one end to see the circular rotation, then turn it sideways and stretch it to see the sine wave as a projection). If we have an RF frequency that matches the precession frequency then the oscillating magnetic field will couple to the precessing nuclei and RF energy will be used to flip the nuclei. If we do this quickly enough, we can see the nuclei realigning with the large static magnetic field. We 'see' this by listening at the RF frequency that we used to flip the nuclei. As the nuclei realign they give off the absorbed energy in the form of a radio signal which decreases with time! Hence, the name Free Induction Decay (FID).
In actual practice, the base RF frequency is mixed with the returning RF signal to get an audio frequency. The FID is much like what you would see if you put a microphone output into an oscilloscope, and then held a tuning fork next to the microphone and tapped it sharply with a small rubber mallet! Conversely, if you feed the NMR FID audio frequency into an audio amplifier with a speaker, you would hear something like a tuning fork sound (assuming of course that you had a pure single peak signal).
Generate an FID Yourself!!! (Java required!)
